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diff --git a/README_FILES/BACKSCATTER_README b/README_FILES/BACKSCATTER_README new file mode 100644 index 0000000..8095df2 --- /dev/null +++ b/README_FILES/BACKSCATTER_README @@ -0,0 +1,281 @@ +PPoossttffiixx BBaacckkssccaatttteerr HHoowwttoo + +------------------------------------------------------------------------------- + +OOvveerrvviieeww + +This document describes features that require Postfix version 2.0 or later. + +Topics covered in this document: + + * What is backscatter mail? + * How do I block backscatter mail to random recipient addresses? + * How do I block backscatter mail to real recipient addresses? + + o Blocking backscatter mail with forged mail server information + o Blocking backscatter mail with forged sender information + o Blocking backscatter mail with other forged information + o Blocking backscatter mail from virus scanners + +The examples use Perl Compatible Regular Expressions (Postfix pcre: tables), +but also provide a translation to POSIX regular expressions (Postfix regexp: +tables). PCRE is preferred primarily because the implementation is often +faster. + +WWhhaatt iiss bbaacckkssccaatttteerr mmaaiill?? + +When a spammer or worm sends mail with forged sender addresses, innocent sites +are flooded with undeliverable mail notifications. This is called backscatter +mail. With Postfix, you know that you're a backscatter victim when your logfile +goes on and on like this: + + Dec 4 04:30:09 hostname postfix/smtpd[58549]: NOQUEUE: reject: + RCPT from xxxxxxx[x.x.x.x]: 550 5.1.1 <yyyyyy@your.domain.here>: + Recipient address rejected: User unknown; from=<> + to=<yyyyyy@your.domain.here> proto=ESMTP helo=<zzzzzz> + +What you see are lots of "user unknown" errors with "from=<>". These are error +reports from MAILER-DAEMONs elsewhere on the Internet, about email that was +sent with a false sender address in your domain. + +HHooww ddoo II bblloocckk bbaacckkssccaatttteerr mmaaiill ttoo rraannddoomm rreecciippiieenntt aaddddrreesssseess?? + +If your machine receives backscatter mail to random addresses, configure +Postfix to reject all mail for non-existent recipients as described in the +LOCAL_RECIPIENT_README and STANDARD_CONFIGURATION_README documentation. + +If your machine runs Postfix 2.0 and earlier, disable the "pause before reject" +feature in the SMTP server. If your system is under stress then it should not +waste time. + + /etc/postfix/main.cf: + # Not needed with Postfix 2.1 and later. + smtpd_error_sleep_time = 0 + + # Not needed with Postfix 2.4 and later. + unknown_local_recipient_reject_code = 550 + +HHooww ddoo II bblloocckk bbaacckkssccaatttteerr mmaaiill ttoo rreeaall rreecciippiieenntt aaddddrreesssseess?? + +When backscatter mail passes the "unknown recipient" barrier, there still is no +need to despair. Many mail systems are kind enough to attach the message +headers of the undeliverable mail in the non-delivery notification. These +message headers contain information that you can use to recognize and block +forged mail. + +BBlloocckkiinngg bbaacckkssccaatttteerr mmaaiill wwiitthh ffoorrggeedd mmaaiill sseerrvveerr iinnffoorrmmaattiioonn + +Although my email address is "wietse@porcupine.org", all my mail systems +announce themselves with the SMTP HELO command as "hostname.porcupine.org". +Thus, if returned mail has a Received: message header like this: + + Received: from porcupine.org ... + +Then I know that this is almost certainly forged mail (almost; see next section +for the fly in the ointment). Mail that is really sent by my systems looks like +this: + + Received: from hostname.porcupine.org ... + +For the same reason the following message headers are very likely to be the +result of forgery: + + Received: from host.example.com ([1.2.3.4] helo=porcupine.org) ... + Received: from [1.2.3.4] (port=12345 helo=porcupine.org) ... + Received: from host.example.com (HELO porcupine.org) ... + Received: from host.example.com (EHLO porcupine.org) ... + +Some forgeries show up in the way that a mail server reports itself in +Received: message headers. Keeping in mind that all my systems have a mail +server name of hostname.porcupine.org, the following is definitely a forgery: + + Received: by porcupine.org ... + Received: from host.example.com ( ... ) by porcupine.org ... + +Another frequent sign of forgery is the Message-ID: header. My systems produce +a Message-ID: of <stuff@hostname.porcupine.org>. The following are forgeries, +especially the first one: + + Message-ID: <1cb479435d8eb9.2beb1.qmail@porcupine.org> + Message-ID: <yulszqocfzsficvzzju@porcupine.org> + +To block such backscatter I use header_checks and body_checks patterns like +this: + + /etc/postfix/main.cf: + header_checks = pcre:/etc/postfix/header_checks + body_checks = pcre:/etc/postfix/body_checks + + /etc/postfix/header_checks: + # Do not indent the patterns between "if" and "endif". + if /^Received:/ + /^Received: +from +(porcupine\.org) +/ + reject forged client name in Received: header: $1 + /^Received: +from +[^ ]+ +\(([^ ]+ +[he]+lo=|[he]+lo +) + (porcupine\.org)\)/ + reject forged client name in Received: header: $2 + /^Received:.* +by +(porcupine\.org)\b/ + reject forged mail server name in Received: header: $1 + endif + /^Message-ID:.* <!&!/ DUNNO + /^Message-ID:.*@(porcupine\.org)/ + reject forged domain name in Message-ID: header: $1 + + /etc/postfix/body_checks: + # Do not indent the patterns between "if" and "endif". + if /^[> ]*Received:/ + /^[> ]*Received: +from +(porcupine\.org) / + reject forged client name in Received: header: $1 + /^[> ]*Received: +from +[^ ]+ +\(([^ ]+ +[he]+lo=|[he]+lo +) + (porcupine\.org)\)/ + reject forged client name in Received: header: $2 + /^[> ]*Received:.* +by +(porcupine\.org)\b/ + reject forged mail server name in Received: header: $1 + endif + /^[> ]*Message-ID:.* <!&!/ DUNNO + /^[> ]*Message-ID:.*@(porcupine\.org)/ + reject forged domain name in Message-ID: header: $1 + +Notes: + + * The example uses pcre: tables mainly for speed; with minor modifications, + you can use regexp: tables as explained below. + + * The example is simplified for educational purposes. In reality my patterns + list multiple domain names, as "(domain|domain|...)". + + * The "\." matches "." literally. Without the "\", the "." would match any + character. + + * The "\(" and "\)" match "(" and ")" literally. Without the "\", the "(" and + ")" would be grouping operators. + + * The "\b" is used here to match the end of a word. If you use regexp: + tables, specify "[[:>:]]" (on some systems you should specify "\>" instead; + for details see your system documentation). + + * The "if /pattern/" and "endif" eliminate unnecessary matching attempts. DO + NOT indent lines starting with /pattern/ between the "if" and "endif"! + + * The two "Message-ID:.* <!&!" rules are workarounds for some versions of + Outlook express, as described in the caveats section below. + +CCaavveeaattss + + * Netscape Messenger (and reportedly, Mozilla) sends a HELO name that is + identical to the sender address domain part. If you have such clients then + the above patterns would block legitimate email. + + My network has only one such machine, and to prevent its mail from being + blocked I have configured it to send mail as user@hostname.porcupine.org. + On the Postfix server, a canonical mapping translates this temporary + address into user@porcupine.org. + + /etc/postfix/main.cf: + canonical_maps = hash:/etc/postfix/canonical + + /etc/postfix/canonical: + @hostname.porcupine.org @porcupine.org + + This is of course practical only when you have very few systems that send + HELO commands like this, and when you never have to send mail to a user on + such a host. + + An alternative would be to remove the hostname from + "hostname.porcupine.org" with address masquerading, as described in the + ADDRESS_REWRITING_README document. + + * Reportedly, Outlook 2003 (perhaps Outlook Express, and other versions as + well) present substantially different Message-ID headers depending upon + whether or not a DSN is requested (via Options "Request a delivery receipt + for this message"). + + When a DSN is requested, Outlook 2003 uses a Message-ID string that ends in + the sender's domain name: + + Message-ID: <!&! ...very long string... ==@example.com> + + where example.com is the domain name part of the email address specified in + Outlook's account settings for the user. Since many users configure their + email addresses as username@example.com, messages with DSN turned on will + trigger the REJECT action in the previous section. + + If you have such clients then you can exclude their Message-ID strings with + the two "Message-ID:.* <!&!" patterns that are shown in the previous + section. Otherwise you will not be able to use the two backscatter rules to + stop forged Message ID strings. Of course this workaround may break the + next time Outlook is changed. + +BBlloocckkiinngg bbaacckkssccaatttteerr mmaaiill wwiitthh ffoorrggeedd sseennddeerr iinnffoorrmmaattiioonn + +Like many people I still have a few email addresses in domains that I used in +the past. Mail for those addresses is forwarded to my current address. Most of +the backscatter mail that I get claims to be sent from these addresses. Such +mail is obviously forged and is very easy to stop. + + /etc/postfix/main.cf: + header_checks = pcre:/etc/postfix/header_checks + body_checks = pcre:/etc/postfix/body_checks + + /etc/postfix/header_checks: + /^(From|Return-Path):.*\b(user@domain\.tld)\b/ + reject forged sender address in $1: header: $2 + + /etc/postfix/body_checks: + /^[> ]*(From|Return-Path):.*\b(user@domain\.tld)\b/ + reject forged sender address in $1: header: $2 + +Notes: + + * The example uses pcre: tables mainly for speed; with minor modifications, + you can use regexp: tables as explained below. + + * The example is simplified for educational purposes. In reality, my patterns + list multiple email addresses as "(user1@domain1\.tld|user2@domain2\.tld)". + + * The two "\b" as used in "\b(user@domain\.tld)\b" match the beginning and + end of a word, respectively. If you use regexp: tables, specify "[[:<:]] + and [[:>:]]" (on some systems you should specify "\< and \>" instead; for + details see your system documentation). + + * The "\." matches "." literally. Without the "\", the "." would match any + character. + +BBlloocckkiinngg bbaacckkssccaatttteerr mmaaiill wwiitthh ootthheerr ffoorrggeedd iinnffoorrmmaattiioonn + +Another sign of forgery can be found in the IP address that is recorded in +Received: headers next to your HELO host or domain name. This information must +be used with care, though. Some mail servers are behind a network address +translator and never see the true client IP address. + +BBlloocckkiinngg bbaacckkssccaatttteerr mmaaiill ffrroomm vviirruuss ssccaannnneerrss + +With all the easily recognizable forgeries eliminated, there is one category of +backscatter mail that remains, and that is notifications from virus scanner +software. Unfortunately, some virus scanning software doesn't know that viruses +forge sender addresses. To make matters worse, the software also doesn't know +how to report a mail delivery problem, so that we cannot use the above +techniques to recognize forgeries. + +Recognizing virus scanner mail is an error prone process, because there is a +lot of variation in report formats. The following is only a small example of +message header patterns. For a large collection of header and body patterns +that recognize virus notification email, see https://web.archive.org/web/ +20100317123907/http://std.dkuug.dk/keld/virus/ or http://www.t29.dk/ +antiantivirus.txt. + + /etc/postfix/header_checks: + /^Subject: *Your email contains VIRUSES/ DISCARD virus notification + /^Content-Disposition:.*VIRUS1_DETECTED_AND_REMOVED/ + DISCARD virus notification + /^Content-Disposition:.*VirusWarning.txt/ DISCARD virus notification + +Note: these documents haven't been updated since 2004, so they are useful only +as a starting point. + +A plea to virus or spam scanner operators: please do not make the problem worse +by sending return mail to forged sender addresses. You're only harassing +innocent people. If you must return mail to the purported sender, please return +the full message headers, so that the sender can filter out the obvious +forgeries. + |