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Running Snort on OpenBSD

30 Nov 2018

I wanted some practice with an IDS, writing custom rules, evading detection, that kind of thing.

I like OpenBSD as a platform for network applications. I use OpenBSD on my local LAN for both authoritative (nsd) and caching (unbound) DNS services and as the primary firewall (pf). I have used Snort before, but not on OpenBSD and that seemed like a good reason to try it for this experiment.

The existing test network looks like this on a single subnet.

outside --- fw --- switch A --- switch B
                       |            |
                    group A      group B

I want to monitor traffic between machines in group A and group B.

Since neither switch supports a network tap, I am going to run the IDS box as a transparent bridge placed here

outside --- fw --- switch A --- IDS --- switch B

This configuration makes it easy to pull the IDS back out when I am done.

OpenBSD Setup

I am using an older dual-core amd64 machine with 3 GigE nics for the hardware.

The operating system is OpenBSD 6.4. I did a standard amd64 installation, no X11 or games. I installed the compiler set, but it’s not necessary.

The 3 nics show up as re0, em0 and em1.

The re0 interface has a static IP used for access.

~$ cat /etc/hostname.re0

The em interfaces used for the bridge do not need IP addresses.

~$ cat /etc/hostname.em0

~$ cat /etc/hostname.em1

Here is the bridge interface configuration

~$ cat /etc/hostname.bridge0
add em0
add em1
blocknonip em0
blocknonip em1

When running it looks like this

~$ ifconfig -a
        lladdr 00:1b:21:a2:a0:52
        index 1 priority 0 llprio 3
        media: Ethernet autoselect (1000baseT full-duplex,rxpause,txpause)
        status: active
        lladdr 00:1b:21:a2:a0:53
        index 2 priority 0 llprio 3
        media: Ethernet autoselect (1000baseT full-duplex,rxpause,txpause)
        status: active
bridge0: flags=41<UP,RUNNING>
        index 6 llprio 3
        groups: bridge
        priority 32768 hellotime 2 fwddelay 15 maxage 20 holdcnt 6 proto rstp
        em0 flags=3<LEARNING,DISCOVER>
                port 1 ifpriority 0 ifcost 0
        em1 flags=3<LEARNING,DISCOVER>
                port 2 ifpriority 0 ifcost 0
pflog0: flags=141<UP,RUNNING,PROMISC> mtu 33136
        index 7 priority 0 llprio 3
        groups: pflog

Pretty simple.

Snort Setup

For more details see this post from the maintainer of the Snort port on OpenBSD.

Snort is in ports and can be installed with the package manager.

~# pkg_add snort

I am using the oinkcode ruleset.

After downloading, install the ruleset like this

~# tar -C /etc/snort -xzf snortrules-snapshot-2990.tar.gz

At a minimum, tell Snort which interface to watch

~$ grep -n em0 /etc/snort/snort.conf
44:config interface: em0

You can use rcctl to control Snort.

# rcctl start snort
# rcctl stop snort
# rcctl restart snort

This will enable starting at boot.

# rcctl enable snort

Snort alerts are logged here


I did not install a GUI for Snort, but tail works fine.

~# tail -f /var/snort/log/alert

After that its just a matter of tuning the rules for the network.

Check It

To verify things are working, here is a quick change to catch noisy portscans.

Edit /etc/snort/snort.conf and modify these two lines

- # preprocessor sfportscan: proto { all } memcap { 10000000 } sense_level { low }
+ preprocessor sfportscan: proto { all } scan_type { all } memcap { 10000000 } sense_level { low }


- # include $PREPROC_RULE_PATH/preprocessor.rules
+ include $PREPROC_RULE_PATH/preprocessor.rules

Restart snort.

Now running an Nmap scan of an A machine from a B machine

~# nmap

produces alerts like this in the snort log

[**] [122:1:1] (portscan) TCP Portscan [**]
[Classification: Attempted Information Leak] [Priority: 2]
11/22-13:45:54.091767 ->
RESERVED TTL:39 TOS:0x0 ID:59253 IpLen:20 DgmLen:165

Or a UDP scan

~# nmap -sU

produces alerts like this

[**] [122:17:1] (portscan) UDP Portscan [**]
[Classification: Attempted Information Leak] [Priority: 2]
11/22-13:46:33.592670 ->
RESERVED TTL:128 TOS:0x0 ID:11208 IpLen:20 DgmLen:166

Firewalling on the bridge

You can add firewall rules to the bridge if you want to expand the experiment.

In my setup, em0 faces the B switch.

A simple example

~# cat /etc/pf.conf

br0_if = "em0"

nameserver = ""

# block a noisy AP in the B group
block in quick on $br0_if proto udp to port ssdp

# force use of a nameserver for B hosts
pass in log quick on $br0_if proto { tcp, udp } from any \
    to ! $nameserver port domain rdr-to $nameserver

The pf firewall logs in pcap format.

To watch real-time events

~# tcpdump -i pflog0 -s 160 -e -n -ttt

The historical log

~# tcpdump -r /var/log/pflog -s 160 -e -n -ttt

As with all things OpenBSD, the man pages are the definitive resource.