[Dnsmasq-discuss] Warning when maximum number of concurrent DNS queries is reached

Simon Kelley simon at thekelleys.org.uk
Thu May 30 10:29:37 BST 2013

On 30/05/13 01:41, Marcelo Salhab Brogliato wrote:
> Dear,
> Today I had a problem on my DNS server because it reached the maximum number of concurrent DNS queries. It was "hard" to discover the cause of the problem because there is no log that it have happened. I took a look at the code and this log is really missing and I guess it is very important.
> Follow my suggestion of improvement for problem detection purpose.
> Version: 2.52
> Patch:
> --- forward.c.original	2013-05-29 21:34:13.836999570 -0300
> +++ forward.c	2013-05-29 21:34:22.837164945 -0300
> @@ -1007,6 +1007,7 @@
>       {
>         if (oldest&&  wait)
>   	*wait = oldest->time + (time_t)TIMEOUT - now;
> +      my_syslog(LOG_WARNING, _("Maximum number of concurrent DNS queries reached (max: %d)"), daemon->ftabsize);
>         return NULL;
>       }
> What do you think about it?

It's many years since this was last looked at.

This code used to to log an error and drop the query. The logging was 
removed when it was changed to not accept new queries when the 
forwarding table is full. The idea is that the UDP packets are queued in 
the kernel until resources become available.

I'm interested to know about the problem you had: this is supposed to be 
handled without drama.

I'm happy in principle to add logging, but some rate-limiting may be 
required. Note that get_new_frec() is called for two purposes: from the 
forwarding path to actually allocate a record, and from the select() 
loop, where it just discovers is the table is full or not, and 
calculates how long it will be until space is available. The second 
could be responsible for a lot of calls and a lot of log messages.



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