Over the last few days I’ve been trying to help one of my users who had an odd connectivity issue for my server. After looking at the obvious issues it started to look more and more like he had triggered one of the fail2ban rules and was being blocked by iptables. This has been the case a few times and normally checking the rules shows the problem, but this time it didn’t show anything obvious.
$ sudo iptables -L ... Chain fail2ban-ssh (1 references) target prot opt source destination REJECT all -- 62-210-205-239.rev.poneytelecom.eu anywhere reject-with icmp-port-unreachable RETURN all -- anywhere anywhere ...
After trying a few other changes and finidng the results to be very intermittent it started to look/feel more like an ipatbles issue, but probably one that was being triggered and then expiring resulting in the intermittent results. But how to view the IP addresses that had been blocked? There was nothing in the logs…
Don’t Do This!
I started looking for answers to enable logging in iptables, and after finding a few places that detailed how it was done I made some changes – only to lock myself out of the server! Yes, iptables is a very powerful tool and getting it wrong results in real problems when you are connected to your server remotely 🙁 However, as I hadn’t configured iptables to load rules at startup a simple reboot would have restored my access had I thought more about it at the time before using a rescue image 🙂
Logging via Fail2ban
The solution turns out to be very straightforward! My jail.conf file had this configuration
Default banning action (e.g. iptables, iptables-new, # iptables-multiport, shorewall, etc) It is used to define # action_* variables. Can be overridden globally or per # section within jail.local file banaction = iptables-multiport
To enable logging of the actions this is simply changed to use the iptables-multiport-log action.
Default banning action (e.g. iptables, iptables-new, # iptables-multiport, shorewall, etc) It is used to define # action_* variables. Can be overridden globally or per # section within jail.local file banaction = iptables-multiport-log
Messages are logged to /var/log/syslog on this system.
... Jun 24 15:01:05 x kernel: [ 3773.082548] fail2ban-ssh:DROP IN=eth0 OUT= MAC=d4:3d:7e:ec:ea:55:cc:e1:7f:ac:56:9e:08:00 SRC=18.104.22.168 DST=aaa.aaa.aaa.aaa LEN=188 TOS=0x00 PREC=0x00 TTL=51 ID=14196 DF PROTO=TCP SPT=53375 DPT=22 WINDOW=296 RES=0x00 ACK PSH FIN URGP=0 ...
Having just started migrating away from Ubunutu to FreeBSD I found the return to using /etc/rc.conf to be “quaint”, but after the issue today I have a new found respect. Rather than having to spend time looking around for how/where the service is started on Ubuntu, it’s all in one place with FreeBSD. Not only that, but when using a rescue image I can mount the drive, find /etc/rc.conf and switch off an offending service quickly and easily.
Thankfully my server is still running 14.04 and so hasn’t been ruined by systemd or this wee adventure would have been far more painful. Another good reason to keep the migration moving forward.