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![]() The IP stack records the largest fragment of a reassembled packet in IPCB(skb)->frag_max_size. When reading a datagram or raw packet that arrived fragmented, expose the value to allow applications to estimate receive path MTU. Tested: Sent data over a veth pair of which the source has a small mtu. Sent data using netcat, received using a dedicated process. Verified that the cmsg IP_RECVFRAGSIZE is returned only when data arrives fragmented, and in that cases matches the veth mtu. ip link add veth0 type veth peer name veth1 ip netns add from ip netns add to ip link set dev veth1 netns to ip netns exec to ip addr add dev veth1 192.168.10.1/24 ip netns exec to ip link set dev veth1 up ip link set dev veth0 netns from ip netns exec from ip addr add dev veth0 192.168.10.2/24 ip netns exec from ip link set dev veth0 up ip netns exec from ip link set dev veth0 mtu 1300 ip netns exec from ethtool -K veth0 ufo off dd if=/dev/zero bs=1 count=1400 2>/dev/null > payload ip netns exec to ./recv_cmsg_recvfragsize -4 -u -p 6000 & ip netns exec from nc -q 1 -u 192.168.10.1 6000 < payload using github.com/wdebruij/kerneltools/blob/master/tests/recvfragsize.c Signed-off-by: Willem de Bruijn <willemb@google.com> Acked-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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