SNMP and external IP

Is there any way to use SNMP to acquire the external IP address of a NAT
router?

I'm after persuading my (homemade) dynamic dns client to function over the
NAT link, and short of abusing the system by auto-updating every five
minutes, or using an external host, I can't think of a way to do it.

The only thoughts I've had have been DHCP (apparently no good by my
reading anyhow) or SNMP.

I'm capable of writing my own agent/client for any useable protocol.

Any clues?

Dave J.

"Dave J" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Is there any way to use SNMP to acquire the external IP address of a NAT
> router?

Yes. snmpwalk starting at 1.3.6.1.2.1.4.20.1.1 lists all IP addresses on my
router.

See http://www.ibr.cs.tu-bs.de/cgi-bin/sbrowser.cgi

HTH,
Alex

"Alex Fraser" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> "Dave J" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> > Is there any way to use SNMP to acquire the external IP address of a
> > NAT router?
>
> Yes. snmpwalk starting at 1.3.6.1.2.1.4.20.1.1 lists all IP addresses on
> my router.
>
> See http://www.ibr.cs.tu-bs.de/cgi-bin/sbrowser.cgi

To expand a little, OIDs under the above should have corresponding ones with
the interface index under 1.3.6.1.2.1.4.20.1.2. AIUI it's permissable for an
interface index to change when the system is reinitialised, but IME that
doesn't happen unless the hardware is changed, and that won't happen in a
router. So, as a one-off, you can manually find the interface index for the
router's WAN port. Then (in your DNS update client) you can poll (walking
....1 and ...2) to find the address associated with that interface.

That said, it's undoubtedly easier and probably satisfactory just to walk
the address entries looking for a routable address.

Alex

In MsgID<(E-Mail Removed)> within uk.comp.home-networking,
'Alex Fraser' wrote:

>> See http://www.ibr.cs.tu-bs.de/cgi-bin/sbrowser.cgi
>
>To expand a little, OIDs under the above should have corresponding ones with
>the interface index under 1.3.6.1.2.1.4.20.1.2. AIUI it's permissable for an
>interface index to change when the system is reinitialised, but IME that
>doesn't happen unless the hardware is changed, and that won't happen in a
>router. So, as a one-off, you can manually find the interface index for the
>router's WAN port. Then (in your DNS update client) you can poll (walking
>...1 and ...2) to find the address associated with that interface.
>
>That said, it's undoubtedly easier and probably satisfactory just to walk
>the address entries looking for a routable address.

Thanks for that. I'm currently trying to find out how the requests are
structured, but only half heartedly, as I'm sporting the latest
fashionable lurgii, the super-intense coldie-fluie one.. (

Slightly annoying as I've seen at least one presentation that described
the packets in lovely detail, discovered by accident when I was revising
for an (almost unrelated) exam, but it seems to have disappeared from my
history file..

--
Dave Johnson - (E-Mail Removed)

"Dave J" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> In MsgID<(E-Mail Removed)> within uk.comp.home-networking,
> 'Alex Fraser' wrote:
[snip]
> Thanks for that. I'm currently trying to find out how the requests are
> structured, but only half heartedly, as I'm sporting the latest
> fashionable lurgii, the super-intense coldie-fluie one.. (

Unless you have some pressing need to construct/parse packets yourself, you
might save time by using the Net-SNMP library, http://www.net-snmp.org/.
Even if you don't want to use it, the command-line tools it comes with will
probably be useful.

Alex