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SubjectRe: IP changes in 2.3.4x make things wierd?
   Date: Sun, 20 Feb 2000 21:09:28 +0000 (GMT)
From: Alan Cox <alan@lxorguk.ukuu.org.uk>

Ok so how does a non setuid application receive the ICMP errors it
needs to make intelligent decisions about load balancing UDP
queries in 2.3.x ? If you can't answer that problem cleanly then
your change is broken.

...

IP_RECVERR is one option.

Not reporting the error is even more stupid than reporting it. Its
always been configurable before. Still it may explain why 2.3.x now
hangs during boot starting userspace apps here.

Accidently reporting the error to the next application which happens
to unluckily get that same port is even more dangerous.

I quote below a mail from Alexey from us discussing just this very
issue, and which made me agree with him that the new behavior is what
we want. Alan, you cannot propose going back to the old behavior
without proposing a solution to this problem.

===============================================================

Let's wait a bit with this problem. I had very good reasons
to make this and want to observe for reaction a bit more before
starting some ado. I know that Alan advocated this Linux specific feature
for years, I advocated it too and even more aggressively.
But at time when Linux (and me) entered to mature state and started
to serve thousands of processes and connections, I started to suspect
something wrong. When IP_RECVERR was done, it were purely theoretical
doubts.

Now I have no doubts that it is wrong.

The last drop was the accident with CommuniGatePRO. It has high preformance
DNS resolver, using not-connected sockets concurrently and at some level
of load it fails to resolve names even for local 100% reliable DNS services.
I ridiculed its vendor for long time, but had to explain, why only
Linux fails yet. The source appered clear, sockets get ICMPs from slow remote
servers, which match new innocent sockets. It was the _end_, I shot
the feature the same day when found that it is real.

We have to shot it as soon as possible simply because it does not match
modern situation. The problem was not seen when nobody really used
Linux as loaded server. Actually, exactly with nis it should occur
very frequently.

Alexey

================================================================

Later,
David S. Miller
davem@redhat.com

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