Date: Wed, 8 May 2019 11:57:33 -0400 From: Luke Shumaker <lukeshu@...awire.io> To: Rich Felker <dalias@...c.org> Cc: musl@...ts.openwall.com Subject: Re: Malformed DNS requests for single-label hostnames with `search .` Rich Felker wrote: > On Tue, May 07, 2019 at 12:29:43PM -0400, Luke Shumaker wrote: > > In some scenarios, musl libc generates invalid DNS queries that are > > discarded by the DNS server. Particularly when `resolv.conf` says > > `search .` and we attempt to resolv a single-label hostname. > > > > / # cat /etc/resolv.conf > > search . > > nameserver 188.8.131.52 > > Note that this is not a good idea, even if it weren't buggy, as it > will just perform all your queries twice. If you don't want to search, > omit the search option or leave it blank. Use-case 1: On macOS, disabling `search`/`domain`-from-DHCP results in `search .`. musl isn't really used on macOS, but it _is_ used in Alpine-on-Docker-on-macOS (by default, Docker just copies the resolv.conf from the host). Use-case 2: Since `search` defaults to `domain`, setting `domain .` will also trigger the bug. Setting `domain .` is a thing that is explicitly mentioned as valid in the Linux man-pages project <https://www.kernel.org/doc/man-pages/> page for resolv.conf. Use-case 3: Some users wish for to use a search suffix, but to try the name-as-requested first, then falling back to the suffix; they can do this by explicitly mentioning `.` before the suffix, setting search . whatever.com > > But if I allow it to use the `search`-path, the query is invalid: > > > > / # time strace -f -e trace=sendto,sendmsg,sendmmsg getent hosts label > > sendto(3, "\16s\1\0\0\1\0\0\0\0\0\0\5label.\0\34\0\1\0", 24, ... > > sendto(3, "\16s\1\0\0\1\0\0\0\0\0\0\5label.\0\34\0\1\0", 24, ... > > sendto(3, "\363\365\1\0\0\1\0\0\0\0\0\0\5label.\0\1\0\1\0", 24, ... > > sendto(3, "\363\365\1\0\0\1\0\0\0\0\0\0\5label.\0\1\0\1\0", 24, ... > > +++ exited with 2 +++ > > Command exited with non-zero status 2 > > real 0m 10.01s > > user 0m 0.00s > > sys 0m 0.00s > ... > > So there are 2 pieces of corruption going on here: > > > > 1. Instead of getting the \0 terminator indicating that there are no > > more lables in the QNAME, it gets an ASCII '.', indicating another > > label of length 46. > > 2. An extra byte is allocated, which appears at the end of the > > message. Oh, I forgot to mention a 3rd piece of corruption: It submits the same request (with the same ID!) twice. > Yes, this is probably a bug, if search is expected to accept trailing > dots, which seems like reasonable-ish functionality. Around line 203 > of lookup_name.c, we'd need to detect this case and replace the search > component with a zero-length one. I don't recall right off if we'd > also need to strip the . separating the query from the search > component; that depends on whether name_from_dns accepts a trailing > dot, which I think it does, so such stripping probably isn't needed. Thanks for the pointers! > Again, I think it's a really bad idea to configure your resolv.conf > like this. As you've done, it will repeat the same query twice in the > case of NxDomain, for no benefit. This will only happen for queries > with fewer than ndots dots in them, which, unless you've increaded > ndots (which has a lot of other problems), will always be NxDomain. > And in the case where you have other nontrivial search components > *after* ".", it will produce a situation where appearance of new > domains in the global namespace will mask local names you might be > using. > > I wonder if it would make more sense to just skip/ignore "." in the > search path... It looks like what GNU libc does is keep track of whether "." appeared in `search`, and skip the no-suffix lookup if it reaches the end of the list (tracking `root_on_list` in `res_query.c`). On the other hand, bind-tools will perform the lookup twice, as you described. -- Happy hacking, ~ Luke Shumaker
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