netdev - Creating FOU tunnels to the same destination IP but different port

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Date:   Fri, 13 Apr 2018 17:57:03 +0100
From:   Kostas Peletidis <kpeletidis@...il.com>
To:     netdev@...r.kernel.org
Subject: Creating FOU tunnels to the same destination IP but different port

Hello,

I am having trouble with a particular case of setting up a fou tunnel
and I would really appreciate your help.

I have a remote multihomed host behind a NAT box and I want to create
a fou tunnel for each of its IP addresses, from my machine.

A typical case would be something like that (output from the local machine):

# ip tun
ipudp09602: ip/ip remote 135.196.22.100 local 172.31.0.140 ttl 225
ipudp00101: ip/ip remote 148.252.129.30 local 172.31.0.140 ttl 225
ipudp09604: ip/ip remote 77.247.11.249 local 172.31.0.140 ttl 225
tunl0: any/ip remote any local any ttl inherit nopmtudisc
ipudp00102: ip/ip remote 213.205.194.18 local 172.31.0.140 ttl 225

However, if the remote end has the same IP address with the remote end
of an existing tunnel (but a different remote port)
tunnel creation fails. In this example there is already a tunnel to
135.196.22.100:32270 and I wanted to create a new tunnel
to 135.196.22.100:24822 as below:

# ip link add name ipudp09603 mtu 1356 type ipip \
  remote 135.196.22.100 \
  local 172.31.0.140 \
  ttl 225 \
  encap fou \
     encap-sport 4500 \
     encap-dport 24822

RTNETLINK answers: File exists

The remote IP addresses in this case are identical because there is a
NAT box in the way, but the port numbers are different. The source
address and port are the same in all cases.

I noticed that ip_tunnel_find() does not check port numbers - being IP
and all - so I am thinking that a not-so-elegant way to do it is to
get the port numbers from the netlink request and have
ip_tunnel_find() compare them against encap.{sport, dport} of existing
tunnels.

Is there a better way to create a second fou tunnel to the same IP
address but a different port? Use of keys as unique tunnel IDs maybe?
Any feedback is appreciated. Thank you.

Regards,
Kostas