IPSec, masquerade and dnat with nftables

[Thomas Bach <t.bach@xxxxxxxxxx>]

Hi,

I have two hosts with public ip addresses running Ubuntu 16.04 with
Kernel version 4.4.0.

I want to interconnect two containers (systemd-nspawn) with veth
interfaces running on these hosts in a server client setup.

So on the first host, where the server in the container runs I have
the following rules:
# nft list ruleset
table ip nat {
  chain prerouting {
    type nat hook prerouting priority 0; policy accept;
    tcp dport { 4506, 4505} dnat 10.0.0.2 
  }

  chain output {
    type nat hook output priority 0; policy accept;
    tcp dport { 4505, 4506} dnat 10.0.0.2
  }

  chain input {
    type nat hook input priority 0; policy accept;
  }

  chain postrouting {
    type nat hook postrouting priority 0; policy accept;
    ip saddr 10.0.0.0/8 oif enp4s0 masquerade 
  }
}

On the second host, where the client runs i have the following:
# nft list ruleset
table ip nat {
  chain prerouting {
    type nat hook prerouting priority 0; policy accept;
  }

  chain output {
    type nat hook output priority 0; policy accept;
  }

  chain input {
    type nat hook input priority 0; policy accept;
  }

  chain postrouting {
    type nat hook postrouting priority 0; policy accept;
    ip saddr 10.0.0.0/8 oif enp0s31f6 masquerade 
  }
}

This works as expected and without any problems at all. Now IPSec
enters the picture. As soon as I setup a policy to encrypt everyting
between the two hosts the following happens:
+ I can still connect from the second host to the server in the
  container without problems,
+ I can still /connect/ (i.e. establish a connection) from the
  container on the second host to the server on the first host, but
+ in tcpdump listening on the interface of the container (on the
  second host) I see lots of TCP Retransmissions and the TCP connection
  is effectively broken.

Can someone give me a hint what is going on here?

Regards

    Thomas Bach.
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