conf.go

Documentation: net

     1  // Copyright 2015 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // +build darwin dragonfly freebsd linux netbsd openbsd solaris
     6  
     7  package net
     8  
     9  import (
    10  	"os"
    11  	"runtime"
    12  	"sync"
    13  	"syscall"
    14  )
    15  
    16  // conf represents a system's network configuration.
    17  type conf struct {
    18  	// forceCgoLookupHost forces CGO to always be used, if available.
    19  	forceCgoLookupHost bool
    20  
    21  	netGo  bool // go DNS resolution forced
    22  	netCgo bool // cgo DNS resolution forced
    23  
    24  	// machine has an /etc/mdns.allow file
    25  	hasMDNSAllow bool
    26  
    27  	goos          string // the runtime.GOOS, to ease testing
    28  	dnsDebugLevel int
    29  
    30  	nss    *nssConf
    31  	resolv *dnsConfig
    32  }
    33  
    34  var (
    35  	confOnce sync.Once // guards init of confVal via initConfVal
    36  	confVal  = &conf{goos: runtime.GOOS}
    37  )
    38  
    39  // systemConf returns the machine's network configuration.
    40  func systemConf() *conf {
    41  	confOnce.Do(initConfVal)
    42  	return confVal
    43  }
    44  
    45  func initConfVal() {
    46  	dnsMode, debugLevel := goDebugNetDNS()
    47  	confVal.dnsDebugLevel = debugLevel
    48  	confVal.netGo = netGo || dnsMode == "go"
    49  	confVal.netCgo = netCgo || dnsMode == "cgo"
    50  
    51  	if confVal.dnsDebugLevel > 0 {
    52  		defer func() {
    53  			switch {
    54  			case confVal.netGo:
    55  				if netGo {
    56  					println("go package net: built with netgo build tag; using Go's DNS resolver")
    57  				} else {
    58  					println("go package net: GODEBUG setting forcing use of Go's resolver")
    59  				}
    60  			case confVal.forceCgoLookupHost:
    61  				println("go package net: using cgo DNS resolver")
    62  			default:
    63  				println("go package net: dynamic selection of DNS resolver")
    64  			}
    65  		}()
    66  	}
    67  
    68  	// Darwin pops up annoying dialog boxes if programs try to do
    69  	// their own DNS requests. So always use cgo instead, which
    70  	// avoids that.
    71  	if runtime.GOOS == "darwin" {
    72  		confVal.forceCgoLookupHost = true
    73  		return
    74  	}
    75  
    76  	// If any environment-specified resolver options are specified,
    77  	// force cgo. Note that LOCALDOMAIN can change behavior merely
    78  	// by being specified with the empty string.
    79  	_, localDomainDefined := syscall.Getenv("LOCALDOMAIN")
    80  	if os.Getenv("RES_OPTIONS") != "" ||
    81  		os.Getenv("HOSTALIASES") != "" ||
    82  		confVal.netCgo ||
    83  		localDomainDefined {
    84  		confVal.forceCgoLookupHost = true
    85  		return
    86  	}
    87  
    88  	// OpenBSD apparently lets you override the location of resolv.conf
    89  	// with ASR_CONFIG. If we notice that, defer to libc.
    90  	if runtime.GOOS == "openbsd" && os.Getenv("ASR_CONFIG") != "" {
    91  		confVal.forceCgoLookupHost = true
    92  		return
    93  	}
    94  
    95  	if runtime.GOOS != "openbsd" {
    96  		confVal.nss = parseNSSConfFile("/etc/nsswitch.conf")
    97  	}
    98  
    99  	confVal.resolv = dnsReadConfig("/etc/resolv.conf")
   100  	if confVal.resolv.err != nil && !os.IsNotExist(confVal.resolv.err) &&
   101  		!os.IsPermission(confVal.resolv.err) {
   102  		// If we can't read the resolv.conf file, assume it
   103  		// had something important in it and defer to cgo.
   104  		// libc's resolver might then fail too, but at least
   105  		// it wasn't our fault.
   106  		confVal.forceCgoLookupHost = true
   107  	}
   108  
   109  	if _, err := os.Stat("/etc/mdns.allow"); err == nil {
   110  		confVal.hasMDNSAllow = true
   111  	}
   112  }
   113  
   114  // canUseCgo reports whether calling cgo functions is allowed
   115  // for non-hostname lookups.
   116  func (c *conf) canUseCgo() bool {
   117  	return c.hostLookupOrder("") == hostLookupCgo
   118  }
   119  
   120  // hostLookupOrder determines which strategy to use to resolve hostname.
   121  func (c *conf) hostLookupOrder(hostname string) (ret hostLookupOrder) {
   122  	if c.dnsDebugLevel > 1 {
   123  		defer func() {
   124  			print("go package net: hostLookupOrder(", hostname, ") = ", ret.String(), "\n")
   125  		}()
   126  	}
   127  	fallbackOrder := hostLookupCgo
   128  	if c.netGo {
   129  		fallbackOrder = hostLookupFilesDNS
   130  	}
   131  	if c.forceCgoLookupHost || c.resolv.unknownOpt || c.goos == "android" {
   132  		return fallbackOrder
   133  	}
   134  	if byteIndex(hostname, '\\') != -1 || byteIndex(hostname, '%') != -1 {
   135  		// Don't deal with special form hostnames with backslashes
   136  		// or '%'.
   137  		return fallbackOrder
   138  	}
   139  
   140  	// OpenBSD is unique and doesn't use nsswitch.conf.
   141  	// It also doesn't support mDNS.
   142  	if c.goos == "openbsd" {
   143  		// OpenBSD's resolv.conf manpage says that a non-existent
   144  		// resolv.conf means "lookup" defaults to only "files",
   145  		// without DNS lookups.
   146  		if os.IsNotExist(c.resolv.err) {
   147  			return hostLookupFiles
   148  		}
   149  		lookup := c.resolv.lookup
   150  		if len(lookup) == 0 {
   151  			// http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5
   152  			// "If the lookup keyword is not used in the
   153  			// system's resolv.conf file then the assumed
   154  			// order is 'bind file'"
   155  			return hostLookupDNSFiles
   156  		}
   157  		if len(lookup) < 1 || len(lookup) > 2 {
   158  			return fallbackOrder
   159  		}
   160  		switch lookup[0] {
   161  		case "bind":
   162  			if len(lookup) == 2 {
   163  				if lookup[1] == "file" {
   164  					return hostLookupDNSFiles
   165  				}
   166  				return fallbackOrder
   167  			}
   168  			return hostLookupDNS
   169  		case "file":
   170  			if len(lookup) == 2 {
   171  				if lookup[1] == "bind" {
   172  					return hostLookupFilesDNS
   173  				}
   174  				return fallbackOrder
   175  			}
   176  			return hostLookupFiles
   177  		default:
   178  			return fallbackOrder
   179  		}
   180  	}
   181  
   182  	// Canonicalize the hostname by removing any trailing dot.
   183  	if stringsHasSuffix(hostname, ".") {
   184  		hostname = hostname[:len(hostname)-1]
   185  	}
   186  	if stringsHasSuffixFold(hostname, ".local") {
   187  		// Per RFC 6762, the ".local" TLD is special. And
   188  		// because Go's native resolver doesn't do mDNS or
   189  		// similar local resolution mechanisms, assume that
   190  		// libc might (via Avahi, etc) and use cgo.
   191  		return fallbackOrder
   192  	}
   193  
   194  	nss := c.nss
   195  	srcs := nss.sources["hosts"]
   196  	// If /etc/nsswitch.conf doesn't exist or doesn't specify any
   197  	// sources for "hosts", assume Go's DNS will work fine.
   198  	if os.IsNotExist(nss.err) || (nss.err == nil && len(srcs) == 0) {
   199  		if c.goos == "solaris" {
   200  			// illumos defaults to "nis [NOTFOUND=return] files"
   201  			return fallbackOrder
   202  		}
   203  		if c.goos == "linux" {
   204  			// glibc says the default is "dns [!UNAVAIL=return] files"
   205  			// http://www.gnu.org/software/libc/manual/html_node/Notes-on-NSS-Configuration-File.html.
   206  			return hostLookupDNSFiles
   207  		}
   208  		return hostLookupFilesDNS
   209  	}
   210  	if nss.err != nil {
   211  		// We failed to parse or open nsswitch.conf, so
   212  		// conservatively assume we should use cgo if it's
   213  		// available.
   214  		return fallbackOrder
   215  	}
   216  
   217  	var mdnsSource, filesSource, dnsSource bool
   218  	var first string
   219  	for _, src := range srcs {
   220  		if src.source == "myhostname" {
   221  			if isLocalhost(hostname) || isGateway(hostname) {
   222  				return fallbackOrder
   223  			}
   224  			hn, err := getHostname()
   225  			if err != nil || stringsEqualFold(hostname, hn) {
   226  				return fallbackOrder
   227  			}
   228  			continue
   229  		}
   230  		if src.source == "files" || src.source == "dns" {
   231  			if !src.standardCriteria() {
   232  				return fallbackOrder // non-standard; let libc deal with it.
   233  			}
   234  			if src.source == "files" {
   235  				filesSource = true
   236  			} else if src.source == "dns" {
   237  				dnsSource = true
   238  			}
   239  			if first == "" {
   240  				first = src.source
   241  			}
   242  			continue
   243  		}
   244  		if stringsHasPrefix(src.source, "mdns") {
   245  			// e.g. "mdns4", "mdns4_minimal"
   246  			// We already returned true before if it was *.local.
   247  			// libc wouldn't have found a hit on this anyway.
   248  			mdnsSource = true
   249  			continue
   250  		}
   251  		// Some source we don't know how to deal with.
   252  		return fallbackOrder
   253  	}
   254  
   255  	// We don't parse mdns.allow files. They're rare. If one
   256  	// exists, it might list other TLDs (besides .local) or even
   257  	// '*', so just let libc deal with it.
   258  	if mdnsSource && c.hasMDNSAllow {
   259  		return fallbackOrder
   260  	}
   261  
   262  	// Cases where Go can handle it without cgo and C thread
   263  	// overhead.
   264  	switch {
   265  	case filesSource && dnsSource:
   266  		if first == "files" {
   267  			return hostLookupFilesDNS
   268  		} else {
   269  			return hostLookupDNSFiles
   270  		}
   271  	case filesSource:
   272  		return hostLookupFiles
   273  	case dnsSource:
   274  		return hostLookupDNS
   275  	}
   276  
   277  	// Something weird. Let libc deal with it.
   278  	return fallbackOrder
   279  }
   280  
   281  // goDebugNetDNS parses the value of the GODEBUG "netdns" value.
   282  // The netdns value can be of the form:
   283  //    1       // debug level 1
   284  //    2       // debug level 2
   285  //    cgo     // use cgo for DNS lookups
   286  //    go      // use go for DNS lookups
   287  //    cgo+1   // use cgo for DNS lookups + debug level 1
   288  //    1+cgo   // same
   289  //    cgo+2   // same, but debug level 2
   290  // etc.
   291  func goDebugNetDNS() (dnsMode string, debugLevel int) {
   292  	goDebug := goDebugString("netdns")
   293  	parsePart := func(s string) {
   294  		if s == "" {
   295  			return
   296  		}
   297  		if '0' <= s[0] && s[0] <= '9' {
   298  			debugLevel, _, _ = dtoi(s)
   299  		} else {
   300  			dnsMode = s
   301  		}
   302  	}
   303  	if i := byteIndex(goDebug, '+'); i != -1 {
   304  		parsePart(goDebug[:i])
   305  		parsePart(goDebug[i+1:])
   306  		return
   307  	}
   308  	parsePart(goDebug)
   309  	return
   310  }
   311  
   312  // isLocalhost reports whether h should be considered a "localhost"
   313  // name for the myhostname NSS module.
   314  func isLocalhost(h string) bool {
   315  	return stringsEqualFold(h, "localhost") || stringsEqualFold(h, "localhost.localdomain") || stringsHasSuffixFold(h, ".localhost") || stringsHasSuffixFold(h, ".localhost.localdomain")
   316  }
   317  
   318  // isGateway reports whether h should be considered a "gateway"
   319  // name for the myhostname NSS module.
   320  func isGateway(h string) bool {
   321  	return stringsEqualFold(h, "gateway")
   322  }
   323
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