golang如何解析域名

最近遇到了一个问题。

/etc/hosts/etc/hosts

/etc/nsswitch.conf/etc/hosts

/etc/nsswitch.conf

hosts:          files dns

即，files优先dns。

顺带完整的理一下linux系统里golang的域名解析。

golang有两种域名解析方法：内置Go解析器；基于cgo的系统解析器。通过环境变量GODEBUG来配置。

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

默认采用的是内置Go解析器，因为当DNS解析阻塞时，内置Go解析器只是阻塞了一个goroutine，而cgo的解析器则是阻塞了一个操作系统级别的线程。

func init() { netGo = true }

resolv.conf

	confVal.resolv = dnsReadConfig("/etc/resolv.conf")
	if confVal.resolv.err != nil && !os.IsNotExist(confVal.resolv.err) &&
		!os.IsPermission(confVal.resolv.err) {
		// If we can't read the resolv.conf file, assume it
		// had something important in it and defer to cgo.
		// libc's resolver might then fail too, but at least
		// it wasn't our fault.
		confVal.forceCgoLookupHost = true
	}

当使用内置Go解析器时，根据解析优先级的不同，还会细分为下面四种。

const (
	// hostLookupCgo means defer to cgo.
	hostLookupCgo      hostLookupOrder = iota
	hostLookupFilesDNS                 // files first
	hostLookupDNSFiles                 // dns first
	hostLookupFiles                    // only files
	hostLookupDNS                      // only DNS
)

/etc/nsswitch.confhostshostLookupDNSFiles

	nss := c.nss
	srcs := nss.sources["hosts"]
	// If /etc/nsswitch.conf doesn't exist or doesn't specify any
	// sources for "hosts", assume Go's DNS will work fine.
	if os.IsNotExist(nss.err) || (nss.err == nil && len(srcs) == 0) {
		if c.goos == "linux" {
			// glibc says the default is "dns [!UNAVAIL=return] files"
			// http://www.gnu.org/software/libc/manual/html_node/Notes-on-NSS-Configuration-File.html.
			return hostLookupDNSFiles
		}
		return hostLookupFilesDNS
    }

nsswitch.conf

	var mdnsSource, filesSource, dnsSource bool
	var first string
	for _, src := range srcs {
		if src.source == "files" || src.source == "dns" {
			if !src.standardCriteria() {
				return fallbackOrder // non-standard; let libc deal with it.
			}
			if src.source == "files" {
				filesSource = true
			} else if src.source == "dns" {
				dnsSource = true
			}
			if first == "" {
				first = src.source
			}
			continue
		}
		// Some source we don't know how to deal with.
		return fallbackOrder
	}

	// Cases where Go can handle it without cgo and C thread
	// overhead.
	switch {
	case filesSource && dnsSource:
		if first == "files" {
			return hostLookupFilesDNS
		} else {
			return hostLookupDNSFiles
		}
	case filesSource:
		return hostLookupFiles
	case dnsSource:
		return hostLookupDNS
	}

hosts:  files dnshostLookupFilesDNS/etc/hosts

详细的解析顺序请参见 hostLookupOrder。