+ /* Find TCP header */
+ int start = ether_size;
+ uint16_t type = packet->data[12] << 8 | packet->data[13];
+
+ if(type == ETH_P_8021Q) {
+ start += 4;
+ type = packet->data[16] << 8 | packet->data[17];
+ }
+
+ if(type == ETH_P_IP && packet->data[start + 9] == 6)
+ start += (packet->data[start] & 0xf) * 4;
+ else if(type == ETH_P_IPV6 && packet->data[start + 6] == 6)
+ start += 40;
+ else
+ return;
+
+ if(packet->len <= start + 20)
+ return;
+
+ /* Use data offset field to calculate length of options field */
+ int len = ((packet->data[start + 12] >> 4) - 5) * 4;
+
+ if(packet->len < start + 20 + len)
+ return;
+
+ /* Search for MSS option header */
+ for(int i = 0; i < len;) {
+ if(packet->data[start + 20 + i] == 0)
+ break;
+
+ if(packet->data[start + 20 + i] == 1) {
+ i++;
+ continue;
+ }
+
+ if(i > len - 2 || i > len - packet->data[start + 21 + i])
+ break;
+
+ if(packet->data[start + 20 + i] != 2) {
+ if(packet->data[start + 21 + i] < 2)
+ break;
+ i += packet->data[start + 21 + i];
+ continue;
+ }
+
+ if(packet->data[start + 21] != 4)
+ break;
+
+ /* Found it */
+ uint16_t oldmss = packet->data[start + 22 + i] << 8 | packet->data[start + 23 + i];
+ uint16_t newmss = mtu - start - 20;
+ uint16_t csum = packet->data[start + 16] << 8 | packet->data[start + 17];
+
+ if(oldmss <= newmss)
+ break;
+
+ ifdebug(TRAFFIC) logger(LOG_INFO, "Clamping MSS of packet from %s to %s to %d", source->name, via->name, newmss);
+
+ /* Update the MSS value and the checksum */
+ packet->data[start + 22 + i] = newmss >> 8;
+ packet->data[start + 23 + i] = newmss & 0xff;
+ csum ^= 0xffff;
+ csum -= oldmss;
+ csum += newmss;
+ csum ^= 0xffff;
+ packet->data[start + 16] = csum >> 8;
+ packet->data[start + 17] = csum & 0xff;
+ break;
+ }
+}
+
+static void swap_mac_addresses(vpn_packet_t *packet) {
+ mac_t tmp;
+ memcpy(&tmp, &packet->data[0], sizeof tmp);
+ memcpy(&packet->data[0], &packet->data[6], sizeof tmp);
+ memcpy(&packet->data[6], &tmp, sizeof tmp);