Filtering ARP traffic with Linux arptables

Most Linux system administrators will be familiar with iptables on Linux. Less known is the arptables utility, which controls filtering arp packets.

Installation

The arptables utility is easy to set-up, as the main functionality is already implemented in the Linux kernel. Just install the arptables package on your favorite Linux distribution.

Red Hat / CentOS / Fedora

yum install arptables

Debian / Ubuntu

apt-get install arptables

Configuration example

To show the effect of filtering traffic, we will show an example by filtering router traffic and blocking it. This way we won’t be able to connect to the internet.

With the arp command we can query the current list of known ARP addresses.

root@ubuntu:/data# arp
 Address                  HWtype  HWaddress           Flags Mask            Iface
 System.cisofy.com        ether   00:a7:22:23:d1:f3   C                     eth0
 Router.cisofy.com        ether   d8:d7:21:22:5a:8d   C                     eth0

Arptables can block traffic by filtering out the IP. So let’s query the arp list again, now in numeric format.

root@ubuntu:/data# arp -n
 Address                  HWtype  HWaddress           Flags Mask            Iface
 192.168.1.20             ether   00:a7:22:23:d1:f3   C                     eth0
 192.168.1.1              ether   d8:d7:21:22:5a:f4   C                     eth0

Time to block the router (192.168.1.1):

root@ubuntu:/data# arptables -A INPUT -s 192.168.1.1 -j DROP

So we dropped traffic to this IP adress, right? Let’s try!

root@ubuntu:/data# ping 192.168.1.1
 PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
 64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=0.645 ms
 64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=0.370 ms
 ^C
 --- 192.168.1.1 ping statistics ---
 2 packets transmitted, 2 received, 0% packet loss, time 1000ms
 rtt min/avg/max/mdev = 0.370/0.507/0.645/0.139 ms

Well, that didn’t work like intended. We dropped ARP related traffic to the IP address, but not on IP level. This is also visible in the arp -n list:

root@ubuntu:/data# arp -n
 Address                  HWtype  HWaddress           Flags Mask            Iface
 192.168.1.20             ether   00:a7:22:23:d1:f3   C                     eth0
 192.168.1.1              ether   d8:d7:21:22:5a:f4   C                     eth0

So to make this work, we simply have to flush the ARP cache. We delete the related ARP entry:

root@ubuntu:/data# arp -d 192.168.1.1
root@ubuntu:/data# arp -n
 Address                  HWtype  HWaddress           Flags Mask            Iface
 192.168.1.20             ether   00:a7:22:23:d1:f3   C                     eth0
 192.168.1.1                      (incomplete)                              eth0

The arp utility will show an incomplete entry. It knows that recently some traffic passed by, but the MAC address is unknown.

Let’s ping again:

root@ubuntu:/data# ping 192.168.1.1
 PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
 From 192.168.1.21 icmp_seq=1 Destination Host Unreachable
 From 192.168.1.21 icmp_seq=2 Destination Host Unreachable

That looks better!

Specific traffic filtering

Back to our original mission: only allow our router to exchange ARP packets.

root@ubuntu:/data# Block ARP traffic from all machines (default: DENY)
arptables -P INPUT DROP

root@ubuntu:/data# Allow router (fixed ARP)
arptables -A INPUT --source-mac d8:d7:21:22:5a:f4 -j ACCEPT

All ARP packets are blocked now. Each system which will transmitting traffic will end up as an (incomplete) entry.

Enable all ARP traffic

If we want to allow traffic again:

root@ubuntu:/data# arptables -P INPUT ACCEPT

root@ubuntu:/data# arptables --flush

Flushing the full ARP cache can be done with ip utility:

root@ubuntu:/data# ip -s neighbour flush all

Conclusion

Arptables is a very powerful utility to filter traffic and avoid an unexpected router taking over our connectivity. However, keep in mind that connectivity is not fully blocked. Only ARP traffic is blocked (layer 2/3 on the OSI model). If someone is able to manually add an entry to the ARP table, traffic is able to flow again.