UDP packet strange behavior

(Fedora Core 4)

I've got a computer A with two Ethernets, connected to two distinct
networks (one corporate, one for development only). I make a socket
for UDP, and I "connect" this socket to a the IP of computer B that's
reached via one of those interfaces.

So I "send" one packet. Recipient B gets it, but it's source IP names
A's other interface! The recipient has no route to get a reply packet
back.

I "send" another packet. This time I get errno=113 - network
unreachable.

What is going on?

(E-Mail Removed) wrote:
> I've got a computer A with two Ethernets, connected to two distinct
> networks (one corporate, one for development only). I make a socket
> for UDP, and I "connect" this socket to a the IP of computer B that's
> reached via one of those interfaces.

> So I "send" one packet. Recipient B gets it, but it's source IP names
> A's other interface! The recipient has no route to get a reply packet
> back.

> I "send" another packet. This time I get errno=113 - network
> unreachable.

> What is going on?

I suspect that when you called connect() on your datagram socket, it
did the implicit bind for you but happened to do it to the other IP on
the system. But you probably already figured-that out.

To be certain of the source IP, you probably need to bind() to it
first before you call connect(). Doing so will make your application
no longer dependent on stack-specific behaviour such as whether or not
it is a "strong" or a "weak" end-system model.

rick jones
--
The computing industry isn't as much a game of "Follow The Leader" as
it is one of "Ring Around the Rosy" or perhaps "Duck Duck Goose."
- Rick Jones
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

(E-Mail Removed) wrote:
> (Fedora Core 4)
>
> I've got a computer A with two Ethernets, connected to two distinct
> networks (one corporate, one for development only). I make a socket
> for UDP, and I "connect" this socket to a the IP of computer B that's
> reached via one of those interfaces.
>
> So I "send" one packet. Recipient B gets it, but it's source IP names
> A's other interface! The recipient has no route to get a reply packet
> back.
>
> I "send" another packet. This time I get errno=113 - network
> unreachable.
>
> What is going on?

How is one computer supposed to know that the other can only reach it
using one particular interface? By what magic would you expect this to
automatically work?

David Schwartz wrote:
> (E-Mail Removed) wrote:
> > (Fedora Core 4)
> >
> > I've got a computer A with two Ethernets, connected to two distinct
> > networks (one corporate, one for development only). I make a socket
> > for UDP, and I "connect" this socket to a the IP of computer B that's
> > reached via one of those interfaces.
> >
> > So I "send" one packet. Recipient B gets it, but it's source IP names
> > A's other interface! The recipient has no route to get a reply packet
> > back.
> >
> > I "send" another packet. This time I get errno=113 - network
> > unreachable.
> >
> > What is going on?
>
> How is one computer supposed to know that the other can only reach it
> using one particular interface? By what magic would you expect this to
> automatically work?

I had expected it to supply the IP of the interface that transmitted
the packet.

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(E-Mail Removed) wrote:
> (Fedora Core 4)
>
> I've got a computer A with two Ethernets, connected to two distinct
> networks (one corporate, one for development only). I make a socket
> for UDP, and I "connect" this socket to a the IP of computer B that's
> reached via one of those interfaces.
>
> So I "send" one packet. Recipient B gets it, but it's source IP names
> A's other interface! The recipient has no route to get a reply packet
> back.
>
> I "send" another packet. This time I get errno=113 - network
> unreachable.
>
> What is going on?

Please, on your Linux "computer A",
echo 1 >/proc/sys/net/ipv4/ip_forward
and try again

- --
Lew Pitcher

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Lew Pitcher wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
>
> (E-Mail Removed) wrote:
> > (Fedora Core 4)
> >
> > I've got a computer A with two Ethernets, connected to two distinct
> > networks (one corporate, one for development only). I make a socket
> > for UDP, and I "connect" this socket to a the IP of computer B that's
> > reached via one of those interfaces.
> >
> > So I "send" one packet. Recipient B gets it, but it's source IP names
> > A's other interface! The recipient has no route to get a reply packet
> > back.
> >
> > I "send" another packet. This time I get errno=113 - network
> > unreachable.
> >
> > What is going on?
>
> Please, on your Linux "computer A",
> echo 1 >/proc/sys/net/ipv4/ip_forward
> and try again

Actually, I probably shouldn't. I'm not allowed to route packets from
one interface to the other. The development network is supposed to be
somewhat isolated from the production network (and my machine's
existence on both is barely tolerated).

OTOH, the IP addresses don't overlap, and I'm sure I could achieve the
same effect with iptables. But ip_forward=0 was just so much easier...

>> Please, on your Linux "computer A",
>> echo 1 >/proc/sys/net/ipv4/ip_forward
>> and try again

> Actually, I probably shouldn't. I'm not allowed to route packets
> from one interface to the other. The development network is
> supposed to be somewhat isolated from the production network (and my
> machine's existence on both is barely tolerated).

Besides, by default, even without ip_forward set, the Linux system
with two NICs will accept traffic addressed to either IP on either
interface. The question is how to get the remote system to know to
reach that "remote" IP it should use the first system's other IP as a
gateway. So, that is where a static route added on the remote system
would come into play.

That, or having the application on the first system make that bind()
call to bind to the IP in the subnet common to both systems.

rick jones
--
Wisdom Teeth are impacted, people are affected by the effects of events.
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

Richard Krehbiel wrote:

> I had expected it to supply the IP of the interface that transmitted
> the packet.

You should think about packets as passing through interfaces.

Most operating systems will use the primary IP address of the "closest
interface" when they originate a packet. They also transmit the packet
on the closest interface. This assumes you didn't do something to
change this, such as 'bind'ing the socket to another address.

DS