Re: positive_dns_ttl

Now bug 2100 <http://www.squid-cache.org/bugs/show_bug.cgi?id=2100>.

WRT negative-ttl being limited; the docs already say that it's a bad
idea to set it to less than 10.

Cheers,

On 2007/10/11, at 11:53 AM, Amos Jeffries wrote:

>>
>> 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.
>>
>
> Given that. I'd agree. However is negative-ttl limited to being 1
> or more
> itself?
>
> Amos
>
>>
>> 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
>>
>>
>>
>
>

--
Mark Nottingham       mnot@yahoo-inc.com