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

Michal Židek mzidek at redhat.com
Mon Jul 22 16:06:39 UTC 2013


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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-Set-default-DNS-resolution-timeout-to-6-seconds.patch
Type: text/x-patch
Size: 1874 bytes
Desc: not available
URL: <https://lists.fedorahosted.org/pipermail/sssd-devel/attachments/20130722/5d81594a/attachment.bin>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0002-Lower-timeout-to-contact-DNS-server.patch
Type: text/x-patch
Size: 930 bytes
Desc: not available
URL: <https://lists.fedorahosted.org/pipermail/sssd-devel/attachments/20130722/5d81594a/attachment-0001.bin>


More information about the sssd-devel mailing list