Linux Routers working

hello,
Is there any way on linux routers to know on which interface
packet is forwarded before actually packet trasmission begins? Also
before packet processed by IP layer can it be possible to know from
which adjecent Router'IP packet came to that router?

(E-Mail Removed) wrote:

> hello,
> Is there any way on linux routers to know on which interface
> packet is forwarded before actually packet trasmission begins? Also
> before packet processed by IP layer can it be possible to know from
> which adjecent Router'IP packet came to that router?
hello,

yes, packets are 'routed' right when they enter the tcp/ip stack (function
route_input). This is necessary, because the system has to figure out what
the destination is, e.g. itself or another host. The incoming interface
will be inserted into the socket buffer by the interrupt handler, that
copies the packet from the NIC to the memory. All of these elements can be
easily accessed using Netfilter. Alex

hello,
I just want to know on which interface packet is routed and get that
local ethernet IP address. Is that possible for me once routing
decision takes place? I want to modify some code in kernel network
stack.

(E-Mail Removed) wrote:

> hello,
> I just want to know on which interface packet is routed and get that
> local ethernet IP address. Is that possible for me once routing
> decision takes place? I want to modify some code in kernel network
> stack.
Hi,

yes, it certainly is possible. But as I pointed out previously, there is no
need to do that. To make the TCP/IP stack do things, the Netfilter hooks
are the thing you are looking for. It is way easier, no need to recompile
the kernel and you can load/unload it any time. To log incoming interface
and IP, you will need hardly more than 4 or 5 lines of code. Try the
prerouting hook and call from there function ip_route_input(). The incoming
interface is sotred as a pointer to a net_device structure in the socket
buffer. The name is simply 'dev'. Lookup the interface BEFORE
ip_route_input(), because this function will change the pointer to the
outgoing device afterwards, or NULL if the packets destination is the local
host.

Alex

simple stupid questions:-
when 2 routers are connected in internet they share direct
communicating link say Ethernet eth1 on router1 and eth3 on router2.
Then that mean Both routers have same NetID and NetMask right?