Re: how ip header added in ip_bulid_xmit function?

[Manu Garg <manu.garg@xxxxxxxxxx>]

linux lover wrote:

> hello ,
>   In ip_output.c file there is ip_build_xmit function
> call. when packet comes from tcp layer to IP layer
> this function is called. In that i found that 
>   skb = sock_alloc_send_skb(sk, length+hh_len+15
> ,flags&MSG_DONTWAIT, &err);
> staement allocates skb for packet. after that i found
> that no iphdr adding statement in ip_build_xmit. i
> want to know where is iphdr push to skb? cause skb
> works using first alloc_skb to allocate memory then
> skb_reserve to reserve headroom then put data in skb
> by skb_put or skb_push for pushing headers in skb. so
> there i found       
>  skb->nh.iph = iph = (struct iphdr *)skb_put(skb,
> length); 
> statement is this does that iphdr adding?
>
> regards,
> linuxlover
>
>
>     
>                 
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>  
A packet comes from TCP layer to IP layer through function call 
tp->af_specific->queue_xmit() (in tcp_output.c:tcp_transmit_skb() ). For 
IP this function is nothing but ip_queue_xmit() defined in
ip_output.c.

If you'll check this function, it clearly allocates ip header and put 
information in it.

----------------------------------------------------
iph = (struct iphdr *) skb_push(skb, sizeof(struct iphdr) + (opt ? 
opt->optlen : 0));
       *((__u16 *)iph) = htons((4 << 12) | (5 << 8) | 
(sk->protinfo.af_inet.tos & 0xff));
iph->tot_len = htons(skb->len);
if (ip_dont_fragment(sk, &rt->u.dst))
   iph->frag_off = htons(IP_DF);
else
   iph->frag_off = 0;
iph->ttl      = sk->protinfo.af_inet.ttl;
iph->protocol = sk->protocol;
iph->saddr    = rt->rt_src;
iph->daddr    = rt->rt_dst;
skb->nh.iph   = iph;
---------------------------------------------------

I hope this answers your question.

regards,
Manu Garg
http://manugarg.freezope.org/notes