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// 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.
// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
package net
import (
"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 // cgo DNS resolution forced
// machine has an /etc/mdns.allow file
hasMDNSAllow bool
goos string // the runtime.GOOS, to ease testing
dnsDebugLevel int
nss *nssConf
resolv *dnsConfig
}
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.dnsDebugLevel > 0 {
defer func() {
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" {
confVal.forceCgoLookupHost = true
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 runtime.GOOS != "openbsd" {
confVal.nss = parseNSSConfFile("/etc/nsswitch.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
}
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 {
return c.hostLookupOrder(nil, "") == hostLookupCgo
}
// hostLookupOrder determines which strategy to use to resolve hostname.
// The provided Resolver is optional. nil means to not consider its options.
func (c *conf) hostLookupOrder(r *Resolver, hostname string) (ret hostLookupOrder) {
if c.dnsDebugLevel > 1 {
defer func() {
print("go package net: hostLookupOrder(", hostname, ") = ", ret.String(), "\n")
}()
}
fallbackOrder := hostLookupCgo
if c.netGo || r.preferGo() {
fallbackOrder = hostLookupFilesDNS
}
if c.forceCgoLookupHost || c.resolv.unknownOpt || c.goos == "android" {
return fallbackOrder
}
if byteIndex(hostname, '\\') != -1 || byteIndex(hostname, '%') != -1 {
// Don't deal with special form hostnames with backslashes
// or '%'.
return fallbackOrder
}
// 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(c.resolv.err) {
return hostLookupFiles
}
lookup := c.resolv.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
}
if len(lookup) < 1 || len(lookup) > 2 {
return fallbackOrder
}
switch lookup[0] {
case "bind":
if len(lookup) == 2 {
if lookup[1] == "file" {
return hostLookupDNSFiles
}
return fallbackOrder
}
return hostLookupDNS
case "file":
if len(lookup) == 2 {
if lookup[1] == "bind" {
return hostLookupFilesDNS
}
return fallbackOrder
}
return hostLookupFiles
default:
return fallbackOrder
}
}
// 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
}
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 == "solaris" {
// illumos defaults to "nis [NOTFOUND=return] files"
return fallbackOrder
}
if c.goos == "linux" {
// glibc says the default is "dns [!UNAVAIL=return] files"
// https://www.gnu.org/software/libc/manual/html_node/Notes-on-NSS-Configuration-File.html.
return hostLookupDNSFiles
}
return hostLookupFilesDNS
}
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
}
var mdnsSource, filesSource, dnsSource bool
var first string
for _, src := range srcs {
if src.source == "myhostname" {
if isLocalhost(hostname) || isGateway(hostname) {
return fallbackOrder
}
hn, err := getHostname()
if err != nil || stringsEqualFold(hostname, hn) {
return fallbackOrder
}
continue
}
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
}
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
}
// 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
}
// 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
}
// Something weird. Let libc deal with it.
return fallbackOrder
}
// 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 := goDebugString("netdns")
parsePart := func(s string) {
if s == "" {
return
}
if '0' <= s[0] && s[0] <= '9' {
debugLevel, _, _ = dtoi(s)
} else {
dnsMode = s
}
}
if i := byteIndex(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")
}
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