17

fail2ban doesn't recognize failed public key ssh logins and I assume that this can be solved by modifying the failregex of /etc/fail2ban/filter.d/sshd.config to match the following line:

<date> <time> <server-hostname> sshd[25917]: Connection closed by <client-ip> [preauth]

But I can't find a decent introduction on how to correctly modify said failregex. As I don't really know what variables like ^%(__prefix_line)s include it's hard to get a working regex.

I understand that the most recent version 0.9.1 includes the regex to matche the "Connection closed by [preauth]" line, but I'm using fai2ban from the Debian repository and the configuration of 0.9.1 is not compatible with the one I have.

apoc
  • 273

6 Answers6

19

No regex hacking is required (at least since fail2ban 0.10.4). In /etc/fail2ban/jail.conf is the following information:

[sshd]

# To use more aggressive sshd modes set filter parameter "mode" in jail.local:
# normal (default), ddos, extra or aggressive (combines all).
# See "tests/files/logs/sshd" or "filter.d/sshd.conf" for usage example and details.
#mode   = normal

So follow the recommendations by creating /etc/fail2ban/jail.local with your other customisations, along with one of the more stringent modes, for example,

[sshd]
mode   = aggressive

This mode now covers failed public keys.

Sparhawk
  • 291
12

This line does it:

^%(__prefix_line)sConnection closed by <HOST> \[preauth\]$

Tested with the following logstring:

Apr 29 12:30:12 sendai sshd[25917]: Connection closed by 127.0.0.1 [preauth]

Successfully tested with:

$ fail2ban-regex ~/ssh.log sshd.conf 

Running tests
=============

Use regex file : sshd.conf
Use log file   : /home/user/ssh.log


Results
=======

Failregex
|- Regular expressions:
[...]
|  [12] ^\s*(?:\S+ )?(?:kernel: \[\d+\.\d+\] )?(?:@vserver_\S+ )?(?:(?:\[\d+\])?:\s+[\[\(]?sshd(?:\(\S+\))?[\]\)]?:?|[\[\(]?sshd(?:\(\S+\))?[\]\)]?:?(?:\[\d+\])?:)?\s*Connection closed by <HOST> \[preauth\]$
|
`- Number of matches:
[...]
   [12] 1 match(es)

Summary
=======

Addresses found:
[...]
[12]
    127.0.0.1 (Wed Apr 29 12:30:12 2015)
[..]

Success, the total number of match is 1
sebix
  • 4,432
6

At least in openssh 7.3 the log messages also contain a port number. So I had to modify sebix's solution to the following:

^%(__prefix_line)sConnection closed by <HOST> port \d+ \[preauth\]$
code-chicken
  • 161
4

On Ubuntu 18.04 with OpenSSH 7.6p1 I had to extend the "cmnfailre" rules in /etc/fail2ban/filter.d/sshd.conf by

^%(__prefix_line)sConnection closed by authenticating user <F-USER>.+</F-USER> <HOST> port \d+ \[preauth\]$

to match ssh login attempts without correct ssh key:

Apr 14 15:27:15 ubuntu-server sshd[8719]: Connection closed by authenticating user root x.x.x.x port 42664 [preauth]
Apr 14 15:27:15 ubuntu-server sshd[8721]: Connection closed by authenticating user root x.x.x.x port 42666 [preauth]
Apr 14 15:27:16 ubuntu-server sshd[8723]: Connection closed by authenticating user root x.x.x.x port 42668 [preauth]
Apr 14 15:27:16 ubuntu-server sshd[8725]: Connection closed by authenticating user root x.x.x.x port 42670 [preauth]
Apr 14 15:27:17 ubuntu-server sshd[8727]: Connection closed by authenticating user root x.x.x.x port 42672 [preauth]
Chris
  • 41
  • 1
1

For me, this works better, as I get tons of legit log entries, matching the "Connection closed" regex, at least in OpenSSH 6.6.1 variations. Believe it also covers the new OpenSSH log format, which includes "port":

^%(__prefix_line)sDid not receive identification string from <HOST>\s*$
^%(__prefix_line)sReceived disconnect from <HOST>: (port \d*: ){,1}11: (Bye Bye){,1} \[preauth\]\s*$

$ cat /etc/*relea* | grep -i desc
DISTRIB_DESCRIPTION="Ubuntu 14.04.5 LTS"

$ fail2ban-regex /var/log/auth.log filter.d/sshd.conf

Running tests
=============

Use   failregex file : filter.d/sshd.conf
Use         log file : /var/log/auth.log


Results
=======

Failregex: 39 total
|-  #) [# of hits] regular expression
|  12) [16] ^\s*(<[^.]+\.[^.]+>)?\s*(?:\S+ )?(?:kernel: \[\d+\.\d+\] )?(?:@vserver_\S+ )?(?:(?:\[\d+\])?:\s+[\[\(]?sshd(?:\(\S+\))?[\]\)]?:?|[\[\(]?sshd(?:\(\S+\))?[\]\)]?:?(?:\[\d+\])?:?)?\s(?:\[ID \d+ \S+\])?\s*Did not receive identification string from <HOST>\s*$
|  13) [23] ^\s*(<[^.]+\.[^.]+>)?\s*(?:\S+ )?(?:kernel: \[\d+\.\d+\] )?(?:@vserver_\S+ )?(?:(?:\[\d+\])?:\s+[\[\(]?sshd(?:\(\S+\))?[\]\)]?:?|[\[\(]?sshd(?:\(\S+\))?[\]\)]?:?(?:\[\d+\])?:?)?\s(?:\[ID \d+ \S+\])?\s*Received disconnect from <HOST>: (port \d*: ){,1}11: (Bye Bye){,1} \[preauth\]\s*$
`-

Ignoreregex: 0 total

Date template hits:
|- [# of hits] date format
|  [62412] MONTH Day Hour:Minute:Second
`-

Lines: 62412 lines, 0 ignored, 39 matched, 62373 missed
Missed line(s):: too many to print.  Use --print-all-missed to print all 62373 lines

$ grep "Did not receive identification string from" /var/log/auth.log | wc -l
16
$ grep "Received disconnect from" /var/log/auth.log | grep -v x.x.x.x | wc -l
23
bofh
  • 61
0

In 2021, in Debian with fail2ban=0.11.2-1 (from Bullseye), all you have to do is to add

mode = aggressive

line to /etc/fail2ban/jail.d/defaults-debian.conf file, under [sshd] section. That bans everyone which attempts to login as valid user using wrong public key. No need to do anything else.

saXh26Ql0zkF
  • 11