Discovering the default gateway without DHCP

Question

A crazy question, but is there anyway to discover the default gateway without DHCP?

This would be for a device on a network which does not use DHCP which does not have an IP address as yet. I was thinking if I could discover the default gateway, then i could try to guess an unused ip address then broadcast on the network to see if it's being used.

I may be asking for trouble i understand. E.g. if there is already a computer which has a static IP and is shutdown.

Answer 1

Packet sniff for a while and then apply heuristics. I'm assuming IPv4 and Ethernet for the rest of this. This won't work so well if Ethernet Switches are used rather than HUBs. More on that at the end.

Create your own RARP (reverse address resolution) table based on the Ethernet (or whatever) and IP headers you see as well as actual ARP packets (ignoring broadcast and multicast IP and MAC addresses). Make sure that your table is able to map multiple IP addresses to a single hardware interface. Any IP addresses in the table that are entered or verified by actual ARP packets should be flagged as such.

The gateway(s) is likely to recieve and send more traffic. This traffic is likely to have many IP addresses from many different networks but have the same hardware address. The gateway will show up in your table as one MAC address with lots of IP addresses. It is likely that you will have observed an ARP transaction involving this MAC address address, so you can look for an IP address that aliases that MAC that also has the seen in ARP packet flag set. That is almost certainly a gateway's IP address.

You will probably be able to guess the network address and subnet mask based on the IP address of the gateway, but this might not be something you can trust. If you want to try to make this work on a network with a non-standard netmask you can try to do the following. Make two IP address sized accumulator variables:

uint32_t acc_and = 0xFFffFFff;
uint32_t acc_or  = 0x00000000;

And then for each ARP verified address in your table you do

acc_and &= ip_addr;
acc_or  |= ip_addr;

You could also use destination IP addresses (that aren't multicast) in packets that came from the gateway for this, but you would have to know the gateway before you could flag them as such.

finding the netmask and network address

The network portion of the address should stay the same in both (after the first operation), but in acc_and the bottom bits should start clearing out while in the acc_or the bottom bits should start filling up. With enough samples you would be able to determine the network address by:

uint32_t net_addr = acc_and & acc_or;

and the netmask by:

uint32_t net_mask = acc_and ^ acc_or;

Getting enough sampled local IP addresses could take too long, so you can start trying to narrow it down by:

uint32_t almost_net_mask = acc_and ^ acc_or;
int i = 0;
while ( 1 & almost_net_mask ) {
   i++;
   almost_net_mask >>= 1;
}
uint32_t net_mask = 0xFFffFFff;
while( i-- ) {
     net_mask <<=1;
}

This would find you the last 1 bit set. The lowest 0 bit is obviously not part of the local portion of the address. Alternately you could get the same i by:

i = ffs( ~ ( acc_and ^ acc_or ) ) ; // assuming that int on your system is 32 bit.  if not use ffsl

If you really really want to be sure of your netmask and network address now you can try some other things. You can try to see if the gateway will attempt to forward packets you send it to what you think should be local addresses to them. For this would would have to go ahead and assign yourself an IP address, which is risky if your netmask isn't right.

choosing an IP for yourself

Try to choose the lowest legal IP address that you haven't seen traffic to or from to increase the chance that it is not out of range. You could spoof an ARP request for that address and look for any replies (you could use your own real MAC address but possibly make up an IP for this. doesn't have to be your IP, which might confuse things for a little bit). After you have found an IP address that you can't ARP claim it for yourself.

It is possible that you ended up with an IP address that is actually too high. This is difficult to know for sure since your netmask is still just a good guess. You're out of luck if this is the case because there is not free slot for you to live.

back to the netmask

To see if your netmask needs adjusting try sending IP packets with destination addresses that you haven't see in the local network but that could be in the local network based on your guesses at the netmask and the network address. If these guesses are off they should have too many bits assigned to the network address, so sending to addresses which change the lower bits from the netmasked portion of the (so far) best guessed network address is what you want. You can try sending these as you normally would by ARPing the IP address and seeing if anyone replies, but since you are guessing addresses and likely to miss, it might be better to try creating the packet(s) with the destination MAC address set to the gateway to see if it would forward it. It may be configured not to, so you could try to see if it would by first doing this for a member of the network which you have already observed in the network and then see if it forwards it for you. If the gateway forwards the packet then you can rely on its idea of the netmask to narrow down your idea of the netmask. If the gateway will not forward for packets destine for known members of the local network then you can either just keep using your idea of the local netmask and network until you have a reason to adjust it or send out ARP requests for addresses in that range (which will only answer your question as either "yes" or "maybe" with no possibility of a sure "no").

if you are using an Ethernet switch, rather than a HUB

If you are using an Ethernet switch things get much more difficult because the switch will not forward Ethernet frames to you if it knows that the frames should go somewhere else, so you will not see them. It will forward many ARP requests, though, since they are broadcast unless the sender still has an entry for that IP in its ARP cache and is just trying for an early renewal of that entry (this may not be done by all systems). This will make building your idea of the network and its members much more difficult, but you will likely be able to do a decent job. You will probably have to rely on the quantity of ARP request for the gateway and from the gateway being higher than the other systems to spot it, though. This is error prone as a file server could have similar traffic.

Answer 2

Could be done but not in easy way (according to my poor knowledge). Solution I would analyze further:

• Detect Gateway - 2 possibilities (all off them related with data sniffing):
  • capture DHCP offer packets and retrieve gateway
  • look for packet which is addressed to outer network (e.g. do arp request on each target IP address - if no answer you have probable mac address of the default gateway - you need to find IP address of this gateway (analyze arp packets, all packets, etc..)
• one you have ip address do a arp requests and find you free ip.

sniffing library I think is libpcap or winpcap.

Never done even something similiar but If I wouldn't find any references on google I would go this way I think.