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

[Giles Pollock]

Seen a similar thing happen, however with outbound SMTP traffic from Amazon
cloud. In that case there was some sort of NAT device in the way which was
messing up the traffic, and the solution was to upgrade them.

Do all emails get broken, or do only ones with a certain amount of content
get knocked out? Wireshark usually ends up being your best friend in these
sort of situations...

On Wed, Jun 3, 2015 at 8:44 AM, Damien Gardner Jnr <rendrag at rendrag.net>
wrote:

> 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
> PING plesk03.rendrag.net.au (103.235.52.251) 1425(1453) bytes of data.
> From 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
> PING plesk03.rendrag.net.au (103.235.52.251) 1424(1452) bytes of data.
> 1432 bytes from plesk03.rendrag.net.au (103.235.52.251): icmp_seq=1
> ttl=111 time=167 ms
> 1432 bytes from 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 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 -  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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>
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