[AusNOG] MTU debugging? (Or possibly just a fault with Amazon SES?)

[Seamus Ryan]

Don’t suppose your client has a spam filtering appliance/device/product that is performing packet/protocol inspection on smtp?

If so, try turning it off temporarily (the packet inspection, not the whole service). Ive seen some nasty issues which sound very similar to your description caused by smtp protocol inspection. Seems to do more harm than good.

Regards,
Seamus

From: AusNOG [mailto:ausnog-bounces at lists.ausnog.net] On Behalf Of Damien Gardner Jnr
Sent: Wednesday, 3 June 2015 8:44 AM
To: ausnog at lists.ausnog.net
Subject: [AusNOG] MTU debugging? (Or possibly just a fault with Amazon SES?)

Hi Folks,

This one is doing my head in somewhat.  I have a customer who needs to receive emails from a body who use Amazon SES in the US to send emails.  I can see the connections coming into the customer mailserver, however they then timeout with no data after connecting.

If I send various sized pings from amazon and linode instances in the US, they work perfectly up until the point where they hit the MTU of our US->AU tunnel, and then get back a Frag-Needed packet, so that's all working perfectly as expected. e.g.:

ubuntu at ip-172-31-4-204:~$ ping -M do -s 1425 plesk03.rendrag.net.au<http://plesk03.rendrag.net.au>
PING plesk03.rendrag.net.au<http://plesk03.rendrag.net.au> (103.235.52.251) 1425(1453) bytes of data.
From rtr01-e0.lax01.ca.rendrag.net.au<http://rtr01-e0.lax01.ca.rendrag.net.au> (174.136.108.50) icmp_seq=1 Frag needed and DF set (mtu = 1452)
ping: local error: Message too long, mtu=1452
ping: local error: Message too long, mtu=1452

ubuntu at ip-172-31-4-204:~$ ping -M do -s 1424 plesk03.rendrag.net.au<http://plesk03.rendrag.net.au>
PING plesk03.rendrag.net.au<http://plesk03.rendrag.net.au> (103.235.52.251) 1424(1452) bytes of data.
1432 bytes from plesk03.rendrag.net.au<http://plesk03.rendrag.net.au> (103.235.52.251): icmp_seq=1 ttl=111 time=167 ms
1432 bytes from plesk03.rendrag.net.au<http://plesk03.rendrag.net.au> (103.235.52.251): icmp_seq=2 ttl=111 time=170 ms

As as far as I can see, things are working as they should.  However the body using Amazon SES has contacted Amazon support and received a 'This usually signifies an MTU misconfiguration on the remote end, we cannot help with this' reply.  Which leaves me at something of a stalemate..

Are there any other tests I can run to make sure it's not my issue?

I can pull down files no problems at all with http, torrents, etc.  Although one interesting exception is that speedtest does not work - https requests to c.speedtest.net<http://c.speedtest.net> just block after the initial request with no response until the connection is brought down by RST.  (Although that happens in multiple regions in my upstreams' network as well, so I've been assuming it was a problem with speedtest for the last 6 months..)

Any ideas?

--

Damien Gardner Jnr
VK2TDG. Dip EE. GradIEAust
rendrag at rendrag.net<mailto:rendrag at rendrag.net> -  http://www.rendrag.net/
--
We rode on the winds of the rising storm,
 We ran to the sounds of thunder.
We danced among the lightning bolts,
 and tore the world asunder
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