Question

I am trying to write a program that scan an ip range and detect if an ip is address of a router or not.

Currently i used traceroute from my computer to all host in the network. However, i believe there must be some way to directly “ask” a host at an ip if it is a router or not?

by the way, do you know any program/ opensource already does this?

Answer 1

Routers are supposed to talk couple of protocols (actually a neat bunch) that regular IP nodes do not, and then there are some which are more common (i.e. even non-router nodes do).

Router-only protocols:

• VRRP
• IGRP / EIGRP
• OSPF
• BGP
• RIP

You could do active-probing on those, i.e. send a packet (behaving as if you are another router, or an end-node) and checking to see what kind of response the router (if at all) sends.

Alternatively you could do passive-probing, like ‘sniffing’, i.e. watching out for the kind of IP packets being sent out by various nodes. There are some which are usually sent out by Routers only (again, mostly from the above list).

Common protocol, but that can actually tell you a lot:

• SNMP (esply the unsecure one’s like v1/v2, are easy to deal with, without having to establish a secure session)

Other ways:

• Portscanning (actually can tell you a real lot), for example all routers have some management ports (although, often they are locked down due to security concerns)

What you want to do is often what many ‘Network Management’ software do, to “discover” capabilities / functionality of other nodes in the network. And, there isn’t a single size-fits all solution. They use bunch of different methods, heuristics to finally figure out what the other node is.