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udp: fix udp checksum

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we did not apply the special case to store 0xFFFF (-0)
in the checksum field when the checksum calculation
returned zero. we survived this for v4 as RFC768 states:

> If the computed checksum is zero, it is transmitted as
> all ones (the equivalent in one's complement arithmetic).
>
> An all zero transmitted checksum value means that the
> transmitter generated no checksum (for debuging or for
> higher level protocols that don't care).

for ipv6 however, the checksum is not optional and receivers
would drop packets with a zero checksum.
This commit is contained in:
cinap_lenrek 2018-06-14 20:48:21 +02:00
parent 39fb26df70
commit 4971db9e32
1 changed files with 9 additions and 4 deletions
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13
sys/src/9/ip/udp.c
View file

@ -190,6 +190,7 @@ udpkick(void *x, Block *bp)
Fs *f;
int version;
Routehint *rh;
ushort csum;
upriv = c->p->priv;
f = c->p->f;
@ -260,8 +261,10 @@ udpkick(void *x, Block *bp)
hnputs(uh4->udplen, ptcllen);
uh4->udpcksum[0] = 0;
uh4->udpcksum[1] = 0;
hnputs(uh4->udpcksum,
ptclcsum(bp, UDP4_PHDR_OFF, dlen+UDP_UDPHDR_SZ+UDP4_PHDR_SZ));
csum = ptclcsum(bp, UDP4_PHDR_OFF, dlen+UDP_UDPHDR_SZ+UDP4_PHDR_SZ);
if(csum == 0)
csum = 0xffff; /* -0 */
hnputs(uh4->udpcksum, csum);
uh4->vihl = IP_VER4;
ipoput4(f, bp, 0, c->ttl, c->tos, rh);
break;
@ -294,8 +297,10 @@ udpkick(void *x, Block *bp)
hnputs(uh6->udplen, ptcllen);
uh6->udpcksum[0] = 0;
uh6->udpcksum[1] = 0;
hnputs(uh6->udpcksum,
ptclcsum(bp, UDP6_PHDR_OFF, dlen+UDP_UDPHDR_SZ+UDP6_PHDR_SZ));
csum = ptclcsum(bp, UDP6_PHDR_OFF, dlen+UDP_UDPHDR_SZ+UDP6_PHDR_SZ);
if(csum == 0)
csum = 0xffff; /* -0 */
hnputs(uh6->udpcksum, csum);
memset(uh6, 0, 8);
uh6->viclfl[0] = IP_VER6;
hnputs(uh6->len, ptcllen);