[Dnsmasq-discuss] Dnsmasq cache does not fetch new value after cache expired for some record

[Simon Kelley]

On 27/02/14 06:43, hu jason wrote:
> HI friends:
>      I am using dnsmasq as the local dns cache. Everything is OK but we
> found a very strange problem for one DNS records : api.m.duoku.com
> <http://api.m.duoku.com>
> 
>      It is ok to resolv this "api.m.duoku.com <http://api.m.duoku.com>"
> as dnsmasq starts up. But after the cache expired(the TTL is 3600),
> dnsmasq do not fetch new record result from upstream servers but return
> a NXDOMAIN result:
> 
>      For now, I have only found problem with his DNS record. It works
> fine for other DNS records!
> 
> 
> --------------------------------------------------
> # dig api.m.duoku.com <http://api.m.duoku.com>
> 
> ; <<>> DiG 9.3.6-P1-RedHat-9.3.6-20.P1.el5 <<>> api.m.duoku.com
> <http://api.m.duoku.com>
> ;; global options:  printcmd
> ;; Got answer:
> ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 33100
> ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
> 
> ;; QUESTION SECTION:
> ;api.m.duoku.com <http://api.m.duoku.com>.        IN    A
> 
> ;; Query time: 0 msec
> ;; SERVER: 127.0.0.1#53(127.0.0.1)
> ;; WHEN: Thu Feb 27 12:39:21 2014
> ;; MSG SIZE  rcvd: 33
> 
> ---------------------------------------------------------
> 
> After I restarted dnsmasq using /etc/init.d/dnsmasq, it returned to normal.
> But after the cache expired, I need to restart again!
> 
> The upstream servers are running Unbound, and they are OK to resolv
> "api.m.duoku.com <http://api.m.duoku.com>", and I never have to restart
> Unbound for the problelm!
> 
> So I highly suspend it is the problem of dnsmasq!
> 
> 
> ------------------------------------------------------------------
> 
> $ cat /etc/resolv.dnsmasq.conf
> nameserver 120.132.147.28
> nameserver 58.83.143.47
> nameserver 216.12.201.200
> nameserver 159.253.130.212
> nameserver 208.43.166.72
> nameserver 103.6.152.201
> 
> ------------------------------------------------------
> 
> # cat /etc/dnsmasq.conf | grep -v '^#' | sed -e '/^$/d'
> resolv-file=/etc/resolv.dnsmasq.conf
> listen-address=127.0.0.1
> no-dhcp-interface=127.0.0.1
> bind-interfaces
> no-hosts
>  
> cache-size=1000
> log-queries
> conf-dir=/etc/dnsmasq.d
> 
> ----------------------------------------------------
> 
> # uname -a
> Linux xxxxxxx.boyaa.com <http://xxxxxxx.boyaa.com> 2.6.18-348.1.1.el5 #1
> SMP Tue Jan 22 16:19:19 EST 2013 x86_64 x86_64 x86_64 GNU/Linux
> 
> 
> 
> --------------------------------------------
> 
> # cat /etc/redhat-release
> CentOS release 5.8 (Final) Tikanga
> 
> # rpm -qa | grep dnsmasq
> dnsmasq-2.45-1.1.el5_3
> 
> -----------------------------------------------------
> 
> So anyone can help this problem?
> 
> Thank you very much!
> 

Is this a one-off occurence, or a repeatable problem? The most likely
explanation is that the TTL expired, and dnsmasq sent the query to an
upstream server which replied with the NXDOMAIN due to some transient
problem with the domain. The NXDOMAIN reply would then be cached by
dnsmasq, so you'd see dnsmasq replying with NXDOMAIN until that record
expired (The TTL for NXDOMAIN is not the same as for extant records, it
comes from the SOA for the zone.)

If you can repeat this, the best thing to do is turn on dns-query
logging (--log-queries) which should give you information about what's
happening. Once you get to the situation where dnsmasq is returning
NXDOMAIN, send the dnsmasq process SIGUSR1, which will make it dump the
contents of the cache to the log too.


Cheers,

Simon.