Date: Mon, 1 Feb 2010 13:26:09 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Lukasz Jaroszewski <lvj@nietykalni.org> Cc: freebsd-hackers@freebsd.org Subject: Re: PFIL: how to get tcp/ip fields from mbuf Message-ID: <alpine.BSF.2.00.1002011322520.23174@fledge.watson.org> In-Reply-To: <3930e0781002010302r3758968cl5f010bd7a585cca4@mail.gmail.com> References: <3930e0781002010302r3758968cl5f010bd7a585cca4@mail.gmail.com>
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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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>
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