Tuesday, October 26, 2010

Fun with Raw Sockets in Erlang: Bridging

Hosts connecting to another system on the same network map the protocol address (e.g., IPv4 address) of the destination host to a hardware address (e.g., ethernet MAC address) using the ARP protocol. Clients on different networks can communicate by having a device forward the packets between networks. These devices, usually multi-homed and spanning networks, forward packets by re-writing the packet headers: for example, the ethernet header (a bridge), the IP header (a router) or the IP and TCP headers (a NAT).

An ethernet II header:

  • Preamble:42
  • Start of Field Delimeter:8
  • Destination Host:48
  • Source Host:48
  • Type:16
  • Protocol Payload:N
  • CRC:16

Not all of these fields will be visible to processes running on the operating system.

  • The Preamble starts the Ethernet frame (not available to the OS)
  • The SOF Delimiter marks the start of the destination host address field (not available to the OS)
  • The Destination Host is the 6 byte MAC address of the target host
  • The Source Host is the 6 byte MAC address of the sending host
  • The Type represents the protocol held in the payload. Common types are IPv4 (ETH_P_IP (16#0800)), ARP (ETH_P_ARP (16#0806)) and IPv6 (ETH_P_IPV6 (16#86DD)).
  • A trailing CRC checksum of the packet (frame check sequence) (usually not available to the OS)

    Ethernet frames can apparently include other trailing data, leading to trailing junk when doing packet captures.

    When capturing data off the network, it's important to properly calculate the size of the packet. For example, for an IPv4 TCP packet:

    IP length - (IP header length * 4) - (TCP offset * 4)

The ethernet header has fixed size fields and so does not explicitly include a field for length. Interestingly, Ethernet 802.3 frames use the Type field for the length (according to Wikipedia 802.3 packets can be distinguished from Ethernet II packets by:

  1. if the value of the Type field is equal to or less than 1500 bytes (maximum frame length), the frame is Ethernet 802.3 and the value represents the length of the frame
  2. if the value of the Type field is equal to or greather than 1536, the frame is Ethernet II and the value represents the protocol type of the encapsulated packet

    The protocol of 802.3 frames is always IPX.

  3. if the value of the Type field is between 1500 and 1536, the behaviour is undefined)

An Erlang binary representation of an Ethernet II frame:

Dhost:6/bytes,        % destination MAC address
Shost:6/bytes,        % source MAC address
Type:16               % protocol type, usually ETH_P_IP

Using Erlang to Bridge Packets

To go along with the ARP poisoning, we need a sort of "one armed bridging" to forward packets from our spoofing host to the real destinations. Once we have the raw ethernet frames, doing the bridging is quite simple. For the complete code, see herp on GitHub (yes, the herp is what you get when you've been promiscuous. I hear like 80% of adults have it).

I won't include much of the code here because it's so trivial. The bridging process captures packets off the network and pattern matches on the headers:

filter(#ether{shost = MAC}, _, #state{mac = MAC}) ->
filter(#ether{type = ?ETH_P_IP}, Packet, State) ->
    {#ipv4{daddr = DA}, _} = epcap_net:ipv4(Packet),
    filter1(DA, Packet, State);
filter(_, _, _) ->

filter1(IP, _, #state{ip = IP}) ->
filter1(IP, Packet, #state{gw = GW}) ->
    MAC = case packet:arplookup(IP) of
        false -> GW;
        {M1,M2,M3,M4,M5,M6} -> <<M1,M2,M3,M4,M5,M6>>
    bridge(MAC, Packet).
  1. Check if the frame has our MAC address
  2. Retrieve the IP address from the frame and check the system ARP cache. A real bridge would monitor the network for ARP packets and cache the results.
  3. If the IP exists in our ARP cache, use the MAC address, otherwise, send it to the gateway (it's possible the IP address does not exist in the system ARP cache and will be wrongly forwarded to the gateway)
If the frame should be bridged, we create a new frame with the source hardware address set to our host's MAC address. The IP header and payload are not touched.
handle_call({packet, DstMAC, Packet}, _From, #state{
        mac = MAC,
        s = Socket,
        i = Ifindex} = State) ->

    Ether = epcap_net:ether(#ether{
            dhost = DstMAC,
            shost = MAC,
            type = ?ETH_P_IP

    packet:send(Socket, Ifindex, list_to_binary([Ether, Packet])),
    {reply, ok, State};

It'd be interesting to experiment with making herp into a traditional network bridge: quite likely very slow, but also redundant, distributed and fault tolerant.

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