dhclient: improve UDP checksum handling

When sending UDP packets:
* compute the checksum in the correct order. This only has an impact
  if the length of the payload is odd.
* don't send packet with a checksum of zero, use 0xffff instead as
  required.
When receiving UDP packets:
* don't do any computations when the checksum is zero.
* compute the checksum in the correct order. This only has an impact
  if the length of the payload is odd.
* when computing the checksum, store the pseudo header checksum
* if the checksum is computed as zero, use 0xffff instead.
* also accept packets, when the checksum in the packet is the pseudo
  header checksum.
The last point fixes a problem when the DHCP client runs in a VM,
the DHCP server runs on the host serving the VM and the network
interface supports transmit checksum offloading. Since dhclient
doesn't use UDP sockets but bpf devices to read the packets, the
checksum will be incorrect and only contain the checksum of the
pseudo header.

PR:			263229
Reviewed by:		markj, Timo Völker
Tested by:		danilo
MFC after:		3 days
Differential Revision:	https://reviews.freebsd.org/D52394

1 files changed, 35 insertions, 18 deletions

diff --git a/sbin/dhclient/packet.c b/sbin/dhclient/packet.c
index 3d7390c06ee0..fc0305a8cb0c 100644
--- a/sbin/dhclient/packet.c
+++ b/sbin/dhclient/packet.c
@@ -135,11 +135,14 @@ assemble_udp_ip_header(unsigned char *buf, int *bufix, u_int32_t from,
 	udp.uh_ulen = htons(sizeof(udp) + len);
 	memset(&udp.uh_sum, 0, sizeof(udp.uh_sum));
 
-	udp.uh_sum = wrapsum(checksum((unsigned char *)&udp, sizeof(udp),
-	    checksum(data, len, checksum((unsigned char *)&ip.ip_src,
+	udp.uh_sum = wrapsum(checksum(data, len, checksum((unsigned char *)&udp,
+	    sizeof(udp), checksum((unsigned char *)&ip.ip_src,
 	    2 * sizeof(ip.ip_src),
 	    IPPROTO_UDP + (u_int32_t)ntohs(udp.uh_ulen)))));
 
+	if (udp.uh_sum == htons(0))
+		udp.uh_sum = htons(0xffff);
+
 	memcpy(&buf[*bufix], &udp, sizeof(udp));
 	*bufix += sizeof(udp);
 }
@@ -166,7 +169,7 @@ decode_udp_ip_header(unsigned char *buf, int bufix, struct sockaddr_in *from,
 	struct ip *ip;
 	struct udphdr *udp;
 	u_int32_t ip_len = (buf[bufix] & 0xf) << 2;
-	u_int32_t sum, usum;
+	u_int32_t sum, usum, pseudo_sum;
 	static int ip_packets_seen;
 	static int ip_packets_bad_checksum;
 	static int udp_packets_seen;
@@ -224,23 +227,37 @@ decode_udp_ip_header(unsigned char *buf, int bufix, struct sockaddr_in *from,
 	}
 
 	usum = udp->uh_sum;
-	udp->uh_sum = 0;
-
-	sum = wrapsum(checksum((unsigned char *)udp, sizeof(*udp),
-	    checksum(data, len, checksum((unsigned char *)&ip->ip_src,
-	    2 * sizeof(ip->ip_src),
-	    IPPROTO_UDP + (u_int32_t)ntohs(udp->uh_ulen)))));
-
 	udp_packets_seen++;
-	if (usum && usum != sum) {
-		udp_packets_bad_checksum++;
-		if (udp_packets_seen > 4 && udp_packets_bad_checksum != 0 &&
-		    (udp_packets_seen / udp_packets_bad_checksum) < 2) {
-			note("%d bad udp checksums in %d packets",
-			    udp_packets_bad_checksum, udp_packets_seen);
-			udp_packets_seen = udp_packets_bad_checksum = 0;
+
+	if (usum != htons(0)) {
+		udp->uh_sum = 0;
+
+		pseudo_sum = checksum((unsigned char *)&ip->ip_src,
+		    2 * sizeof(ip->ip_src),
+		    IPPROTO_UDP + (u_int32_t)ntohs(udp->uh_ulen));
+		sum = wrapsum(checksum(data, len,
+		    checksum((unsigned char *)udp, sizeof(*udp), pseudo_sum)));
+		if (sum == htons(0))
+			sum = htons(0xffff);
+
+		/*
+		 * In addition to accepting UDP packets with the correct
+		 * checksum in the checksum field, accept also the ones which
+		 * have the correct pseudo header checksum in the checksum
+		 * field. This allows to process UDP packets, which have been
+		 * marked for transmit checksum offloading by the sender side.
+		 */
+		if (usum != sum && usum != htons(pseudo_sum & 0x0000ffff)) {
+			udp_packets_bad_checksum++;
+			if (udp_packets_seen > 4 &&
+			    udp_packets_bad_checksum != 0 &&
+			    (udp_packets_seen / udp_packets_bad_checksum) < 2) {
+				note("%d bad udp checksums in %d packets",
+				    udp_packets_bad_checksum, udp_packets_seen);
+				udp_packets_seen = udp_packets_bad_checksum = 0;
+			}
+			return (-1);
 		}
-		return (-1);
 	}
 
 	memcpy(&from->sin_port, &udp->uh_sport, sizeof(udp->uh_sport));