fail2ban/config/filter.d/sendmail-reject.conf

# Fail2Ban filter for sendmail spam/relay type failures
#
# Some of the below failregex will only work properly, when the following
# options are set in the .mc file (see your Sendmail documentation on how
# to modify it and generate the corresponding .cf file):
#
# FEATURE(`delay_checks')
# FEATURE(`greet_pause', `500')
# FEATURE(`ratecontrol', `nodelay', `terminate')
# FEATURE(`conncontrol', `nodelay', `terminate')
#
# ratecontrol and conncontrol also need corresponding options ClientRate:
# and ClientConn: in the access file, see documentation for ratecontrol and
# conncontrol in the sendmail/cf/README file.

[INCLUDES]

before = common.conf

[Definition]

_daemon = (?:(sm-(mta|acceptingconnections)|sendmail))
__prefix_line = %(known/__prefix_line)s(?:\w{14,20}: )?

prefregex = ^<F-MLFID>%(__prefix_line)s</F-MLFID><F-CONTENT>.+</F-CONTENT>$

cmnfailre = ^ruleset=check_rcpt, arg1=(?P<email><\S+@\S+>), relay=(\S+ )?\[(?:IPv6:<IP6>|<IP4>)\](?: \(may be forged\))?, reject=(550 5\.7\.1 (?P=email)\.\.\. Relaying denied\. (IP name possibly forged \[(\d+\.){3}\d+\]|Proper authentication required\.|IP name lookup failed \[(\d+\.){3}\d+\])|553 5\.1\.8 (?P=email)\.\.\. Domain of sender address \S+ does not exist|550 5\.[71]\.1 (?P=email)\.\.\. (Rejected: .*|User unknown))$
            ^ruleset=check_relay, arg1=(?P<dom>\S+), arg2=(?:IPv6:<IP6>|<IP4>), relay=((?P=dom) )?\[(\d+\.){3}\d+\](?: \(may be forged\))?, reject=421 4\.3\.2 (Connection rate limit exceeded\.|Too many open connections\.)$
            ^rejecting commands from (\S* )?\[(?:IPv6:<IP6>|<IP4>)\] due to pre-greeting traffic after \d+ seconds$
            ^(?:\S+ )?\[(?:IPv6:<IP6>|<IP4>)\]: (?:(?i)expn|vrfy) \S+ \[rejected\]$
            ^<[^@]+@[^>]+>\.\.\. No such user here$
            ^<F-NOFAIL>from=<[^@]+@[^>]+></F-NOFAIL>, size=\d+, class=\d+, nrcpts=\d+, bodytype=\w+, proto=E?SMTP, daemon=MTA, relay=\S+ \[(?:IPv6:<IP6>|<IP4>)\]$

mdre-normal =

mdre-extra = ^(?:\S+ )?\[(?:IPv6:<IP6>|<IP4>)\](?: \(may be forged\))? did not issue (?:[A-Z]{4}[/ ]?)+during connection to (?:TLS)?M(?:TA|S[PA])(?:-\w+)?$

mdre-aggressive = %(mdre-extra)s

failregex = %(cmnfailre)s
            <mdre-<mode>>

# Parameter "mode": normal (default), extra or aggressive
# Usage example (for jail.local):
#   [sendmail-reject]
#   filter = sendmail-reject[mode=extra]
#
mode = normal

ignoreregex = 

journalmatch = SYSLOG_IDENTIFIER=sm-mta + _SYSTEMD_UNIT=sendmail.service

# DEV NOTES:
# 
# Regarding the multiline regex:
#
# "No such user" lines generate a failure and needs to be matched together with
# another line with the HOST, therefore no-failure line was added as regex, that
# contains HOST (see line with tag <F-NOFAIL>).
#
# Note the capture <F-MLFID>, includes both the __prefix_lines (which includes
# the sendmail PID), but also the `\w{14}` which the the sendmail assigned 
# mail ID (todo: check this is necessary, possible obsolete).
#
# Author: Daniel Black, Fabian Wenk and Sergey Brester aka sebres.
# Rewritten using prefregex by Serg G. Brester.