Hello,

I am not asking how to do this (although always more info is welcome), but If is it possible to guess the gateway address of a network in a interface.

Case a)
Computer A connected directly through ethernet to another Computer B. Computer B has its eth0 in a specific network (e.g., 10.0.0.1, etc)


Case b)
Computer A connected through WLAN or Ethernet switch to a network created by Computer B (also connected).


In both cases:
Computer A wants to know which is the network to can "communicate" with Computer B.


There is no DHCP, just static IP.

Doing ping is not possible "Network is unreachable". But I don't know with nmap, arp-scan,...

Thanks!

neither of your examples have anything to do with "gateways" at all... this is just flat ethernet networking. You can sniff traffic and see what networks are being asked for, but past that, no there's no formal way to discover a the address details for a subnet officially, it's not part of the spec at all.

Maybe I express myself wrong.
I said gateway because I was thinking that behind the Computer B, it is connected to another network that provides Internet connection. And it is forwarded to eth0 of Computer B (through, lets say, its 10.0.0.1).

So, the only way is through a packet analyzer like Wireshark, and trying to obtain packets.

But in the case A would be impossible, isn't it? Because you are the only computer connected to B.

So, there is no way to do something like:

Test 1:
sudo ip addr add 10.0.0.120/24 dev enp2s0
sudo ip link set up dev enp2s0
ping -c 1 10.0.0.1

Test 2:
ping -c 1 10.0.0.2

etc...
Until receive packet.


And the same for all IP.

That is the only way?

I know two methods of network investigating - scanning and sniffing.

As you mentioned nmap and arp-scan, I think that you know about scanning. In case you don't know actual networks parameters, you can use something to start - e.g. 169.254.x.y. To send packets to any address directly you should set default gw to the address of own interface.

I prefer sniffing. Actually, hosts and network equipment often send packets to discover something via broadcast. How often - it depends. Maybe you'll never see any packets. But in fact any host should at least check for duplicate IP, while starting up network interfaces. So you may sniff something.