{"id":20,"date":"2008-03-24T12:15:58","date_gmt":"2008-03-24T05:15:58","guid":{"rendered":"http:\/\/blog.iwayvietnam.com\/tuanta\/2008\/03\/24\/sendmails-greet_pause\/"},"modified":"2008-03-27T14:50:36","modified_gmt":"2008-03-27T07:50:36","slug":"sendmails-greet_pause","status":"publish","type":"post","link":"https:\/\/blog.iwayvietnam.com\/tuanta\/2008\/03\/sendmails-greet_pause\/","title":{"rendered":"Sendmail’s Greet_Pause"},"content":{"rendered":"

Slamming<\/em> is a popular spammer tactic in which the spammer quickly fires off SMTP messages without waiting for responses from the receiving server. A poorly behaved MTA will then accept traffic from the spammer, instead of rejecting it as it should. But even well-behaved MTAs are affected because of the sheer volume of traffic with which they are forced to deal. The venerable sendmail, as of version 8.13, has a nifty feature called “greet_pause<\/strong>” that not only rejects incorrect SMTP transactions, but also discourages re-sends.<\/p>\n

In a normal SMTP transaction, the client first connects and the server is supposed to send back a “220” greeting, something like:<\/p>\n

$ telnet mail.foo.org 25
\nTrying 12.34.56.78...
\nConnected to foo.com.
\nEscape character is '^]'.
\n220-host6.foo.org ESMTP Sendmail 8.13.6\/8.13.6; Wed, 14 Jun 2006 18:04:49 -0600
\n220-We do not authorize the use of this system to transport unsolicited,
\n220 and\/or bulk e-mail.
\n<\/code><\/p>\n

Then, the client says “ehlo” or “helo,” and the transaction continues. When the client is an impatient spammer and sends more commands without listening, the greet_pause feature detects this, marks the connection bad, and responds to anything else that tries to come over that connection with a 554 (transaction failed) message. It works by pausing briefly before sending out its 220 messages.<\/p>\n

The pause interval is configurable, so you can tune it as needed.<\/p>\n

Interestingly, you’ll probably find that your total spam attempts drop significantly after implementing greet_pause, possibly because the spammer’s software thinks it’s hitting a bad server or bad addresses, or otherwise getting stuck somehow. It’s an ingenious and simple method with a low-overhead that discourages significant amounts of spam.<\/p>\n

As always, be sure to whitelist all of your important addresses. Visit sendmail.org\/doc\/<\/a> to learn more.<\/p>\n","protected":false},"excerpt":{"rendered":"

Slamming is a popular spammer tactic in which the spammer quickly fires off SMTP messages without waiting for responses from the receiving server. A poorly behaved MTA will then accept traffic from the spammer, instead of rejecting it as it should. But even well-behaved MTAs are affected because of the sheer volume of traffic withContinue reading “Sendmail’s Greet_Pause”<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[18,4],"tags":[],"class_list":["post-20","post","type-post","status-publish","format-standard","hentry","category-sendmail","category-tips","entry"],"_links":{"self":[{"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/posts\/20"}],"collection":[{"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/comments?post=20"}],"version-history":[{"count":0,"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/posts\/20\/revisions"}],"wp:attachment":[{"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/media?parent=20"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/categories?post=20"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.iwayvietnam.com\/tuanta\/wp-json\/wp\/v2\/tags?post=20"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}