determine outgoing interface (eth0,eth1) for a packet according to the dest IP

determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[John Que]

Hello,
What is the right way to determine on which interface card
(eth0 or eth1) will a packet be sent (according to the dest IP)?

I have a machine with two NICS (eth0 and eth1).
I have 2 different gateways.
I need to know on which interface (eth0 or eth1) will the
packet be send according to the dest IP.
I want to do in in a User Space function.

I think of one way to do this is:
Parse the routing table (/proc/net/route) as is done in
net-tools. (to be more specific rprint_fib() in lib/inet_gr.c
of net-tools.
Than according to the IP address and the netmask check if the
dest IP is for one gatway or the second.
If not - than it is the default gateway.

Is there another way ? Is it the correct way ?
Regards,
John

Re: determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[Andi Kleen]

On Tuesday 25 April 2006 09:31, John Que wrote:
> Hello,
> What is the right way to determine on which interface card
> (eth0 or eth1) will a packet be sent (according to the dest IP)?

You can send a rtnetlink RTM_GETROUTE message to ask the kernel.
Result is the interface index in RTA_OIF, which can be converted
into a name.

-Andi

Re: determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[David S. Miller]

From: Andi Kleen <ak@suse.de>
Date: Tue, 25 Apr 2006 09:43:49 +0200

> On Tuesday 25 April 2006 09:31, John Que wrote:
> > Hello,
> > What is the right way to determine on which interface card
> > (eth0 or eth1) will a packet be sent (according to the dest IP)?
> 
> You can send a rtnetlink RTM_GETROUTE message to ask the kernel.
> Result is the interface index in RTA_OIF, which can be converted
> into a name.

That scheme does not handle netfilter nor packet scheduler
classifier mangling and redirection of the packet.

Re: determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[John Que]

Thanks a lot !

  I had tried the sending RTM_GETROUTE message using a NETLINK_ROUTE
	socket in a User Space program and it went OK.
	
	It gaves correct routing struct which I could parse.
	In fact it gave the rotuing table.
	But in sending that message I did not specify a certain
	dest IP.
	Consider the follwing simple scenario: I have 2 gateways
	(one on eth0,one on eth1), and I am sending
	a packet to some dest IP ; I want to know according to
	that ip on which interface (or gw) it will be out
	
	But where do I specify that certain dest IP ?
Regards,
John
On 4/25/06, Andi Kleen <ak@suse.de> wrote:
> On Tuesday 25 April 2006 09:31, John Que wrote:
> > Hello,
> > What is the right way to determine on which interface card
> > (eth0 or eth1) will a packet be sent (according to the dest IP)?
>
> You can send a rtnetlink RTM_GETROUTE message to ask the kernel.
> Result is the interface index in RTA_OIF, which can be converted
> into a name.
>
> -Andi
>
>

Re: determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[Andi Kleen]

On Tuesday 25 April 2006 16:44, John Que wrote:
> Thanks a lot !
> 
>   I had tried the sending RTM_GETROUTE message using a NETLINK_ROUTE
> 	socket in a User Space program and it went OK.
> 	
> 	It gaves correct routing struct which I could parse.
> 	In fact it gave the rotuing table.
> 	But in sending that message I did not specify a certain
> 	dest IP.
> 	Consider the follwing simple scenario: I have 2 gateways
> 	(one on eth0,one on eth1), and I am sending
> 	a packet to some dest IP ; I want to know according to
> 	that ip on which interface (or gw) it will be out
> 	
> 	But where do I specify that certain dest IP ?

You add RTA_SRC and RTA_DST attributes to the query.

-Andi

Re: determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[John Que]

Hello,

Does anybody knows where suh a code example
which gets routing info according to the destination IP using
GETROUTE netlink sockets
can be found ?

I had spent time trying to write something which
uses RTA_SRC and RTA_DST and got some problems.
(I probably made something stupid.)
I googled for such examples and did not get much.
I saw that the zebra project and iproute uses
netlinks but it is complex.,

TIA
John

On 4/25/06, Andi Kleen <ak@suse.de> wrote:
> On Tuesday 25 April 2006 16:44, John Que wrote:
> > Thanks a lot !
> >
> >   I had tried the sending RTM_GETROUTE message using a NETLINK_ROUTE
> >       socket in a User Space program and it went OK.
> >
> >       It gaves correct routing struct which I could parse.
> >       In fact it gave the rotuing table.
> >       But in sending that message I did not specify a certain
> >       dest IP.
> >       Consider the follwing simple scenario: I have 2 gateways
> >       (one on eth0,one on eth1), and I am sending
> >       a packet to some dest IP ; I want to know according to
> >       that ip on which interface (or gw) it will be out
> >
> >       But where do I specify that certain dest IP ?
>
> You add RTA_SRC and RTA_DST attributes to the query.
>
> -Andi
>

Re: determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[Herbert Xu]

John Que <qwejohn@gmail.com> wrote:
> 
> I had spent time trying to write something which
> uses RTA_SRC and RTA_DST and got some problems.
> (I probably made something stupid.)
> I googled for such examples and did not get much.
> I saw that the zebra project and iproute uses
> netlinks but it is complex.,

Have a look at

http://gondor.apana.org.au/~herbert/findsaddr-linux.c

which I wrote for traceroute some years back.  It should be easy
to adapt to get the interface instead of the source address.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[Lennert Buytenhek]

On Wed, Apr 26, 2006 at 11:58:00PM +1000, Herbert Xu wrote:

> > I had spent time trying to write something which
> > uses RTA_SRC and RTA_DST and got some problems.
> > (I probably made something stupid.)
> > I googled for such examples and did not get much.
> > I saw that the zebra project and iproute uses
> > netlinks but it is complex.,
> 
> Have a look at
> 
> http://gondor.apana.org.au/~herbert/findsaddr-linux.c
> 
> which I wrote for traceroute some years back.

A fragile and ugly but easier/shorter way of getting the source address
(which I've used a couple of times in the past) is to open a SOCK_DGRAM
socket, connect() it to the intended destination, and then do
getsockname().

(That doesn't give you the interface, though.)


--L

Re: determine outgoing interface (eth0,eth1) for a packet according to the dest IP

[Herbert Xu]

On Sun, Apr 30, 2006 at 11:17:11AM +0200, Lennert Buytenhek wrote:
> 
> A fragile and ugly but easier/shorter way of getting the source address
> (which I've used a couple of times in the past) is to open a SOCK_DGRAM
> socket, connect() it to the intended destination, and then do
> getsockname().

Yes you're quite right that this would've been a much simpler solution.
I suppose I was quite fond of the netlink hammer at the time :)
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt