Proxy problem: "imap-login: Error: proxy(USERNAME): connect(10.x.x.178, 993) failed: Cannot assign requested address (after 0 secs, local=10.x.x.104)"

Ralf Hildebrandt r at
Tue Oct 21 10:29:26 UTC 2014

> >FYI: It was a kernel bug.
> Do you mind sharing the actual technical background. Which kernel
> was affected and how?

We didn't track it down to a specific bug, but we finally decided that
our setup using multiple IPs for source and destination was OK and
should work as intended - but it didn't.

So we switched from Debians 3.2 kernel to a 3.14 from backports and -
surprise - it worked as designed. Something in the 3.2 kernel limited
the number of established connections to the frame given by the local
port range, even incoming ones. I still don't know what, but I am quite
sure it's not a new feature in 3.14 which makes our setup work, as it
should be possible to have many connects from different source IPs
on basically every linux kernel.

There are other limits (as some TCP hashtable sizes) which can be tuned,
but that was not the limit we were hitting...

Old (not working): 
linux-image-3.2.0-4-amd64           3.2.63-2

New (working OK):
linux-image-3.14-0.bpo.2-rt-amd64   3.14.15-2~bpo70+1

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