go-dns-playgroound

Play with Go DNS names resolution and take some notes about it.

Go Name Resolution

Go resolves domain names using various methods that depends on the operating system.

Name resolution can be indirect (e.g. Dial function) or direct (e.g. LookupHost and LookupAddr functions).

Unix

The resolver has two options for resolving names:

Use a pure Go resolver that sends DNS requests directly to the server listed in the /etc/resolv.conf file
Use a cgo-based resolver that calls C library routines such as getaddrinfo and getnameinfo

By default the pure Go resolver is used, because a blocked DNS request consumes only a goroutine, while a blocked C call consumes an operating system thread.

When a cgo is available, the cgo-based resolver is used instead under a variety of conditions:

On systems that do not let programs to perform direct DNS requests (macOS)
When the LOCALDOMAIN environment variable is present (even empty)
When the RES_OPTIONS or HOSTALIASES environment variable is non-empty
When the ASR_CONFIG environment variable is non-empty (Open-BSD only)
When /etc/resolv.conf or /etc/nsswitch.conf specify the use of features that the Go resolver does not implement
When the name being looked up ends in .local or is a mDNS name

The resolver decision can be overridden by setting the netdns value of the GODEBUG environment variable

export GODEBUG=netdns=go    # force pure Go resolver
export GODEBUG=netdns=cgo   # force cgo resolver

The decision can also be forced while building the Go source tree by setting the netgo or netcgo build tag.

A numeric netdns setting, as in GODEBUG=netdns=1, causes the resolver to print debugging information about its decisions.

Windows

On Windows, the resolver always uses C library functions, such as GetAddrInfo and DnsQuery.

Run

macOS

As described above, macOS doesn't let programs to perform direct DNS requests, so if you run the main function in the main.go file with the debug enabled

GODEBUG=netdns=1 go run dns.go

you will obtain the following output

❯ GODEBUG=netdns=1 go run dns.go
go package net: using cgo DNS resolver
2020/04/25 17:02:44 dial took 12.584126ms
2020/04/25 17:02:44 lookupHost took 554.512µs
2020/04/25 17:02:44 lookupAddr took 245.715µs

where the net package tells you that the cgo DNS resolver has been selected to resolve the domain name, as expected. If you try to force the cgo resolver the program doesn't fail, but it takes a lot of time to perform the rresolution of the domain names (why?)

❯ GODEBUG=netdns=go go run dns.go
2020/04/25 17:12:30 dial took 54.249965ms
2020/04/25 17:12:30 lookupHost took 41.536915ms
2020/04/25 17:12:30 lookupAddr took 21.286505ms

Linux

On Linux you can use both methods by setting the GODEBUG=netdns environment variable.

Go

$ GODEBUG=netdns=go go run main.go
2020/04/25 16:58:24 dial took 2.553256ms
2020/04/25 16:58:24 lookupHost took 1.652838ms
2020/04/25 16:58:24 lookupAddr took 697.97µs

cgo

$ GODEBUG=netdns=go go run main.go
2020/04/25 16:58:30 dial took 3.19819ms
2020/04/25 16:58:30 lookupHost took 1.261135ms
2020/04/25 16:58:30 lookupAddr took 792.498µs

If you run your program without forcing the DNS resolver, the DNS resolver will be dynamically accorrding to the conditions mentioned aboove

go package net: dynamic selection of DNS resolver
2020/04/25 17:15:48 dial took 4.355174ms
2020/04/25 17:15:48 lookupHost took 2.970379ms
2020/04/25 17:15:48 lookupAddr took 662.399µs

trampfox / go-dns-playground