1 files changed, 357 insertions, 0 deletions
diff --git a/src/net/conf.go b/src/net/conf.go
new file mode 100644
index 0000000..4119604
--- /dev/null
+++ b/src/net/conf.go
@@ -0,0 +1,357 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !js
+
+package net
+
+import (
+	"internal/bytealg"
+	"internal/godebug"
+	"os"
+	"runtime"
+	"sync"
+	"syscall"
+)
+
+// conf represents a system's network configuration.
+type conf struct {
+	// forceCgoLookupHost forces CGO to always be used, if available.
+	forceCgoLookupHost bool
+
+	netGo  bool // go DNS resolution forced
+	netCgo bool // non-go DNS resolution forced (cgo, or win32)
+
+	// machine has an /etc/mdns.allow file
+	hasMDNSAllow bool
+
+	goos          string // the runtime.GOOS, to ease testing
+	dnsDebugLevel int
+}
+
+var (
+	confOnce sync.Once // guards init of confVal via initConfVal
+	confVal  = &conf{goos: runtime.GOOS}
+)
+
+// systemConf returns the machine's network configuration.
+func systemConf() *conf {
+	confOnce.Do(initConfVal)
+	return confVal
+}
+
+func initConfVal() {
+	dnsMode, debugLevel := goDebugNetDNS()
+	confVal.dnsDebugLevel = debugLevel
+	confVal.netGo = netGo || dnsMode == "go"
+	confVal.netCgo = netCgo || dnsMode == "cgo"
+	if !confVal.netGo && !confVal.netCgo && (runtime.GOOS == "windows" || runtime.GOOS == "plan9") {
+		// Neither of these platforms actually use cgo.
+		//
+		// The meaning of "cgo" mode in the net package is
+		// really "the native OS way", which for libc meant
+		// cgo on the original platforms that motivated
+		// PreferGo support before Windows and Plan9 got support,
+		// at which time the GODEBUG=netdns=go and GODEBUG=netdns=cgo
+		// names were already kinda locked in.
+		confVal.netCgo = true
+	}
+
+	if confVal.dnsDebugLevel > 0 {
+		defer func() {
+			if confVal.dnsDebugLevel > 1 {
+				println("go package net: confVal.netCgo =", confVal.netCgo, " netGo =", confVal.netGo)
+			}
+			switch {
+			case confVal.netGo:
+				if netGo {
+					println("go package net: built with netgo build tag; using Go's DNS resolver")
+				} else {
+					println("go package net: GODEBUG setting forcing use of Go's resolver")
+				}
+			case confVal.forceCgoLookupHost:
+				println("go package net: using cgo DNS resolver")
+			default:
+				println("go package net: dynamic selection of DNS resolver")
+			}
+		}()
+	}
+
+	// Darwin pops up annoying dialog boxes if programs try to do
+	// their own DNS requests. So always use cgo instead, which
+	// avoids that.
+	if runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
+		confVal.forceCgoLookupHost = true
+		return
+	}
+
+	if runtime.GOOS == "windows" || runtime.GOOS == "plan9" {
+		return
+	}
+
+	// If any environment-specified resolver options are specified,
+	// force cgo. Note that LOCALDOMAIN can change behavior merely
+	// by being specified with the empty string.
+	_, localDomainDefined := syscall.Getenv("LOCALDOMAIN")
+	if os.Getenv("RES_OPTIONS") != "" ||
+		os.Getenv("HOSTALIASES") != "" ||
+		confVal.netCgo ||
+		localDomainDefined {
+		confVal.forceCgoLookupHost = true
+		return
+	}
+
+	// OpenBSD apparently lets you override the location of resolv.conf
+	// with ASR_CONFIG. If we notice that, defer to libc.
+	if runtime.GOOS == "openbsd" && os.Getenv("ASR_CONFIG") != "" {
+		confVal.forceCgoLookupHost = true
+		return
+	}
+
+	if _, err := os.Stat("/etc/mdns.allow"); err == nil {
+		confVal.hasMDNSAllow = true
+	}
+}
+
+// canUseCgo reports whether calling cgo functions is allowed
+// for non-hostname lookups.
+func (c *conf) canUseCgo() bool {
+	ret, _ := c.hostLookupOrder(nil, "")
+	return ret == hostLookupCgo
+}
+
+// hostLookupOrder determines which strategy to use to resolve hostname.
+// The provided Resolver is optional. nil means to not consider its options.
+// It also returns dnsConfig when it was used to determine the lookup order.
+func (c *conf) hostLookupOrder(r *Resolver, hostname string) (ret hostLookupOrder, dnsConfig *dnsConfig) {
+	if c.dnsDebugLevel > 1 {
+		defer func() {
+			print("go package net: hostLookupOrder(", hostname, ") = ", ret.String(), "\n")
+		}()
+	}
+	fallbackOrder := hostLookupCgo
+	if c.netGo || r.preferGo() {
+		switch c.goos {
+		case "windows":
+			// TODO(bradfitz): implement files-based
+			// lookup on Windows too? I guess /etc/hosts
+			// kinda exists on Windows. But for now, only
+			// do DNS.
+			fallbackOrder = hostLookupDNS
+		default:
+			fallbackOrder = hostLookupFilesDNS
+		}
+	}
+	if c.forceCgoLookupHost || c.goos == "android" || c.goos == "windows" || c.goos == "plan9" {
+		return fallbackOrder, nil
+	}
+	if bytealg.IndexByteString(hostname, '\\') != -1 || bytealg.IndexByteString(hostname, '%') != -1 {
+		// Don't deal with special form hostnames with backslashes
+		// or '%'.
+		return fallbackOrder, nil
+	}
+
+	conf := getSystemDNSConfig()
+	if conf.err != nil && !os.IsNotExist(conf.err) && !os.IsPermission(conf.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.
+		return fallbackOrder, conf
+	}
+
+	if conf.unknownOpt {
+		return fallbackOrder, conf
+	}
+
+	// OpenBSD is unique and doesn't use nsswitch.conf.
+	// It also doesn't support mDNS.
+	if c.goos == "openbsd" {
+		// OpenBSD's resolv.conf manpage says that a non-existent
+		// resolv.conf means "lookup" defaults to only "files",
+		// without DNS lookups.
+		if os.IsNotExist(conf.err) {
+			return hostLookupFiles, conf
+		}
+
+		lookup := conf.lookup
+		if len(lookup) == 0 {
+			// https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5
+			// "If the lookup keyword is not used in the
+			// system's resolv.conf file then the assumed
+			// order is 'bind file'"
+			return hostLookupDNSFiles, conf
+		}
+		if len(lookup) < 1 || len(lookup) > 2 {
+			return fallbackOrder, conf
+		}
+		switch lookup[0] {
+		case "bind":
+			if len(lookup) == 2 {
+				if lookup[1] == "file" {
+					return hostLookupDNSFiles, conf
+				}
+				return fallbackOrder, conf
+			}
+			return hostLookupDNS, conf
+		case "file":
+			if len(lookup) == 2 {
+				if lookup[1] == "bind" {
+					return hostLookupFilesDNS, conf
+				}
+				return fallbackOrder, conf
+			}
+			return hostLookupFiles, conf
+		default:
+			return fallbackOrder, conf
+		}
+	}
+
+	// Canonicalize the hostname by removing any trailing dot.
+	if stringsHasSuffix(hostname, ".") {
+		hostname = hostname[:len(hostname)-1]
+	}
+	if stringsHasSuffixFold(hostname, ".local") {
+		// Per RFC 6762, the ".local" TLD is special. And
+		// because Go's native resolver doesn't do mDNS or
+		// similar local resolution mechanisms, assume that
+		// libc might (via Avahi, etc) and use cgo.
+		return fallbackOrder, conf
+	}
+
+	nss := getSystemNSS()
+	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 == "solaris" {
+			// illumos defaults to "nis [NOTFOUND=return] files"
+			return fallbackOrder, conf
+		}
+
+		return hostLookupFilesDNS, conf
+	}
+	if nss.err != nil {
+		// We failed to parse or open nsswitch.conf, so
+		// conservatively assume we should use cgo if it's
+		// available.
+		return fallbackOrder, conf
+	}
+
+	var mdnsSource, filesSource, dnsSource bool
+	var first string
+	for _, src := range srcs {
+		if src.source == "myhostname" {
+			if isLocalhost(hostname) || isGateway(hostname) || isOutbound(hostname) {
+				return fallbackOrder, conf
+			}
+			hn, err := getHostname()
+			if err != nil || stringsEqualFold(hostname, hn) {
+				return fallbackOrder, conf
+			}
+			continue
+		}
+		if src.source == "files" || src.source == "dns" {
+			if !src.standardCriteria() {
+				return fallbackOrder, conf // 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
+		}
+		if stringsHasPrefix(src.source, "mdns") {
+			// e.g. "mdns4", "mdns4_minimal"
+			// We already returned true before if it was *.local.
+			// libc wouldn't have found a hit on this anyway.
+			mdnsSource = true
+			continue
+		}
+		// Some source we don't know how to deal with.
+		return fallbackOrder, conf
+	}
+
+	// We don't parse mdns.allow files. They're rare. If one
+	// exists, it might list other TLDs (besides .local) or even
+	// '*', so just let libc deal with it.
+	if mdnsSource && c.hasMDNSAllow {
+		return fallbackOrder, conf
+	}
+
+	// Cases where Go can handle it without cgo and C thread
+	// overhead.
+	switch {
+	case filesSource && dnsSource:
+		if first == "files" {
+			return hostLookupFilesDNS, conf
+		} else {
+			return hostLookupDNSFiles, conf
+		}
+	case filesSource:
+		return hostLookupFiles, conf
+	case dnsSource:
+		return hostLookupDNS, conf
+	}
+
+	// Something weird. Let libc deal with it.
+	return fallbackOrder, conf
+}
+
+var netdns = godebug.New("netdns")
+
+// goDebugNetDNS parses the value of the GODEBUG "netdns" value.
+// The netdns value can be of the form:
+//
+//	1       // debug level 1
+//	2       // debug level 2
+//	cgo     // use cgo for DNS lookups
+//	go      // use go for DNS lookups
+//	cgo+1   // use cgo for DNS lookups + debug level 1
+//	1+cgo   // same
+//	cgo+2   // same, but debug level 2
+//
+// etc.
+func goDebugNetDNS() (dnsMode string, debugLevel int) {
+	goDebug := netdns.Value()
+	parsePart := func(s string) {
+		if s == "" {
+			return
+		}
+		if '0' <= s[0] && s[0] <= '9' {
+			debugLevel, _, _ = dtoi(s)
+		} else {
+			dnsMode = s
+		}
+	}
+	if i := bytealg.IndexByteString(goDebug, '+'); i != -1 {
+		parsePart(goDebug[:i])
+		parsePart(goDebug[i+1:])
+		return
+	}
+	parsePart(goDebug)
+	return
+}
+
+// isLocalhost reports whether h should be considered a "localhost"
+// name for the myhostname NSS module.
+func isLocalhost(h string) bool {
+	return stringsEqualFold(h, "localhost") || stringsEqualFold(h, "localhost.localdomain") || stringsHasSuffixFold(h, ".localhost") || stringsHasSuffixFold(h, ".localhost.localdomain")
+}
+
+// isGateway reports whether h should be considered a "gateway"
+// name for the myhostname NSS module.
+func isGateway(h string) bool {
+	return stringsEqualFold(h, "_gateway")
+}
+
+// isOutbound reports whether h should be considered a "outbound"
+// name for the myhostname NSS module.
+func isOutbound(h string) bool {
+	return stringsEqualFold(h, "_outbound")
+}