DOKK / manpages / debian 10 / socat / procan.1.en
socat(1) socat(1)

socat - Multipurpose relay (SOcket CAT)

socat [options] <address> <address>
socat -V
socat -h[h[h]] | -?[?[?]]
filan
procan

Socat is a command line based utility that establishes two bidirectional byte streams and transfers data between them. Because the streams can be constructed from a large set of different types of data sinks and sources (see address types), and because lots of address options may be applied to the streams, socat can be used for many different purposes.

Filan is a utility that prints information about its active file descriptors to stdout. It has been written for debugging socat, but might be useful for other purposes too. Use the -h option to find more infos.

Procan is a utility that prints information about process parameters to stdout. It has been written to better understand some UNIX process properties and for debugging socat, but might be useful for other purposes too.

The life cycle of a socat instance typically consists of four phases.

In the init phase, the command line options are parsed and logging is initialized.

During the open phase, socat opens the first address and afterwards the second address. These steps are usually blocking; thus, especially for complex address types like socks, connection requests or authentication dialogs must be completed before the next step is started.

In the transfer phase, socat watches both streams’ read and write file descriptors via select() , and, when data is available on one side and can be written to the other side, socat reads it, performs newline character conversions if required, and writes the data to the write file descriptor of the other stream, then continues waiting for more data in both directions.

When one of the streams effectively reaches EOF, the closing phase begins. Socat transfers the EOF condition to the other stream, i.e. tries to shutdown only its write stream, giving it a chance to terminate gracefully. For a defined time socat continues to transfer data in the other direction, but then closes all remaining channels and terminates.

Socat provides some command line options that modify the behaviour of the program. They have nothing to do with so called address options that are used as parts of address specifications.

Print version and available feature information to stdout, and exit.
Print a help text to stdout describing command line options and available address types, and exit.
Like -h, plus a list of the short names of all available address options. Some options are platform dependend, so this output is helpful for checking the particular implementation.
Like -hh, plus a list of all available address option names.
Without this option, only fatal and error messages are generated; applying this option also prints warning messages. See DIAGNOSTICS for more information.
Prints fatal, error, warning, and notice messages.
Prints fatal, error, warning, notice, and info messages.
Prints fatal, error, warning, notice, info, and debug messages.
Logs information about file descriptors before starting the transfer phase.
Writes messages to syslog instead of stderr; severity as defined with -d option. With optional <facility>, the syslog type can be selected, default is "daemon". Third party libraries might not obey this option.
Writes messages to <logfile> [filename] instead of stderr. Some third party libraries, in particular libwrap, might not obey this option.
Writes messages to stderr (this is the default). Some third party libraries might not obey this option, in particular libwrap appears to only log to syslog.
Overrides the program name printed in error messages and used for constructing environment variable names.
Extends the timestamp of error messages to microsecond resolution. Does not work when logging to syslog.
Mixed log mode. During startup messages are printed to stderr; when socat starts the transfer phase loop or daemon mode (i.e. after opening all streams and before starting data transfer, or, with listening sockets with fork option, before the first accept call), it switches logging to syslog. With optional <facility>, the syslog type can be selected, default is "daemon".
Adds hostname to log messages. Uses the value from environment variable HOSTNAME or the value retrieved with uname() if HOSTNAME is not set.
Writes the transferred data not only to their target streams, but also to stderr. The output format is text with some conversions for readability, and prefixed with "> " or "< " indicating flow directions.
Writes the transferred data not only to their target streams, but also to stderr. The output format is hexadecimal, prefixed with "> " or "< " indicating flow directions. Can be combined with -v .
Sets the data transfer block <size> [size_t]. At most <size> bytes are transferred per step. Default is 8192 bytes.
By default, socat terminates when an error occurred to prevent the process from running when some option could not be applied. With this option, socat is sloppy with errors and tries to continue. Even with this option, socat will exit on fatals, and will abort connection attempts when security checks failed.
When one channel has reached EOF, the write part of the other channel is shut down. Then, socat waits <timeout> [timeval] seconds before terminating. Default is 0.5 seconds. This timeout only applies to addresses where write and read part can be closed independently. When during the timeout interval the read part gives EOF, socat terminates without awaiting the timeout.
Total inactivity timeout: when socat is already in the transfer loop and nothing has happened for <timeout> [timeval] seconds (no data arrived, no interrupt occurred...) then it terminates. Useful with protocols like UDP that cannot transfer EOF.
Uses unidirectional mode. The first address is only used for reading, and the second address is only used for writing (example).
Uses unidirectional mode in reverse direction. The first address is only used for writing, and the second address is only used for reading.
During address option parsing, don’t check if the option is considered useful in the given address environment. Use it if you want to force, e.g., appliance of a socket option to a serial device.
If lockfile exists, exits with error. If lockfile does not exist, creates it and continues, unlinks lockfile on exit.
If lockfile exists, waits until it disappears. When lockfile does not exist, creates it and continues, unlinks lockfile on exit.
-4
Use IP version 4 in case that the addresses do not implicitly or explicitly specify a version; this is the default.
-6
Use IP version 6 in case that the addresses do not implicitly or explicitly specify a version.

With the address command line arguments, the user gives socat instructions and the necessary information for establishing the byte streams.

An address specification usually consists of an address type keyword, zero or more required address parameters separated by ’:’ from the keyword and from each other, and zero or more address options separated by ’,’.

The keyword specifies the address type (e.g., TCP4, OPEN, EXEC). For some keywords there exist synonyms (’-’ for STDIO, TCP for TCP4). Keywords are case insensitive. For a few special address types, the keyword may be omitted: Address specifications starting with a number are assumed to be FD (raw file descriptor) addresses; if a ’/’ is found before the first ’:’ or ’,’, GOPEN (generic file open) is assumed.

The required number and type of address parameters depend on the address type. E.g., TCP4 requires a server specification (name or address), and a port specification (number or service name).

Zero or more address options may be given with each address. They influence the address in some ways. Options consist of an option keyword or an option keyword and a value, separated by ’=’. Option keywords are case insensitive. For filtering the options that are useful with an address type, each option is member of one option group. For each address type there is a set of option groups allowed. Only options belonging to one of these address groups may be used (except with option -g).

Address specifications following the above schema are also called single address specifications. Two single addresses can be combined with "!!" to form a dual type address for one channel. Here, the first address is used by socat for reading data, and the second address for writing data. There is no way to specify an option only once for being applied to both single addresses.

Usually, addresses are opened in read/write mode. When an address is part of a dual address specification, or when option -u or -U is used, an address might be used only for reading or for writing. Considering this is important with some address types.

With socat version 1.5.0 and higher, the lexical analysis tries to handle quotes and parenthesis meaningfully and allows escaping of special characters. If one of the characters ( { [ ’ is found, the corresponding closing character - ) } ] ’ - is looked for; they may also be nested. Within these constructs, socats special characters and strings : , !! are not handled specially. All those characters and strings can be escaped with \ or within ""

This section describes the available address types with their keywords, parameters, and semantics.

Opens <filename> with creat() and uses the file descriptor for writing. This address type requires write-only context, because a file opened with creat cannot be read from.
Flags like O_LARGEFILE cannot be applied. If you need them use OPEN with options create,create.
<filename> must be a valid existing or not existing path. If <filename> is a named pipe, creat() might block; if <filename> refers to a socket, this is an error.
Option groups: FD,REG,NAMED
Useful options: mode, user, group, unlink-early, unlink-late, append
See also: OPEN, GOPEN
Forks a sub process that establishes communication with its parent process and invokes the specified program with execvp() . <command-line> is a simple command with arguments separated by single spaces. If the program name contains a ’/’, the part after the last ’/’ is taken as ARGV[0]. If the program name is a relative path, the execvp() semantics for finding the program via $PATH apply. After successful program start, socat writes data to stdin of the process and reads from its stdout using a UNIX domain socket generated by socketpair() per default. (example)
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty, setsid, pipes, login, sigint, sigquit
See also: SYSTEM
Uses the file descriptor <fdnum>. It must already exist as valid UN*X file descriptor.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: STDIO, STDIN, STDOUT, STDERR
(Generic open) This address type tries to handle any file system entry except directories usefully. <filename> may be a relative or absolute path. If it already exists, its type is checked. In case of a UNIX domain socket, socat connects; if connecting fails, socat assumes a datagram socket and uses sendto() calls. If the entry is not a socket, socat opens it applying the O_APPEND flag. If it does not exist, it is opened with flag O_CREAT as a regular file (example).
Option groups: FD,REG,SOCKET,NAMED,OPEN
See also: OPEN, CREATE, UNIX-CONNECT
Opens a raw IP socket. Depending on host specification or option pf, IP protocol version 4 or 6 is used. It uses <protocol> to send packets to <host> [IP address] and receives packets from host, ignores packets from other hosts. Protocol 255 uses the raw socket with the IP header being part of the data.
Option groups: FD,SOCKET,IP4,IP6
Useful options: pf, ttl
See also: IP4-SENDTO, IP6-SENDTO, IP-RECVFROM, IP-RECV, UDP-SENDTO, UNIX-SENDTO
Communicates with a network connected on an interface using raw packets including link level data. <interface> is the name of the network interface. Currently only available on Linux. Option groups: FD,SOCKET
Useful options: pf, type
See also: ip-recv
Like IP-SENDTO, but always uses IPv4.
Option groups: FD,SOCKET,IP4
Like IP-SENDTO, but always uses IPv6.
Option groups: FD,SOCKET,IP6

Sends outgoing data to the specified address which may in particular be a broadcast or multicast address. Packets arriving on the local socket are checked if their source addresses match RANGE or TCPWRAP options. This address type can for example be used for implementing symmetric or asymmetric broadcast or multicast communications.
Option groups: FD, SOCKET, IP4, IP6, RANGE
Useful options: bind, range, tcpwrap, broadcast, ip-multicast-loop, ip-multicast-ttl, ip-multicast-if, ip-add-membership, ttl, tos, pf
See also: IP4-DATAGRAM, IP6-DATAGRAM, IP-SENDTO, IP-RECVFROM, IP-RECV, UDP-DATAGRAM
Like IP-DATAGRAM, but always uses IPv4. (example)
Option groups: FD,SOCKET,IP4,RANGE
Like IP-DATAGRAM, but always uses IPv6. Please note that IPv6 does not know broadcasts.
Option groups: FD,SOCKET,IP6,RANGE

Opens a raw IP socket of <protocol>. Depending on option pf, IP protocol version 4 or 6 is used. It receives one packet from an unspecified peer and may send one or more answer packets to that peer. This mode is particularly useful with fork option where each arriving packet - from arbitrary peers - is handled by its own sub process. This allows a behaviour similar to typical UDP based servers like ntpd or named.
Please note that the reply packets might be fetched as incoming traffic when sender and receiver IP address are identical because there is no port number to distinguish the sockets.
This address works well with IP-SENDTO address peers (see above). Protocol 255 uses the raw socket with the IP header being part of the data.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: pf, fork, range, ttl, broadcast
See also: IP4-RECVFROM, IP6-RECVFROM, IP-SENDTO, IP-RECV, UDP-RECVFROM, UNIX-RECVFROM
Like IP-RECVFROM, but always uses IPv4.
Option groups: FD,SOCKET,IP4,CHILD,RANGE
Like IP-RECVFROM, but always uses IPv6.
Option groups: FD,SOCKET,IP6,CHILD,RANGE

Opens a raw IP socket of <protocol>. Depending on option pf, IP protocol version 4 or 6 is used. It receives packets from multiple unspecified peers and merges the data. No replies are possible. It can be, e.g., addressed by socat IP-SENDTO address peers. Protocol 255 uses the raw socket with the IP header being part of the data.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: pf, range
See also: IP4-RECV, IP6-RECV, IP-SENDTO, IP-RECVFROM, UDP-RECV, UNIX-RECV
Like IP-RECV, but always uses IPv4.
Option groups: FD,SOCKET,IP4,RANGE
Like IP-RECV, but always uses IPv6.
Option groups: FD,SOCKET,IP6,RANGE

Opens <filename> using the open() system call (example). This operation fails on UNIX domain sockets.
Note: This address type is rarely useful in bidirectional mode.
Option groups: FD,REG,NAMED,OPEN
Useful options: creat, excl, noatime, nofollow, append, rdonly, wronly, lock, readbytes, ignoreeof
See also: CREATE, GOPEN, UNIX-CONNECT
Tries to establish a SSL connection to <port> [TCP service] on <host> [IP address] using TCP/IP version 4 or 6 depending on address specification, name resolution, or option pf.
NOTE: Up to version 1.7.2.4 the server certificate was only checked for validity against the system certificate store or cafile or capath, but not for match with the server’s name or its IP address. Since version 1.7.3.0 socat checks the peer certificate for match with the <host> parameter or the value of the openssl-commonname option. Socat tries to match it against the certificates subject commonName, and the certifications extension subjectAltName DNS names. Wildcards in the certificate are supported.
Option groups: FD,SOCKET,IP4,IP6,TCP,OPENSSL,RETRY
Useful options: cipher, method, verify, commonname cafile, capath, certificate, key, compress, bind, pf, connect-timeout, sourceport, retry
See also: OPENSSL-LISTEN, TCP
Listens on tcp <port> [TCP service]. The IP version is 4 or the one specified with pf. When a connection is accepted, this address behaves as SSL server.
Note: You probably want to use the certificate option with this address.
NOTE: The client certificate is only checked for validity against cafile or capath, but not for match with the client’s name or its IP address!
Option groups: FD,SOCKET,IP4,IP6,TCP,LISTEN,OPENSSL,CHILD,RANGE,RETRY
Useful options: pf, cipher, method, verify, commonname cafile, capath, certificate, key, compress, fork, bind, range, tcpwrap, su, reuseaddr, retry
See also: OPENSSL, TCP-LISTEN
If <filename> already exists, it is opened. If it does not exist, a named pipe is created and opened. Beginning with socat version 1.4.3, the named pipe is removed when the address is closed (but see option unlink-close
Note: When a pipe is used for both reading and writing, it works as echo service.
Note: When a pipe is used for both reading and writing, and socat tries to write more bytes than the pipe can buffer (Linux 2.4: 2048 bytes), socat might block. Consider using socat option, e.g., -b 2048
Option groups: FD,NAMED,OPEN
Useful options: rdonly, nonblock, group, user, mode, unlink-early
See also: unnamed pipe
Creates an unnamed pipe and uses it for reading and writing. It works as an echo, because everything written to it appeares immediately as read data.
Note: When socat tries to write more bytes than the pipe can queue (Linux 2.4: 2048 bytes), socat might block. Consider, e.g., using option -b 2048
Option groups: FD
See also: named pipe
Connects to an HTTP proxy server on port 8080 using TCP/IP version 4 or 6 depending on address specification, name resolution, or option pf, and sends a CONNECT request for hostname:port. If the proxy grants access and succeeds to connect to the target, data transfer between socat and the target can start. Note that the traffic need not be HTTP but can be an arbitrary protocol.
Option groups: FD,SOCKET,IP4,IP6,TCP,HTTP,RETRY
Useful options: proxyport, ignorecr, proxyauth, resolve, crnl, bind, connect-timeout, mss, sourceport, retry
See also: SOCKS, TCP
Generates a pseudo terminal (pty) and uses its master side. Another process may open the pty’s slave side using it like a serial line or terminal. (example). If both the ptmx and the openpty mechanisms are available, ptmx is used (POSIX).
Option groups: FD,NAMED,PTY,TERMIOS
Useful options: link, openpty, wait-slave, mode, user, group
See also: UNIX-LISTEN, PIPE, EXEC, SYSTEM
Uses GNU readline and history on stdio to allow editing and reusing input lines (example).
Due to licensing restrictions the readline feature is disabled in Debian. See BUGS.
You can use STDIO instead.
Establishes an SCTP stream connection to the specified <host> [IP address] and <port> [TCP service] using TCP/IP version 4 or 6 depending on address specification, name resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,SCTP,CHILD,RETRY
Useful options: bind, pf, connect-timeout, tos, mtudiscover, sctp-maxseg, sctp-nodelay, nonblock, sourceport, retry, readbytes
See also: SCTP4-CONNECT, SCTP6-CONNECT, SCTP-LISTEN, TCP-CONNECT
Like SCTP-CONNECT, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,SCTP,CHILD,RETRY
Like SCTP-CONNECT, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,SCTP,CHILD,RETRY
Listens on <port> [TCP service] and accepts a TCP/IP connection. The IP version is 4 or the one specified with address option pf, socat option (-4, -6), or environment variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,SCTP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf, max-children, backlog, sctp-maxseg, sctp-nodelay, su, reuseaddr, retry, cool-write
See also: SCTP4-LISTEN, SCTP6-LISTEN, TCP-LISTEN, SCTP-CONNECT
Like SCTP-LISTEN, but only supports IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,SCTP,RETRY
Like SCTP-LISTEN, but only supports IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,SCTP,RETRY
Creates a stream socket using the first and second given socket parameters and SOCK_STREAM (see man socket(2)) and connects to the remote-address. The two socket parameters have to be specified by int numbers. Consult your OS documentation and include files to find the appropriate values. The remote-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components.
Please note that you can - beyond the options of the specified groups - also use options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,CHILD,RETRY
Useful options: bind, setsockopt-int, setsockopt-bin, setsockopt-string
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO
Creates a datagram socket using the first three given socket parameters (see man socket(2)) and sends outgoing data to the remote-address. The three socket parameters have to be specified by int numbers. Consult your OS documentation and include files to find the appropriate values. The remote-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components.
Please note that you can - beyond the options of the specified groups - also use options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,RANGE
Useful options: bind, range, setsockopt-int, setsockopt-bin, setsockopt-string
See also: UDP-DATAGRAM, IP-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV, SOCKET-RECVFROM
Creates a stream socket using the first and second given socket parameters and SOCK_STREAM (see man socket(2)) and waits for incoming connections on local-address. The two socket parameters have to be specified by int numbers. Consult your OS documentation and include files to find the appropriate values. The local-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components.
Please note that you can - beyond the options of the specified groups - also use options of higher level protocols when you apply socat option -g.
Option groups: FD,SOCKET,LISTEN,RANGE,CHILD,RETRY
Useful options: setsockopt-int, setsockopt-bin, setsockopt-string
See also: TCP, UDP-CONNECT, UNIX-CONNECT, SOCKET-LISTEN, SOCKET-SENDTO, SOCKET-SENDTO
Creates a socket using the three given socket parameters (see man socket(2)) and binds it to <local-address>. Receives arriving data. The three parameters have to be specified by int numbers. Consult your OS documentation and include files to find the appropriate values. The local-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET,RANGE
Useful options: range, setsockopt-int, setsockopt-bin, setsockopt-string
See also: UDP-RECV, IP-RECV, UNIX-RECV, SOCKET-DATAGRAM, SOCKET-SENDTO, SOCKET-RECVFROM
Creates a socket using the three given socket parameters (see man socket(2)) and binds it to <local-address>. Receives arriving data and sends replies back to the sender. The first three parameters have to be specified as int numbers. Consult your OS documentation and include files to find the appropriate values. The local-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET,CHILD,RANGE
Useful options: fork, range, setsockopt-int, setsockopt-bin, setsockopt-string
See also: UDP-RECVFROM, IP-RECVFROM, UNIX-RECVFROM, SOCKET-DATAGRAM, SOCKET-SENDTO, SOCKET-RECV
Creates a socket using the three given socket parameters (see man socket(2)). Sends outgoing data to the given address and receives replies. The three parameters have to be specified as int numbers. Consult your OS documentation and include files to find the appropriate values. The remote-address must be the data representation of a sockaddr structure without sa_family and (BSD) sa_len components.
Option groups: FD,SOCKET
Useful options: bind, setsockopt-int, setsockopt-bin, setsockopt-string
See also: UDP-SENDTO, IP-SENDTO, UNIX-SENDTO, SOCKET-DATAGRAM, SOCKET-RECV SOCKET-RECVFROM
Connects via <socks-server> [IP address] to <host> [IPv4 address] on <port> [TCP service], using socks version 4 protocol over IP version 4 or 6 depending on address specification, name resolution, or option pf (example).
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
Useful options: socksuser, socksport, sourceport, pf, retry
See also: SOCKS4A, PROXY, TCP
like SOCKS4, but uses socks protocol version 4a, thus leaving host name resolution to the socks server.
Option groups: FD,SOCKET,IP4,IP6,TCP,SOCKS4,RETRY
Uses file descriptor 2.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD
Uses file descriptor 0.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD
Uses file descriptor 0 for reading, and 1 for writing.
Option groups: FD (TERMIOS,REG,SOCKET)
Useful options: readbytes
See also: FD
Uses file descriptor 1.
Option groups: FD (TERMIOS,REG,SOCKET)
See also: FD
Forks a sub process that establishes communication with its parent process and invokes the specified program with system() . Please note that <shell-command> [string] must not contain ’,’ or "!!", and that shell meta characters may have to be protected. After successful program start, socat writes data to stdin of the process and reads from its stdout.
Option groups: FD,SOCKET,EXEC,FORK,TERMIOS
Useful options: path, fdin, fdout, chroot, su, su-d, nofork, pty, stderr, ctty, setsid, pipes, sigint, sigquit
See also: EXEC
Connects to <port> [TCP service] on <host> [IP address] using TCP/IP version 4 or 6 depending on address specification, name resolution, or option pf.
Option groups: FD,SOCKET,IP4,IP6,TCP,RETRY
Useful options: crnl, bind, pf, connect-timeout, tos, mtudiscover, mss, nodelay, nonblock, sourceport, retry, readbytes
See also: TCP4, TCP6, TCP-LISTEN, UDP, SCTP-CONNECT, UNIX-CONNECT
Like TCP, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,IP4,TCP,RETRY
Like TCP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,TCP,RETRY
Listens on <port> [TCP service] and accepts a TCP/IP connection. The IP version is 4 or the one specified with address option pf, socat option (-4, -6), or environment variable SOCAT_DEFAULT_LISTEN_IP. Note that opening this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6,TCP,RETRY
Useful options: crnl, fork, bind, range, tcpwrap, pf, max-children, backlog, mss, su, reuseaddr, retry, cool-write
See also: TCP4-LISTEN, TCP6-LISTEN, UDP-LISTEN, SCTP-LISTEN, UNIX-LISTEN, OPENSSL-LISTEN, TCP-CONNECT
Like TCP-LISTEN, but only supports IPv4 protocol (example).
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,TCP,RETRY
Like TCP-LISTEN, but only supports IPv6 protocol.
Additional useful option: ipv6only
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6,TCP,RETRY
Creates a Linux TUN/TAP device and optionally assignes it the address and netmask given by the parameters. The resulting network interface is almost ready for use by other processes; socat serves its "wire side". This address requires read and write access to the tunnel cloning device, usually /dev/net/tun , as well as permission to set some ioctl()s. Option iff-up is required to immediately activate the interface!
Option groups: FD,NAMED,OPEN,TUN
Useful options: iff-up, tun-device, tun-name, tun-type, iff-no-pi
See also: ip-recv
Connects to <port> [UDP service] on <host> [IP address] using UDP/IP version 4 or 6 depending on address specification, name resolution, or option pf.
Please note that, due to UDP protocol properties, no real connection is established; data has to be sent for `connecting’ to the server, and no end-of-file condition can be transported.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4, UDP6, UDP-LISTEN, TCP, IP
Like UDP, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4
Like UDP, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6
Sends outgoing data to the specified address which may in particular be a broadcast or multicast address. Packets arriving on the local socket are checked for the correct remote port and if their source addresses match RANGE or TCPWRAP options. This address type can for example be used for implementing symmetric or asymmetric broadcast or multicast communications.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: bind, range, tcpwrap, broadcast, ip-multicast-loop, ip-multicast-ttl, ip-multicast-if, ip-add-membership, ttl, tos, sourceport, pf
See also: UDP4-DATAGRAM, UDP6-DATAGRAM, UDP-SENDTO, UDP-RECVFROM, UDP-RECV, UDP-CONNECT, UDP-LISTEN, IP-DATAGRAM
Like UDP-DATAGRAM, but only supports IPv4 protocol (example1, example2).
Option groups: FD,SOCKET,IP4, RANGE
Like UDP-DATAGRAM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE
Waits for a UDP/IP packet arriving on <port> [UDP service] and `connects’ back to sender. The accepted IP version is 4 or the one specified with option pf. Please note that, due to UDP protocol properties, no real connection is established; data has to arrive from the peer first, and no end-of-file condition can be transported. Note that opening this address usually blocks until a client connects.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4,IP6
Useful options: fork, bind, range, pf
See also: UDP, UDP4-LISTEN, UDP6-LISTEN, TCP-LISTEN
Like UDP-LISTEN, but only support IPv4 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP4
Like UDP-LISTEN, but only support IPv6 protocol.
Option groups: FD,SOCKET,LISTEN,CHILD,RANGE,IP6
Communicates with the specified peer socket, defined by <port> [UDP service] on <host> [IP address], using UDP/IP version 4 or 6 depending on address specification, name resolution, or option pf. It sends packets to and receives packets from that peer socket only. This address effectively implements a datagram client. It works well with socat UDP-RECVFROM and UDP-RECV address peers.
Option groups: FD,SOCKET,IP4,IP6
Useful options: ttl, tos, bind, sourceport, pf
See also: UDP4-SENDTO, UDP6-SENDTO, UDP-RECVFROM, UDP-RECV, UDP-CONNECT, UDP-LISTEN, IP-SENDTO
Like UDP-SENDTO, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4
Like UDP-SENDTO, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6
Creates a UDP socket on <port> [UDP service] using UDP/IP version 4 or 6 depending on option pf. It receives one packet from an unspecified peer and may send one or more answer packets to that peer. This mode is particularly useful with fork option where each arriving packet - from arbitrary peers - is handled by its own sub process. This allows a behaviour similar to typical UDP based servers like ntpd or named. This address works well with socat UDP-SENDTO address peers.
Option groups: FD,SOCKET,IP4,IP6,CHILD,RANGE
Useful options: fork, ttl, tos, bind, sourceport, pf
See also: UDP4-RECVFROM, UDP6-RECVFROM, UDP-SENDTO, UDP-RECV, UDP-CONNECT, UDP-LISTEN, IP-RECVFROM, UNIX-RECVFROM
Like UDP-RECVFROM, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,CHILD,RANGE
Like UDP-RECVFROM, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,CHILD,RANGE
Creates a UDP socket on <port> [UDP service] using UDP/IP version 4 or 6 depending on option pf. It receives packets from multiple unspecified peers and merges the data. No replies are possible. It works well with, e.g., socat UDP-SENDTO address peers; it behaves similar to a syslog server.
Option groups: FD,SOCKET,IP4,IP6,RANGE
Useful options: fork, pf, bind, sourceport, ttl, tos
See also: UDP4-RECV, UDP6-RECV, UDP-SENDTO, UDP-RECVFROM, UDP-CONNECT, UDP-LISTEN, IP-RECV, UNIX-RECV
Like UDP-RECV, but only supports IPv4 protocol.
Option groups: FD,SOCKET,IP4,RANGE
Like UDP-RECV, but only supports IPv6 protocol.
Option groups: FD,SOCKET,IP6,RANGE
Connects to <filename> assuming it is a UNIX domain socket. If <filename> does not exist, this is an error; if <filename> is not a UNIX domain socket, this is an error; if <filename> is a UNIX domain socket, but no process is listening, this is an error.
Option groups: FD,SOCKET,NAMED,RETRY,UNIX
) Useful options: bind
See also: UNIX-LISTEN, UNIX-SENDTO, TCP
Listens on <filename> using a UNIX domain stream socket and accepts a connection. If <filename> exists and is not a socket, this is an error. If <filename> exists and is a UNIX domain socket, binding to the address fails (use option unlink-early!). Note that opening this address usually blocks until a client connects. Beginning with socat version 1.4.3, the file system entry is removed when this address is closed (but see option unlink-close) (example).
Option groups: FD,SOCKET,NAMED,LISTEN,CHILD,RETRY,UNIX
Useful options: fork, umask, mode, user, group, unlink-early
See also: UNIX-CONNECT, UNIX-RECVFROM, UNIX-RECV, TCP-LISTEN
Communicates with the specified peer socket, defined by [<filename>] assuming it is a UNIX domain datagram socket. It sends packets to and receives packets from that peer socket only. Please note that it might be necessary to bind the local socket to an address (e.g. /tmp/sock1, which must not exist before). This address type works well with socat UNIX-RECVFROM and UNIX-RECV address peers.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-RECVFROM, UNIX-RECV, UNIX-CONNECT, UDP-SENDTO, IP-SENDTO
Creates a UNIX domain datagram socket [<filename>]. Receives one packet and may send one or more answer packets to that peer. This mode is particularly useful with fork option where each arriving packet - from arbitrary peers - is handled by its own sub process. This address works well with socat UNIX-SENDTO address peers.
Option groups: FD,SOCKET,NAMED,CHILD,UNIX
Useful options: fork
See also: UNIX-SENDTO, UNIX-RECV, UNIX-LISTEN, UDP-RECVFROM, IP-RECVFROM
Creates a UNIX domain datagram socket [<filename>]. Receives packets from multiple unspecified peers and merges the data. No replies are possible. It can be, e.g., addressed by socat UNIX-SENDTO address peers. It behaves similar to a syslog server. Option groups: FD,SOCKET,NAMED,UNIX
See also: UNIX-SENDTO, UNIX-RECVFROM, UNIX-LISTEN, UDP-RECV, IP-RECV
Communicates with the specified peer socket, defined by [<filename>] assuming it is a UNIX domain socket. It first tries to connect and, if that fails, assumes it is a datagram socket, thus supporting both types.
Option groups: FD,SOCKET,NAMED,UNIX
Useful options: bind
See also: UNIX-CONNECT, UNIX-SENDTO, GOPEN
The ABSTRACT addresses are almost identical to the related UNIX addresses except that they do not address file system based sockets but an alternate UNIX domain address space. To archieve this the socket address strings are prefixed with "\0" internally. This feature is available (only?) on Linux. Option groups are the same as with the related UNIX addresses, except that the ABSTRACT addresses are not member of the NAMED group.

Address options can be applied to address specifications to influence the process of opening the addresses and the properties of the resulting data channels.

For technical reasons not every option can be applied to every address type; e.g., applying a socket option to a regular file will fail. To catch most useless combinations as early as in the open phase, the concept of option groups was introduced. Each option belongs to one or more option groups. Options can be used only with address types that support at least one of their option groups (but see option -g).

Address options have data types that their values must conform to. Every address option consists of just a keyword or a keyword followed by "=value", where value must conform to the options type. Some address options manipulate parameters of system calls; e.g., option sync sets the O_SYNC flag with the open() call. Other options cause a system or library call; e.g., with option `ttl=value’ the setsockopt(fd, SOL_IP, IP_TTL, value, sizeof(int)) call is applied. Other options set internal socat variables that are used during data transfer; e.g., `crnl’ causes explicit character conversions. A few options have more complex implementations; e.g., su-d (substuser-delayed) inquires some user and group infos, stores them, and applies them later after a possible chroot() call.

If multiple options are given to an address, their sequence in the address specification has (almost) no effect on the sequence of their execution/application. Instead, socat has built in an option phase model that tries to bring the options in a useful order. Some options exist in different forms (e.g., unlink, unlink-early, unlink-late) to control the time of their execution.

If the same option is specified more than once within one address specification, with equal or different values, the effect depends on the kind of option. Options resulting in function calls like setsockopt() cause multiple invocations. With options that set parameters for a required call like open() or set internal flags, the value of the last option occurrence is effective.

The existence or semantics of many options are system dependent. Socat usually does NOT try to emulate missing libc or kernel features, it just provides an interface to the underlying system. So, if an operating system lacks a feature, the related option is simply not available on this platform.

The following paragraphs introduce just the more common address options. For a more comprehensive reference and to find information about canonical option names, alias names, option phases, and platforms see file xio.help.

FD option group

This option group contains options that are applied to a UN*X style file descriptor, no matter how it was generated. Because all current socat address types are file descriptor based, these options may be applied to any address.
Note: Some of these options are also member of another option group, that provides another, non-fd based mechanism. For these options, it depends on the actual address type and its option groups which mechanism is used. The second, non-fd based mechanism is prioritized.

Sets the FD_CLOEXEC flag with the fcntl() system call to value <bool>. If set, the file descriptor is closed on exec() family function calls. Socat internally handles this flag for the fds it controls, so in most cases there will be no need to apply this option.
Tries to set a discretionary write lock to the whole file using the fcntl(fd, F_SETLK, ...) system call. If the file is already locked, this call results in an error. On Linux, when the file permissions for group are "S" (g-x,g+s), and the file system is locally mounted with the "mand" option, the lock is mandatory, i.e. prevents other processes from opening the file.
Tries to set a discretionary waiting write lock to the whole file using the fcntl(fd, F_SETLKW, ...) system call. If the file is already locked, this call blocks. See option setlk for information about making this lock mandatory.
Tries to set a discretionary read lock to the whole file using the fcntl(fd, F_SETLK, ...) system call. If the file is already write locked, this call results in an error. See option setlk for information about making this lock mandatory.
Tries to set a discretionary waiting read lock to the whole file using the fcntl(fd, F_SETLKW, ...) system call. If the file is already write locked, this call blocks. See option setlk for information about making this lock mandatory.
Tries to set a blocking exclusive advisory lock to the file using the flock(fd, LOCK_EX) system call. Socat hangs in this call if the file is locked by another process.
Tries to set a nonblocking exclusive advisory lock to the file using the flock(fd, LOCK_EX|LOCK_NB) system call. If the file is already locked, this option results in an error.
Tries to set a blocking shared advisory lock to the file using the flock(fd, LOCK_SH) system call. Socat hangs in this call if the file is locked by another process.
Tries to set a nonblocking shared advisory lock to the file using the flock(fd, LOCK_SH|LOCK_NB) system call. If the file is already locked, this option results in an error.
Sets a blocking lock on the file. Uses the setlk or flock mechanism depending on availability on the particular platform. If both are available, the POSIX variant (setlkw) is used.
Sets the <user> (owner) of the stream. If the address is member of the NAMED option group, socat uses the chown() system call after opening the file or binding to the UNIX domain socket (race condition!). Without filesystem entry, socat sets the user of the stream using the fchown() system call. These calls might require root privilege.
user-late=<user>
Sets the owner of the fd to <user> with the fchown() system call after opening or connecting the channel. This is useful only on file system entries.
Sets the <group> of the stream. If the address is member of the NAMED option group, socat uses the chown() system call after opening the file or binding to the UNIX domain socket (race condition!). Without filesystem entry, socat sets the group of the stream with the fchown() system call. These calls might require group membership or root privilege.
group-late=<group>
Sets the group of the fd to <group> with the fchown() system call after opening or connecting the channel. This is useful only on file system entries.
Sets the <mode> [mode_t] (permissions) of the stream. If the address is member of the NAMED option group and uses the open() or creat() call, the mode is applied with these. If the address is member of the NAMED option group without using these system calls, socat uses the chmod() system call after opening the filesystem entry or binding to the UNIX domain socket (race condition!). Otherwise, socat sets the mode of the stream using fchmod() . These calls might require ownership or root privilege.
Sets the permissions of the fd to value <mode> [mode_t] using the fchmod() system call after opening or connecting the channel. This is useful only on file system entries.
Always writes data to the actual end of file. If the address is member of the OPEN option group, socat uses the O_APPEND flag with the open() system call (example). Otherwise, socat applies the fcntl(fd, F_SETFL, O_APPEND) call.
Tries to open or use file in nonblocking mode. Its only effects are that the connect() call of TCP addresses does not block, and that opening a named pipe for reading does not block. If the address is member of the OPEN option group, socat uses the O_NONBLOCK flag with the open() system call. Otherwise, socat applies the fcntl(fd, F_SETFL, O_NONBLOCK) call.
Opens the file in binary mode to avoid implicit line terminator conversions (Cygwin).
Opens the file in text mode to force implicit line terminator conversions (Cygwin).
Does not keep this file open in a spawned process (Cygwin).
Takes it easy when write fails with EPIPE or ECONNRESET and logs the message with notice level instead of error. This prevents the log file from being filled with useless error messages when socat is used as a high volume server or proxy where clients often abort the connection.
This option is experimental.
Changes the (address dependent) method of ending a connection to just close the file descriptors. This is useful when the connection is to be reused by or shared with other processes (example).
Normally, socket connections will be ended with shutdown(2) which terminates the socket even if it is shared by multiple processes. close(2) "unlinks" the socket from the process but keeps it active as long as there are still links from other processes.
Similarly, when an address of type EXEC or SYSTEM is ended, socat usually will explicitly kill the sub process. With this option, it will just close the file descriptors.
Changes the (address dependent) method of shutting down the write part of a connection to not do anything.
Changes the (address dependent) method of shutting down the write part of a connection to shutdown(fd, SHUT_WR). Is only useful with sockets.
Changes the (address dependent) method of shutting down the write part of a connection to close(fd).
When one address indicates EOF, socat will send a zero sized packet to the write channel of the other address to transfer the EOF condition. This is useful with UDP and other datagram protocols. Has been tested against netcat and socat with option null-eof.
Normally socat will ignore empty (zero size payload) packets arriving on datagram sockets, so it survives port scans. With this option socat interprets empty datagram packets as EOF indicator (see shut-null).
Calls ioctl() with the request value as second argument and NULL as third argument. This option allows utilizing ioctls that are not explicitly implemented in socat.
Calls ioctl() with the request value as second argument and the integer value as third argument.
Calls ioctl() with the request value as second argument and a pointer to the integer value as third argument.
Calls ioctl() with the request value as second argument and a pointer to the given data value as third argument. This data must be specified in <dalan> form.
Calls ioctl() with the request value as second argument and a pointer to the given string as third argument. <dalan> form.

NAMED option group

These options work on file system entries.
See also options user, group, and mode.

user-early=<user>
Changes the <user> (owner) of the file system entry before accessing it, using the chown() system call. This call might require root privilege.
group-early=<group>
Changes the <group> of the file system entry before accessing it, using the chown() system call. This call might require group membership or root privilege.
Changes the <mode> [mode_t] of the file system entry before accessing it, using the chmod() system call. This call might require ownership or root privilege.
Sets the umask of the process to <mode> [mode_t] before accessing the file system entry (useful with UNIX domain sockets!). This call might affect all further operations of the socat process!
Unlinks (removes) the file before opening it and even before applying user-early etc.
Unlinks (removes) the file before accessing it, but after user-early etc.
Unlinks (removes) the file after opening it to make it inaccessible for other processes after a short race condition.
Removes the addresses file system entry when closing the address. For named pipes, listening unix domain sockets, and the symbolic links of pty addresses, the default is 1; for created files, opened files, generic opened files, and client unix domain sockets the default is 0.

OPEN option group

The OPEN group options allow setting flags with the open() system call. E.g., option `creat’ sets the O_CREAT flag.
See also options append and nonblock.

Creates the file if it does not exist (example).
Blocks write() calls until metainfo is physically written to media.
With option creat, if file exists this is an error.
On 32 bit systems, allows a file larger than 2^31 bytes.
Sets the O_NOATIME options, so reads do not change the access timestamp.
Does not make this file the controlling terminal.
Does not follow symbolic links.
Does not allow sharing this file with other processes.
Does not allow other processes to open this file for writing.
Blocks write() until metainfo is physically written to media.
Blocks write() until data is physically written to media.
Opens the file for reading only.
Opens the file for writing only.
Truncates the file to size 0 during opening it.

REG and BLK option group

These options are usually applied to a UN*X file descriptor, but their semantics make sense only on a file supporting random access.

Applies the lseek(fd, <offset>, SEEK_SET) (or lseek64 ) system call, thus positioning the file pointer absolutely to <offset> [off_t or off64_t]. Please note that a missing value defaults to 1, not 0.
Applies the lseek(fd, <offset>, SEEK_CUR) (or lseek64 ) system call, thus positioning the file pointer <offset> [off_t or off64_t] bytes relatively to its current position (which is usually 0). Please note that a missing value defaults to 1, not 0.
Applies the lseek(fd, <offset>, SEEK_END) (or lseek64 ) system call, thus positioning the file pointer <offset> [off_t or off64_t] bytes relatively to the files current end. Please note that a missing value defaults to 1, not 0.
Applies the ftruncate(fd, <offset>) (or ftruncate64 if available) system call, thus truncating the file at the position <offset> [off_t or off64_t]. Please note that a missing value defaults to 1, not 0.
These options change non standard file attributes on operating systems and file systems that support these features, like Linux with ext2fs, ext3fs, or reiserfs. See man 1 chattr for information on these options. Please note that there might be a race condition between creating the file and applying these options.

PROCESS option group

Options of this group change the process properties instead of just affecting one data channel. For EXEC and SYSTEM addresses and for LISTEN and CONNECT type addresses with option FORK, these options apply to the child processes instead of the main socat process.

Performs a chroot() operation to <directory> after processing the address (example). This call might require root privilege.
Performs a chroot() operation to <directory> before opening the address. This call might require root privilege.
Changes the primary <group> of the process after processing the address. This call might require root privilege. Please note that this option does not drop other group related privileges.
Like setgit but is performed before opening the address.
Changes the <user> (owner) of the process after processing the address. This call might require root privilege. Please note that this option does not drop group related privileges. Check if option su better fits your needs.
Like setuid but is performed before opening the address.
Changes the <user> (owner) and groups of the process after processing the address (example). This call might require root privilege.
Short name for substuser-delayed. Changes the <user> (owner) and groups of the process after processing the address (example). The user and his groups are retrieved before a possible chroot() . This call might require root privilege.
Makes the process a member of the specified process group <pid_t>. If no value is given, or if the value is 0 or 1, the process becomes leader of a new process group.
Makes the process the leader of a new session (example).

READLINE option group

Due to licensing restrictions the readline feature is disabled in Debian (see BUGS).
These options apply to the readline address type.

Reads and writes history from/to <filename> (example).
Since version 1.4.0, socat per default tries to determine a prompt - that is then passed to the readline call - by remembering the last incomplete line of the output. With this option, socat does not pass a prompt to readline, so it begins line editing in the first column of the terminal.
Specifies a regular pattern for a prompt that prevents the following input line from being displayed on the screen and from being added to the history. The prompt is defined as the text that was output to the readline address after the lastest newline character and before an input character was typed. The pattern is a regular expression, e.g. "^[Pp]assword:.*$" or "([Uu]ser:|[Pp]assword:)". See regex(7) for details. (example)
Passes the string as prompt to the readline function. readline prints this prompt when stepping through the history. If this string matches a constant prompt issued by an interactive program on the other socat address, consistent look and feel can be archieved.

APPLICATION option group

This group contains options that work at data level. Note that these options only apply to the "raw" data transferred by socat, but not to protocol data used by addresses like PROXY.

Converts the default line termination character NL (’\n’, 0x0a) to/from CR (’\r’, 0x0d) when writing/reading on this channel.
Converts the default line termination character NL (’\n’, 0x0a) to/from CRNL ("\r\n", 0x0d0a) when writing/reading on this channel (example). Note: socat simply strips all CR characters.
When EOF occurs on this channel, socat ignores it and tries to read more data (like "tail -f") (example).
socat reads only so many bytes from this address (the address provides only so many bytes for transfer and pretends to be at EOF afterwards). Must be greater than 0.
If lockfile exists, exits with error. If lockfile does not exist, creates it and continues, unlinks lockfile on exit.
If lockfile exists, waits until it disappears. When lockfile does not exist, creates it and continues, unlinks lockfile on exit.
Specifies the numeric code of a character that triggers EOF on the input stream. It is useful with a terminal in raw mode (example).

SOCKET option group

These options are intended for all kinds of sockets, e.g. IP or UNIX domain. Most are applied with a setsockopt() call.

Binds the socket to the given socket address using the bind() system call. The form of <sockname> is socket domain dependent: IP4 and IP6 allow the form [hostname|hostaddress][:(service|port)] (example), UNIX domain sockets require <filename>.
Abort the connection attempt after <seconds> [timeval] with error status.
Binds the socket to the given <interface>. This option might require root privilege.
For datagram sockets, allows sending to broadcast addresses and receiving packets addressed to broadcast addresses.
Enables socket debugging.
Only communicates with directly connected peers, does not use routers.
Enables sending keepalives on the socket.
Blocks shutdown() or close() until data transfers have finished or the given timeout [int] expired.
Places out-of-band data in the input data stream.
Sets the protocol defined <priority> [<int>] for outgoing packets.
Sets the size of the receive buffer after the socket() call to <bytes> [int]. With TCP sockets, this value corresponds to the socket’s maximal window size.
Sets the size of the receive buffer when the socket is already connected to <bytes> [int]. With TCP sockets, this value corresponds to the socket’s maximal window size.
Specifies the minimum number of received bytes [int] until the socket layer will pass the buffered data to socat.
Allows other sockets to bind to an address even if parts of it (e.g. the local port) are already in use by socat (example).
Sets the size of the send buffer after the socket() call to <bytes> [int].
Sets the size of the send buffer when the socket is connected to <bytes> [int].
Specifies the minimum number of bytes in the send buffer until the socket layer will send the data to <bytes> [int].
Forces the use of the specified IP version or protocol. <string> can be something like "ip4" or "ip6". The resulting value is used as first argument to the socket() or socketpair() calls. This option affects address resolution and the required syntax of bind and range options.
Sets the type of the socket, specified as second argument to the socket() or socketpair() calls, to <type> [int]. Address resolution is not affected by this option. Under Linux, 1 means stream oriented socket, 2 means datagram socket, and 3 means raw socket.
Sets the protocol of the socket, specified as third argument to the socket() or socketpair() calls, to <prototype> [int]. Address resolution is not affected by this option. 6 means TCP, 17 means UDP.
Set the SO_REUSEPORT socket option.
Sets the SO_TIMESTAMP socket option. This enables receiving and logging of timestamp ancillary messages.
Invokes setsockopt() for the socket with the given parameters. level [int] is used as second argument to setsockopt() and specifies the layer, e.g. SOL_TCP for TCP (6 on Linux), or SOL_SOCKET for the socket layer (1 on Linux). optname [int] is the third argument to setsockopt() and tells which socket option is to be set. For the actual numbers you might have to look up the appropriate include files of your system. The 4th setsockopt() parameter, value [int], is passed to the function per pointer, and for the length parameter sizeof(int) is taken implicitely.
Like setsockopt-int, but <optval> must be provided in dalan format and specifies an arbitrary sequence of bytes; the length parameter is automatically derived from the data.
Like setsockopt-int, but <optval> must be a string. This string is passed to the function with trailing null character, and the length parameter is automatically derived from the data.

UNIX option group

These options apply to UNIX domain based addresses.

On socket operations, pass a socket address length that does not include the whole struct sockaddr_un record but (besides other components) only the relevant part of the filename or abstract string. Default is 1.

IP4 and IP6 option groups

These options can be used with IPv4 and IPv6 based sockets.

Sets the TOS (type of service) field of outgoing packets to <tos> [byte] (see RFC 791).
Sets the TTL (time to live) field of outgoing packets to <ttl> [byte].
Sets IP options like source routing. Must be given in binary form, recommended format is a leading "x" followed by an even number of hex digits. This option may be used multiple times, data are appended. E.g., to connect to host 10.0.0.1 via some gateway using a loose source route, use the gateway as address parameter and set a loose source route using the option ip-options=x8307040a000001 .
IP options are defined in RFC 791.
Takes 0, 1, 2 to never, want, or always use path MTU discover on this socket.
Sets the IP_PKTINFO socket option. This enables receiving and logging of ancillary messages containing destination address and interface (Linux) (example).
Sets the IP_RECVERR socket option. This enables receiving and logging of ancillary messages containing detailed error information.
Sets the IP_RECVOPTS socket option. This enables receiving and logging of IP options ancillary messages (Linux, *BSD).
Sets the IP_RECVTOS socket option. This enables receiving and logging of TOS (type of service) ancillary messages (Linux).
Sets the IP_RECVTTL socket option. This enables receiving and logging of TTL (time to live) ancillary messages (Linux, *BSD).
Sets the IP_RECVDSTADDR socket option. This enables receiving and logging of ancillary messages containing destination address (*BSD) (example).
Sets the IP_RECVIF socket option. This enables receiving and logging of interface ancillary messages (*BSD) (example).
Makes the socket member of the specified multicast group. This is currently only implemented for IPv4. The option takes the IP address of the multicast group and info about the desired network interface. The most common syntax is the first one, while the others are only available on systems that provide struct mreqn (Linux).
The indices of active network interfaces can be shown using the utility procan.
Specifies hostname or address of the network interface to be used for multicast traffic.
Specifies if outgoing multicast traffic should loop back to the interface.
Sets the TTL used for outgoing multicast traffic. Default is 1.
These options set the corresponding resolver (name resolution) option flags. Append "=0" to clear a default option. See man resolver(5) for more information on these options. Note: these options are valid only for the address they are applied to.

IP6 option group

These options can only be used on IPv6 based sockets. See IP options for options that can be applied to both IPv4 and IPv6 sockets.

Sets the IPV6_V6ONLY socket option. If 0, the TCP stack will also accept connections using IPv4 protocol on the same port. The default is system dependent.
Sets the IPV6_RECVDSTOPTS socket option. This enables receiving and logging of ancillary messages containing the destination options.
Sets the IPV6_RECVHOPLIMIT socket option. This enables receiving and logging of ancillary messages containing the hoplimit.
Sets the IPV6_RECVHOPOPTS socket option. This enables receiving and logging of ancillary messages containing the hop options.
Sets the IPV6_RECVPKTINFO socket option. This enables receiving and logging of ancillary messages containing destination address and interface.
Sets the IPV6_UNICAST_HOPS socket option. This sets the hop count limit (TTL) for outgoing unicast packets.
Sets the IPV6_RECVRTHDR socket option. This enables receiving and logging of ancillary messages containing routing information.
Sets the IPV6_TCLASS socket option. This sets the transfer class of outgoing packets.
Sets the IPV6_RECVTCLASS socket option. This enables receiving and logging of ancillary messages containing the transfer class.

TCP option group

These options may be applied to TCP sockets. They work by invoking setsockopt() with the appropriate parameters.

Doesn’t send packets smaller than MSS (maximal segment size).
While listening, accepts connections only when data from the peer arrived.
Sets the number of keepalives before shutting down the socket to <count> [int].
Sets the idle time before sending the first keepalive to <seconds> [int].
Sets the interval between two keepalives to <seconds> [int].
Sets the time to keep the socket in FIN-WAIT-2 state to <seconds> [int].
Sets the MSS (maximum segment size) after the socket() call to <bytes> [int]. This value is then proposed to the peer with the SYN or SYN/ACK packet (example).
Sets the MSS of the socket after connection has been established to <bytes> [int].
Turns off the Nagle algorithm for measuring the RTT (round trip time).
Enables RFC1323 TCP options: TCP window scale, round-trip time measurement (RTTM), and protect against wrapped sequence numbers (PAWS) (AIX).
Enables RFC1122 compliant urgent pointer handling (AIX).
Sets the maximal number of SYN retransmits during connect to <count> [int].
Enables generation of MD5 digests on the packets (FreeBSD).
Disables use of TCP options (FreeBSD, MacOSX).
sets the TCP_NOPUSH socket option (FreeBSD, MacOSX).
Disables use the selective acknowledge feature (OpenBSD).
Enables generation of MD5 digests on the packets (OpenBSD).
Sets the time to wait for an answer of the peer on an established connection (HP-UX).
Sets the time to wait for an answer of the server during the initial connect (HP-UX).
Sets the time to wait for an answer of the server during connect() before giving up. Value in half seconds, default is 150 (75s) (Tru64).
Enables the "protect against wrapped sequence numbers" feature (Tru64).
Enables selective acknowledge (Tru64).
Enables the time stamp option that allows RTT recalculation on existing connections (Tru64).

SCTP option group

These options may be applied to SCTP stream sockets.

Sets the SCTP_NODELAY socket option that disables the Nagle algorithm.
Sets the SCTP_MAXSEG socket option to <bytes> [int]. This value is then proposed to the peer with the SYN or SYN/ACK packet.

UDP, TCP, and SCTP option groups

Here we find options that are related to the network port mechanism and thus can be used with UDP, TCP, and SCTP client and server addresses.

For outgoing (client) TCP and UDP connections, it sets the source <port> using an extra bind() call. With TCP or UDP listen addresses, socat immediately shuts down the connection if the client does not use this sourceport (example).
Outgoing (client) TCP and UDP connections with this option use an unused random source port between 640 and 1023 incl. On UNIX class operating systems, this requires root privilege, and thus indicates that the client process is authorized by local root. TCP and UDP listen addresses with this option immediately shut down the connection if the client does not use a sourceport <= 1023. This mechanism can provide limited authorization under some circumstances.

SOCKS option group

When using SOCKS type addresses, some socks specific options can be set.

Overrides the default "socks" service or port 1080 for the socks server port with <TCP service>.
Sends the <user> [string] in the username field to the socks server. Default is the actual user name ($LOGNAME or $USER) (example).

HTTP option group

Options that can be provided with HTTP type addresses. The only HTTP address currently implemented is proxy-connect.

Overrides the default HTTP proxy port 8080 with <TCP service>.
The HTTP protocol requires the use of CR+NL as line terminator. When a proxy server violates this standard, socat might not understand its answer. This option directs socat to interprete NL as line terminator and to ignore CR in the answer. Nevertheless, socat sends CR+NL to the proxy.
Provide "basic" authentication to the proxy server. The argument to the option is used with a "Proxy-Authorization: Base" header in base64 encoded form.
Note: username and password are visible for every user on the local machine in the process list; username and password are transferred to the proxy server unencrypted (base64 encoded) and might be sniffed.
Per default, socat sends to the proxy a CONNECT request containing the target hostname. With this option, socat resolves the hostname locally and sends the IP address. Please note that, according to RFC 2396, only name resolution to IPv4 addresses is implemented.

RANGE option group

These options check if a connecting client should be granted access. They can be applied to listening and receiving network sockets. tcp-wrappers options fall into this group.

After accepting a connection, tests if the peer is within range. For IPv4 addresses, address-range takes the form address/bits, e.g. 10.0.0.0/8, or address:mask, e.g. 10.0.0.0:255.0.0.0 (example); for IPv6, it is [ip6-address/bits], e.g. [::1/128]. If the client address does not match, socat issues a warning and keeps listening/receiving.
Uses Wietse Venema’s libwrap (tcpd) library to determine if the client is allowed to connect. The configuration files are /etc/hosts.allow and /etc/hosts.deny per default, see "man 5 hosts_access" for more information. The optional <name> (type string) is passed to the wrapper functions as daemon process name (example). If omitted, the basename of socats invocation (argv[0]) is passed. If both tcpwrap and range options are applied to an address, both conditions must be fulfilled to allow the connection.
Takes the specified file instead of /etc/hosts.allow.
Takes the specified file instead of /etc/hosts.deny.
Looks for hosts.allow and hosts.deny in the specified directory. Is overridden by options hosts-allow and hosts-deny.

LISTEN option group

Options specific to listening sockets.

Sets the backlog value passed with the listen() system call to <count> [int]. Default is 5.
Limits the number of concurrent child processes [int]. Default is no limit.

CHILD option group

Options for addresses with multiple connections via child processes.

After establishing a connection, handles its channel in a child process and keeps the parent process attempting to produce more connections, either by listening or by connecting in a loop (example).
OPENSSL-CONNECT and OPENSSL-LISTEN differ in when they actually fork off the child: OPENSSL-LISTEN forks before the SSL handshake, while OPENSSL-CONNECT forks afterwards. RETRY and FOREVER options are not inherited by the child process.
On some operating systems (e.g. FreeBSD) this option does not work for UDP-LISTEN addresses.

EXEC option group

Options for addresses that invoke a program.

Overrides the PATH environment variable for searching the program with <string>. This $PATH value is effective in the child process too.
Prefixes argv[0] for the execvp() call with ’-’, thus making a shell behave as login shell.

FORK option group

EXEC or SYSTEM addresses invoke a program using a child process and transfer data between socat and the program. The interprocess communication mechanism can be influenced with the following options. Per default, a socketpair() is created and assigned to stdin and stdout of the child process, while stderr is inherited from the socat process, and the child process uses file descriptors 0 and 1 for communicating with the main socat process.

Does not fork a subprocess for executing the program, instead calls execvp() or system() directly from the actual socat instance. This avoids the overhead of another process between the program and its peer, but introduces a lot of restrictions:
this option can only be applied to the second socat address.
it cannot be applied to a part of a dual address.
the first socat address cannot be OPENSSL or READLINE
socat options -b, -t, -D, -l, -v, -x become useless
for both addresses, options ignoreeof, cr, and crnl become useless
for the second address (the one with option nofork), options append, cloexec, flock, user, group, mode, nonblock, perm-late, setlk, and setpgid cannot be applied. Some of these could be used on the first address though.
Creates a pair of unnamed pipes for interprocess communication instead of a socket pair.
Establishes communication with the sub process using a pseudo terminal created with openpty() instead of the default (socketpair or ptmx).
Establishes communication with the sub process using a pseudo terminal created by opening /dev/ptmx or /dev/ptc instead of the default (socketpair).
Establishes communication with the sub process using a pseudo terminal instead of a socket pair. Creates the pty with an available mechanism. If openpty and ptmx are both available, it uses ptmx because this is POSIX compliant (example).
Makes the pty the controlling tty of the sub process (example).
Directs stderr of the sub process to its output channel by making stderr a dup() of stdout (example).
Assigns the sub processes input channel to its file descriptor <fdnum> instead of stdin (0). The program started from the subprocess has to use this fd for reading data from socat (example).
Assigns the sub processes output channel to its file descriptor <fdnum> instead of stdout (1). The program started from the subprocess has to use this fd for writing data to socat (example).
Has socat pass signals of this type to the sub process. If no address has this option, socat terminates on these signals.

TERMIOS option group

For addresses that work on a tty (e.g., stdio, file:/dev/tty, exec:...,pty), the terminal parameters defined in the UN*X termios mechanism are made available as address option parameters. Please note that changes of the parameters of your interactive terminal remain effective after socat’s termination, so you might have to enter "reset" or "stty sane" in your shell afterwards. For EXEC and SYSTEM addresses with option PTY, these options apply to the pty by the child processes.

Disconnects the terminal.
Sets the serial line speed to 19200 baud. Some other rates are possible; use something like socat -hh |grep ’ b[1-9]’ to find all speeds supported by your implementation.
Note: On some operating systems, these options may not be available. Use ispeed or ospeed instead.
Enables or disables local echo.
Sets or clears canonical mode, enabling line buffering and some special characters.
Sets raw mode, thus passing input and output almost unprocessed. This option is obsolete, use option rawer or cfmakeraw instead.
Makes terminal rawer than raw option. This option implicitly turns off echo. (example).
Sets raw mode by invoking cfmakeraw() or by simulating this call. This option implicitly turns off echo.
Ignores or interpretes the BREAK character (e.g., ^C)

cr0
cr1
cr2
cr3
Sets the carriage return delay to 0, 1, 2, or 3, respectively. 0 means no delay, the other values are terminal dependent.

cs5 
cs6 
cs7 
cs8
Sets the character size to 5, 6, 7, or 8 bits, respectively.
Sets two stop bits, rather than one.
Sets the value for the VDSUSP character that suspends the current foreground process and reactivates the shell (all except Linux).
Echos control characters in hat notation (e.g. ^A)
Set the baud rate for incoming data on this line.
See also: ospeed, b19200
Sets the newline delay to 0.
Enables or disables output processing; e.g., converts NL to CR-NL.
Set the baud rate for outgoing data on this line.
See also: ispeed, b19200
Enable parity generation on output and parity checking for input.
Brings the terminal to something like a useful default state.
With UNIX System V STREAMS, removes all drivers from the stack.
With UNIX System V STREAMS, pushes the driver (module) with the given name (string) onto the stack. For example, to make sure that a character device on Solaris supports termios etc, use the following options: i-pop-all,i-push=ptem,i-push=ldterm,i-push=ttcompat

PTY option group

These options are intended for use with the pty address type.

Generates a symbolic link that points to the actual pseudo terminal (pty). This might help to solve the problem that ptys are generated with more or less unpredictable names, making it difficult to directly access the socat generated pty automatically. With this option, the user can specify a "fix" point in the file hierarchy that helps him to access the actual pty (example). Beginning with socat version 1.4.3, the symbolic link is removed when the address is closed (but see option unlink-close).
Blocks the open phase until a process opens the slave side of the pty. Usually, socat continues after generating the pty with opening the next address or with entering the transfer loop. With the wait-slave option, socat waits until some process opens the slave side of the pty before continuing. This option only works if the operating system provides the poll() system call. And it depends on an undocumented behaviour of pty’s, so it does not work on all operating systems. It has successfully been tested on Linux, FreeBSD, NetBSD, and on Tru64 with openpty.
When the wait-slave option is set, socat periodically checks the HUP condition using poll() to find if the pty’s slave side has been opened. The default polling interval is 1s. Use the pty-interval option [timeval] to change this value.

OPENSSL option group

These options apply to the openssl and openssl-listen address types.

Selects the list of ciphers that may be used for the connection. See the man page of ciphers , section CIPHER LIST FORMAT, for detailed information about syntax, values, and default of <cipherlist>.
Several cipher strings may be given, separated by ’:’. Some simple cipher strings:
3DES
Uses a cipher suite with triple DES.
Uses a cipher suite with MD5.
Uses a cipher suite without authentication.
Does not use encryption.
Uses a cipher suite with "high" encryption. Note that the peer must support the selected property, or the negotiation will fail.
Sets the protocol version to be used. Valid strings (not case sensitive) are:
Select SSL protocol version 2.
Select SSL protocol version 3.
Select the best available SSL or TLS protocol. This is the default when this option is not provided.
Select TLS protocol version 1.
Select TLS protocol version 1.1.
Select TLS protocol version 1.2.
Select DTLS protocol version 1.
Controls check of the peer’s certificate. Default is 1 (true). Disabling verify might open your socket for everyone, making the encryption useless!
Specifies the file with the certificate and private key for authentication. The certificate must be in OpenSSL format (*.pem). With openssl-listen, use of this option is strongly recommended. Except with cipher aNULL, "no shared ciphers" error will occur when no certificate is given.
Specifies the file with the private key. The private key may be in this file or in the file given with the cert option. The party that has to proof that it is the owner of a certificate needs the private key.
Specifies the file with the Diffie Hellman parameters. These parameters may also be in the file given with the cert option in which case the dhparams option is not needed.
Specifies the file with the trusted (root) authority certificates. The file must be in PEM format and should contain one or more certificates. The party that checks the authentication of its peer trusts only certificates that are in this file.
Specifies the directory with the trusted (root) certificates. The directory must contain certificates in PEM format and their hashes (see OpenSSL documentation)
On some systems, openssl requires an explicit source of random data. Specify the socket name where an entropy gathering daemon like egd provides random data, e.g. /dev/egd-pool.
On systems where openssl cannot find an entropy source and where no entropy gathering daemon can be utilized, this option activates a mechanism for providing pseudo entropy. This is archieved by taking the current time in microseconds for feeding the libc pseudo random number generator with an initial value. openssl is then feeded with output from random() calls.
NOTE:This mechanism is not sufficient for generation of secure keys!
Enable or disable the use of compression for a connection. Setting this to "none" disables compression, setting it to "auto" lets OpenSSL choose the best available algorithm supported by both parties. The default is to not touch any compression-related settings. NOTE: Requires OpenSSL 0.9.8 or higher and disabling compression with OpenSSL 0.9.8 affects all new connections in the process.
Specify the commonname that the peer certificate must match. With OPENSSL-CONNECT address this overrides the given hostname or IP target address; with OPENSSL-LISTEN this turns on check of peer certificates commonname. This option has only meaning when option verify is not disabled and the chosen cipher provides a peer certificate.
Enables FIPS mode if compiled in. For info about the FIPS encryption implementation standard see http://oss-institute.org/fips-faq.html. This mode might require that the involved certificates are generated with a FIPS enabled version of openssl. Setting or clearing this option on one socat address affects all OpenSSL addresses of this process.

RETRY option group

Options that control retry of some system calls, especially connection attempts.

Number of retries before the connection or listen attempt is aborted. Default is 0, which means just one attempt.
Time between consecutive attempts (seconds, [timespec]). Default is 1 second.
Performs an unlimited number of retry attempts.

TUN option group

Options that control Linux TUN/TAP interface device addresses.

Instructs socat to take another path for the TUN clone device. Default is /dev/net/tun.
Gives the resulting network interface a specific name instead of the system generated (tun0, tun1, etc.)
Sets the type of the TUN device; use this option to generate a TAP device. See the Linux docu for the difference between these types. When you try to establish a tunnel between two TUN devices, their types should be the same.
Sets the IFF_NO_PI flag which controls if the device includes additional packet information in the tunnel. When you try to establish a tunnel between two TUN devices, these flags should have the same values.
Sets the TUN network interface status UP. Strongly recommended.
Sets the BROADCAST flag of the TUN network interface.
Sets the DEBUG flag of the TUN network interface.
Sets the LOOPBACK flag of the TUN network interface.
Sets the POINTOPOINT flag of the TUN device.
Sets the NOTRAILERS flag of the TUN device.
Sets the RUNNING flag of the TUN device.
Sets the NOARP flag of the TUN device.
Sets the PROMISC flag of the TUN device.
Sets the ALLMULTI flag of the TUN device.
Sets the MASTER flag of the TUN device.
Sets the SLAVE flag of the TUN device.
Sets the MULTICAST flag of the TUN device.
Sets the PORTSEL flag of the TUN device.
Sets the AUTOMEDIA flag of the TUN device.
Sets the DYNAMIC flag of the TUN device.

This section explains the different data types that address parameters and address options can take.

Is currently only implemented for IPv4 and IPv6. See address-option `range’
"0" or "1"; if value is omitted, "1" is taken.
An unsigned int number, read with strtoul() , lower or equal to UCHAR_MAX .
A string specifying a program name and its arguments, separated by single spaces.
A raw data specification following dalan syntax. Currently the only valid form is a string starting with ’x’ followed by an even number of hex digits, specifying a sequence of bytes.
A string with usual UN*X directory name semantics.
The name of a syslog facility in lower case characters.
An unsigned int type, read with strtoul() , specifying a UN*X file descriptor.
A string with usual UN*X filename semantics.
If the first character is a decimal digit, the value is read with strtoul() as unsigned integer specifying a group id. Otherwise, it must be an existing group name.
A number following the rules of the strtol() function with base "0", i.e. decimal number, octal number with leading "0", or hexadecimal number with leading "0x". The value must fit into a C int.
A string specifying the device name of a network interface as shown by ifconfig or procan, e.g. "eth0".
An IPv4 address in numbers-and-dots notation, an IPv6 address in hex notation enclosed in brackets, or a hostname that resolves to an IPv4 or an IPv6 address.
Examples: 127.0.0.1, [::1], www.dest-unreach.org, dns1
An IPv4 address in numbers-and-dots notation or a hostname that resolves to an IPv4 address.
Examples: 127.0.0.1, www.dest-unreach.org, dns2
An iPv6 address in hexnumbers-and-colons notation enclosed in brackets, or a hostname that resolves to an IPv6 address.
Examples: [::1], [1234:5678:9abc:def0:1234:5678:9abc:def0], ip6name.domain.org
A number read with strtol() . The value must fit into a C long.
long long
A number read with strtoll() . The value must fit into a C long long.
An implementation dependend signed number, usually 32 bits, read with strtol or strtoll.
An implementation dependend signed number, usually 64 bits, read with strtol or strtoll.
An unsigned integer, read with strtoul() , specifying mode (permission) bits.
A number, read with strtol() , specifying a process id.
A uint16_t (16 bit unsigned number) specifying a TCP or UDP port, read with strtoul() .
An unsigned 8 bit number, read with strtoul() .
An unsigned number with size_t limitations, read with strtoul .
A socket address. See address-option `bind’
A sequence of characters, not containing ’\0’ and, depending on the position within the command line, ’:’, ’,’, or "!!". Note that you might have to escape shell meta characters in the command line.
A service name, not starting with a digit, that is resolved by getservbyname() , or an unsigned int 16 bit number read with strtoul() .
A double float specifying seconds; the number is mapped into a struct timeval, consisting of seconds and microseconds.
A double float specifying seconds; the number is mapped into a struct timespec, consisting of seconds and nanoseconds.
A service name, not starting with a digit, that is resolved by getservbyname() , or an unsigned int 16 bit number read with strtoul() .
A number read with strtoul() . The value must fit into a C unsigned int.
If the first character is a decimal digit, the value is read with strtoul() as unsigned integer specifying a user id. Otherwise, it must be an existing user name.

transfers data between STDIO (-) and a TCP4 connection to port 80 of host www.domain.org. This example results in an interactive connection similar to telnet or netcat. The stdin terminal parameters are not changed, so you may close the relay with ^D or abort it with ^C.

socat -d -d READLINE,history=$HOME/.http_history \ 
TCP4:www.domain.org:www,crnl
this is similar to the previous example, but you can edit the current line in a bash like manner (READLINE) and use the history file .http_history; socat prints messages about progress (-d -d). The port is specified by service name (www), and correct network line termination characters (crnl) instead of NL are used.

installs a simple TCP port forwarder. With TCP4-LISTEN it listens on local port "www" until a connection comes in, accepts it, then connects to the remote host (TCP4) and starts data transfer. It will not accept a second connection.

socat -d -d -lmlocal2 \ 
TCP4-LISTEN:80,bind=myaddr1,reuseaddr,fork,su=nobody,range=10.0.0.0/8 \ 
TCP4:www.domain.org:80,bind=myaddr2
TCP port forwarder, each side bound to another local IP address (bind). This example handles an almost arbitrary number of parallel or consecutive connections by fork’ing a new process after each accept() . It provides a little security by su’ing to user nobody after forking; it only permits connections from the private 10 network (range); due to reuseaddr, it allows immediate restart after master process’s termination, even if some child sockets are not completely shut down. With -lmlocal2, socat logs to stderr until successfully reaching the accept loop. Further logging is directed to syslog with facility local2.

socat TCP4-LISTEN:5555,fork,tcpwrap=script \ 
EXEC:/bin/myscript,chroot=/home/sandbox,su-d=sandbox,pty,stderr
a simple server that accepts connections (TCP4-LISTEN) and fork’s a new child process for each connection; every child acts as single relay. The client must match the rules for daemon process name "script" in /etc/hosts.allow and /etc/hosts.deny, otherwise it is refused access (see "man 5 hosts_access"). For EXEC’uting the program, the child process chroot’s to /home/sandbox, su’s to user sandbox, and then starts the program /home/sandbox/bin/myscript. Socat and myscript communicate via a pseudo tty (pty); myscript’s stderr is redirected to stdout, so its error messages are transferred via socat to the connected client.

socat EXEC:"mail.sh target@domain.com",fdin=3,fdout=4 \ 
TCP4:mail.relay.org:25,crnl,bind=alias1.server.org,mss=512
mail.sh is a shell script, distributed with socat, that implements a simple SMTP client. It is programmed to "speak" SMTP on its FDs 3 (in) and 4 (out). The fdin and fdout options tell socat to use these FDs for communication with the program. Because mail.sh inherits stdin and stdout while socat does not use them, the script can read a mail body from stdin. Socat makes alias1 your local source address (bind), cares for correct network line termination (crnl) and sends at most 512 data bytes per packet (mss).

opens an interactive connection via the serial line, e.g. for talking with a modem. rawer sets the console’s and ttyS0’s terminal parameters to practicable values, crnl converts to correct newline characters. escape allows terminating the socat process with character control-O.

socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork \ 
SOCKS4:host.victim.org:127.0.0.1:6000,socksuser=nobody,sourceport=20
with UNIX-LISTEN, socat opens a listening UNIX domain socket /tmp/.X11-unix/X1. This path corresponds to local XWindow display :1 on your machine, so XWindow client connections to DISPLAY=:1 are accepted. Socat then speaks with the SOCKS4 server host.victim.org that might permit sourceport 20 based connections due to an FTP related weakness in its static IP filters. Socat pretends to be invoked by socksuser nobody, and requests to be connected to loopback port 6000 (only weak sockd configurations will allow this). So we get a connection to the victims XWindow server and, if it does not require MIT cookies or Kerberos authentication, we can start work. Please note that there can only be one connection at a time, because TCP can establish only one session with a given set of addresses and ports.

this is an example for unidirectional data transfer (-u). Socat transfers data from file /tmp/readdata (implicit address GOPEN), starting at its current end (seek-end=0 lets socat start reading at current end of file; use seek=0 or no seek option to first read the existing data) in a "tail -f" like mode (ignoreeof). The "file" might also be a listening UNIX domain socket (do not use a seek option then).

(sleep 5; echo PASSWORD; sleep 5; echo ls; sleep 1) | 
socat - EXEC:'ssh -l user server',pty,setsid,ctty
EXEC’utes an ssh session to server. Uses a pty for communication between socat and ssh, makes it ssh’s controlling tty (ctty), and makes this pty the owner of a new process group (setsid), so ssh accepts the password from socat.

socat -u TCP4-LISTEN:3334,reuseaddr,fork \ 
OPEN:/tmp/in.log,creat,append
implements a simple network based message collector. For each client connecting to port 3334, a new child process is generated (option fork). All data sent by the clients are append’ed to the file /tmp/in.log. If the file does not exist, socat creat’s it. Option reuseaddr allows immediate restart of the server process.

socat PTY,link=$HOME/dev/vmodem0,rawer,wait-slave \
EXEC:"ssh modemserver.us.org socat - /dev/ttyS0,nonblock,rawer"
generates a pseudo terminal device (PTY) on the client that can be reached under the symbolic link $HOME/dev/vmodem0. An application that expects a serial line or modem can be configured to use $HOME/dev/vmodem0; its traffic will be directed to a modemserver via ssh where another socat instance links it to /dev/ttyS0.

socat TCP4-LISTEN:2022,reuseaddr,fork \ 
PROXY:proxy:www.domain.org:22,proxyport=3128,proxyauth=user:pass
starts a forwarder that accepts connections on port 2022, and directs them through the proxy daemon listening on port 3128 (proxyport) on host proxy, using the CONNECT method, where they are authenticated as "user" with "pass" (proxyauth). The proxy should establish connections to host www.domain.org on port 22 then.

is an OpenSSL client that tries to establish a secure connection to an SSL server. Option cafile specifies a file that contains trust certificates: we trust the server only when it presents one of these certificates and proofs that it owns the related private key. Otherwise the connection is terminated. With cert a file containing the client certificate and the associated private key is specified. This is required in case the server wishes a client authentication; many Internet servers do not.
The first address (’-’) can be replaced by almost any other socat address.

is an OpenSSL server that accepts TCP connections, presents the certificate from the file server.pem and forces the client to present a certificate that is verified against cafile.crt.
The second address (’PIPE’) can be replaced by almost any other socat address.
For instructions on generating and distributing OpenSSL keys and certificates see the additional socat docu socat-openssl.txt.

creates a 100GB sparse file; this requires a file system type that supports this (ext2, ext3, reiserfs, jfs; not minix, vfat). The operation of writing 1 byte might take long (reiserfs: some minutes; ext2: "no" time), and the resulting file can consume some disk space with just its inodes (reiserfs: 2MB; ext2: 16KB).

listens for incoming TCP connections on port 7777. For each accepted connection, invokes a shell. This shell has its stdin and stdout directly connected to the TCP socket (nofork). The shell starts filan and lets it print the socket addresses to stderr (your terminal window).

functions as primitive binary editor: it writes the 4 bytes 000 014 000 000 to the executable /usr/bin/squid at offset 0x00074420 (this is a real world patch to make the squid executable from Cygwin run under Windows, actual per May 2004).

connects to an unknown service and prevents being flooded.

merges data arriving from different TCP streams on port 8888 to just one stream to target:9999. The end-close option prevents the child processes forked off by the second address from terminating the shared connection to 9999 (close(2) just unlinks the inode which stays active as long as the parent process lives; shutdown(2) would actively terminate the connection).

sends a broadcast to the network 192.168.1.0/24 and receives the replies of the timeservers there. Ignores NTP packets from hosts outside this network.
is semantically equivalent to the previous example, but all parameters are specified in generic form. the value 6 of setsockopt-int is the Linux value for SO_BROADCAST.

sends a broadcast to the local network(s) using protocol 44. Accepts replies from the private address range only.

transfers data from stdin to the specified multicast address using UDP. Both local and remote ports are 6666. Tells the interface eth0 to also accept multicast packets of the given group. Multiple hosts on the local network can run this command, so all data sent by any of the hosts will be received by all the other ones. Note that there are many possible reasons for failure, including IP-filters, routing issues, wrong interface selection by the operating system, bridges, or a badly configured switch.

establishes one side of a virtual (but not private!) network with host2 where a similar process might run, with UDP-L and tun address 192.168.255.2. They can reach each other using the addresses 192.168.255.1 and 192.168.255.2. Note that streaming eg. via TCP or SSL does not guarantee to retain packet boundaries and may thus cause packet loss.

circumvents the problem that pppd requires a serial device and thus might not be able to work on a synchronous line that is represented by a network device. socat creates a PTY to make pppd happy, binds to the network interface hdlc0, and can transfer data between both devices. Use pppd on device /var/run/ppp then.
creates a simple HTTP echo server: each HTTP client that connects gets a valid HTTP reply that contains information about the client address and port as it is seen by the server host, the host address (which might vary on multihomed servers), and the original client request.

waits for an incoming UDP packet on port 9999 and prints the environment variables provided by socat. On BSD based systems you have to replace ip-pktinfo with ip-recvdstaddr,ip-recvif. Especially interesting is SOCAT_IP_DSTADDR: it contains the target address of the packet which may be a unicast, multicast, or broadcast address.

Socat uses a logging mechanism that allows filtering messages by severity. The severities provided are more or less compatible to the appropriate syslog priority. With one or up to four occurrences of the -d command line option, the lowest priority of messages that are issued can be selected. Each message contains a single uppercase character specifying the messages severity (one of F, E, W, N, I, or D)

Conditions that require unconditional and immediate program termination.
Conditions that prevent proper program processing. Usually the program is terminated (see option -s).
Something did not function correctly or is in a state where correct further processing cannot be guaranteed, but might be possible.
Interesting actions of the program, e.g. for supervising socat in some kind of server mode.
Description of what the program does, and maybe why it happens. Allows monitoring the lifecycles of file descriptors.
Description of how the program works, all system or library calls and their results.

Log messages can be written to stderr, to a file, or to syslog.

On exit, socat gives status 0 if it terminated due to EOF or inactivity timeout, with a positive value on error, and with a negative value on fatal error.

/usr/bin/socat
/usr/bin/filan
/usr/bin/procan

Input variables carry information from the environment to socat, output variables are set by socat for use in executed scripts and programs.

In the output variables beginning with "SOCAT" this prefix is actually replaced by the upper case name of the executable or the value of option -lp.

(Values 4 or 6) Sets the IP version to be used for listen, recv, and recvfrom addresses if no pf (protocol-family) option is given. Is overridden by socat options -4 or -6.
(Values 0, 4, or 6) Sets the IP version to be used when resolving target host names when version is not specified by address type, option pf (protocol-family), or address format. If name resolution does not return a matching entry, the first result (with differing IP version) is taken. With value 0, socat always selects the first record and its IP version.
Specifies the time (seconds) to sleep the parent and child processes after successful fork(). Useful for debugging.
Socat sets this variable to its version string, e.g. "1.7.0.0" for released versions or e.g. "1.6.0.1+envvar" for temporary versions; can be used in scripts invoked by socat.
Socat sets this variable to its process id. In case of fork address option, SOCAT_PID gets the child processes id. Forking for exec and system does not change SOCAT_PID.
Socat sets this variable to its process id. In case of fork, SOCAT_PPID keeps the pid of the master process.
With passive socket addresses (all LISTEN and RECVFROM addresses), this variable is set to a string describing the peers socket address. Port information is not included.
With appropriate passive socket addresses (TCP, UDP, and SCTP - LISTEN and RECVFROM), this variable is set to a string containing the number of the peer port.
With all LISTEN addresses, this variable is set to a string describing the local socket address. Port information is not included example
With TCP-LISTEN, UDP-LISTEN, and SCTP-LISTEN addresses, this variable is set to the local port.
With all RECVFROM addresses where address option so-timestamp is applied, socat sets this variable to the resulting timestamp.
With all IPv4 based RECVFROM addresses where address option ip-recvopts is applied, socat fills this variable with the IP options of the received packet.
With all IPv4 based RECVFROM addresses where address option ip-recvdstaddr (BSD) or ip-pktinfo (other platforms) is applied, socat sets this variable to the destination address of the received packet. This is particularly useful to identify broadcast and multicast addressed packets.
With all IPv4 based RECVFROM addresses where address option ip-recvif (BSD) or ip-pktinfo (other platforms) is applied, socat sets this variable to the name of the interface where the packet was received.
With all IPv4 based RECVFROM addresses where address option ip-pktinfo is applied, socat sets this variable to the address of the interface where the packet was received.
With all IPv4 based RECVFROM addresses where address option ip-recvtos is applied, socat sets this variable to the TOS (type of service) of the received packet.
With all IPv4 based RECVFROM addresses where address option ip-recvttl is applied, socat sets this variable to the TTL (time to live) of the received packet.
With all IPv6 based RECVFROM addresses where address option ipv6-recvhoplimit is applied, socat sets this variable to the hoplimit value of the received packet.
With all IPv6 based RECVFROM addresses where address option ipv6-recvpktinfo is applied, socat sets this variable to the destination address of the received packet.
With all IPv6 based RECVFROM addresses where address option ipv6-recvtclass is applied, socat sets this variable to the transfer class of the received packet.
Issuer field from peer certificate
Subject field from peer certificate
commonName entries from peer certificates subject. Multiple values are separated by " // ".
all other entries from peer certificates subject
DNS entries from peer certificates extensions - subjectAltName field. Multiple values are separated by " // ".
Is used to determine the hostname for logging (see -lh).
Is used as name for the socks client user name if no socksuser is given.
With options su and su-d, LOGNAME is set to the given user name.
Is used as name for the socks client user name if no socksuser is given and LOGNAME is empty.
With options su and su-d, USER is set to the given user name.
With options su and su-d, SHELL is set to the login shell of the given user.
Can be set with option path for exec and system addresses.
With options su and su-d, HOME is set to the home directory of the given user.

The work of the following groups and organizations was invaluable for this project:

The FSF (GNU, http://www.fsf.org/ project with their free and portable development software and lots of other useful tools and libraries.

The Linux developers community (http://www.linux.org/) for providing a free, open source operating system.

The Open Group (http://www.unix-systems.org/) for making their standard specifications available on the Internet for free.

This man page describes version 1.7.3 of socat.

Addresses cannot be nested, so a single socat process cannot, e.g., drive ssl over socks.

Address option ftruncate without value uses default 1 instead of 0.

Verbose modes (-x and/or -v) display line termination characters inconsistently when address options cr or crnl are used: They show the data after conversion in either direction.

The licenses of OpenSSL and GNU Readline are incompatible. Therefore readline support is disabled in Debian.

Send bug reports to <socat@dest-unreach.org>

nc(1), rinetd(8), openssl(1), stunnel(8), rlwrap(1), setsid(1)

Socat home page http://www.dest-unreach.org/socat/

Gerhard Rieger <rieger@dest-unreach.org>