golang解析域名的步骤全纪录

hosts: files dns

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

func init() { netGo = true }

	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
	}

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

	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
 }

	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
	}