Fix up checksums instead of recalculating them.

Currently the checksums of translated packets are calculated
from scratch by checksumming the translated packet. This is slow
and does not work in the case of fragments, because the whole
packet is not available. Instead, calculate the checksum by
adjusting the checksum of the original packet.

Bug: 11542311
Bug: 12116252
Change-Id: I6b78a94ca5bd96b13ee2653b6200551193b3dcc1

1 files changed, 5 insertions, 4 deletions

diff --git a/ipv6.c b/ipv6.c
index 79303ec..e4a73fe 100644
--- a/ipv6.c
+++ b/ipv6.c
@@ -88,7 +88,7 @@ int ipv6_packet(clat_packet out, int pos, const char *packet, size_t len) {
   uint8_t protocol;
   const char *next_header;
   size_t len_left;
-  uint32_t checksum;
+  uint32_t old_sum, new_sum;
   int iov_len;
 
   if(len < sizeof(struct ip6_hdr)) {
@@ -133,16 +133,17 @@ int ipv6_packet(clat_packet out, int pos, const char *packet, size_t len) {
   out[pos].iov_len = sizeof(struct iphdr);
 
   // Calculate the pseudo-header checksum.
-  checksum = ipv4_pseudo_header_checksum(0, ip_targ, len_left);
+  old_sum = ipv6_pseudo_header_checksum(0, ip6, len_left);
+  new_sum = ipv4_pseudo_header_checksum(0, ip_targ, len_left);
 
   // does not support IPv6 extension headers, this will drop any packet with them
   if (protocol == IPPROTO_ICMP) {
     iov_len = icmp6_packet(out, pos + 1, (const struct icmp6_hdr *) next_header, len_left);
   } else if (ip6->ip6_nxt == IPPROTO_TCP) {
-    iov_len = tcp_packet(out, pos + 1, (const struct tcphdr *) next_header, checksum,
+    iov_len = tcp_packet(out, pos + 1, (const struct tcphdr *) next_header, old_sum, new_sum,
                          len_left);
   } else if (ip6->ip6_nxt == IPPROTO_UDP) {
-    iov_len = udp_packet(out, pos + 1, (const struct udphdr *) next_header, checksum,
+    iov_len = udp_packet(out, pos + 1, (const struct udphdr *) next_header, old_sum, new_sum,
                          len_left);
   } else if (ip6->ip6_nxt == IPPROTO_GRE) {
     iov_len = generic_packet(out, pos + 1, next_header, len_left);