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...; I have no experience with Ubuntu, but I find it hard to believe it would > > block ICMP Fragmentation Needed packets out of the box. > > I can confirm you that this is the case. Ubuntu ignores those ICMP > packets... :( (rp_filter settings) > > You can see it here : https://mellowd.co.uk/ccie/?p=5662 Ok... well that's just silly then. > > Can you check what tinc thinks the PMTU is in each of these cases? With > > tinc 1.0.24, you do "tincd -n <netname> -kUSR2", and it will send a list > > of nodes to the syslog. The MTU should be in it....

...; I have no experience with Ubuntu, but I find it hard to believe it would > > block ICMP Fragmentation Needed packets out of the box. > > I can confirm you that this is the case. Ubuntu ignores those ICMP > packets... :( (rp_filter settings) > > You can see it here : https://mellowd.co.uk/ccie/?p=5662 As I read that page, Ubuntu has rp_filter on by default, which causes it to ignore ICMP messages that arrive from an interface different from the one it expects that sending IP to be using. Does that actually apply to the Ubuntu client that is having problems here? From the or...