This is active banning of IPs that are brute forcing login attempts
to SSH, versus the passive banning of 10,000 abusive IPs from the
abuseipdb.com blacklist. For now I am banning IPs that fail to log
in successfully more than twelve times in a one-hour period, but
these settings might change, and I can override them at the group
and host level if needed.
Currently this works for CentOS 7, Ubuntu 16.04, and Ubuntu 18.04,
with minor differences in the systemd configuration due to older
versions on some distributions.
You can see the status of the jail like this:
# fail2ban-client status sshd
Status for the jail: sshd
|- Filter
| |- Currently failed: 0
| |- Total failed: 0
| `- Journal matches: _SYSTEMD_UNIT=sshd.service + _COMM=sshd
`- Actions
|- Currently banned: 1
|- Total banned: 1
`- Banned IP list: 106.13.112.20
You can unban IPs like this:
# fail2ban-client set sshd unbanip 106.13.112.20
Now that I'm blocking ~10,000 malicious IPs from AbuseIPDB I feel
more comfortable using a more relaxed rate limit for SSH. A limit
of 12 per minute is about one every five seconds.
If a user has RSA, ECDSA, and ED25519 private keys present on their
system then the ssh client will offer all of these to the server
and they may not get a chance to try password auth before it fails.
There is a bug in iptables 1.8.2 in Debian 10 "Buster" that causes
firewalld to fail when restoring rules. The bug has been fixed in
iptables 1.8.3, which is currently in buster-backports.
See: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=914694
The default in later OpenSSH is 6, which seems too high. If you can't
get your password correct after 3 tries then I think you need help.
Eventually I'd like an easy way to enable blocking of repeated login
attempts at the firewall level. I think it's possible in firewalld.
Vanilla Ubuntu (and Debian actually) defaults to using the official
mirror for security updates rather than country or regional mirrors.
Also, for what it's worth, Ubuntu mirrors didn't always sync these
security archives. I'd prefer to stay closer to vanilla Ubuntu but
also it kinda makes sense to get security updates from the official
source than a mirror (in case of delay or errors).
I'm surprised this worked all these years actually. Since Ansible
version 1.6 it has been possible to use the version_compare filter
instead of doing math logic on strings.
See: https://docs.ansible.com/ansible/playbooks_tests.html
I had removed them from Debian 8 and Ubuntu 14.04 configs last year
when the NSA's Suite B crypto guidelines dropped 128-bit algorithms
but those changes didn't make it to my new Ubuntu 16.04 config.
It is probably overkill and paranoid, but this server is mine, so I
can make those decisions (and I only connect from modern clients).
Automatically uses the best mirror for your location, see:
http://httpredir.debian.org/demo.html
Should be much better than any hardcoded default for most hosts.
Signed-off-by: Alan Orth <alan.orth@gmail.com>
