Re: positive_dns_ttl

On Wed, 10 Oct 2007, Mark Nottingham wrote:

> From ipcache.c;
>
>> if (ttl == 0 || ttl > Config.positiveDnsTtl)
>> ttl = Config.positiveDnsTtl;
>> if (ttl < Config.negativeDnsTtl)
>> ttl = Config.negativeDnsTtl;
>> i->expires = squid_curtime + ttl;
>
> As I read this, if the TTL from an upstream resolver happens to be '0', it
> changes it to whatever positive_dns_ttl is -- even though that also acts as a
> ceiling for DNS TTLs.

I think this is partly left over from the old days when Squid always
used the external dnsserver programs. 'dnsserver' could only report
TTLs if the O/S had the libresolv _dns_ttl hack. So "ttl == 0"
meant that dnsserver didn't have any TTL value, so it should be set
to positive_dns_ttl.

> The problem is that this plays havoc with DNS-based load balancers, which
> will be '0' more often than other DNS entries by nature. Any chance of
> either;

The only thing I'm worried about is that with true 0 TTL squid will
have to make multiple lookups for a single HTTP request. For
example, if someone had a long list of 'dst' ACLs then each one
could result in a new DNS lookup.

AFAIK, the ipcache is the only place where DNS lookups are cached
and Squid may refer to the ipcache multiple times for a given HTTP
transaction.

DW
Received on Wed Oct 10 2007 - 11:06:31 MDT