--- name: "proxsmtpd" section: "8" platform: "linux" package: "proxsmtp" version: "1.10-5" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/proxsmtpd" --- # proxsmtpd(8) 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)](/proxsmtpd.conf/5.md) file. See the man page for [proxsmtpd.conf(5)](/proxsmtpd.conf/5.md) 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)](/proxsmtpd.conf/5.md) 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). ## SEE ALSO [proxsmtpd.conf(5)](/proxsmtpd.conf/5.md) ## AUTHOR Stef Walter ⟨stef@memberwebs.com⟩