Fail2ban: optional regex filter

Optional regex filter:

SSH, edit or replace /etc/fail2ban/filter.d/sshd.conf or /etc/fail2ban/filter.d/sshd-ddos.conf
^%(__prefix_line)s(?:error: PAM: )?[aA]uthentication (?:failure|error|failed) for .* from <HOST>( via \S+)?\s*$
^%(__prefix_line)s(?:error: PAM: )?User not known to the underlying authentication module for .* from <HOST>\s*$
^%(__prefix_line)sFailed \S+ for .*? from <HOST>(?: port \d*)?(?: ssh\d*)?(: (ruser .*|(\S+ ID \S+ \(serial \d+\) CA )?\S+ %(__md5hex)s(, client user ".*", client host ".*")?))?\s*$
^%(__prefix_line)sROOT LOGIN REFUSED.* FROM <HOST>\s*$
^%(__prefix_line)s[iI](?:llegal|nvalid) user .* from <HOST>\s*$
^%(__prefix_line)sUser .+ from <HOST> not allowed because not listed in AllowUsers\s*$
^%(__prefix_line)sUser .+ from <HOST> not allowed because listed in DenyUsers\s*$
^%(__prefix_line)sUser .+ from <HOST> not allowed because not in any group\s*$
^%(__prefix_line)srefused connect from \S+ \(<HOST>\)\s*$
^%(__prefix_line)s(?:error: )?Received disconnect from <HOST>: 3: .*: Auth fail(?: \[preauth\])?$
^%(__prefix_line)sUser .+ from <HOST> not allowed because a group is listed in DenyGroups\s*$
^%(__prefix_line)sUser .+ from <HOST> not allowed because none of user's groups are listed in AllowGroups\s*$
^(?P<__prefix>%(__prefix_line)s)User .+ not allowed because account is locked<SKIPLINES>(?P=__prefix)(?:error: )?Received disconnect from <HOST>: 11: .+ \[preauth\]$
^(?P<__prefix>%(__prefix_line)s)Disconnecting: Too many authentication failures for .+? \[preauth\]<SKIPLINES>(?P=__prefix)(?:error: )?Connection closed by <HOST> \[preauth\]$
^(?P<__prefix>%(__prefix_line)s)Connection from <HOST> port \d+(?: on \S+ port \d+)?<SKIPLINES>(?P=__prefix)Disconnecting: Too many authentication failures for .+? \[preauth\]$
^%(__prefix_line)s(error: )?maximum authentication attempts exceeded for .* from <HOST>(?: port \d*)?(?: ssh\d*)? \[preauth\]$
^%(__prefix_line)spam_unix\(sshd:auth\):\s+authentication failure;\s*logname=\S*\s*uid=\d*\s*euid=\d*\s*tty=\S*\s*ruser=\S*\s*rhost=<HOST>\s.*$
^%(__prefix_line)sConnection closed by <HOST> \[preauth\]$
^%(__prefix_line)sReceived disconnect from <HOST>: 11: (Bye Bye)? \[preauth\]$
^%(__prefix_line)sReceived disconnect from <HOST>: 3: \S+: Auth fail$
Postfix-sasl, edit or replace /etc/fail2ban/filter.d/postfix-sasl.conf
^%(__prefix_line)swarning: [-._\w]+\[<HOST>\]: SASL ((?i)LOGIN|PLAIN|(?:CRAM|DIGEST)-MD5) authentication failed(:[ A-Za-z0-9+/:]*={0,2})?\s*$
lost connection after AUTH from (.*)\[<HOST>\] 
Postfix, edit or replace /etc/fail2ban/filter.d/postfix.conf
^%(__prefix_line)sNOQUEUE: reject: RCPT from \S+\[<HOST>\]: 554 5\.7\.1 .*$
^%(__prefix_line)sNOQUEUE: reject: RCPT from \S+\[<HOST>\]: 450 4\.7\.1 Client host rejected: cannot find your hostname, (\[\S*\]); from=<\S*> to=<\S+> proto=ESMTP helo=<\S*>$
^%(__prefix_line)sNOQUEUE: reject: RCPT from \S+\[<HOST>\]: 450 4\.7\.1 : Helo command rejected: Host not found; from=<> to=<> proto=ESMTP helo= *$
^%(__prefix_line)sNOQUEUE: reject: EHLO from \S+\[<HOST>\]: 504 5\.5\.2 <\S+>: Helo command rejected: need fully-qualified hostname;
^%(__prefix_line)sNOQUEUE: reject: VRFY from \S+\[<HOST>\]: 550 5\.1\.1 .*$
^%(__prefix_line)sNOQUEUE: reject: RCPT from \S+\[<HOST>\]: 450 4\.1\.8 <\S*>: Sender address rejected: Domain not found; from=<\S*> to=<\S+> proto=ESMTP helo=<\S*>$
^%(__prefix_line)simproper command pipelining after \S+ from [^[]*\[<HOST>\]:?$
lost connection after (AUTH|UNKNOWN|EHLO) from (.*)\[<HOST>\]
connect from unknown\[<HOST>\]
disconnect from unknown\[<HOST>\]?$
statistics: max connection rate 1/60s for (smtp:<HOST>)?$
statistics: max connection rate 1 for (smtp:<HOST>)?$
ProFTP, edit or replace /etc/fail2ban/filter.d/proftpd.conf 
^%(__prefix_line)s%(__hostname)s \(\S+\[<HOST>\]\)[: -]+ USER .*: no such user found from \S+ \[\S+\] to \S+:\S+ *$
^%(__prefix_line)s%(__hostname)s \(\S+\[<HOST>\]\)[: -]+ USER .* \(Login failed\): %(__suffix_failed_login)s\s*$
^%(__prefix_line)s%(__hostname)s \(\S+\[<HOST>\]\)[: -]+ SECURITY VIOLATION: .* login attempted\. *$
^%(__prefix_line)s%(__hostname)s \(\S+\[<HOST>\]\)[: -]+ Maximum login attempts \(\d+\) exceeded *$
USER \S+: no such user found from \S* ?\[<HOST>\] to \S+\s*$
\(\S+\[<HOST>\]\)[: -]+ USER \S+: no such user found from \S+ \[[0-9.]+\] to \S+:\S+.*$
\(\S+\[<HOST>\]\)[: -]+ USER \S+ \(Login failed\): Incorrect password.*$
\(\S+\[<HOST>\]\)[: -]+ SSH2 session closed.

References: