Date: Wed, 7 Aug 2002 15:04:52 +0300 From: Giorgos Keramidas <keramida@ceid.upatras.gr> To: soheil h <soheil_h_y@hotmail.com> Cc: freebsd-questions@FreeBSD.ORG, hassas@ce.sharif.ac.ir Subject: Re: accessing tcp header in the ip_input.c::ip_forward() !!!!! Message-ID: <20020807120452.GA4760@hades.hell.gr> In-Reply-To: <F170mpN7JvVAabAQJ6j0000c5ab@hotmail.com> References: <F170mpN7JvVAabAQJ6j0000c5ab@hotmail.com>
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On 2002-08-05 22:50 +0000, soheil h wrote:
> i made a change in ip_forward() in ip_input.c to print the window
> sizes passes through my gateway. but it doesn't work properly and
> doesn't show the true th_win (window size) and the th_dport they
> doesn't mach the tcpdump output but shows the true ip->ip_p for TCP
> it means that i make a wrong cast in ip packet means not at the
> place of the tcphdr some byte more or less i don't know please
> verify me if my code is true and if not please help me !!!!!
[...]
> static void
> ip_forward(m, srcrt)
> struct mbuf *m;
> int srcrt;
> {
> register struct ip *ip = mtod(m, struct ip *);
[...]
> int s_len; /* the ip length */
[...]
> s_len = IP_VHL_HL(ip->ip_vhl) << 2;
> if(ip->ip_p == IPPROTO_TCP)
> {
> s_hdr = (struct tcpShdr *)( (caddr_t) ip + s_len);
If my understanding of the relevant issues is not flawed, you have
missed just a very minor detail, and used the `ip' pointer to print
the contents of random memory instead of the true tcphdr->th_win
values you think you are printing.
The packet is stored in a chain of mbufs. The first mbuf might
contain less data than you'd normally expect. You will need to use
m_pullup() to ensure that the proper amount of data is available at
the head of the packet's mbuf chain. Note that m_pullup() might be an
expensive operation that involves memory copies. You will almost
invariably find that it's a good idea to call m_pullup() only if it is
absolutely and unavoidably necessary, i.e. you should call it only if:
m->m_len < (sizeof(struct iphdr) + sizeof(struct tcphdr))
with code similar to:
int hlen;
struct mbuf *p;
hlen = sizeof(struct iphdr) + sizeof(struct tcphdr);
if (m->m_len < hlen) {
if ((p = m_pullup(m, hlen)) == NULL) {
/* Failed. Handle the error. */
}
}
/* Work on the `m' chain of mbufs like you do until now. */
- Giorgos
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