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>

proxsmtp                                         September, 2004                                    proxsmtpd(8)