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From: Robert Watson <rwatson@FreeBSD.org>
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To: Lukasz Jaroszewski <lvj@nietykalni.org>
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Cc: freebsd-hackers@freebsd.org
Subject: Re: PFIL: how to get tcp/ip fields from mbuf
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On Mon, 1 Feb 2010, Lukasz Jaroszewski wrote:

> I am wondering about most elegant and proper way to get IP header fields 
> from mbuf, using PFILs. I have read Murat Balaban paper on PFIL_HOOKS where 
> I found some example function. Question is how can I access IP header field 
> in such manner.

The best reference here is probably firewall source code that already exists 
in the tree.  For IP-layer hooks, you'll need to use the m_pullup() call to 
ensure the bytes you want are contiguously stored, and then mtod() to cast the 
mbuf pointer appropriately.  Although I notice ipfw, at least, doesn't call 
m_pullup() for the base header, as it assumes the calling context will already 
have arranged for it to be contiguous:

static int
ipfw_check_hook(void *arg, struct mbuf **m0, struct ifnet *ifp, int dir,
     struct inpcb *inp)
{
...
                if (mtod(*m0, struct ip *)->ip_v == 4)
                         ret = ip_dn_io_ptr(m0, dir, &args);
...

Robert

>
> static int
> hisar_chkinput(void *arg, struct mbuf **m, struct ifnet *ifp, int dir,
> struct inpcb *inp)
> {
>        in_bytes += (*m)->m_len;
>        return 0;
> }
> Regards
> LVJ.
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