Dear All,

I'm doing linux kernel programming for my senior year project at college. I've studied the tcp/ip implementation in kernel 2.4 to some extent, since i'm working on 2.4. So i was trying to transmit a test IP packet from scratch. This is being done as an extension to the kernel, in a file of my own. My implementation follows the way a packet is being transmitted in the kernel, specifically in the ip_output.c file. The problem i'm having is that when i try to find out the route for my destination using the ip_route_output() function i always get a null and hence the packet transmission doesn't occur. Here's where the problem occurs:

/* Get dst_entry if not present */
ip_route_output(&rt, dest, src, 0, dev-ifindex);

if(rt == NULL)
{
// some error and exit code
}

It turns out that rt is null. src and dest are the source and destination ip addresses in network byte order in hex. rt is defined as struct rtable* and dev is the transmitting device..that is eth0 in this case.

Does anyone have any clue about why rt is turning out to be null?

Hi
Just guessing here...but you have initialized all your pointers..right and there are no "null" fields in the call itself?.For the last parameter check if you are using the right interface ; coz that parameter does look like an interface.Print out individual fields and isolate the problem and see what field is printing out as null.
gud luck
Arvind

Yea, i did initialize the fields as such:

__u32 src, dest; and then equated them to the hex values in net byte order

struct rtable* rt = NULL; and also tried not equating it to NULL...either way it doesn't make a difference.

rt has a lot many fields....i'll try the printing out approach. Thanks.

Didn't work. rt is coming out to be NULL persistently. It might be a network byte order issue, i dunno. Does this seem right:

I tried printing out rt->rt_src and rt->rt_dest but they send the kernel in panic!

__u32 src = 0xc0a80204; // for 192.168.2.4 i.e. the source address
__u32 dest = 0xc0a80203; // for 192.168.2.3 i.e. the destination address

I think this is in network byte order, am i wrong. So then isn't it correct if i pass it as this in:

ip_route_output( &rt, dest, src, 0, dev->ifindex);

Any one please tell me....i really need to get this done. Thanks in advance.