path: root/doc/proxsmtpd.8

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.\" CONTRIBUTORS
.\"  Stef Walter <stef@memberwebs.com>
.\"
.Dd September, 2004
.Dt proxsmtpd 8
.Os proxsmtp 
.Sh NAME
.Nm proxsmtpd
.Nd an SMTP server for performing filtering
.Sh SYNOPSIS
.Nm
.Op Fl d Ar level
.Op Fl f Ar configfile
.Op Fl p Ar pidfile
.Nm 
.Fl v
.Sh DESCRIPTION
.Nm
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. 
.Pp
The DATA email body is intercepted and scanned before forwarding. Email can be 
altered, bounced, or silently dropped. 
.Pp
.Nm
aims to be lightweight and simple rather than have a myriad of options. The options
it does have are configured by editing the 
.Xr proxsmtpd.conf 5
file. See the man page for 
.Xr proxsmtpd.conf 5
for more info on the default location of the configuration file.
.Sh OPTIONS
The options are as follows. 
.Bl -tag -width Fl
.It Fl d 
Don't detach from the console and run as a daemon. In addition the 
.Ar level
argument specifies what level of error messages to display. 0 being 
the least, 4 the most.
.It Fl f 
.Ar configfile 
specifies an alternate location for the 
.Nm
configuration file. See 
.Xr proxsmtpd.conf 5
for more details on where the configuration file is located by default.
.It Fl p
.Ar pidfile
specifies a location for the a process id file to be written to. This file 
contains the process id of 
.Nm 
and can be used to stop the daemon.
.It Fl v
Prints the proxsmtp version number and exits.
.El
.Sh FILTER SCRIPTS
The filter script is specified using the 
.Ar 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.
.Pp
If the
.Ar 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 
.Ar EMAIL
environment variable. Your script can change the file as needed. Standard error 
is still processed as outlined below.
.Pp
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. 
.Pp
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. 
.Pp
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.
.Pp
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.
.Bl -tag -width Fl
.It Ar CLIENT
The network address of the SMTP client connected.
.It Ar EMAIL
When the
.Ar FilterType
option is set to 'file', this specifies the file that the email was saved to.
.It Ar RECIPIENTS
The email addresses of the email recipients. These are specified one per 
line, in standard address format. 
.It Ar REMOTE
If 
.Nm
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.
.It Ar SENDER
The email address for the sender of the email. 
.It Ar SERVER
The network address of the SMTP server we're connected to.
.It Ar TMPDIR
The path to the temp directory in use. This is the same as the 
.Ar TempDirectory
option. 
.El
.Sh LOGGING
.Nm
logs to 
.Xr syslogd 
by default under the 'mail' facility. You can also output logs to the console
using the 
.Fl d 
option.
.Sh LOOPBACK FEATURE
In some cases it's advantageous to consolidate the filtering for several mail 
servers on one machine. 
.Nm
allows this by providing a loopback feature to connect back to the IP that an 
SMTP connection comes in from. 
.Pp
To use this feature specify only a port number (no IP address) for the 
.Ar OutAddress
setting in the configuration file. This will cause 
.Nm
to pass the email back to the said port on the incoming IP address.
.Pp
Make sure the 
.Ar MaxConnections
setting is set high enough to handle the mail from all the servers without refusing
connections. 
.Sh 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. 
.Nm
has support for transparent proxying of SMTP traffic by enabling the 
.Ar TransparentProxy
setting. This type of setup usually involves firewall rules which redirect traffic to 
.Nm 
and the setup varies from OS to OS. The SMTP traffic will be forwarded to it's 
original destination after being scanned. 
.Pp
Note that some features (such as SSL/TLS) will not be available
when going through the transparent proxy. 
.Pp
Make sure that the
.Ar MaxConnections 
setting is set high enough for your transparent proxying. Because 
.Nm 
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. 
.Sh 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. 
.Pp
Care should be taken with the directory that 
.Nm
writes its temporary files to. In order to be secure, it should not be a world
writeable location. Specify the directory using the 
.Ar TempDirectory
setting.
.Pp
Make sure you understand the issues involved with escaping external data. The
environment variables such as 
.Ar SENDER
or
.Ar RECIPIENTS
need to be treated with care. 
.Pp
If running 
.Nm
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).
.Sh SEE ALSO
.Xr proxsmtpd.conf 5
.Sh AUTHOR
.An Stef Walter Aq stef@memberwebs.com