Getting destination address for UDP packets

Getting destination address for UDP packets

[Petru Paler]

Hi,

I'm working on a program which binds on all the available interfaces (0.0.0.0)
and listens for/replies with UDP packets.

The problem is that I need to send back responses from the same IP address that
the query arrived to, and this is not usually happening.

Example: supposing I have 1.1.1.2 and 1.1.1.3 aliased on the same interface, and
a query arrives on 1.1.1.3, it's mandatory that the reply packet goes out from
1.1.1.3.

The question is: how do I get (from user space, if possible) the destination
IP address of an UDP packet?

Thanks

Petru

RE: Getting destination address for UDP packets

[David Schwartz]

> I'm working on a program which binds on all the available
> interfaces (0.0.0.0)
> and listens for/replies with UDP packets.

	You need to bind to each interface *address*.

> The problem is that I need to send back responses from the same
> IP address that
> the query arrived to, and this is not usually happening.

	Right, so bind each port to exactly one IP address.

> Example: supposing I have 1.1.1.2 and 1.1.1.3 aliased on the same
> interface, and
> a query arrives on 1.1.1.3, it's mandatory that the reply packet
> goes out from
> 1.1.1.3.

	Then bind one socket to each IP and send the reply from the same socket
that received it.

> The question is: how do I get (from user space, if possible) the
> destination
> IP address of an UDP packet?

	There are actually ways to do this, but the most portable way (and the way
NTP, bind, and others do it), is to bind to each IP that the program needs
to listen on.

	DS