lookup_unix.go

Documentation: net

     1  // Copyright 2011 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  	"context"
    11  	"sync"
    12  )
    13  
    14  var onceReadProtocols sync.Once
    15  
    16  // readProtocols loads contents of /etc/protocols into protocols map
    17  // for quick access.
    18  func readProtocols() {
    19  	file, err := open("/etc/protocols")
    20  	if err != nil {
    21  		return
    22  	}
    23  	defer file.close()
    24  
    25  	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
    26  		// tcp    6   TCP    # transmission control protocol
    27  		if i := byteIndex(line, '#'); i >= 0 {
    28  			line = line[0:i]
    29  		}
    30  		f := getFields(line)
    31  		if len(f) < 2 {
    32  			continue
    33  		}
    34  		if proto, _, ok := dtoi(f[1]); ok {
    35  			if _, ok := protocols[f[0]]; !ok {
    36  				protocols[f[0]] = proto
    37  			}
    38  			for _, alias := range f[2:] {
    39  				if _, ok := protocols[alias]; !ok {
    40  					protocols[alias] = proto
    41  				}
    42  			}
    43  		}
    44  	}
    45  }
    46  
    47  // lookupProtocol looks up IP protocol name in /etc/protocols and
    48  // returns correspondent protocol number.
    49  func lookupProtocol(_ context.Context, name string) (int, error) {
    50  	onceReadProtocols.Do(readProtocols)
    51  	return lookupProtocolMap(name)
    52  }
    53  
    54  func (r *Resolver) dial(ctx context.Context, network, server string) (dnsConn, error) {
    55  	// Calling Dial here is scary -- we have to be sure not to
    56  	// dial a name that will require a DNS lookup, or Dial will
    57  	// call back here to translate it. The DNS config parser has
    58  	// already checked that all the cfg.servers are IP
    59  	// addresses, which Dial will use without a DNS lookup.
    60  	var c Conn
    61  	var err error
    62  	if r.Dial != nil {
    63  		c, err = r.Dial(ctx, network, server)
    64  	} else {
    65  		var d Dialer
    66  		c, err = d.DialContext(ctx, network, server)
    67  	}
    68  	if err != nil {
    69  		return nil, mapErr(err)
    70  	}
    71  	if _, ok := c.(PacketConn); ok {
    72  		return &dnsPacketConn{c}, nil
    73  	}
    74  	return &dnsStreamConn{c}, nil
    75  }
    76  
    77  func (r *Resolver) lookupHost(ctx context.Context, host string) (addrs []string, err error) {
    78  	order := systemConf().hostLookupOrder(host)
    79  	if !r.PreferGo && order == hostLookupCgo {
    80  		if addrs, err, ok := cgoLookupHost(ctx, host); ok {
    81  			return addrs, err
    82  		}
    83  		// cgo not available (or netgo); fall back to Go's DNS resolver
    84  		order = hostLookupFilesDNS
    85  	}
    86  	return r.goLookupHostOrder(ctx, host, order)
    87  }
    88  
    89  func (r *Resolver) lookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
    90  	if r.PreferGo {
    91  		return r.goLookupIP(ctx, host)
    92  	}
    93  	order := systemConf().hostLookupOrder(host)
    94  	if order == hostLookupCgo {
    95  		if addrs, err, ok := cgoLookupIP(ctx, host); ok {
    96  			return addrs, err
    97  		}
    98  		// cgo not available (or netgo); fall back to Go's DNS resolver
    99  		order = hostLookupFilesDNS
   100  	}
   101  	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, host, order)
   102  	return
   103  }
   104  
   105  func (r *Resolver) lookupPort(ctx context.Context, network, service string) (int, error) {
   106  	if !r.PreferGo && systemConf().canUseCgo() {
   107  		if port, err, ok := cgoLookupPort(ctx, network, service); ok {
   108  			if err != nil {
   109  				// Issue 18213: if cgo fails, first check to see whether we
   110  				// have the answer baked-in to the net package.
   111  				if port, err := goLookupPort(network, service); err == nil {
   112  					return port, nil
   113  				}
   114  			}
   115  			return port, err
   116  		}
   117  	}
   118  	return goLookupPort(network, service)
   119  }
   120  
   121  func (r *Resolver) lookupCNAME(ctx context.Context, name string) (string, error) {
   122  	if !r.PreferGo && systemConf().canUseCgo() {
   123  		if cname, err, ok := cgoLookupCNAME(ctx, name); ok {
   124  			return cname, err
   125  		}
   126  	}
   127  	return r.goLookupCNAME(ctx, name)
   128  }
   129  
   130  func (r *Resolver) lookupSRV(ctx context.Context, service, proto, name string) (string, []*SRV, error) {
   131  	var target string
   132  	if service == "" && proto == "" {
   133  		target = name
   134  	} else {
   135  		target = "_" + service + "._" + proto + "." + name
   136  	}
   137  	cname, rrs, err := r.lookup(ctx, target, dnsTypeSRV)
   138  	if err != nil {
   139  		return "", nil, err
   140  	}
   141  	srvs := make([]*SRV, len(rrs))
   142  	for i, rr := range rrs {
   143  		rr := rr.(*dnsRR_SRV)
   144  		srvs[i] = &SRV{Target: rr.Target, Port: rr.Port, Priority: rr.Priority, Weight: rr.Weight}
   145  	}
   146  	byPriorityWeight(srvs).sort()
   147  	return cname, srvs, nil
   148  }
   149  
   150  func (r *Resolver) lookupMX(ctx context.Context, name string) ([]*MX, error) {
   151  	_, rrs, err := r.lookup(ctx, name, dnsTypeMX)
   152  	if err != nil {
   153  		return nil, err
   154  	}
   155  	mxs := make([]*MX, len(rrs))
   156  	for i, rr := range rrs {
   157  		rr := rr.(*dnsRR_MX)
   158  		mxs[i] = &MX{Host: rr.Mx, Pref: rr.Pref}
   159  	}
   160  	byPref(mxs).sort()
   161  	return mxs, nil
   162  }
   163  
   164  func (r *Resolver) lookupNS(ctx context.Context, name string) ([]*NS, error) {
   165  	_, rrs, err := r.lookup(ctx, name, dnsTypeNS)
   166  	if err != nil {
   167  		return nil, err
   168  	}
   169  	nss := make([]*NS, len(rrs))
   170  	for i, rr := range rrs {
   171  		nss[i] = &NS{Host: rr.(*dnsRR_NS).Ns}
   172  	}
   173  	return nss, nil
   174  }
   175  
   176  func (r *Resolver) lookupTXT(ctx context.Context, name string) ([]string, error) {
   177  	_, rrs, err := r.lookup(ctx, name, dnsTypeTXT)
   178  	if err != nil {
   179  		return nil, err
   180  	}
   181  	txts := make([]string, len(rrs))
   182  	for i, rr := range rrs {
   183  		txts[i] = rr.(*dnsRR_TXT).Txt
   184  	}
   185  	return txts, nil
   186  }
   187  
   188  func (r *Resolver) lookupAddr(ctx context.Context, addr string) ([]string, error) {
   189  	if !r.PreferGo && systemConf().canUseCgo() {
   190  		if ptrs, err, ok := cgoLookupPTR(ctx, addr); ok {
   191  			return ptrs, err
   192  		}
   193  	}
   194  	return r.goLookupPTR(ctx, addr)
   195  }
   196
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