Date: Thu, 9 Jan 2014 22:43:09 +0100 From: Luigi Rizzo <rizzo@iet.unipi.it> To: Adrian Chadd <adrian@freebsd.org> Cc: "current@freebsd.org" <current@freebsd.org>, "net@freebsd.org" <net@freebsd.org> Subject: Re: unused in_cksum_update() ? Message-ID: <20140109214309.GA51309@onelab2.iet.unipi.it> In-Reply-To: <CAJ-VmokVngHsZLQUwkDqjHe0Waeb=5S8g1MfZ0A62HCqfTowzw@mail.gmail.com> References: <20140109192114.GA49934@onelab2.iet.unipi.it> <CAJ-VmokVngHsZLQUwkDqjHe0Waeb=5S8g1MfZ0A62HCqfTowzw@mail.gmail.com>
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On Thu, Jan 09, 2014 at 01:27:52PM -0800, Adrian Chadd wrote:
> It's likely used elsewhere; it's the kind of thing you abuse when
> doing header rewriting and reinjection. So, what's the NAT and such
> code using?
natd/libalias has its own code with a DifferentialChecksum() function
in sys/netinet/libalias/alias_util.c which adds and subtracts
the chunks in the old and new packet.
ip_fastfwd.c does it inline:
/*
* Decrement the TTL and incrementally change the IP header checksum.
* Don't bother doing this with hw checksum offloading, it's faster
* doing it right here.
*/
ip->ip_ttl -= IPTTLDEC;
if (ip->ip_sum >= (u_int16_t) ~htons(IPTTLDEC << 8))
ip->ip_sum -= ~htons(IPTTLDEC << 8);
else
ip->ip_sum += htons(IPTTLDEC << 8);
ip_forward() relies on the recomputation done in ip_output().
And there is no trace of in_cksum_update() in the entire source
tree apart from its definition.
cheers
luigi
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