Date: Thu, 16 Nov 2000 09:19:54 -0600 From: Jonathan Lemon <jlemon@flugsvamp.com> To: Jonathan Lemon <jlemon@FreeBSD.org>, net@FreeBSD.org Subject: Re: Delayed checksums commit broke UDP checksum calculation Message-ID: <20001116091954.A19895@prism.flugsvamp.com> In-Reply-To: <20001116120936.A45755@sunbay.com> References: <20001116120936.A45755@sunbay.com>
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On Thu, Nov 16, 2000 at 12:09:36PM +0200, Ruslan Ermilov wrote:
> Hi!
>
> RFC768> If the computed checksum is zero, it is transmitted as all ones
> RFC768> (the equivalent in one's complement arithmetic). An all zero
> RFC768> transmitted checksum value means that the transmitter generated
> RFC768> no checksum.
>
> This (0x0000 -> 0xFFFF) apparently got broken in udp_usrreq.c,v 1.65.
Actually, it got broken in a different fashion; I had originally moved
the above logic within in_cksum itself, and when that got taken out,
the UDP case wasn't updated.
> Index: ip_output.c
> ===================================================================
> RCS file: /home/ncvs/src/sys/netinet/ip_output.c,v
> retrieving revision 1.116
> diff -u -p -r1.116 ip_output.c
> --- ip_output.c 2000/11/01 01:59:28 1.116
> +++ ip_output.c 2000/11/16 10:05:06
> @@ -974,6 +974,8 @@ in_delayed_cksum(struct mbuf *m)
> ip = mtod(m, struct ip *);
> offset = IP_VHL_HL(ip->ip_vhl) << 2 ;
> csum = in_cksum_skip(m, ip->ip_len, offset);
> + if (m->m_pkthdr.csum_flags & CSUM_UDP && csum == 0)
> + csum = 0xffff; /* per RFC768 */
> offset += m->m_pkthdr.csum_data; /* checksum offset */
>
> if (offset + sizeof(u_short) > m->m_len) {
This would work. Alternatively, you could change it to:
if (csum == 0)
csum = 0xffff;
So that the same logic applies to TCP packets as well. Currently, we
can send a TCP packet with a checksum of 0, which is legal. Of possible
interest is that Linux doesn't do this; they alwyas send a non-zero
checksum in the TCP case, if a checksum was computed.
--
Jonathan
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