One possible use for this DNS feature is to identify cluster members for OVSDB clusters. That might be an appropriate use for a low TTL.
On Mon, Oct 30, 2017 at 10:52:14AM +0100, Miguel Angel Ajo Pelayo wrote: > I don't believe rounding up TTLs would be a good practice. The > administrators > are aware of the risks of having a very low TTL, so if they decide to pick > a low TTL they may have good reasons (like the possibility of a very close > event of the IP being moved, or some sort of failover mechanism, or a > way for dealing with dynamic dns). If you round it up you add unexpected > extra downtime. > > > > On Fri, Oct 27, 2017 at 6:38 PM, Yifeng Sun <[email protected]> wrote: > > > Thanks a lot. I will keep your guidance in mind. > > > > On Fri, Oct 27, 2017 at 9:29 AM, Mark Michelson <[email protected]> > > wrote: > > > > > Usually the software that performs DNS lookups and caches results is > > > referred to as a "DNS forwarder". You configure resolv.conf's nameserver > > as > > > 127.0.0.1. This way, DNS queries go to the DNS forwarder running on > > > localhost. The forwarder then has real nameservers configured to send the > > > queries to. The forwarder receives the DNS response (or lack of response > > if > > > the nameserver is unreachable), caches the result, and sends the result > > > back to the system resolver. From the perspective of OVS, all it is doing > > > is performing a simple DNS lookup. Further DNS lookups hit the forwarder, > > > which has cached the previous queries and can immediately return the > > cached > > > records (or cached failure). > > > > > > One common DNS forwarder that works this way is dnsmasq. There are likely > > > lots of others. I believe that bind can be configured to act as a DNS > > > forwarder as well. I recommend searching for "DNS forwarder" and seeing > > > what you can find. The common thread here, though, is that OVS would not > > > actually package or depend on a DNS forwarder. Rather, we would add > > > documentation that strongly suggests that applications that consider DNS > > > resolution to be critical configure their favorite DNS forwarder on > > systems > > > running OVS. > > > > > > Since we are looking into using a third party library for DNS resolution, > > > it may also be worth looking into what sort of caching those libraries > > > provide. I am not familiar with that information off the top of my head. > > > > > > Mark > > > > > > On Fri, Oct 27, 2017 at 11:14 AM Yifeng Sun <[email protected]> > > > wrote: > > > > > >> Mark, thanks a lot for the detailed and thorough explanation. > > >> > > >> Do you happen to know any other projects that we can take a peek? > > >> > > >> On Fri, Oct 27, 2017 at 8:57 AM, Mark Michelson <[email protected]> > > >> wrote: > > >> > > >>> Yep, that makes good sense. I'd recommend having some min and max > > >>> threshold though. That way, if a record has a TTL of multiple days, > > you can > > >>> round that down to something more reasonable. Similarly, a > > ridiculously low > > >>> TTL can be rounded up. > > >>> > > >>> There's another aspect to DNS caching that I briefly mentioned in my > > >>> last e-mail: negative caching. Consider that you attempt to look up > > >>> exampledomain.com, and for some reason, you either can't reach your > > DNS > > >>> server or the DNS server returns NXDOMAIN (or some other error). Each > > >>> additional lookup of exampledomain.com will likely hit this same > > >>> problem until either you can restore the link to your DNS server or the > > >>> proper records get added to DNS. If exampledomain.com is a frequent > > >>> destination for traffic, you don't want to be doing full DNS lookups > > of it > > >>> each time, just to find out you can't send to it. This is especially > > true > > >>> in the case that the DNS server is unreachable. resolv.conf by default > > will > > >>> wait 5 seconds on an attempt and will attempt the query twice before > > giving > > >>> up. This means each DNS query will take 10 seconds just to ultimately > > fail. > > >>> This can lead to large backlogs of queries. With negative caching, you > > >>> cache the fact that an attempt to reach exampledomain.com failed, and > > >>> for some amount of time, any further attempts to query that domain will > > >>> immediately fail rather than attempting the query again. > > >>> > > >>> I mention negative caching because in some ways, it's more important > > >>> than positive caching. I've seen certain applications get completely > > >>> crippled by having the DNS server unavailable since they'll keep > > attempting > > >>> to perform DNS lookups for domains that will ultimately fail. Caching > > >>> positive results based on TTL of records won't help in a case like > > this. > > >>> > > >>> I'm not going to take a hard-line stand that DNS caching absolutely > > >>> should not be added to OVS. But I point out negative caching as one of > > >>> those things that may seem non-obvious at first but that is very > > important. > > >>> If we can implement it successfully, then that is fantastic. But I just > > >>> know that other projects have already gone through the pains of trying > > to > > >>> implement this properly and if we can piggyback on their efforts, we'll > > >>> probably end up happier in the long run. > > >>> > > >>> Mark > > >>> > > >>> PS There's actually an RFC dedicated to negative caching of DNS > > >>> responses: https://tools.ietf.org/html/rfc2308 > > >>> > > >>> On Fri, Oct 27, 2017 at 10:19 AM Ben Pfaff <[email protected]> wrote: > > >>> > > >>>> Does it make sense to cache the entry until its declared TTL expires? > > >>>> > > >>>> On Fri, Oct 27, 2017 at 01:30:41PM +0000, Mark Michelson wrote: > > >>>> > This opens the can of worms that is DNS caching. > > >>>> > > > >>>> > On one end of the spectrum, you can always perform a full DNS lookup > > >>>> of a > > >>>> > target and never store the result. If the result of the lookup > > >>>> changes, > > >>>> > then you will know about it as soon as possible. However, the > > >>>> repeated DNS > > >>>> > lookups are very expensive, and most of the time, those lookups will > > >>>> be > > >>>> > redundant. > > >>>> > > > >>>> > On the other end of the spectrum, you can look up a target once and > > >>>> cache > > >>>> > the result forever. It is much less expensive since you only ever do > > >>>> one > > >>>> > DNS lookup, but if the DNS record ever changes, you will send > > traffic > > >>>> to > > >>>> > the wrong address. > > >>>> > > > >>>> > Usually what works best is some middle ground. Essentially, cache > > the > > >>>> > result of a DNS lookup for some configured time. After that time is > > >>>> up, > > >>>> > perform the lookup again. It's a compromise. You don't have to > > perform > > >>>> > lookups as often, which is good. However, most of the lookups you > > >>>> still > > >>>> > perform will be redundant since it is unlikely that results will > > >>>> change. > > >>>> > And if the address does change, then you will not detect it until > > the > > >>>> > current cached result is stale. > > >>>> > > > >>>> > Having said all that, caching is hard. If we can avoid having to do > > it > > >>>> > ourselves, that's a good thing. One way to go with this is to go > > with > > >>>> the > > >>>> > extreme of always performing DNS lookups. We can then recommend that > > >>>> users > > >>>> > of OVS that will be performing frequent DNS lookups also run a DNS > > >>>> > forwarder that has built-in caching (as well as negative caching. > > >>>> That's > > >>>> > another headache). That separates concerns more evenly. > > >>>> > > > >>>> > Anyways, that's just my opinion. If you decide that a DNS caching > > >>>> layer in > > >>>> > OVS is appropriate, then that's fine too. > > >>>> > > > >>>> > Mark > > >>>> > > > >>>> > On Thu, Oct 26, 2017 at 5:14 PM Yifeng Sun <[email protected]> > > >>>> wrote: > > >>>> > > > >>>> > > Thanks Mark for your reply. > > >>>> > > > > >>>> > > There is one more thing. If we bring DNS into play, we may need a > > >>>> > > mechanism to watch for changes of ip addresses that were already > > >>>> resolved > > >>>> > > and being used. > > >>>> > > > > >>>> > > Thanks, > > >>>> > > Yifeng > > >>>> > > > > >>>> > > On Thu, Oct 26, 2017 at 12:10 PM, Mark Michelson < > > >>>> [email protected]> > > >>>> > > wrote: > > >>>> > > > > >>>> > >> On Wed, Oct 25, 2017 at 4:16 PM Yifeng Sun < > > [email protected] > > >>>> > > > >>>> > >> wrote: > > >>>> > >> > > >>>> > >>> I feel that unbound stands out in the available open source DNS > > >>>> resolver. > > >>>> > >>> > > >>>> > >>> Below is the summary for unbound: > > >>>> > >>> * The actual resolving work is done by a background process or > > >>>> thread. A > > >>>> > >>> background process or thread seems unavoidable. Linux's > > >>>> getaddrinfo_a > > >>>> > >>> clones a thread similarly. > > >>>> > >>> > > >>>> > >> * It is ported on Linux, BSD, Windows, MacOS/X and Solaris/SPARC. > > >>>> This is > > >>>> > >>> good because OVS runs on a large range of platforms. > > >>>> > >>> > > >>>> > >> * It complies to the standard, with optional DNSSEC support. Some > > >>>> of its > > >>>> > >>> features may not be needed in our case. > > >>>> > >>> * The unbound context is thread-safe. Its internal locks may > > >>>> bring some > > >>>> > >>> overhead. But since the DNS resolving is not frequent in OVS, I > > >>>> suppose > > >>>> > >>> this small overhead is not an issue. > > >>>> > >>> > > >>>> > >>> Unbound looks like a good option. Another option is to create a > > >>>> > >>> background thread which processes DNS resolving requests from > > the > > >>>> main > > >>>> > >>> thread and sends back the resulting events to the main thread. > > >>>> This method > > >>>> > >>> is quite simple and straightforward. > > >>>> > >>> > > >>>> > >>> The above are what I got so far. Please give your thoughts, > > >>>> thanks a lot. > > >>>> > >>> > > >>>> > >> > > >>>> > >> If portability to all of the systems you mentioned in your second > > >>>> bullet > > >>>> > >> point is important, then you can rule out a couple of options: > > >>>> > >> * getaddrinfo_a is a GNU extension and is only available with > > glibc > > >>>> > >> * The resolver functions[1] are a BSD specification so they'd be > > >>>> > >> available on most platforms, but not on Windows. I don't > > personally > > >>>> > >> recommend these because of the need to manually parse the DNS > > >>>> responses you > > >>>> > >> receive. > > >>>> > >> > > >>>> > >> That leaves two options: > > >>>> > >> * Run a background thread uses getaddrinfo() to perform > > resolution. > > >>>> > >> * Use a third-party library (like unbound). > > >>>> > >> > > >>>> > >> Of these two options, I feel like the third-party library is the > > >>>> better > > >>>> > >> option. The only downside I can think of is the extra dependency > > >>>> for OVS. > > >>>> > >> And as far as what third-party library to use, I was the one that > > >>>> suggested > > >>>> > >> unbound in the first place, so obviously I'm fine with using it > > :) > > >>>> > >> > > >>>> > >> Mark > > >>>> > >> > > >>>> > >> [1] http://man7.org/linux/man-pages/man3/resolver.3.html > > >>>> > >> > > >>>> > >> > > >>>> > >>> > > >>>> > >>> Below is the link for original discussion: > > >>>> > >>> https://mail.openvswitch.org/pipermail/ovs-dev/2017-August/ > > >>>> 337038.html > > >>>> > >>> > > >>>> > >>> > > >>>> > >>> > > >>>> > >>> On Wed, Oct 25, 2017 at 2:11 PM, Ben Pfaff <[email protected]> wrote: > > >>>> > >>> > > >>>> > >>>> Hello everyone, please allow me to introduce Yifeng Sun (CCed), > > >>>> who > > >>>> > >>>> recently joined VMware's Open vSwitch team. I've asked Yifeng > > >>>> to start > > >>>> > >>>> out by working on DNS support for Open vSwitch. Yifeng, can > > you > > >>>> tell us > > >>>> > >>>> about what you've discovered so far, based on this thread from > > >>>> August > > >>>> > >>>> that I'm reviving, and your tentative conclusions? > > >>>> > >>>> > > >>>> > >>>> Thanks, > > >>>> > >>>> > > >>>> > >>>> Ben. > > >>>> > >>>> > > >>>> > >>> > > >>>> > >>> > > >>>> > > > > >>>> > > >>> > > >> > > _______________________________________________ > > dev mailing list > > [email protected] > > https://mail.openvswitch.org/mailman/listinfo/ovs-dev > > > _______________________________________________ > dev mailing list > [email protected] > https://mail.openvswitch.org/mailman/listinfo/ovs-dev _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
