| proxsmtpd(8) | System Manager's Manual | proxsmtpd(8) |
proxsmtpd — an
SMTP server for performing filtering
proxsmtpd |
[-d level]
[-f configfile]
[-p pidfile] |
proxsmtpd |
-v |
proxsmtpd is an SMTP filter that allows
you to perform arbitrary filtering on email. It accepts SMTP connections and
forwards the SMTP commands and responses to another SMTP server.
The DATA email body is intercepted and scanned before forwarding. Email can be altered, bounced, or silently dropped.
proxsmtpd aims to be lightweight and
simple rather than have a myriad of options. The options it does have are
configured by editing the proxsmtpd.conf(5) file. See the
man page for proxsmtpd.conf(5) for more info on the
default location of the configuration file.
The options are as follows.
-d-fproxsmtpd configuration file. See
proxsmtpd.conf(5) for more details on where the
configuration file is located by default.-pproxsmtpd and can be used to stop the daemon.-vThe filter script is specified using the FilterCommand option. By default the email is piped through the script on standard input. Standard output is read for the filtered email. Standard error is also read for error messages.
If the FilterType option is set to 'file', your filter will operate on a file rather than processing standard in and standard out. The file name will be passed to your filter command using the EMAIL environment variable. Your script can change the file as needed. Standard error is still processed as outlined below.
If the filter command returns a successful exit code (ie: 0), then the filtered email is sent to the destination mail server as usual. When a error exit code (ie: anything but 0) a failure message is sent back to the sending server. In this case the email is not sent.
You can customize the error message sent back. The last line of output printed to standard error will be used in this case. If you specify a full SMTP error code then it will be used (ie: '550 Bad Email'). If it's just a text message then a 550 SMTP error code will be used.
You can silently drop messages by using an error message with a 250 SMTP code. This gives the illusion to the sending server that the email was accepted.
Various environment variables will be present when your script is run. You may need to escape them properly before use in your favorite scripting language. Failure to do this could lead to a REMOTE COMPROMISE of your machine.
proxsmtpd is being used to filter email between
SMTP servers, then this is the IP address of the original client. In order
for this information to be present (a) the SMTP client (sending server)
must an send an XFORWARD command and (b) the SMTP server (receiving
server) must accept that XFORWARD command without error.proxsmtpd is being used to filter email between
SMTP servers, then this is the HELO/EHLO banner of the original client. In
order for this information to be present (a) the SMTP client (sending
server) must an send an XFORWARD command and (b) the SMTP server
(receiving server) must accept that XFORWARD command without error.proxsmtpd logs to
syslogd by default under the 'mail' facility. You can also
output logs to the console using the -d option.
In some cases it's advantageous to consolidate the filtering for
several mail servers on one machine. proxsmtpd
allows this by providing a loopback feature to connect back to the IP that
an SMTP connection comes in from.
To use this feature specify only a port number (no IP address) for
the OutAddress setting in the configuration file. This
will cause proxsmtpd to pass the email back to the
said port on the incoming IP address.
Make sure the MaxConnections setting is set high enough to handle the mail from all the servers without refusing connections.
A transparent proxy is a configuration on a gateway that routes
certain types of traffic through a proxy server without any changes on the
client computers. proxsmtpd has support for
transparent proxying of SMTP traffic by enabling the
TransparentProxy setting. This type of setup usually
involves firewall rules which redirect traffic to
proxsmtpd and the setup varies from OS to OS. The
SMTP traffic will be forwarded to it's original destination after being
scanned.
Note that some features (such as SSL/TLS) will not be available when going through the transparent proxy.
Make sure that the MaxConnections setting is
set high enough for your transparent proxying. Because
proxsmtpd is not being used as a filter inside a
queue, which usually throttles the amount of email going through, this
setting may need to be higher than usual.
There's no reason to run this daemon as root. It is meant as a filter and should listen on a high TCP port.
Care should be taken with the directory that
proxsmtpd writes its temporary files to. In order to
be secure, it should not be a world writeable location. Specify the
directory using the TempDirectory setting.
Make sure you understand the issues involved with escaping external data. The environment variables such as SENDER or RECIPIENTS need to be treated with care.
If running proxsmtpd on a publicly
accessible IP address or without a firewall please be sure to understand all
the possible security issues. This is especially true if the loopback
feature is used (see above).
Stef Walter ⟨stef@memberwebs.com⟩
| September, 2004 | proxsmtp |