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How I saved myself 3-months of frustration by finding and fixing a sendmail bug on a vendors appliance
I recently upgraded a vendors appliance to a new version of code to address a couple of known isues. After the upgrade I noticed that the appliance wasn't sending e-mail (yes some vendors still use e-mail) alerts when problems were detected. Luckily the appliance runs Linux and provides shell access through SSH if you enable it. My previous support experiences have been less than stellar so I figured I would root cause the problem and send them a bug report instead of opening a support case…
$ read more →Using the python SMTP DebuggingServer to test SMTP communications
During the development of the dns-domain-expiration-checker script I needed a way to test SMTP mail delivery w/o relying on an actual mail exchanger. While reading through the smtplib and snmpd documentation I came across the SMTP debugging server. This nifty module allows you to run a local mail relay which will print the messages it receives to standard out. To enable it you can load the smtpd module and instruct it to run the DebuggingServer command on the IP and port passed as arguments: This will fire up a local mail server on localhost:8025 and each messaged received will be printed to STDOUT…
$ read more →Gathering statistics from your postfix mail logs
I have been supporting postfix mail relays for several years. When you are tasked with ensuring that your SMTP relays work without issue, you need to stay on top of the logs and the current operational state of each server to prevent major problems from occurring. Problems can take the form of blacklisted relays, mail routing loops, spam attacks or people trying to exploit a known issue in the SMTP software. All of these items suck for a site that relies on mail to function…
$ read more →Testing postfix restrictions
The postfix mail delivery system allows one or more restrictions to be placed on incoming messages. These restrictions allow you to block messages when clients don't use the SMTP handshake as outlined in RFC 821 during delivery, if the name supplied in the HELO is not fully qualified or fails to resolve, or if the connecting host resides on one or more blacklists. Several restrictions come enabled by default, but others need to be manually enabled by adding additional directives to the smtpd_helo_restrictions, smtpd_sender_restrictions and smtpd_recipient_restrictions variables. Since new restrictions can have unexpected results (e.g., lost E-mail), it is beneficial to test these rules prior to enforcing them…
$ read more →Filtering attachments with procmail
I have been performing some basic statistical analysis on the spam and viruses I receive, and it seems like more and more Email comes littered with silly attachments. While I really appreciate people sending me executables, help files, compressed archives, VB script and screen savers, I should alert you now that these messages go directly to the bit bucket ( /dev/null ). This is accomplished with the following procmail filter: This checks the message body for a MIME header with the filename type, and attempts to match the value against a variety of extensions. If it finds a match, the file is written to /dev/null…
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