Unique IP address for PPP interface?

[Chris Nelson]

I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0
interfaces but the system I was given to work with -- which ostensibily
works -- has the same address for both:

eth0 Link encap:Ethernet HWaddr 00:A0:1D:2E:2E:1F
inet addr:10.93.0.190 Bcast:10.255.255.255 Mask:255.0.0.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:75 errors:0 dropped:0 overruns:0 frame:0
TX packets:36 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:100
RX bytes:5360 (5.2 kb) TX bytes:2693 (2.6 kb)
Base address:0xe00
...
ppp0 Link encap:Point-Point Protocol
inet addr:10.93.0.190 P-t-P:10.192.8.11 Mask:255.255.255.255
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:5 errors:0 dropped:0 overruns:0 frame:0
TX packets:5 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:78 (78.0 b) TX bytes:72 (72.0 b)

This seems quite wrong but if, as I'm told, this is working then it
must be right and I'm confused. I don't know enough PPP to be sure.

[James Carlson]

Post by Chris Nelson
I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0
interfaces but the system I was given to work with -- which ostensibily

The local address on a point-to-point link may be identical to one or
more other local addresses. It's the remote address that must be
unique.

Post by Chris Nelson
eth0 Link encap:Ethernet HWaddr 00:A0:1D:2E:2E:1F
inet addr:10.93.0.190 Bcast:10.255.255.255 Mask:255.0.0.0

[...]

Post by Chris Nelson
ppp0 Link encap:Point-Point Protocol
inet addr:10.93.0.190 P-t-P:10.192.8.11 Mask:255.255.255.255

That's fine. You have a distinct remote address. The system uses the
remote address to distinguish among point-to-point interfaces, not the
local address.

Post by Chris Nelson
This seems quite wrong but if, as I'm told, this is working then it
must be right and I'm confused. I don't know enough PPP to be sure.

It looks fine to me. What problem are you trying to solve?

[Chris Nelson]

Post by James Carlson

Post by Chris Nelson
I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0
interfaces but the system I was given to work with -- which ostensibily

The local address on a point-to-point link may be identical to one or
more other local addresses. It's the remote address that must be
unique.

Thanks.

Post by James Carlson
...
It looks fine to me. What problem are you trying to solve?

I've got my PPP client and server sitting on a tiny little LAN on my
desk and I'm trying to test PPP with a null modem cable. Maybe that's
a bad thing. I can ping from the server to the client via PPP because
the IP address chooses the ppp0 interface. But I can't ping from the
client to the server because the server can be reached -- with the same
IP -- on both eth0 and ppp0. I guess that situation is unlikely to
arise in practice.

[James Carlson]

Post by Chris Nelson
I've got my PPP client and server sitting on a tiny little LAN on my
desk and I'm trying to test PPP with a null modem cable. Maybe that's
a bad thing.

That should work fine.

Post by Chris Nelson
I can ping from the server to the client via PPP because
the IP address chooses the ppp0 interface. But I can't ping from the
client to the server because the server can be reached -- with the same
IP -- on both eth0 and ppp0. I guess that situation is unlikely to
arise in practice.

Then just shut down eth0 for purposes of testing, or use different
addresses.

The situation you're talking about is really just fine from a
networking point of view. Each system is doing *exactly* what it's
supposed to do -- finding the best way to send packets to the other.
That the decision is different in the two cases is pretty much
immaterial.

IP is supposed to get the packets through, not make testing easy. ;-}

[Unruh]

Post by Chris Nelson

Post by James Carlson

Post by Chris Nelson
I"m 99.7% sure that I need a separate IP address for my ppp0 and eth0
interfaces but the system I was given to work with -- which ostensibily

The local address on a point-to-point link may be identical to one or
more other local addresses. It's the remote address that must be
unique.

Thanks.

Post by James Carlson
...
It looks fine to me. What problem are you trying to solve?

I've got my PPP client and server sitting on a tiny little LAN on my
desk and I'm trying to test PPP with a null modem cable. Maybe that's
a bad thing. I can ping from the server to the client via PPP because
the IP address chooses the ppp0 interface. But I can't ping from the
client to the server because the server can be reached -- with the same
IP -- on both eth0 and ppp0. I guess that situation is unlikely to
arise in practice.

Yes, Just tell your server to route that address over ppp.
The routing tables will always take the most specific address.

route add -host 111.222.333.444 ppp0