| SSHD(8) | System Manager's Manual | SSHD(8) |
sshd — OpenSSH
daemon
sshd |
[-46DdeiqTt]
[-C connection_spec]
[-c host_certificate_file]
[-E log_file]
[-f config_file]
[-g login_grace_time]
[-h host_key_file]
[-o option]
[-p port]
[-u len] |
sshd (OpenSSH Daemon) is the daemon
program for ssh(1). Together these programs replace rlogin
and rsh, and provide secure encrypted communications between two untrusted
hosts over an insecure network.
sshd listens for connections from clients.
It is normally started at boot from /etc/init.d/ssh.
It forks a new daemon for each incoming connection. The forked daemons
handle key exchange, encryption, authentication, command execution, and data
exchange.
sshd can be configured using command-line
options or a configuration file (by default
sshd_config(5)); command-line options override values
specified in the configuration file. sshd rereads
its configuration file when it receives a hangup signal,
SIGHUP, by executing itself with the name and
options it was started with, e.g.
/usr/sbin/sshd.
The options are as follows:
-4sshd to use IPv4 addresses only.-6sshd to use IPv6 addresses only.-C
connection_spec-T extended test mode. If provided, any
Match directives in the configuration file that
would apply are applied before the configuration is written to standard
output. The connection parameters are supplied as keyword=value pairs and
may be supplied in any order, either with multiple
-C options or as a comma-separated list. The
keywords are “addr,” “user”,
“host”, “laddr”, “lport”, and
“rdomain” and correspond to source address, user, resolved
source host name, local address, local port number and routing domain
respectively.-c
host_certificate_filesshd during key exchange. The certificate file
must match a host key file specified using the -h
option or the HostKey configuration
directive.-Dsshd will not
detach and does not become a daemon. This allows easy monitoring of
sshd.-d-d options
increase the debugging level. Maximum is 3.-E
log_file-e-f
config_filesshd
refuses to start if there is no configuration file.-g
login_grace_time-h
host_key_filesshd is not run as root (as the normal host key
files are normally not readable by anyone but root). The default is
/etc/ssh/ssh_host_ecdsa_key,
/etc/ssh/ssh_host_ed25519_key and
/etc/ssh/ssh_host_rsa_key. It is possible to have
multiple host key files for the different host key algorithms.-isshd is being run from
inetd(8).-o
option-p
portPort option are
ignored when a command-line port is specified. Ports specified using the
ListenAddress option override command-line
ports.-q-TMatch rules may be applied by specifying the
connection parameters using one or more -C
options.-tsshd
reliably as configuration options may change.-u
lenutmp structure that holds the remote host name. If
the resolved host name is longer than len, the
dotted decimal value will be used instead. This allows hosts with very
long host names that overflow this field to still be uniquely identified.
Specifying -u0 indicates that only dotted decimal
addresses should be put into the utmp file.
-u0 may also be used to prevent
sshd from making DNS requests unless the
authentication mechanism or configuration requires it. Authentication
mechanisms that may require DNS include
HostbasedAuthentication and using a
from="pattern-list" option in a key
file. Configuration options that require DNS include using a USER@HOST
pattern in AllowUsers or
DenyUsers.The OpenSSH SSH daemon supports SSH protocol 2 only. Each host has a host-specific key, used to identify the host. Whenever a client connects, the daemon responds with its public host key. The client compares the host key against its own database to verify that it has not changed. Forward secrecy is provided through a Diffie-Hellman key agreement. This key agreement results in a shared session key. The rest of the session is encrypted using a symmetric cipher. The client selects the encryption algorithm to use from those offered by the server. Additionally, session integrity is provided through a cryptographic message authentication code (MAC).
Finally, the server and the client enter an authentication dialog. The client tries to authenticate itself using host-based authentication, public key authentication, challenge-response authentication, or password authentication.
Regardless of the authentication type, the account is checked to
ensure that it is accessible. An account is not accessible if it is locked,
listed in DenyUsers or its group is listed in
DenyGroups . The definition of a locked account is
system dependent. Some platforms have their own account database (eg AIX)
and some modify the passwd field (
‘*LK*’ on Solaris and UnixWare,
‘*’ on HP-UX, containing
‘Nologin’ on Tru64, a leading
‘*LOCKED*’ on FreeBSD and a leading
‘!’ on most Linuxes). If there is a
requirement to disable password authentication for the account while
allowing still public-key, then the passwd field should be set to something
other than these values (eg ‘NP’ or
‘*NP*’ ).
If the client successfully authenticates itself, a dialog for preparing the session is entered. At this time the client may request things like allocating a pseudo-tty, forwarding X11 connections, forwarding TCP connections, or forwarding the authentication agent connection over the secure channel.
After this, the client either requests a shell or execution of a command. The sides then enter session mode. In this mode, either side may send data at any time, and such data is forwarded to/from the shell or command on the server side, and the user terminal in the client side.
When the user program terminates and all forwarded X11 and other connections have been closed, the server sends command exit status to the client, and both sides exit.
When a user successfully logs in, sshd
does the following:
PermitUserEnvironment option in
sshd_config(5).PermitUserRC
option is set, runs it; else if /etc/ssh/sshrc
exists, runs it; otherwise runs xauth. The “rc” files are
given the X11 authentication protocol and cookie in standard input. See
SSHRC, below.If the file ~/.ssh/rc exists,
sh(1) runs it after reading the environment files but
before starting the user's shell or command. It must not produce any output
on stdout; stderr must be used instead. If X11 forwarding is in use, it will
receive the "proto cookie" pair in its standard input (and
DISPLAY in its environment). The script must call
xauth(1) because sshd will not run
xauth automatically to add X11 cookies.
The primary purpose of this file is to run any initialization routines which may be needed before the user's home directory becomes accessible; AFS is a particular example of such an environment.
This file will probably contain some initialization code followed by something similar to:
if read proto cookie && [ -n "$DISPLAY" ]; then if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then # X11UseLocalhost=yes echo add unix:`echo $DISPLAY | cut -c11-` $proto $cookie else # X11UseLocalhost=no echo add $DISPLAY $proto $cookie fi | xauth -q - fi
If this file does not exist, /etc/ssh/sshrc is run, and if that does not exist either, xauth is used to add the cookie.
AuthorizedKeysFile specifies the files
containing public keys for public key authentication; if this option is not
specified, the default is ~/.ssh/authorized_keys and
~/.ssh/authorized_keys2. Each line of the file
contains one key (empty lines and lines starting with a
‘#’ are ignored as comments). Public
keys consist of the following space-separated fields: options, keytype,
base64-encoded key, comment. The options field is optional. The supported
key types are:
The comment field is not used for anything (but may be convenient for the user to identify the key).
Note that lines in this file can be several hundred bytes long (because of the size of the public key encoding) up to a limit of 8 kilobytes, which permits RSA keys up to 16 kilobits. You don't want to type them in; instead, copy the id_dsa.pub, id_ecdsa.pub, id_ecdsa_sk.pub, id_ed25519.pub, id_ed25519_sk.pub, or the id_rsa.pub file and edit it.
sshd enforces a minimum RSA key modulus
size of 1024 bits.
The options (if present) consist of comma-separated option specifications. No spaces are permitted, except within double quotes. The following option specifications are supported (note that option keywords are case-insensitive):
agent-forwardingrestrict option.Certificates may encode access restrictions similar to these key options. If both certificate restrictions and key options are present, the most restrictive union of the two is applied.
command="command"no-pty. A quote may be
included in the command by quoting it with a backslash.
This option might be useful to restrict certain public keys to
perform just a specific operation. An example might be a key that
permits remote backups but nothing else. Note that the client may
specify TCP and/or X11 forwarding unless they are explicitly prohibited,
e.g. using the restrict key option.
The command originally supplied by the client is available in
the SSH_ORIGINAL_COMMAND environment variable.
Note that this option applies to shell, command or subsystem execution.
Also note that this command may be superseded by a
sshd_config(5) ForceCommand
directive.
If a command is specified and a forced-command is embedded in a certificate used for authentication, then the certificate will be accepted only if the two commands are identical.
environment="NAME=value"PermitUserEnvironment option.expiry-time="timespec"from="pattern-list"In addition to the wildcard matching that may be applied to
hostnames or addresses, a from stanza may match
IP addresses using CIDR address/masklen notation.
The purpose of this option is to optionally increase security: public key authentication by itself does not trust the network or name servers or anything (but the key); however, if somebody somehow steals the key, the key permits an intruder to log in from anywhere in the world. This additional option makes using a stolen key more difficult (name servers and/or routers would have to be compromised in addition to just the key).
no-agent-forwardingno-port-forwardingcommand option.no-ptyno-user-rcno-X11-forwardingpermitlisten="[host:]port"-R option such that it may only listen on the
specified host (optional) and port. IPv6 addresses can be specified by
enclosing the address in square brackets. Multiple
permitlisten options may be applied separated by
commas. Hostnames may include wildcards as described in the PATTERNS
section in ssh_config(5). A port specification of
* matches any port. Note that the setting of
GatewayPorts may further restrict listen
addresses. Note that ssh(1) will send a hostname of
“localhost” if a listen host was not specified when the
forwarding was requested, and that this name is treated differently to the
explicit localhost addresses “127.0.0.1” and
“::1”.permitopen="host:port"-L option such that it may only connect to the
specified host and port. IPv6 addresses can be specified by enclosing the
address in square brackets. Multiple permitopen
options may be applied separated by commas. No pattern matching or name
lookup is performed on the specified hostnames, they must be literal host
names and/or addresses. A port specification of *
matches any port.port-forwardingrestrict option.principals="principals"cert-authority line, specifies allowed
principals for certificate authentication as a comma-separated list. At
least one name from the list must appear in the certificate's list of
principals for the certificate to be accepted. This option is ignored for
keys that are not marked as trusted certificate signers using the
cert-authority option.ptyrestrict option.no-touch-requiredecdsa-sk and
ed25519-sk.verify-requiredecdsa-sk and
ed25519-sk.restricttunnel="n"user-rcrestrict option.X11-forwardingrestrict option.An example authorized_keys file:
# Comments allowed at start of line ssh-rsa AAAAB3Nza...LiPk== user@example.net from="*.sales.example.net,!pc.sales.example.net" ssh-rsa AAAAB2...19Q== john@example.net command="dump /home",no-pty,no-port-forwarding ssh-rsa AAAAC3...51R== example.net permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-rsa AAAAB5...21S== permitlisten="localhost:8080",permitopen="localhost:22000" ssh-rsa AAAAB5...21S== tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== jane@example.net restrict,command="uptime" ssh-rsa AAAA1C8...32Tv== user@example.net restrict,pty,command="nethack" ssh-rsa AAAA1f8...IrrC5== user@example.net no-touch-required sk-ecdsa-sha2-nistp256@openssh.com AAAAInN...Ko== user@example.net
The /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts files contain host public keys for all known hosts. The global file should be prepared by the administrator (optional), and the per-user file is maintained automatically: whenever the user connects to an unknown host, its key is added to the per-user file.
Each line in these files contains the following fields: markers (optional), hostnames, keytype, base64-encoded key, comment. The fields are separated by spaces.
The marker is optional, but if it is present then it must be one of “@cert-authority”, to indicate that the line contains a certification authority (CA) key, or “@revoked”, to indicate that the key contained on the line is revoked and must not ever be accepted. Only one marker should be used on a key line.
Hostnames is a comma-separated list of patterns
(‘*’ and
‘?’ act as wildcards); each pattern in
turn is matched against the host name. When sshd is
authenticating a client, such as when using
HostbasedAuthentication, this will be the canonical
client host name. When ssh(1) is authenticating a server,
this will be the host name given by the user, the value of the
ssh(1) HostkeyAlias if it was
specified, or the canonical server hostname if the ssh(1)
CanonicalizeHostname option was used.
A pattern may also be preceded by
‘!’ to indicate negation: if the host
name matches a negated pattern, it is not accepted (by that line) even if it
matched another pattern on the line. A hostname or address may optionally be
enclosed within ‘[’ and
‘]’ brackets then followed by
‘:’ and a non-standard port
number.
Alternately, hostnames may be stored in a hashed form which hides
host names and addresses should the file's contents be disclosed. Hashed
hostnames start with a ‘|’ character.
Only one hashed hostname may appear on a single line and none of the above
negation or wildcard operators may be applied.
The keytype and base64-encoded key are taken directly from the host key; they can be obtained, for example, from /etc/ssh/ssh_host_rsa_key.pub. The optional comment field continues to the end of the line, and is not used.
Lines starting with ‘#’ and
empty lines are ignored as comments.
When performing host authentication, authentication is accepted if any matching line has the proper key; either one that matches exactly or, if the server has presented a certificate for authentication, the key of the certification authority that signed the certificate. For a key to be trusted as a certification authority, it must use the “@cert-authority” marker described above.
The known hosts file also provides a facility to mark keys as revoked, for example when it is known that the associated private key has been stolen. Revoked keys are specified by including the “@revoked” marker at the beginning of the key line, and are never accepted for authentication or as certification authorities, but instead will produce a warning from ssh(1) when they are encountered.
It is permissible (but not recommended) to have several lines or different host keys for the same names. This will inevitably happen when short forms of host names from different domains are put in the file. It is possible that the files contain conflicting information; authentication is accepted if valid information can be found from either file.
Note that the lines in these files are typically hundreds of characters long, and you definitely don't want to type in the host keys by hand. Rather, generate them by a script, ssh-keyscan(1) or by taking, for example, /etc/ssh/ssh_host_rsa_key.pub and adding the host names at the front. ssh-keygen(1) also offers some basic automated editing for ~/.ssh/known_hosts including removing hosts matching a host name and converting all host names to their hashed representations.
An example ssh_known_hosts file:
# Comments allowed at start of line closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= # A hashed hostname |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa AAAA1234.....= # A revoked key @revoked * ssh-rsa AAAAB5W... # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
PrintLastLog
and PrintMotd, respectively, are enabled. It does
not suppress printing of the banner specified by
Banner.
sshd reads it as root.
Additionally, this file must be owned by the user, and must not have write
permissions for anyone else. The recommended permission for most machines
is read/write for the user, and not accessible by others.
If this file, the ~/.ssh directory, or
the user's home directory are writable by other users, then the file
could be modified or replaced by unauthorized users. In this case,
sshd will not allow it to be used unless the
StrictModes option has been set to
“no”.
#’), and assignment lines of the
form name=value. The file should be writable only by the user; it need not
be readable by anyone else. Environment processing is disabled by default
and is controlled via the PermitUserEnvironment
option.
sshd refuses to let anyone
except root log in. The contents of the file are displayed to anyone
trying to log in, and non-root connections are refused. The file should be
world-readable.
sshd does not start if these
files are group/world-accessible.
sshd. The file
format and configuration options are described in
sshd_config(5).
sshd
during privilege separation in the pre-authentication phase. The directory
should not contain any files and must be owned by root and not group or
world-writable.
sshd listening for
connections (if there are several daemons running concurrently for
different ports, this contains the process ID of the one started last).
The content of this file is not sensitive; it can be world-readable.scp(1), sftp(1), ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), chroot(2), hosts_access(5), moduli(5), sshd_config(5), inetd(8), sftp-server(8)
OpenSSH is a derivative of the original and free ssh 1.2.12 release by Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo de Raadt and Dug Song removed many bugs, re-added newer features and created OpenSSH. Markus Friedl contributed the support for SSH protocol versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support for privilege separation.
| August 27, 2020 | Debian |