sshd.conf

# Fail2Ban filter for openssh
#
# If you want to protect OpenSSH from being bruteforced by password
# authentication then get public key authentication working before disabling
# PasswordAuthentication in sshd_config.
#
#
# "Connection from <HOST> port \d+" requires LogLevel VERBOSE in sshd_config
#

[INCLUDES]

# Read common prefixes. If any customizations available -- read them from
# common.local
before = common.conf

[DEFAULT]

_daemon = sshd

# optional prefix (logged from several ssh versions) like "error: ", "error: PAM: " or "fatal: "
__pref = (?:(?:error|fatal): (?:PAM: )?)?
# optional suffix (logged from several ssh versions) like " [preauth]"
__suff = (?: \[preauth\])?\s*
__on_port_opt = (?: port \d+)?(?: on \S+(?: port \d+)?)?

# single line prefix:
__prefix_line_sl = %(__prefix_line)s%(__pref)s
# multi line prefixes (for first and second lines):
__prefix_line_ml1 = (?P<__prefix>%(__prefix_line)s)%(__pref)s
__prefix_line_ml2 = %(__suff)s$<SKIPLINES>^(?P=__prefix)%(__pref)s

mode = %(normal)s

normal = ^%(__prefix_line_sl)s[aA]uthentication (?:failure|error|failed) for .* from <HOST>( via \S+)?\s*%(__suff)s$
         ^%(__prefix_line_sl)sUser not known to the underlying authentication module for .* from <HOST>\s*%(__suff)s$
         ^%(__prefix_line_sl)sFailed \S+ for (?P<cond_inv>invalid user )?(?P<user>(?P<cond_user>\S+)|(?(cond_inv)(?:(?! from ).)*?|[^:]+)) from <HOST>%(__on_port_opt)s(?: ssh\d*)?(?(cond_user): |(?:(?:(?! from ).)*)$)
         ^%(__prefix_line_sl)sROOT LOGIN REFUSED.* FROM <HOST>\s*%(__suff)s$
         ^%(__prefix_line_sl)s[iI](?:llegal|nvalid) user .*? from <HOST>%(__on_port_opt)s\s*$
         ^%(__prefix_line_sl)sUser .+ from <HOST> not allowed because not listed in AllowUsers\s*%(__suff)s$
         ^%(__prefix_line_sl)sUser .+ from <HOST> not allowed because listed in DenyUsers\s*%(__suff)s$
         ^%(__prefix_line_sl)sUser .+ from <HOST> not allowed because not in any group\s*%(__suff)s$
         ^%(__prefix_line_sl)srefused connect from \S+ \(<HOST>\)\s*%(__suff)s$
         ^%(__prefix_line_sl)sReceived disconnect from <HOST>%(__on_port_opt)s:\s*3: .*: Auth fail%(__suff)s$
         ^%(__prefix_line_sl)sUser .+ from <HOST> not allowed because a group is listed in DenyGroups\s*%(__suff)s$
         ^%(__prefix_line_sl)sUser .+ from <HOST> not allowed because none of user's groups are listed in AllowGroups\s*%(__suff)s$
         ^%(__prefix_line_sl)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.*%(__suff)s$
         ^%(__prefix_line_sl)s(error: )?maximum authentication attempts exceeded for .* from <HOST>%(__on_port_opt)s(?: ssh\d*)? \[preauth\]$
         ^%(__prefix_line_ml1)sUser .+ not allowed because account is locked%(__prefix_line_ml2)sReceived disconnect from <HOST>: 11: .+%(__suff)s$
         ^%(__prefix_line_ml1)sDisconnecting: Too many authentication failures for .+?%(__prefix_line_ml2)sConnection closed by <HOST>%(__suff)s$
         ^%(__prefix_line_ml1)sConnection from <HOST>%(__on_port_opt)s%(__prefix_line_ml2)sDisconnecting: Too many authentication failures for .+%(__suff)s$

ddos =   ^%(__prefix_line_sl)sDid not receive identification string from <HOST>%(__suff)s$
         ^%(__prefix_line_sl)sReceived disconnect from <HOST>%(__on_port_opt)s:\s*14: No supported authentication methods available%(__suff)s$
         ^%(__prefix_line_sl)sUnable to negotiate with <HOST>%(__on_port_opt)s: no matching (?:cipher|key exchange method) found.
         ^%(__prefix_line_ml1)sConnection from <HOST>%(__on_port_opt)s%(__prefix_line_ml2)sUnable to negotiate a (?:cipher|key exchange method)%(__suff)s$
         ^%(__prefix_line_ml1)sSSH: Server;Ltype: (?:Authname|Version|Kex);Remote: <HOST>-\d+;[A-Z]\w+:.*%(__prefix_line_ml2)sRead from socket failed: Connection reset by peer%(__suff)s$

aggressive = %(normal)s
             %(ddos)s

[Definition]

failregex = %(mode)s

ignoreregex = 

[Init]

# "maxlines" is number of log lines to buffer for multi-line regex searches
maxlines = 10

journalmatch = _SYSTEMD_UNIT=sshd.service + _COMM=sshd

# DEV Notes:
#
#   "Failed \S+ for .*? from <HOST>..." failregex uses non-greedy catch-all because
#   it is coming before use of <HOST> which is not hard-anchored at the end as well,
#   and later catch-all's could contain user-provided input, which need to be greedily
#   matched away first.
#
# Author: Cyril Jaquier, Yaroslav Halchenko, Petr Voralek, Daniel Black