BIND listen backlog too small

Barry Margolin barmar at
Thu Oct 16 21:26:54 UTC 2014

In article <mailman.1083.1413494517.26362.bind-users at>,
 Shawn Zhou <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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