[nsd-users] NSD doing 3 IXFR queries in rapid succession

[W.C.A. Wijngaards]

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Hi Anand,

On 20/12/15 13:15, Anand Buddhdev wrote:
> Hello NSD users and developers,
> 
> I was just looking at some BIND logs on one of our servers. It 
> feeds a downstream NSD 4.1.7 slave. In the BIND logs, I often see 
> this:
> 
> 20-Dec-2015 11:50:45.011 xfer-out: client 10.64.0.12#47701/key 
> main.ripe.net (132.72.185.in-addr.arpa): transfer of 
> '132.72.185.in-addr.arpa/IN': IXFR ended 20-Dec-2015 11:50:45.078 
> xfer-out: client 10.64.0.12#47704/key main.ripe.net 
> (132.72.185.in-addr.arpa): transfer of 
> '132.72.185.in-addr.arpa/IN': IXFR ended 20-Dec-2015 11:50:45.146 
> xfer-out: client 10.64.0.12#47707/key main.ripe.net 
> (132.72.185.in-addr.arpa): transfer of 
> '132.72.185.in-addr.arpa/IN': IXFR ended
> 
> Notice that NSD appears to be doing 3 IXFR queries for the same 
> zone in rapid succession.
> 
> I captured some packets using tcpdump, and then put them through 
> PacketQ with the filter:
> 
> select src_addr,src_port,qname,qtype,rcode,aname,atype from dns
> 
> The results corresponding to the above log is:
> 
> ["10.64.0.12",47701,"132.72.185.in-addr.arpa.",251,0,"",0], 
> ["93.175.159.250",53,"132.72.185.in-addr.arpa.",251,0,"132.72.185.in-a
ddr.arpa.",6],
>["10.64.0.12",47704,"132.72.185.in-addr.arpa.",251,0,"",0],
>
> 
["93.175.159.250",53,"132.72.185.in
> addr.arpa.",251,0,"132.72.185.inaddr.arpa.",6], 
> ["10.64.0.12",47707,"132.72.185.in-addr.arpa.",251,0,"",0], 
> ["93.175.159.250",53,"132.72.185.in-addr.arpa.",251,0,"132.72.185.in-a
ddr.arpa.",6]
>
>
>
> 
The packet capture also shows shows the same thing: 3 IXFR
> queries in rapid succession, with the same responses.
> 
> Does anyone have any idea why NSD is doing 3 queries per zone like
>  this?

NSD wants to fetch an update for the zone.  The server does not
provide one.  NSD tries all masters several times to find the update.
 This is where this server sees multiple requests.

The reason for fetching it may be the SOA refresh timer or a NOTIFY.

Load balancers, and the server may be in the process of loading the
data, so the third query may return a different result.  Also, the
code, currently, just retries in the face of no result, which makes
for simpler code.

Best regards, Wouter
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