Fw: [Bug 213669] New: PMTU dicovery not working for IPsec

[Stephen Hemminger <stephen@networkplumber.org>]

Begin forwarded message:

Date: Wed, 07 Jul 2021 09:08:07 +0000
From: bugzilla-daemon@bugzilla.kernel.org
To: stephen@networkplumber.org
Subject: [Bug 213669] New: PMTU dicovery not working for IPsec


https://bugzilla.kernel.org/show_bug.cgi?id=213669

            Bug ID: 213669
           Summary: PMTU dicovery not working for IPsec
           Product: Networking
           Version: 2.5
    Kernel Version: 5.12.13
          Hardware: x86-64
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: IPV4
          Assignee: stephen@networkplumber.org
          Reporter: marek.gresko@protonmail.com
        Regression: No

Hello,

I have two sites interconnected using ipsec (libreswan)

the situation is as follows:

X <=> (a) <=> (Internet) <=> (b) <=> Y

So you have two gateways a and b connected to the internet and their
corresponding internal subnets X and Y. The gateway a is connected to the
provider p using pppoe. The ipsec tunnel is created between a and b to
interconnect subnets X and Y. When gateway b with internal address y itself is
communication to the gateway a using its internal address x. Addresses x and y
are defined by leftsourceif and rightsourceip in the libreswan configuration,
you get this behavior:

b# ping -M do x -s 1392 -c 1
PING x (x.x.x.x) 1392(1420) bytes of data.

--- ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms

b# ping -M do a -s 1460 -c 3
PING a (a.a.a.a) 1460(1488) bytes of data.
From p (p.p.p.p) icmp_seq=1 Frag needed and DF set (mtu = 1480)
ping: local error: message too long, mtu=1480
ping: local error: message too long, mtu=1480

--- ping statistics ---
3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2014ms

b# ping -M do x -s 1392 -c 3
PING x (x.x.x.x) 1392(1420) bytes of data.
ping: local error: message too long, mtu=1418
ping: local error: message too long, mtu=1418
ping: local error: message too long, mtu=1418

--- ping statistics ---
3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2046ms


Legend:
x.x.x.x is an inner ip address if the gateway (a) (or x from the inside).
a.a.a.a is an outer address of the gateway (a).
p.p.p.p is some address in the provider's network of the (a) side.

So definitely the ipsec tunnel is aware of the mtu only when some outer
communication is in progress. The inner communication itself is not aware of
icmp packets using for PMTU discovery. I had also a situation when also the
outer pings did not help the ipsec to be aware of the MTU and after reboot it
started to behave like discribed again.

Did I describe it understandably or should I clarify things?

Thanks

Marek

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