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net: document GODEBUG=netdns=xxx settings

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Fixes #12191.

Change-Id: I5c7659ccb0566dad3613041d9e76be87ceacae61
Reviewed-on: https://go-review.googlesource.com/13700
Reviewed-by: Rob Pike <r@golang.org>
This commit is contained in:
Russ Cox 2015-08-18 23:19:41 -04:00
parent 54eb9efb70
commit b711f5ad8f
1 changed files with 40 additions and 0 deletions
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src/net/net.go
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@ -35,6 +35,46 @@ The Listen function creates servers:
}
go handleConnection(conn)
}
Name Resolution
The method for resolving domain names, whether indirectly with functions like Dial
or directly with functions like LookupHost and LookupAddr, varies by operating system.
On Unix systems, the resolver has two options for resolving names.
It can use a pure Go resolver that sends DNS requests directly to the servers
listed in /etc/resolv.conf, or it can 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 cgo is available, the cgo-based resolver is used instead under a variety of
conditions: on systems that do not let programs make direct DNS requests (OS X),
when the LOCALDOMAIN environment variable is present (even if empty),
when the RES_OPTIONS or HOSTALIASES environment variable is non-empty,
when the ASR_CONFIG environment variable is non-empty (OpenBSD only),
when /etc/resolv.conf or /etc/nsswitch.conf specify the use of features that the
Go resolver does not implement, and when the name being looked up ends in .local
or is an mDNS name.
The resolver decision can be overridden by setting the netdns value of the
GODEBUG environment variable (see package runtime) to go or cgo, as in:
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.
To force a particular resolver while also printing debugging information,
join the two settings by a plus sign, as in GODEBUG=netdns=go+1.
On Plan 9, the resolver always accesses /net/cs and /net/dns.
On Windows, the resolver always uses C library functions, such as GetAddrInfo and DnsQuery.
*/
package net