proxsmtpd - an SMTP server for performing filtering
proxsmtpd [-d level] [-f configfile] [-p pidfile]
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 Don’t detach from the console and run as a daemon. In
addition the level argument specifies what level of error
messages to display. 0 being the least, 4 the most.
-f configfile specifies an alternate location for the proxsmtpd
configuration file. See proxsmtpd.conf(5) for more details on
where the configuration file is located by default.
-p pidfile specifies a location for the a process id file to be
written to. This file contains the process id of proxsmtpd
and can be used to stop the daemon.
-v Prints the proxsmtp version number and exits.
The 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
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
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.
CLIENT The network address of the SMTP client connected.
EMAIL When the FilterType option is set to ’file’, this specifies
the file that the email was saved to.
RECIPIENTS The email addresses of the email recipients. These are
specified one per line, in standard address format.
REMOTE If 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.
If 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.
SENDER The email address for the sender of the email.
SERVER The network address of the SMTP server we’re connected to.
TMPDIR The path to the temp directory in use. This is the same as
the TempDirectory option.
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
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.
TRANSPARENT PROXY FEATURE
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
Make sure you understand the issues involved with escaping external data.
The environment variables such as SENDER or RECIPIENTS need to be treated
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 〈email@example.com〉