Determine Router / Firewall external IP address

[DonU]

I have a router/firewall combo and I need to know the external IP address of the router (obtained from my ISP via DHCP).

Is there a way I can determine this without accessing the router administration? Idealy, I would like to be able determine this in a program from inside the router.

 

[pansophic]

Finding the external address of your router is simple if the router is performing NAT. Just go to

http://grc.com

and go to Shields UP. It will tell you the NAT address that is being used. There are other websites out there that do the same thing, I just can't think of any off hand.

If you have routable addresses on your internal network (highly unlikely, especially if your router is getting a DHCP address), then you can use a Looking Glass server to perform a traceroute back to your machine. The router will be the last address that returns if you are blocking ICMP and will be the second to the last address if you are allowing ICMP.

Go to:

http://www.traceroute.org/#Looking

Glass

for a list of Looking Glass servers.
pansophic

 

[StarTAC]

u can also perform a traceroute out to the net.. u may land on your external interface as one of the hops.. but grc.com is a definite bet..

good luck...

 

[DonU]

Unfortunately neither of the above suggestions works (Grc.com or traceroute) as I really need to be able to record this address for users that are "less than technically challenged"!

 

[pansophic]

Why are you trying to get this address and what do you intend to do with it?

Traceroute will not work because you only receive a response from the interface that receives the packet, not the one that sends it (unless they are the same interface). With an Internet router, they are not the same interface.

I am assuming that grc.com worked, it just isn't a workable solution for you?
pansophic

 

[Accessdabbler]

I think he wants to get the address without logging into the router and without using a website such as grc.com. He wants to know (and so do I) if there is another way to determine his router's WAN IP.

 

[pansophic]

Why not use your router's Dynamic DNS feature to contact a DNS server on the net, or one that you set up? I've never used the Dynamic DNS stuff before, but it should be relatively easy to set one up yourself (the static ones are).

With Dynamic DNS, each time your router is forced to update its IP address, it will contact the DNS server of your choice, and then tell the DNS server your new IP address.

You can get the service for relatively cheap from someone like TZO.NET. The all you have to do is an nslookup for your hostname.
pansophic

 

[peterve]

why would your users want to know the external IP address of your router when they are sitting behind it ?
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[DonU]

We have these routers at many remote locations that we need to contact via PC AnyWhere. In order to do this we need the current IP address of the outside of the router. The users in these locations are less that computer literate in many cases and are also often extremely busy running their business. An automated way of getting this info would be best.

 

[pansophic]

You definitely want to use a Dynamic DNS provider. Each time the router gets a new IP, the DNS records will update and you will be able to contact the remote offices by name again. You will have to set up some address translation rules for incoming connections though, and you won't be able to have more than one remotely controlled system on each site unless you change the port number that you connect to in PC Anywhere because only one incoming port can be rerouted to one IP/port.
pansophic