VOIP with a linksys PAP2

[Kevin J. Cummings]

THUFIR HAWAT wrote:
> On 6/14/05, Tony Nelson <tonynelson at georgeanelson.com> wrote:
> ...
> 
>>No, what he said was clear to me.  He knows both interfaces work because
>>he's tried each of them in turn.  Of course, when he did that, each one was
>>automatically configured by DHCP in his router, and the other one would
>>have been unconfigured.
>>
>>For his setup to work, he needs to either run a DHCP server on the
>>downstream i/f (usually his eth1) or set up that i/f with a static IP (or
>>probably both so his 'phone thing gets an address).
>>
>>Or he could put the hub between his upstream i/f and the router, and put
>>the 'phone on the hub, but he doesn't want to do that.
> 
> ...
> 
> heh, glad I made sense to at least one person ;)

OK, I misunderstood.  I was assuming that both ethernet cards were 
capable of working, but only after being configured!  You have to ensure 
that they get configured properly.  The DHCP from your internet 
connection is configuring eth0 (because system-config-network has 
configured eth0 to use DHCP).  You still need to do something to 
configure eth1 so it will work correctly.

> I turned everything off, plugged things in like so:
> 
> internet-->router-->wireless-->asus network adapter-->hub
> 
> hub-->PAP2-->telephone
> 
> hub-->eth0
> 
> on reboot I got messages about 100% packet loss.  I'm looking over
> /var/log/messages and dmesg, can't find those error messages.  anyhow,
> as I understand there's a problem with MAC address's which prevents
> that setup.  (ultimately, this'd be nice, for now I'm just looking for
> a phone.)

Yes, unfortunately, DHCP is most easily set up to allocate an IP address 
to a single network interface (each network interface has a unique MAC 
address).  If it has allocated an address (to your PAP2?) it won't 
allocate one to your computer.  I have the same problem with my cable 
modem.  I got around that by power cycling my cable modem between 
different connections.

> I need a DHCP router on the downstream, meaning between eth0 and eth1
> to provide the masquerading?

No, you need to run a DCHP server (software daemon) on your computer 
(and it should allocate IP addresses to devices attached to the network 
via eth1) in order for your PAP2 to be configured correctly.  You also 
need to setup eth1!  If you do it right, you can add this to your DHCP 
configuration as well, though its probably easiest to just use a static 
private network address (in the same subnet as your DHCP server so 
everything on eth1 is in the same subnet).  In addition, if you set up 
eth1 as a different network than eth0 (recommended), then you need to 
masquerade (ie, NAT) the eth1 network to the eth0 network.  This is in 
addition to the IP forwarding you need to do between the 2 networks. 
This will make your computer into a (NAT) router between the 2 networks. 
  In that case, the packets from your PAP2 telephones will be routed to 
your computer, which will then take them from eth1, masqerade them, and 
send them out on eth0 as if the originated on your computer.  When the 
return packets come back from the internet, they will go to eth0, be 
recognized by the masquerading software as intended for your PAP2, 
un-masquerade them, and send them back to the PAP2 which will convert 
them back into telephony.

[I have a question for the community in general:  Will he be able to NAT 
between eth0 and eth1 and then have his wireless network NAT *that* to 
the internet?  ie, does nested NATing work?  I've not ever tried it.]

-- 
Kevin J. Cummings
kjchome at rcn.com
cummings at kjchome.homeip.net
cummings at kjc386.framingham.ma.us