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Date: Wed, 30 Apr 2014 21:18:14 -0400
From: Rich Felker <>
Subject: Re: Requirement for proper DNS resolving / source port = 0

On Thu, May 01, 2014 at 03:08:26AM +0200, Stefan de Konink wrote:
> Hi,
> I am setting up an embedded system, and consider that I have not set
> up anything other then I have explicitly described.
> I am using a Linux kernel 3.14.2, and have set up kernel
> autoconfiguration (ip=dhcp). That having said, I am able to ping
> external hosts using busybox.
> My current problem is that I am unable to resolve hostnames in my
> system. The file /etc/resolv.conf is in place, and I am observing
> outgoing DNS requests. The strange thing about them is that the
> source port is actually 0, not illegal, but it isn't getting any
> replies.
> I have tried to add /etc/hosts and /etc/services. I am able to ping
> localhost, which does resolve to an IP address using the entry from
> /etc/hosts.
> Is this a bad kernel configuration? Does anyone have a hint where to
> look for?
> My guess it is related to here... and I guess it is expecting udp
> port randomization, which does not seem to happen. Any clues?

Binding port 0 and calling connect should yield a kernel-assigned
source port. If this isn't happening, I think something is wrong with
your kernel, but I don't know exactly what. If there's an easy way we
could detect and work around this situation I wouldn't mind trying to
handle it, but I don't know how and didn't even expect for it to be


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