3 files changed, 28 insertions, 3 deletions

diff --git a/checksum.c b/checksum.c
index f35c948..175381d 100644
--- a/checksum.c
+++ b/checksum.c
@@ -52,6 +52,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <linux/udp.h>
 #include <linux/icmp.h>
 #include <linux/icmpv6.h>
 
@@ -124,6 +125,27 @@ void csum_icmp4(struct icmphdr *icmp4hr, const void *payload, size_t len)
 }
 
 /**
+ * csum_udp6() - Calculate and set checksum for a UDP over IPv6 packet
+ * @udp6hr:	UDP header, initialised apart from checksum
+ * @payload:	UDP packet payload
+ * @len:	Length of @payload (not including UDP header)
+ */
+void csum_udp6(struct udphdr *udp6hr,
+	       const struct in6_addr *saddr, const struct in6_addr *daddr,
+	       const void *payload, size_t len)
+{
+	/* Partial checksum for the pseudo-IPv6 header */
+	uint32_t psum = sum_16b(saddr, sizeof(*saddr)) +
+		        sum_16b(daddr, sizeof(*daddr)) +
+		        htons(len + sizeof(*udp6hr)) + htons(IPPROTO_UDP);
+
+	udp6hr->check = 0;
+	/* Add in partial checksum for the UDP header alone */
+	psum += sum_16b(udp6hr, sizeof(*udp6hr));
+	udp6hr->check = csum_unaligned(payload, len, psum);
+}
+
+/**
  * csum_icmp6() - Calculate and set checksum for an ICMPv6 packet
  * @icmp6hr:	ICMPv6 header, initialised apart from checksum
  * @saddr:	IPv6 source address
diff --git a/checksum.h b/checksum.h
index bf0620f..2bb2ff9 100644
--- a/checksum.h
+++ b/checksum.h
@@ -6,6 +6,7 @@
 #ifndef CHECKSUM_H
 #define CHECKSUM_H
 
+struct udphdr;
 struct icmphdr;
 struct icmp6hdr;
 
@@ -13,6 +14,9 @@ uint32_t sum_16b(const void *buf, size_t len);
 uint16_t csum_fold(uint32_t sum);
 uint16_t csum_unaligned(const void *buf, size_t len, uint32_t init);
 void csum_icmp4(struct icmphdr *ih, const void *payload, size_t len);
+void csum_udp6(struct udphdr *udp6hr,
+	       const struct in6_addr *saddr, const struct in6_addr *daddr,
+	       const void *payload, size_t len);
 void csum_icmp6(struct icmp6hdr *icmp6hr,
 		const struct in6_addr *saddr, const struct in6_addr *daddr,
 		const void *payload, size_t len);
diff --git a/tap.c b/tap.c
index f082901..9c197cb 100644
--- a/tap.c
+++ b/tap.c
@@ -183,9 +183,8 @@ void tap_ip_send(const struct ctx *c, const struct in6_addr *src, uint8_t proto,
 		} else if (proto == IPPROTO_UDP) {
 			struct udphdr *uh = (struct udphdr *)(ip6h + 1);
 
-			uh->check = 0;
-			uh->check = csum_unaligned(ip6h, len + sizeof(*ip6h),
-						   0);
+			csum_udp6(uh, &ip6h->saddr, &ip6h->daddr,
+				  uh + 1, len - sizeof(*uh));
 		} else if (proto == IPPROTO_ICMPV6) {
 			struct icmp6hdr *ih = (struct icmp6hdr *)(ip6h + 1);