shithub: riscv

Info  •  Files  •  Log  •  Branches

udp: fix udp checksum

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.

--- a/sys/src/9/ip/udp.c

+++ b/sys/src/9/ip/udp.c

@@ -190,6 +190,7 @@

 	Fs *f;

 	int version;

 	Routehint *rh;

+	ushort csum;

 	upriv = c->p->priv;

 	f = c->p->f;

@@ -260,8 +261,10 @@

 		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 @@

 		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);