net: dns lookup sometimes fails with "no such host" when there are too many open files

What did you do?

My network-intensive code would sometimes fail resolving domain names, returning "no such host" using the default resolver (pure go).The domains that fail resolving are valid domains that are successfully resolved using dig and the same dns server address.

I noticed that increasing the number of open files limit fix the issue.

I managed to reproduce the failure with: https://play.golang.org/p/MEZUS8h-o5

go build test.go
ulimit -n 20 # Set open file limit to something low
$ ./test 
2017/01/10 11:01:11 lookup munic.io on 127.0.1.1:53: dial udp 127.0.1.1:53: socket: too many open files
$ ./test 
2017/01/10 11:01:11 lookup munic.io on 127.0.1.1:53: dial udp 127.0.1.1:53: socket: too many open files
$ ./test 
2017/01/10 12:01:12 lookup munic.io on 127.0.1.1:53: no such host

What did you expect to see?

Always "too many open files"

What did you see instead?

Sometimes Lookup returns "no such host" when it shouldn't.

Does this issue reproduce with the latest release (go1.7.4)?

I first noticed the problem with 1.7.3. I reproduced the problem on 1.8beta2.

System details

go version go1.8beta2 linux/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/costanzj/projects/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build335463609=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
GOROOT/bin/go version: go version go1.8beta2 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.8beta2 X:framepointer
uname -sr: Linux 4.4.0-38-generic
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.1 LTS
Release:	16.04
Codename:	xenial
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu GLIBC 2.23-0ubuntu5) stable release version 2.23, by Roland McGrath et al.
gdb --version: GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.04) 7.11.1