Fix unable to send big UDP packets when SNAT rules are configured for nat46 #39
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… nat46 In the IPv4->IPv6 direction, if SNAT rules are configured for nat46 (i.e. "iptables -A POSTROUTING -t nat -o clatd -j SNAT --to-source 182.0.0.111"), when the host sends a big udp packet (8000 bytes), the IP layer will fragment the packet. However, the fragments must be reassembled before doing NAT. The length of reassembled packet is less than nat46's mtu 16384, so it is not fragmented after NAT. Then nat46 converts it into an IPv6 packet, which causes the packet to be dropped by the network protocol stack and send an icmpv6 with "Fragmentation needed" to the source since the length of the packet is greater than the mtu of next hop and it is not sent locally. In order to send this packet, we should tell the network protocol stack that this is a locally sent IPv6 packet and fragment it if necessary.
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In the IPv4->IPv6 direction, if SNAT rules are configured for nat46 (i.e. "iptables -A POSTROUTING -t nat -o clatd -j SNAT --to-source 182.0.0.111"), when the host sends a big udp packet (8000 bytes), the IP layer will fragment the packet. However, the fragments must be reassembled before doing NAT. The length of reassembled packet is less than nat46's mtu 16384, so it is not fragmented after NAT. Then nat46 converts it into an IPv6 packet, which causes the packet to be dropped by the network protocol stack and send an icmpv6 with "Fragmentation needed" to the source since the length of the packet is greater than the mtu of next hop and it is not sent locally. In order to send this packet, we should tell the network protocol stack that this is a locally sent IPv6 packet and fragment it if necessary.