Ubuntu Manpage: proxsmtpd — an SMTP server for performing filtering

NAME

     proxsmtpd — an SMTP server for performing filtering

SYNOPSIS

     proxsmtpd [-d level] [-f configfile] [-p pidfile]
     proxsmtpd -v

DESCRIPTION

     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.

OPTIONS

     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.

FILTER SCRIPTS

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 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.

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.

REMOTE_HELO
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.

LOGGING

     proxsmtpd logs to syslogd by default under the 'mail' facility. You can also output logs to the console
     using the -d option.

LOOPBACK FEATURE

     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.

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.

SECURITY

     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).

AUTHOR

     Stef Walter <stef@memberwebs.com>