proposal: net: add API to override DNS servers

[teknoraver]

What version of Go are you using (go version)?

go version go1.8 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/matteo/go"
GORACE=""
GOROOT="/usr/lib/go-1.8"
GOTOOLDIR="/usr/lib/go-1.8/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build344172046=/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"

Feature request

It could be useful to set the DNS servers used for queries, both globally or in net.Lookup*

draft implementation: https://go-review.googlesource.com/37434

[bradfitz]

If we do this, why just the DNS servers? What about options? Perhaps an API to set the string contents of a /etc/resolv.conf file, which we already have a parser for?

/cc @mdempsky

[nightlyone]

This will lead to hard to discover bugs in DNS configuration which can not be observed from the outside of a binary. I expect this to lead to Go binaries getting a bad reputation among people operating systems with such binaries.

Adding environment variables to achieve these effects can be observed and discovered from the outside and can implement the same API via self exec with those environment variables set.

[mdempsky]

Perhaps an API to set the string contents of a /etc/resolv.conf file, which we already have a parser for?

I think this would be unfortunate, since /etc/resolv.conf is semi OS-specific. Also, some OSes use multiple files for config (e.g., gai.conf and nsswitch.conf on Linux).

I'd rather expose dnsConfig itself somehow, though I also worry that would limit our flexibility in supporting new OS-specific resolver behavior in the future.

[teknoraver]

Or just expose some basic info like the DNS addresses, and allow to replace them.
Actually there is no way in Go to do a DNS query using a custom DNS.
Even some argument in net.Lookup* or functions which allows this could be usefult too

[rsc]

Does the C library allow this? If so, how?

[bradfitz]

Adding a field to net.Resolver seems like the right place if we add something. The top-level Lookup funcs use DefaultResolver.

[mdempsky]

Does the C library allow this?

I don't see any functionality like this in glibc. If it exists, it's unlikely to be portable to other OSes.

[rsc]

Actually there is no way in Go to do a DNS query using a custom DNS.

But if there's no way in C either, then I'm not too worried. There already exist third-party DNS clients for Go. https://github.com/miekg/dns in particular is very good.

I think we know how we'd do this. What's missing so far is a strong justification for adding that API complexity. Why is this important to users? In what context does it make sense to override the DNS config in Go but not other languages, or in one program but not others? Why not just always override system-wide?

[ianlancetaylor]

In C, you can change the set of name servers to query by calling res_init and then changing the nscount and nsaddr_list fields of the _res global variable declared in <resolv.h>. This is not seriously documented, but programs do it, and books refer to it; e.g., the O'Reilly DNS and Bind book: http://docstore.mik.ua/orelly/networking_2ndEd/dns/ch15_02.htm .

[teknoraver]

So it could be done with something like:

// #include <resolv.h>
import "C"
...
net.DefaultResolver.PreferGo = false

C.res_init()
C.__res_state().nscount = 1
C.__res_state().nsaddr_list[0].sin_addr.s_addr = 0xDEDE43D0 // resolver1.opendns.com

net.LookupIP("example.com")

[bradfitz]

PreferGo = false doesn't do what you think. False is the default (zero) value. That boolean only prefers Go. It doesn't prefer C if false.

[teknoraver]

ok, so it could be done with that snippet and GODEBUG=netdns=cgo