[SSSD] [PATCH 1/2] Set default DNS resolution timeout to 15 seconds.

Michal Židek mzidek at redhat.com
Fri Aug 9 14:33:43 UTC 2013


On 08/09/2013 11:51 AM, Jakub Hrozek wrote:
> On Wed, Jul 24, 2013 at 02:00:38PM +0200, Jakub Hrozek wrote:
>> On Tue, Jul 23, 2013 at 06:57:08PM +0200, Jakub Hrozek wrote:
>>> On Mon, Jul 22, 2013 at 06:06:39PM +0200, Michal Židek wrote:
>>>> On 07/19/2013 12:15 PM, Jakub Hrozek wrote:
>>>>> On Mon, Jul 08, 2013 at 09:43:42PM +0200, Michal Židek wrote:
>>>>>> On 07/04/2013 07:45 PM, Simo Sorce wrote:
>>>>>>> On Thu, 2013-07-04 at 16:23 +0200, Michal Židek wrote:
>>>>>>>> https://fedorahosted.org/sssd/ticket/1966 (SSSD failover doesn't work if
>>>>>>>> the first DNS server in resolv.conf is unavailable).
>>>>>>>>
>>>>>>>> The problem here is, that if first nameserver in resolv.conf is down,
>>>>>>>> the resolution is too slow and SSSD will not wait for the result of
>>>>>>>> ares_search and go offline. In my case the resolution was sometimes more
>>>>>>>> than a minute, because all search domains in resolv.conf were searched
>>>>>>>> inside ares_search() call using the first (not working) nameserver in
>>>>>>>> the first place and then with the working nameserver (and before that,
>>>>>>>> SSSD tried to figure out the domain name from my incorrectly set
>>>>>>>> hostname, which added more unnecessary DNS lookups).
>>>>>>>>
>>>>>>>> To avoid this problem, the option dns_discovery_domain must be set
>>>>>>>> properly, so that only the correct domain is searched, but even that is
>>>>>>>> not enough, because the default timeout for dns resolver operation in
>>>>>>>> sssd is too low. This patch rises the default value to 15 seconds
>>>>>>>> (instead of 5 seconds).
>>>>>>>>
>>>>>>>> Another option might be to lower the amount of time ares waits for a
>>>>>>>> nameserver to respond (currently it is 5 seconds, that is why 5 second
>>>>>>>> for the entire dns resolution is not sufficient), but I do not want to
>>>>>>>> do this.
>>>>>>>
>>>>>>> Why not ?
>>>>>>>
>>>>>>> 5 seconds these days is quite awfully high all in all.
>>>>>>> I think 1 second should be plenty in most cases, so maybe a default of 2
>>>>>>> seconds should be considered.
>>>>>>>
>>>>>>
>>>>>> I also think that 5 seconds is high value, but in general, I have bad
>>>>>> feeling when lowering not configurable timeouts (theoretically it could
>>>>>> cause some DNS servers that are reachable now to become unreachable).
>>>>>>
>>>>>
>>>>> Right, but in practice I can't think of a valid network configuration
>>>>> where the name server would take so long to reply. I mean, if the
>>>>> network was so slow that just resolving name would take 5 seconds, then
>>>>> chances are we wouldn't be able to download the entry from LDAP at all.
>>>>> And if the nameserver is slow, then let's rather skip it and fail over
>>>>> to another one.
>>>>>
>>>>>> Another possibility would be to make this value configurable. Then I
>>>>>> would have no problem lowering the default value. But, should this be
>>>>>> configurable? We already have one option to configure timeout for the
>>>>>> entire DNS resolution, adding another timeout for subtaks of this
>>>>>> operation does not sound nice to me. But I do not have strong opinion
>>>>>> on this.
>>>>>>
>>>>>
>>>>> Here is what we discussed with Michal today:
>>>>>      * make the per-nameserver timeout lower a little. Now it's 5
>>>>>        seconds, which is too long especially considering the failover
>>>>>        timeout is 5 seconds as well. We could change it to 2 or 3
>>>>>        seconds.
>>>>>      * the glibc allows 3 nameservers. The failover request timeut could
>>>>>        then default to 3*nameserver_timeout to give the resolver a chance
>>>>>        to iterate over all the configured name servers.
>>>>>
>>>>>> Btw, once the correct data is returned from DNS servers, we no longer
>>>>>> require any DNS operation, so it only happens first time the service
>>>>>> is required (for example the first 'getent' call will be slow, but
>>>>>> after that the service is fast). This is not so bad IMO.
>>>>>>
>>>>>>>> These patches also change man pages, so probably master only (string
>>>>>>>> freeze)? Even if this is a really small change.
>>>>>
>>>>> We accept changes again now that we've released 1.10.1.
>>>>>
>>>>> But this change would be OK even for a string-frozen release, because it
>>>>> only changes the number, not translatable strings.
>>>>>
>>>>>>>>
>>>>>>>> I was also thinking, would it make sense to write a warning to the logs
>>>>>>>> if the dns_discovery_domain option is not set? It seem to be important
>>>>>>>> to set it properly for cases like this one.
>>>>>>>
>>>>>>> I wonder, should we use the machine name to automatically set the
>>>>>>> discovery domain, and only if that fails fall back to the ones defined
>>>>>>> in resolv.conf ?
>>>>>>
>>>>>> Actually, we do this. Wireshark shows that the first srv lookup was
>>>>>> constructed from my hostname. But the whole process is unclear to me,
>>>>>> for example I do not know why we call the resolv_gethostbyname_send()
>>>>>> inside of resolv_get_domain_send(). It adds another DNS lookups
>>>>>> for A records (which causes another slow-down in the case of bad
>>>>>> nameserver searched first). In wireshark I also see searches for A/AAA
>>>>>> records for names like:
>>>>>> mzidek.example.com
>>>>>> mzidek.example.com.searchdom1.com
>>>>>> mzidek.example.com.searchdom2.com
>>>>>> mzidek.example.com.searchdom3.com
>>>>>
>>>>> IIRC we try to canonicalize the host name before chopping of the first
>>>>> part and using the rest as domain name.
>>>>>
>>>>> Would using gethostname() be safe in real world environments?
>>>>> I know that host names are often set to complete garbage and can't be
>>>>> relied on..but is it OK to just say "fix your environment" in that case?
>>>>>
>>>>> Personally I would argue the current canonicalization doesn't add much,
>>>>> because if the host name is set to complete garbage then the canonicalization
>>>>> wouldn't magically fix it..
>>>>>
>>>>>>
>>>>>> where mzidek.example.com is my hostname and searchdom1/2/3 are search
>>>>>> domains specified in resolv.conf. Why do we even try to resolve these
>>>>>> names? It seems like waste of time to me. If the dns_discovery_domain is
>>>>>> properly set, these lookups do not appear.
>>>>>>
>>>>>> Thanks
>>>>>> Michal
>>>>>>
>>>>>> PS: Patches are the same as before.
>>>>>>
>>>>>>>
>>>>>>> Simo.
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>> New patches attached. As discussed above, I changed the timeout to
>>>> contact DNS server from 5s to 2s, and timeout for the entire DNS resolution
>>>> from 5 to 6 seconds (instead of 15s as proposed originally)
>>>> (so it has enough time to contact 3 nameservers).
>>>>
>>>> I tested it with one and two unreachable nameservers.
>>>>
>>>> Thanks
>>>> Michal
>>>
>>> Works for me, too, so I'll ACK the patches. But I'll leave them on the
>>> list for a day longer in case somebody else would like to voice their
>>> concern about changing the default timeout value. I personally think
>>> it's OK.
>>
>> Pushed to master and sssd-1-10
>
> Hi,
>
> attached is a 1.9 bacport of the patches. Michal, do you think we need
> to lower the FO_DEFAULT_SVC_TIMEOUT as well? I guess so, but you know
> the details better..
>


FO_DEFAULT_SVC_TIMEOUT is not used anywhere. It is actually removed in 
the two minor fixes you posted earlier .

Ack to both patches.

Michal




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