HOWTO Setup fail2ban: Difference between revisions
No edit summary |
|||
| Line 166: | Line 166: | ||
<br> | <br> | ||
==Extra Fun!== | |||
You can use this one-liner to parse through your '''/var/log/messages''' file and totalize the number of times a specific IP-address has been attempting to access your machine: | |||
awk '($(NF-7) = /invalid user/){print $(NF-3)}' /var/log/messages | sort | uniq -c | sort | |||
Once you have this listing, you can manually add IP-blocks of the form: | |||
iptables -I INPUT -p tcp -s 83.103.96.33 --dport ssh -j REJECT --reject-with tcp-reset | |||
Revision as of 17:33, 9 December 2009
What fail2ban does
fail2ban parses logfiles, and finds repeated-access-failures for various services. Once a specified number of failures (5) within a given time (10min) is reached, fail2ban makes an iptables-entry automatically, banning (blocking) that IP-address. After a configurable length of time (2 weeks), the IP-address is automatically unblocked. The status (number of hosts banned; number un-banned) is extracted from /var/log/fail2ban.log by logwatch, and summarized in our daily logwatch-email.
Why we want fail2ban
Our Gentoo systems use syslog-ng, sometimes with separate SSH auth.log file (many times, though SSH logging is integrated into /var/log/messages). Occasionally we also supply vsftpd connectivity for users, which uses yet another separate log-file. We may also add other services (apache2, postfix, and others). Fail2ban is capable of working with multiple log-files, and multiple services. The flexibility and integration with iptables/netfilter is a major benefit of fail2ban, and filtering is performed at the kernel-level. For most servers, this is the way to go.
One alternative to fail2ban is denyhosts, which requires tcpwrappers (tcpd USE-flag), and makes banned-host-IP entries in your /etc/hosts.deny file. I would recommend DenyHosts for a desktop/workstation, where SSH is the only externally-accessible service. It is simpler than fail2ban, and informal testing has shown it works very well.
- DenyHosts Advantages
- doesn't require iptables
- scans your log-files for SSH failures only, and doesn't (appear) to care about the actual port. This is very handy for SSH running on multiple, or non-standard ports.
- can use an Internet server + database of abusive IP's, so it offers a bit more-proactive protection, rather than being purely reactionary
- it's just a Python script, and easily adapted to some of our legacy systems (SuSE 9.3 is a working example)
- DenyHosts Disadvantages
- filtering is performed at the user-level.
- only capable of scanning a single log-file
- only supplied with SSH scanning support
Yet another alternative to fail2ban is a simple iptable-rule, in which you specify a time-window and number-of-failures you will tolerate within that time-window. If you exercise some patience and wait a bit, the block goes away; most-effective against simplest automated attacks:
$IPTABLES -A INPUT -i eth0 -p tcp --dport 22 -m state --state NEW -m recent --set --name SSH $IPTABLES -A INPUT -i eth0 -p tcp --dport 22 -m state --state NEW -m recent --update --seconds 60 --hitcount 10 --rttl --name SSH -j DROP
Pre-Requisites for fail2ban
- Python
- iptables
- logrotate (watch out! Some stuff doesn't rotate; follow the link to fix it!)
- gamin (if you're following my suggestion, below)
Installing fail2ban
hostname ~ # emerge -v gamin fail2ban
Configuring fail2ban
edit /etc/fail2ban/jail.conf
hostname ~ # emacs -nw /etc/fail2ban/jail.conf
- set the bantime to 2 weeks, measured in seconds:
bantime = 1209600
- set the findtime to 30 minutes, again measured in seconds:
findtime = 1800
- scroll to section "[ssh-iptables]" and enable it:
###enabled = false enabled = true
- change the the SSH logfile we want to scan, if necessary (often it is not; sometimes it is):
###logpath = /var/log/sshd.log logpath = /var/log/auth.log
- tighten up the max failures we'll tolerate; remember, on-campus and a select few others are ignored and not subject to this (allows fumble-fingered admins to successfully auth)
maxretry = 3
- comment out the "mail-whois" actions, in order to reduce email-clutter. If you do wish to be notified each time an IP-address is blocked, edit jail.conf and replace you@mail.com with your own email-address.
# mail-whois[name=SSH, dest=yourmail@mail.com]
Repeat the enabling and configuring for all the other services you want, such as vsftpd, apache, etc.
# Don't block SFU-local folks, nor SHAW / Telus ignoreip = 142.58.0.0/16 209.87.56.0/24 24.64.0.0/13 24.76.0.0/14 24.80.0.0/13 24.108.0.0/15 64.59.128.0/18 70.64.0.0/12 137.186.0.0/16 206.75.0.0/16 207.81.0.0/16 207.134.0.0/16
Running fail2ban
hostname ~ # /etc/init.d/fail2ban start * Starting fail2ban ... [ ok ]
hostname ~ # rc-update add fail2ban default * fail2ban added to runlevel default
Monitoring and Verifying fail2ban
Check the log file:
hostname ~ # tail /var/log/fail2ban.log
and, for scrolling/real-time monitoring of additions to the log-file:
hostname ~ # tail -f /var/log/fail2ban.log
Check Iptables:
hostname ~ # iptables -L
Example output, for a web-server (port 80 open), with SSH and FTP services too. Note the banned SSH user, resulting from too many failed-login-attempts:
Chain INPUT (policy DROP) target prot opt source destination fail2ban-VSFTPD tcp -- anywhere anywhere tcp dpt:ftp fail2ban-SSH tcp -- anywhere anywhere tcp dpt:ssh ACCEPT all -- anywhere anywhere DROP all -- anywhere anywhere state INVALID ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:ftp-data ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:ftp ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:ssh ACCEPT tcp -- anywhere anywhere state NEW tcp dpt:http Chain FORWARD (policy DROP) target prot opt source destination Chain OUTPUT (policy ACCEPT) target prot opt source destination ACCEPT all -- anywhere anywhere ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED Chain fail2ban-SSH (1 references) target prot opt source destination DROP all -- sr-01504.iat.sfu.ca anywhere RETURN all -- anywhere anywhere Chain fail2ban-VSFTPD (1 references) target prot opt source destination RETURN all -- anywhere anywhere
Troubleshooting fail2ban
One of the best ways of troubleshooting is to use the fail2ban-regex tool. Feed this with the log-file you want to watch, and point it to the filter you want applied:
hostname ~ # /usr/bin/fail2ban-regex /var/log/messages /etc/fail2ban/filter.d/sshd.conf
Example output:
Unable to find a corresponding IP address for eges.esstel.ru
Unable to find a corresponding IP address for adsl-75-22-172-193.dsl.sndg02.sbcglobal.net
Unable to find a corresponding IP address for adsl-75-22-172-193.dsl.sndg02.sbcglobal.net
Running tests
=============
Use regex file : /etc/fail2ban/filter.d/sshd.conf
Use log file : /var/log/messages
Results
=======
Failregex:
[1] Authentication failure for .* from <HOST>$
[2] Failed [-/\w]+ for .* from <HOST>$
[3] ROOT LOGIN REFUSED .* FROM <HOST>$
[4] [iI](?:llegal|nvalid) user .* from <HOST>$
Number of matches:
[1] 3161 match(es)
[2] 0 match(es)
[3] 0 match(es)
[4] 932 match(es)
Addresses found:
[1]
203.98.175.182 (Wed Nov 19 06:04:16 2008)
59.125.226.213 (Wed Nov 19 06:07:05 2008)
80.153.123.179 (Wed Nov 19 06:10:13 2008)
76.255.174.78 (Wed Nov 19 06:13:03 2008)
66.158.139.11 (Wed Nov 19 06:15:54 2008)
66.158.139.11 (Wed Nov 19 06:18:54 2008)
<truncated the rest of the output, for brevity's sake>
This verifies that the logfile you are parsing does indeed contain appropriate information for fail2ban, and it shows the filter-rules and numbers of matches for each rule. Some attacks try to sneak under a fail2ban threshold, so this kind of regex-test may show only a single, or perhaps 2 failures from a given IP-address... often (depending on configuration), this isn't enough to trigger a ban (the default config may require 5 attempts, or a tweaked version may reduce this threshold to 3 failures before banning).
Another useful tool is fail2ban-client to view the current status of a jail:
hostname ~ # /usr/bin/fail2ban-client status ssh-iptables
Example output:
Status for the jail: ssh-iptables |- filter | |- Currently failed: 11 | `- Total failed: 207 `- action |- Currently banned: 3 | `- IP list: 211.139.168.226 140.120.108.187 83.208.170.100 `- Total banned: 3
Extra Fun!
You can use this one-liner to parse through your /var/log/messages file and totalize the number of times a specific IP-address has been attempting to access your machine:
awk '($(NF-7) = /invalid user/){print $(NF-3)}' /var/log/messages | sort | uniq -c | sort
Once you have this listing, you can manually add IP-blocks of the form:
iptables -I INPUT -p tcp -s 83.103.96.33 --dport ssh -j REJECT --reject-with tcp-reset