Re: positive_dns_ttl

Makes sense. In the current code, if the test for ttl == 0 is
removed, it should set the '0' case to the negative TTL rather than
the positive one, which would be better in all cases, I think.

On 2007/10/11, at 3:06 AM, Duane Wessels wrote:

>
>
>
> 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

--
Mark Nottingham       mnot@yahoo-inc.com