[Dnsmasq-discuss] Multiple PTR permutation

dnsmasq user dnsmasq at linuxnet.ca
Fri Jan 18 07:22:19 GMT 2008

While recently diagnosing some DNS idiosyncrasies where ptr
lookups were failing for some and returning what at first
appeared to be random results for others I came across the

A dns server for which I have no control over has a somewhat
large amount of ptr records assigned to a single IP address.
262 to be exact.

gethostbyaddress doesn't want to work with the large amount of
data being returned, no fall over to tcp is what I'm assuming,
hence this being brought to my attention initially as ptr
lookups on this IP were failing on all the servers, but ptr
records were being resolved for said IP on all workstations
on multiple subnets, albeit what appeared to be random results
per subnet. 

After a little bit of head scratching what I discovered was
the workstations were resolving from dnsmasq while the servers
were resolving from named servers. Looking into it further 
what I found was dnsmasq was returning only one ptr record
result and the ptr lookup result would change every time
dnsmasq was restarted, which explained what appeared to be
randomness as the individual subnets have their own dnsmasq
servers to resolve from.

Setting cache-size=0 and restarting dnsmasq allowed the
lookups on the workstations to return the entire ptr
lists assigned to that IP.

Discussing this with a colleague it was thought that perhaps
dnsmasq was never tested against such a large ptr data result
so I figured I would run it by the list to see if anyone had
any thoughts on this.

dnsmasq 2.40


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