What is the DHCP server address?

[Lolita_Magnum]

Does anyone know how to find the DHCP server address on the (WAN) side of JioFiber? I tried using the routers inbuilt packet-tracer but was unable to find anything useful.

 

[vishalrao]

Try sending a DHCP discover request to the broadcast IP on the wan side? Maybe nmap tool could do this?

 

[Lolita_Magnum]

Cool but how do you expect me to broadcast this on WAN? DHCP-broadcast is 255.255.255.255.

 

[vishalrao]

I mean if you know that wan ip a.b.c.d via traceroute then try simply send DHCP discover request to a.b.c.255 using some tools like nmap or wireshark?

I'm just trying to generalise off the top of my head but you could google for specifics?

 

[vishalrao]

Or even the dhclient command on linux ?

 

[Lolita_Magnum]

@vishalrao Yea.... that is the problem, I don't know what the DHCP server's address is... Hence, the post.
Discovery Request only discovers local LAN's DHCP server, I'm not sure how will I broadcast this on JioFiber router's wan interface.

 

[vishalrao]

Hang on...

If you do a traceroute and try sending DHCP discover requests to some of the IPs in the trace that could be the internal "WAN" IP not the actual WAN public IP but a few hops before?

So let's say one of the potential internal IP is 1.2.3.4 then you send the request to 1.2.3.255 ?

I'm sure their DHCP server, if they have any, will not be on the public WAN IP subnet.

Disclaimer: I actually failed my first attempt at my computer networking exam back during my BE degree final semester so I'm not sure what I'm suggesting would even work.

 

[vishalrao]

If the DHCP discover request actually reaches out to the x.y.z.255 broadcast address then you will see a response from the DHCP server including its IP either in the output of the tool (nmap or dhclient) or you can try wireshark to observe the response.

First test your approach with your local network and see if you are able to get valid response then try on the IPs you get from traceroute. I mean if your router is 192.168.1.1 then send DHCP discover request to 192.168.1.255 and if you observe response from 192.168.1.1 itself then you know at least your steps are correct.

 

[wheatbread]

You can't send DHCP Discover packets to an IP, even if you could somehow they won't reply without authentication success with their AAA system. Also, most large scale DHCP servers don't have a routable IP address because they don't need one. They can make DHCP offer with just link-local address.

 

[vishalrao]

@wheatbread if not to an IP then how? Something like ARP?

This is an interesting topic I might try myself this weekend.