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

Michal Židek mzidek at redhat.com
Fri Aug 9 15:13:43 UTC 2013


On 08/09/2013 04:51 PM, Jakub Hrozek wrote:
> On Fri, Aug 09, 2013 at 04:33:43PM +0200, Michal Židek wrote:
>> 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
>
> Actually these two patches are only intended for the 1.9 branch and the
> other two patches I sent are for master. In 1.9, FO_DEFAULT_SVC_TIMEOUT
> is still used:
>
>  From src/providers/data_provider_fo.c:
>
> 67 static int be_fo_get_options(struct be_ctx *ctx,
> 68                              struct fo_options *opts)
> 69 {
> 70     errno_t ret;
> 71
> 72     ret = confdb_get_int(ctx->cdb, ctx->conf_path,
> 73                          CONFDB_DOMAIN_RESOLV_TIMEOUT,
> 74                          FO_DEFAULT_SVC_TIMEOUT,
> 75                          &opts->service_resolv_timeout);
> 76     if (ret != EOK) {
> 77         return ret;
> 78     }
>
>
> So FO_DEFAULT_SVC_TIMEOUT is used as the per-service (aka per-request)
> timeout and in 1.9 it's set to 10 seconds. I remember that the your patches
> set both the c-ares timeout and the per-service timeout to the same value,
> so I was wondering if in 1.9 we should do the same.

Ah, I see it now. Well, 10s is OK. Anything less than 6 seconds would be
a problem. But yes, we can change it to 6 seconds to have the same
defaults in master and 1.9.



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