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

Jakub Hrozek jhrozek at redhat.com
Tue Jul 23 16:57:08 UTC 2013


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.



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