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     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          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
     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

     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 〈