Re: XFRM / DF Flag / Fragmentation Needed

Thomas DuBuisson [off-list ref] wrote:

After A establishes an SSH connection with C and tries to transfer the
patches the size of a packet from A destined for C is quickly reaches 1500
while the MTU
to A is ~1400.  At this point A sends an ICMP 'Fragmentation Needed'
packet to its self (see xfrm_output.c xfrm4_tunnel_check_size(...)).  It
seems this packet is never acted on - it just disappears into the
loopback interface.  The proper mtu trial/error process never takes
place.

There is a known problem in xfrm4_tunnel_check_size if your underlying
path MTU is a multiple of 8.  So if your path MTU is 1480, you'll need
to lower it to 1476 before it will work.

You can query the path MTU using "ip r g <remote-gateway>".  If it
is a multiple of 8, you can change it by doing

ip r a <remote-gateway> ... mtu <current-mtu - 4>
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