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Old 04-09-2005, 02:19 AM   #1
sidra
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Registered: Oct 2004
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Packet Transmission in the Kernel


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?
 
Old 04-10-2005, 02:20 PM   #2
live_dont_exist
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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
 
Old 04-10-2005, 04:18 PM   #3
sidra
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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.
 
Old 04-17-2005, 10:48 AM   #4
sidra
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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.
 
  


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