Date: Thu, 22 Oct 2015 17:56:08 -0400 From: Rich Felker <dalias@...c.org> To: Tim Hockin <thockin@...gle.com> Cc: musl@...ts.openwall.com Subject: Re: Re: Would love to see reconsideration for domain and search On Thu, Oct 22, 2015 at 02:24:11PM -0700, Tim Hockin wrote: > Hi all, > > I saw this thread on the web archive but am not sure how to respond to > the thread directly as a new joinee of the ML. I hope this finds its > way... No problem; just starting a new thread like this and quoting the old one is fine. > I am one of the developers of Kubernetes and I own the DNS portion, in > particular. I desperately want to use Alpine Linux (based on musl) > but for now I have to warn people NOT to use it because of this issue. > > On Fri, Sep 04, 2015 at 02:04:29PM -0400, Rich Felker wrote: > > On Fri, Sep 04, 2015 at 12:11:36PM -0500, Andy Shinn wrote: > >> I'm writing the wonderful musl project today to open discussion > >> about the future possibility of DNS search and domain keyword > >> support. We've been using musl libc (by way of Alpine Linux) for > >> new development of applications as containers that discover each > >> other through DNS and other software defined networking. > >> > >> In particular, we are starting to use applications like SkyDNS, > >> Consul, and Kubernetes, all of which rely on local name > >> resolution in some way using search paths. Many users of the > >> Alpine Linux container image have also expressed their desire for > >> this feature at > >> https://github.com/gliderlabs/docker-alpine/issues/8. > >> > >> On the functional differences between glibc page, the domain and > >> search keyword "Support may be added in the future if there is > >> demand". So please consider this request an addition to whatever > >> demand for the feature already exists. > >> > >> Thank you for your time and great work on the musl libc project! > > > > I think this is a reasonable request. I'll look into it more. > > > > One property I do not want to break is deterministic results, so > > when a search is performed, if any step of the search ends with > > an error rather than a positive or negative result, the whole > > lookup needs to stop and report the error rather than falling > > back. Falling back is not safe and creates a situation where DoS > > can be used to control which results are returned. > > I understand your point, though the world at large tends to disagree. > Everyone has a primary and secondary `nameserver` record (or should). > If the first one times out, try the second. Most resolver libs seem > to accept a SERVFAIL response or a timeout as a signal to try the next > server, and I would encourage you to do the same. musl intentionally does not do this because it yields abysmal performance. If the first nameserver is overloaded or the packet is lost, you suffer several-second lookup latency. > Stopping on positive response or NXDOMAIN seems to be commonly > accepted with a caveat. You can't query all nameservers and just take > the first NXDOMAIN to respond. You can only accept NXDOMAIN if all of > the higher-priority (listed first in resolv.conf) nameservers have > timed out or SERVFAIL'ed. You can issue queries in parallel, but you > must process responses in order, which is what you describe below. Timeout or servfail is not sufficient to accept an nxdomain from a lower-priority server. To preserve consistency of results under transient failure, you actually have to wait for the nxdomain from the higher-priority server. Either way, this very much pessimizes usage cases like running "netstat" with huge numbers of connections where many of the ip addresses fail to reverse. Being able to return immediately as soon as any one of the nameservers responds with nxdomain makes the difference between a <1s netstat run and a 5-10s netstat run. Thus, if we add extensions to support the kind of result unioning you want across multiple nameservers, I think they should be configurable and off-by-default. A simple option in resolv.conf could turn them on. And there could be options for requiring nxdomain from all servers (true union) or just for highest-priority when accepting negative results. > > While it would be possible to parallelize the search while > > serializing the results (i.e. waiting to accept a result from the > > second query until the first query finished with a negative > > result), I think the consensus during the last round of > > discussion of this topic was that the complexity cost is too > > great and the benefit too small. Ideally, the first query should > > always succeed, anyway. > > The real world is not ideal. Not all nameservers are identically > scoped - you MUST respect the ordering in resolv.conf - to do > otherwise is semantically broken. If implementation simplicity means > literally doing queries in serial, then that is what you should do. Even legacy resolvers had the option to rotate the nameservers for load-balancing, so I think it's a stretch to say the ordering is supposed to be semantic. My view has always been that multiple nameservers in resolv.conf are for redundancy, not for serving conflicting records. > Similarly, you can't just search all search domains in parallel and > take the first response. The ordering is meaningful. Indeed, search domains are like that, because they inherently produce ambiguity/overlapping namespaces with different definitions. This is why myself and others who weighed in on the original question of supporting them were against, but left the option open to revisit the topic if users who need them show up. > > I also have a few questions: > > > > 1. Do you need multiple search items, or just a single domain? > > Any setup with multiple searches necessarily has suboptimal > > performance because ndots is not sufficient to make the right > > initial choice of query. If you do need this functionality, a > > unioning proxy dns server may be a better option than resolv.conf > > domain search; it would give much better performance. > > We use multiple search paths and ndots > 1. I'm not sure what you > mean by "unioning" here. Search path ordering is as meaningful as > nameserver ordering. You can't avoid making the query for each search > suffix in the worst case, and it has the same restriction as > nameserver - the search order must be respected. > > There does seem to be some different implementations that search for > the "naked" query first vs last, though. I think the semantically > correct (but pessimal performance) is to search for that last. The traditional behavior is to do the naked query first if the query string has at least 'ndots' dots, and to do the search domains first otherwise. Also I believe a final dot always suppresses search. My point was that with ndots=1 (default) and only a single search domain, the _expected_ result is that the first query succeed. But if you have ndots>1 or multiple search domains, you expect a portion of your queries to fall back at least once. This adds significant latency. In such a situation, you can avoid the additional latency (except on the first query of a given record) by running a local caching nameserver that does the search and unioning for you, rather than having the stub resolver in libc do it. Then subsequent queries succeed immediately using the cache. The reason I asked about usage case (ndots=1 vs ndots>1, single vs multiple search) is that, in the multi-fallback case, it might make more sense (from a performance and clean design standpoint) to implement this with a caching nameserver on localhost rather than in musl. > > 2. For your intended applications, is there a need to support > > ndots>1? Such configurations are generally not friendly to > > applications that expect to be able to resolve normal internet > > domain lookups, and performance for such lookups will be very bad > > (because the search domains first have to fail). > > DNS is a very lightweight protocol. We have not measured any > practical detriment for having 6 search domains and ndots=5. In the > normal case it fails very quickly. That aside, it should be my > business if I want to (mis)configure my system that way :) I suspect we have different definitions of quick... :) > > 3. The glibc behavior is just to swap the order of search when > > the query string has >=ndots dots in it, but would it be > > acceptable never to try the search domains at all in this case? > > That would yield much better performance for nxdomain results and > > avoid unexpected positive results due to weird subdomains > > existing in your search domain (e.g. a wildcard for > > *.us.example.com would cause *.us to wrongly resolve for > > non-existant .us domains). > > I think that would be correct. If I have 3 dots and ndots=2, search > paths should be ignored. Glad we agree on this. I hope you feel like this conversation is productive. I don't want to rule out anything/"say no" right away, but rather try to get a better understanding of your requirements first and figure out what makes the most sense to do on musl's side. Rich
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