BIND listen backlog too small

Darcy Kevin (FCA) kevin.darcy at
Thu Oct 16 22:08:49 UTC 2014

Yeah, in that case you might see higher-than-normal TCP traffic ☺

                                                                        - Kevin

From: bind-users-bounces at [mailto:bind-users-bounces at] On Behalf Of Shawn Zhou
Sent: Thursday, October 16, 2014 6:08 PM
To: Barry Margolin; comp-protocols-dns-bind at
Subject: Re: BIND listen backlog too small

This is for one of our masters which has about 20K zones and handles zone transfer traffic from few hundred of our slaves.

On Thursday, October 16, 2014 2:27 PM, Barry Margolin <barmar at<mailto:barmar at>> wrote:

In article <mailman.1083.1413494517.26362.bind-users at<mailto:mailman.1083.1413494517.26362.bind-users at>>,
Shawn Zhou <shawnzhou00 at<mailto:shawnzhou00 at>> wrote:

> Hello,
> While I was investigating potential SYN flooding warning messages on my Linux
> box for our DNS traffic,I was very surprised to see the backlog was set to
> very small numbers for BIND tcp sockets.
> strace showed backlog was '10' for listening socket for port 53 and '128' for
> listening socket for port 953 (rdnc traffic).
> I've restarted BIND after I updated somaxconn but BIND didn't pick up the
> value.
> Why doesn't BIND set the backlog to a huge number and let OSes reduce it to
> whatever somaxconn is? Or just set backlog to whatever is is set for
> somaxconn?

Since TCP queries should be infrequent, why does it need a high backlog?

It seems like it's already increasing it, IIRC the default is 5.

Barry Margolin
Arlington, MA
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