Source file src/net/lookup.go

  // Copyright 2012 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.
  
  package net
  
  import (
  	"context"
  	"internal/nettrace"
  	"internal/singleflight"
  )
  
  // protocols contains minimal mappings between internet protocol
  // names and numbers for platforms that don't have a complete list of
  // protocol numbers.
  //
  // See http://www.iana.org/assignments/protocol-numbers
  //
  // On Unix, this map is augmented by readProtocols via lookupProtocol.
  var protocols = map[string]int{
  	"icmp":      1,
  	"igmp":      2,
  	"tcp":       6,
  	"udp":       17,
  	"ipv6-icmp": 58,
  }
  
  // services contains minimal mappings between services names and port
  // numbers for platforms that don't have a complete list of port numbers
  // (some Solaris distros, nacl, etc).
  // On Unix, this map is augmented by readServices via goLookupPort.
  var services = map[string]map[string]int{
  	"udp": {
  		"domain": 53,
  	},
  	"tcp": {
  		"ftp":    21,
  		"ftps":   990,
  		"gopher": 70, // ʕ◔ϖ◔ʔ
  		"http":   80,
  		"https":  443,
  		"imap2":  143,
  		"imap3":  220,
  		"imaps":  993,
  		"pop3":   110,
  		"pop3s":  995,
  		"smtp":   25,
  		"ssh":    22,
  		"telnet": 23,
  	},
  }
  
  const maxProtoLength = len("RSVP-E2E-IGNORE") + 10 // with room to grow
  
  func lookupProtocolMap(name string) (int, error) {
  	var lowerProtocol [maxProtoLength]byte
  	n := copy(lowerProtocol[:], name)
  	lowerASCIIBytes(lowerProtocol[:n])
  	proto, found := protocols[string(lowerProtocol[:n])]
  	if !found || n != len(name) {
  		return 0, &AddrError{Err: "unknown IP protocol specified", Addr: name}
  	}
  	return proto, nil
  }
  
  const maxServiceLength = len("mobility-header") + 10 // with room to grow
  
  func lookupPortMap(network, service string) (port int, error error) {
  	switch network {
  	case "tcp4", "tcp6":
  		network = "tcp"
  	case "udp4", "udp6":
  		network = "udp"
  	}
  
  	if m, ok := services[network]; ok {
  		var lowerService [maxServiceLength]byte
  		n := copy(lowerService[:], service)
  		lowerASCIIBytes(lowerService[:n])
  		if port, ok := m[string(lowerService[:n])]; ok && n == len(service) {
  			return port, nil
  		}
  	}
  	return 0, &AddrError{Err: "unknown port", Addr: network + "/" + service}
  }
  
  // DefaultResolver is the resolver used by the package-level Lookup
  // functions and by Dialers without a specified Resolver.
  var DefaultResolver = &Resolver{}
  
  // A Resolver looks up names and numbers.
  //
  // A nil *Resolver is equivalent to a zero Resolver.
  type Resolver struct {
  	// PreferGo controls whether Go's built-in DNS resolver is preferred
  	// on platforms where it's available. It is equivalent to setting
  	// GODEBUG=netdns=go, but scoped to just this resolver.
  	PreferGo bool
  
  	// TODO(bradfitz): optional interface impl override hook
  	// TODO(bradfitz): Timeout time.Duration?
  }
  
  // LookupHost looks up the given host using the local resolver.
  // It returns a slice of that host's addresses.
  func LookupHost(host string) (addrs []string, err error) {
  	return DefaultResolver.LookupHost(context.Background(), host)
  }
  
  // LookupHost looks up the given host using the local resolver.
  // It returns a slice of that host's addresses.
  func (r *Resolver) LookupHost(ctx context.Context, host string) (addrs []string, err error) {
  	// Make sure that no matter what we do later, host=="" is rejected.
  	// ParseIP, for example, does accept empty strings.
  	if host == "" {
  		return nil, &DNSError{Err: errNoSuchHost.Error(), Name: host}
  	}
  	if ip := ParseIP(host); ip != nil {
  		return []string{host}, nil
  	}
  	return r.lookupHost(ctx, host)
  }
  
  // LookupIP looks up host using the local resolver.
  // It returns a slice of that host's IPv4 and IPv6 addresses.
  func LookupIP(host string) ([]IP, error) {
  	addrs, err := DefaultResolver.LookupIPAddr(context.Background(), host)
  	if err != nil {
  		return nil, err
  	}
  	ips := make([]IP, len(addrs))
  	for i, ia := range addrs {
  		ips[i] = ia.IP
  	}
  	return ips, nil
  }
  
  // LookupIPAddr looks up host using the local resolver.
  // It returns a slice of that host's IPv4 and IPv6 addresses.
  func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, error) {
  	// Make sure that no matter what we do later, host=="" is rejected.
  	// ParseIP, for example, does accept empty strings.
  	if host == "" {
  		return nil, &DNSError{Err: errNoSuchHost.Error(), Name: host}
  	}
  	if ip := ParseIP(host); ip != nil {
  		return []IPAddr{{IP: ip}}, nil
  	}
  	trace, _ := ctx.Value(nettrace.TraceKey{}).(*nettrace.Trace)
  	if trace != nil && trace.DNSStart != nil {
  		trace.DNSStart(host)
  	}
  	// The underlying resolver func is lookupIP by default but it
  	// can be overridden by tests. This is needed by net/http, so it
  	// uses a context key instead of unexported variables.
  	resolverFunc := r.lookupIP
  	if alt, _ := ctx.Value(nettrace.LookupIPAltResolverKey{}).(func(context.Context, string) ([]IPAddr, error)); alt != nil {
  		resolverFunc = alt
  	}
  
  	ch := lookupGroup.DoChan(host, func() (interface{}, error) {
  		return testHookLookupIP(ctx, resolverFunc, host)
  	})
  
  	select {
  	case <-ctx.Done():
  		// The DNS lookup timed out for some reason. Force
  		// future requests to start the DNS lookup again
  		// rather than waiting for the current lookup to
  		// complete. See issue 8602.
  		err := mapErr(ctx.Err())
  		lookupGroup.Forget(host)
  		if trace != nil && trace.DNSDone != nil {
  			trace.DNSDone(nil, false, err)
  		}
  		return nil, err
  	case r := <-ch:
  		if trace != nil && trace.DNSDone != nil {
  			addrs, _ := r.Val.([]IPAddr)
  			trace.DNSDone(ipAddrsEface(addrs), r.Shared, r.Err)
  		}
  		return lookupIPReturn(r.Val, r.Err, r.Shared)
  	}
  }
  
  // lookupGroup merges LookupIPAddr calls together for lookups
  // for the same host. The lookupGroup key is is the LookupIPAddr.host
  // argument.
  // The return values are ([]IPAddr, error).
  var lookupGroup singleflight.Group
  
  // lookupIPReturn turns the return values from singleflight.Do into
  // the return values from LookupIP.
  func lookupIPReturn(addrsi interface{}, err error, shared bool) ([]IPAddr, error) {
  	if err != nil {
  		return nil, err
  	}
  	addrs := addrsi.([]IPAddr)
  	if shared {
  		clone := make([]IPAddr, len(addrs))
  		copy(clone, addrs)
  		addrs = clone
  	}
  	return addrs, nil
  }
  
  // ipAddrsEface returns an empty interface slice of addrs.
  func ipAddrsEface(addrs []IPAddr) []interface{} {
  	s := make([]interface{}, len(addrs))
  	for i, v := range addrs {
  		s[i] = v
  	}
  	return s
  }
  
  // LookupPort looks up the port for the given network and service.
  func LookupPort(network, service string) (port int, err error) {
  	return DefaultResolver.LookupPort(context.Background(), network, service)
  }
  
  // LookupPort looks up the port for the given network and service.
  func (r *Resolver) LookupPort(ctx context.Context, network, service string) (port int, err error) {
  	port, needsLookup := parsePort(service)
  	if needsLookup {
  		port, err = r.lookupPort(ctx, network, service)
  		if err != nil {
  			return 0, err
  		}
  	}
  	if 0 > port || port > 65535 {
  		return 0, &AddrError{Err: "invalid port", Addr: service}
  	}
  	return port, nil
  }
  
  // LookupCNAME returns the canonical name for the given host.
  // Callers that do not care about the canonical name can call
  // LookupHost or LookupIP directly; both take care of resolving
  // the canonical name as part of the lookup.
  //
  // A canonical name is the final name after following zero
  // or more CNAME records.
  // LookupCNAME does not return an error if host does not
  // contain DNS "CNAME" records, as long as host resolves to
  // address records.
  func LookupCNAME(host string) (cname string, err error) {
  	return DefaultResolver.lookupCNAME(context.Background(), host)
  }
  
  // LookupCNAME returns the canonical name for the given host.
  // Callers that do not care about the canonical name can call
  // LookupHost or LookupIP directly; both take care of resolving
  // the canonical name as part of the lookup.
  //
  // A canonical name is the final name after following zero
  // or more CNAME records.
  // LookupCNAME does not return an error if host does not
  // contain DNS "CNAME" records, as long as host resolves to
  // address records.
  func (r *Resolver) LookupCNAME(ctx context.Context, host string) (cname string, err error) {
  	return r.lookupCNAME(ctx, host)
  }
  
  // LookupSRV tries to resolve an SRV query of the given service,
  // protocol, and domain name. The proto is "tcp" or "udp".
  // The returned records are sorted by priority and randomized
  // by weight within a priority.
  //
  // LookupSRV constructs the DNS name to look up following RFC 2782.
  // That is, it looks up _service._proto.name. To accommodate services
  // publishing SRV records under non-standard names, if both service
  // and proto are empty strings, LookupSRV looks up name directly.
  func LookupSRV(service, proto, name string) (cname string, addrs []*SRV, err error) {
  	return DefaultResolver.lookupSRV(context.Background(), service, proto, name)
  }
  
  // LookupSRV tries to resolve an SRV query of the given service,
  // protocol, and domain name. The proto is "tcp" or "udp".
  // The returned records are sorted by priority and randomized
  // by weight within a priority.
  //
  // LookupSRV constructs the DNS name to look up following RFC 2782.
  // That is, it looks up _service._proto.name. To accommodate services
  // publishing SRV records under non-standard names, if both service
  // and proto are empty strings, LookupSRV looks up name directly.
  func (r *Resolver) LookupSRV(ctx context.Context, service, proto, name string) (cname string, addrs []*SRV, err error) {
  	return r.lookupSRV(ctx, service, proto, name)
  }
  
  // LookupMX returns the DNS MX records for the given domain name sorted by preference.
  func LookupMX(name string) ([]*MX, error) {
  	return DefaultResolver.lookupMX(context.Background(), name)
  }
  
  // LookupMX returns the DNS MX records for the given domain name sorted by preference.
  func (r *Resolver) LookupMX(ctx context.Context, name string) ([]*MX, error) {
  	return r.lookupMX(ctx, name)
  }
  
  // LookupNS returns the DNS NS records for the given domain name.
  func LookupNS(name string) ([]*NS, error) {
  	return DefaultResolver.lookupNS(context.Background(), name)
  }
  
  // LookupNS returns the DNS NS records for the given domain name.
  func (r *Resolver) LookupNS(ctx context.Context, name string) ([]*NS, error) {
  	return r.lookupNS(ctx, name)
  }
  
  // LookupTXT returns the DNS TXT records for the given domain name.
  func LookupTXT(name string) ([]string, error) {
  	return DefaultResolver.lookupTXT(context.Background(), name)
  }
  
  // LookupTXT returns the DNS TXT records for the given domain name.
  func (r *Resolver) LookupTXT(ctx context.Context, name string) ([]string, error) {
  	return r.lookupTXT(ctx, name)
  }
  
  // LookupAddr performs a reverse lookup for the given address, returning a list
  // of names mapping to that address.
  //
  // When using the host C library resolver, at most one result will be
  // returned. To bypass the host resolver, use a custom Resolver.
  func LookupAddr(addr string) (names []string, err error) {
  	return DefaultResolver.lookupAddr(context.Background(), addr)
  }
  
  // LookupAddr performs a reverse lookup for the given address, returning a list
  // of names mapping to that address.
  func (r *Resolver) LookupAddr(ctx context.Context, addr string) (names []string, err error) {
  	return r.lookupAddr(ctx, addr)
  }
  

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