Some things that force Go to call the C library for name resolution on Linux
Traditionally on Unix systems there is no official standard for how to do various sorts of network name lookups, or rather the official standard is 'call the functions in the system C library'. There is generally no actual specification for how name lookup works at the level that would permit you to create an independent implementation (although there is generally documentation on how to configure it). This presents a problem for people who are creating non-C languages; they must either arrange to call the C library (through the C library's limited interfaces for this) or write their own versions that may not resolve things exactly like the C library does, so that you get inconsistent behavior between C programs and programs written in the other language.
Go kind of takes both approaches. As covered in the net package's documentation, Go can both call the C library routines (using cgo) and do its own name lookups in pure Go. Go normally tries to use the pure Go approach as much as possible because it's considered better (partly because cgo calls can be relatively expensive). In theory the pure Go approach should give you the same results as the cgo approach; in practice, the two can behave somewhat differently in some situations, sometimes because of oversights.
(Although the net package's documentation talks only about DNS
related lookups, this also affects how at least
Go normally attempts to be pretty hyper-intelligent about whether or
not it can use its pure Go lookup functions. It makes this decision
in part by reading through your
to see if you're using anything that it doesn't think it can handle.
This raises the question of what things in either of these files
can accidentally force Go to use cgo calls to the C library, instead
of its own more efficient (and more consistent across systems) pure
Go version. For
/etc/resolv.conf, Go understands all of the common
things but anything else will force it to cgo, including any mistakes
you may have lurking in there. For
/etc/nsswitch.conf, Go looks at
hosts' line and a few complications can be common on modern
- if your
myhostname, only lookups of names with dots in them can be done in pure Go. Because of an implementation quirk, this currently means that
net.LookupPort()is forced to use the C library.
(Some other things are also forced to use the C library, but arguably they should in this situation because they involve hostnames.)
- if your
mymachines, all lookups go to the C library. This is probably common on modern systemd-based Linux distributions.
If you're using Go programs and you don't use containers or don't
need the magic functionality of
mymachines, you may want to
strip it out of your
nsswitch.conf. If you're like me, you may
even be surprised to find it there in the first place. You may not
myhostname either, especially if your host has IP aliases
that are most definitely not included in what a name to IP lookup
for its hostname should return.
Note that contrary to what you might think,
things that call it to get ports, like
not look at the
services line in
/etc/nsswitch.conf, only the
hosts line. And of course the pure Go port lookup only looks at
/etc/services (and may not parse it exactly like the C library
does). At the moment a missing or unreadable
not to force a fallback to the C library but instead uses a tiny
built in default version.
(This probably doesn't affect anyone. I can't imagine that there
are very many people who use NIS or otherwise do something funny
services lookups and not
You can see what sort of lookup is used for specific queries by
GODEBUG environment variable to a verbosity of 2 or
more, ie '
GODEBUG=netdns=2'. The resulting report may look something
go package net: dynamic selection of DNS resolver go package net: hostLookupOrder() = cgo go package net: hostLookupOrder(smtp.cs) = files,dns
This is on a Linux machine where I set
hosts to include
mymachines. The first
hostLookupOrder() is for looking
up the port number for
smtp; here the presence of
forced it to resort to cgo. A blank argument to
is used by several sorts of lookups, including
(This is of course an implementation detail and may change at some point.)
Comments on this page:Written on 14 December 2015.