FreeBSD: How to block port scanners from enumerating open ports on your server, by using fail2ban and an ASCII representation of pf logs.


I use fail2ban to keep away attackers and bots alike that attempt to scan my websites or brute force my mailboxes. Fail2ban works by scanning log files for specific patterns and keeping a count of matches per IP, and allows the systems administrator to define what to do when that count exceeds a defined threshold.

The patterns are indicative of malicious activity, such as attempting to guess a mailbox password, or attempt to scan a web site space for vulnerabilities.

The action to perform is most of the time to block the offending IP address via the machine firewall, but fail2ban supports any mechanism that you can conceive, as long as it can be enacted by a UNIX command.

PF and its logs

On my FreeBSD server I use the excellent pf packet filter to policy incoming traffic and to perform traffic normalization.

The PF logging mechanism is very UNIX-y, as it provides a virtual network interface (pflog0) onto which the initial bytes of packets blocked by a rule that has the log specifier are forwarded, so that real-time block logs can be inspected via a simple:

# tcpdump -eni pflog0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on pflog0, link-type PFLOG (OpenBSD pflog file), capture size 262144 bytes
01:48:13.748353 rule 1/0(match): block in on vtnet0: > Flags [S], seq 1929621329, win 29200, options [mss 1460,sackOK,TS val 840989709 ecr 0,nop,wscale 7], length 0
01:48:15.726215 rule 1/0(match): block in on vtnet0: > UDP, length 3
01:48:17.993439 rule 1/0(match): block in on vtnet0: > Flags [S], seq 3365362952, win 1024, length 0
3 packets captured
3 packets received by filter
0 packets dropped by kernel

These logs can be saved by pflogd into a pcap format file in /var/log/pflog, that can be used for async troubleshooting and inspection, as well using tcpdump or anything that can parse pcap files (such as wireshark).

Limits of binary logs

I had already configured fail2ban to parse postfix, dovecot and nginx logs, so that if you try to brute an SMTP or IMAP passwd on my box or you try to run something like nikto against my web site you’ll soon be banned by fail2ban and your incoming connections will be dropped by pf.

However I could not ask fail2ban to read the binary pflog produced by pflogd, as fail2ban is regex-based and only understands text input.

Python to the rescue

I thought of a software that would:

  • Start an async loop
  • Run tcpdump and attach to its stdout and stderr
  • Write the stdout and stderr to a file
  • Trap a HUP and USR1 signal and re-open the file, to aid log rotation

Can I haz it?

Sure thing! Head over to github and check out pfasciilogd and the supporting fail2ban configuration.

I hope you find this useful.

Have fun!