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
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