Date: Fri, 30 Mar 2018 14:44:44 -0500 From: William Pitcock <nenolod@...eferenced.org> To: musl@...ts.openwall.com Subject: Re: [PATCH] resolver: only exit the search path loop there are a positive number of results given Hello, On Fri, Mar 30, 2018 at 2:35 PM, Rich Felker <dalias@...c.org> wrote: > On Fri, Mar 30, 2018 at 02:19:48PM -0500, William Pitcock wrote: >> Hello, >> >> On Fri, Mar 30, 2018 at 2:14 PM, Rich Felker <dalias@...c.org> wrote: >> > On Fri, Mar 30, 2018 at 06:52:25PM +0000, William Pitcock wrote: >> >> In the event of no results being given by any of the lookup modules, EAI_NONAME will still >> >> be thrown. >> >> >> >> This is intended to mitigate problems that occur when zones are hosted by weird DNS servers, >> >> such as the one Cloudflare have implemented, and appear in the search path. >> >> --- >> >> src/network/lookup_name.c | 2 +- >> >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> >> >> diff --git a/src/network/lookup_name.c b/src/network/lookup_name.c >> >> index 209c20f0..b068bb92 100644 >> >> --- a/src/network/lookup_name.c >> >> +++ b/src/network/lookup_name.c >> >> @@ -202,7 +202,7 @@ static int name_from_dns_search(struct address buf[static MAXADDRS], char canon[ >> >> memcpy(canon+l+1, p, z-p); >> >> canon[z-p+1+l] = 0; >> >> int cnt = name_from_dns(buf, canon, canon, family, &conf); >> >> - if (cnt) return cnt; >> >> + if (cnt > 0) return cnt; >> >> } >> >> } >> > >> > This patch is incorrect, and the reason should be an FAQ item if it's >> > not already. Only a return value of 0 means that the requested name >> > does not exist and that it's permissible to continue search. Other >> > nonpositive return values indicate either that the name does exist but >> > does not have a record of the quested type, or that a transient error >> > occurred, making it impossible to determine whether the search can be >> > continued and thus requiring the error to be reported to the caller. >> > Anything else results in one or both of the following bugs: >> > >> > - Nondeterministically returning different results for the same query >> > depending on transient unavailability of the nameservers to answer >> > on time. >> > >> > - Returning inconsistent results (for different search components) >> > depending on whether AF_INET, AF_INET6, or AF_UNSPEC was requested. >> > >> > I'm aware that at least rancher-dns and Cloudflare's nameservers have >> > had bugs related to this issue. I'm not sure what the status on >> > getting them fixed is, and for Cloudflare I don't know exactly what it >> > is they're doing wrong or why. But I do know the problem is that >> > they're returning semantically incorrect dns replies. >> >> Kubernetes imposes a default search path with the cluster domain last, so: >> >> - local.prod.svc.whatever >> - prod.svc.whatever >> - svc.whatever >> - yourdomain.com >> >> The cloudflare issue is that they send SUCCESS code with 0 replies, >> which causes musl to error when it hits the yourdomain.com. > > Yes, that makes sense. Do you know why they're doing it? If they > refuse to fix it, the only clean fix I know is a local proxy > configured to fix the records for the specific broken domains you care > about. But of course that's not convenient. My contacts at cloudflare indicate that their environment depends on this behaviour, so they have no interest in fixing it. A local proxy isn't going to be workable, because most people are going to just say "but Debian or Fedora doesn't require this," and then just go use a glibc distribution. There is a talk in a few weeks at Kubecon (the Kubernetes conference), explicitly titled "Don't Use Alpine If You Care About DNS." The talk largely centers around how musl's overly strict behaviour makes Alpine a bad choice for "the real world." I would like to turn this into a story where we can announce that Alpine 3.8 mitigates this problem instead, doing such will be good for both Alpine and the musl ecosystem as a whole, as it is defanging a point of possible FUD. > >> Do you have any suggestions on a mitigation which would be more >> palatable? We need to ship a mitigation for this in Alpine 3.8 >> regardless. I would much rather carry a patch that is upstreamable, >> but I am quite willing to carry one that isn't, in order to solve this >> problem. > > A theoretically-non-horrible (but somewhat costly) solution is to > always query both A and AAAA, rather than only doing it for AF_UNSPEC. > Then if you see a reply with 0 (total, between both) records, you can > opt to interpret that the same way as NxDomain without breaking > consistency properties. If Cloudflare refuses to fix the bug, maybe we > should consider adding an _option_ (in the resolv.conf options line) > to do this. I don't think it should be the default behavior because it > mildly slows down lookups, especially if you have nontrivial packet > loss since probability of failure is now 1-(1-p)²=2p-p² rather than p > (where p is the packet loss rate). It seems to me we could just send ANY and filter out the records we don't care about. This is what I did with charybdis's asynchronous DNS resolver when it had a similar problem. What are your thoughts on that? William
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