On Wed, Nov 08, 2006 at 10:13:32PM +0000, Al Viro wrote:
> AFAICS, the rules are:
>
> (1) checksum is 16-bit one's complement of the one's complement sum of
> relevant 16bit words.
>
> (2) for v4 UDP all-zeroes has special meaning - no checksum; if you get
> it from (1), send all-ones instead.
>
> (3) for v6 UDP we have the same remapping as in (2), but all-zeroes has
> different meaning - not "ignore checksum" as in v4, but "reject the
> packet".
>
> (4) there is no (4).
>
> IOW, nobody except UDP has any business doing that 0->0xffff
> replacement. However, we have
> if (icmp6h->icmp6_cksum == 0)
> icmp6h->icmp6_cksum = -1;
> and similar in net/ipv6/raw.c
>
> AFAICS, both went in with (commit ID by linux-hist repository)
> commit 6bac90985c8b65ffc25839c001aa7ef4831d2915
> Author: Kazunori Miyazawa <[EMAIL PROTECTED]>
> Date: Mon May 12 00:21:19 2003 -0700
>
> [IPV4]: Introduce ip6_append_data.
>
> That changeset has very similar changes done in udp.c, icmp.c and raw.c;
> remapping of UDP checksum used to be in the area affected by the changeset
> and it looks like it got not just preserved in udp.c (as it should), but
> copied to icmp.c and raw.c instances.
>
> So... is it really needed there?
While we are at it, shouldn't ip_nat_mangle_udp_packet() do the same
remapping in
} else
udph->check = nf_proto_csum_update(*pskb,
htons(oldlen) ^
htons(0xFFFF),
htons(datalen),
udph->check, 1);
branch? Note that udp_manip_pkt() does it in the same situation...
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