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bug#23547: MTU / icmp black hole issue

From: Chong Yidong
Subject: bug#23547: MTU / icmp black hole issue
Date: Mon, 16 May 2016 10:47:51 +0800


Just received the following bug report, which I'm forwarding here.

---------- Forwarded message ----------
From: Janne Snabb <address@hidden>
Date: Sun, May 15, 2016 at 5:28 PM
Subject: MTU / icmp black hole issue


I was wondering why my email to was stuck.

After researching with tcpdump and ping it appears that this server is
using or is behind a network which uses MTU which is smaller than normal
but it is not advertising that correctly in TCP MSS and not informing
the client about blackholed backets with icmp. The advertised TCP MSS
value is too high. Packets that big end up in a black hole and the email
delivery TCP connection stalls. There may be some icmp blackholing
involved as well.

Just FYI. It would be a good idea to fix this. The current setup is very

I was able to work around this issue and get my email (including this
one) delivered by using sysctl -w net.ipv4.tcp_mtu_probing=1 on client side.

Janne Snabb

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