Source: lookup_unix.go in package net

+build aix darwin dragonfly freebsd linux netbsd openbsd solaris


package net

import (
	"context"
	"internal/bytealg"
	"sync"
	"syscall"

	"golang.org/x/net/dns/dnsmessage"
)

var onceReadProtocols sync.Once

readProtocols loads contents of /etc/protocols into protocols map for quick access.

func readProtocols() {
	file, err := open("/etc/protocols")
	if err != nil {
		return
	}
	defer file.close()

tcp 6 TCP # transmission control protocol

		if i := bytealg.IndexByteString(line, '#'); i >= 0 {
			line = line[0:i]
		}
		f := getFields(line)
		if len(f) < 2 {
			continue
		}
		if proto, _, ok := dtoi(f[1]); ok {
			if _, ok := protocols[f[0]]; !ok {
				protocols[f[0]] = proto
			}
			for _, alias := range f[2:] {
				if _, ok := protocols[alias]; !ok {
					protocols[alias] = proto
				}
			}
		}
	}
}

lookupProtocol looks up IP protocol name in /etc/protocols and returns correspondent protocol number.

func lookupProtocol(_ context.Context, name string) (int, error) {
	onceReadProtocols.Do(readProtocols)
	return lookupProtocolMap(name)
}

Calling Dial here is scary -- we have to be sure not to dial a name that will require a DNS lookup, or Dial will call back here to translate it. The DNS config parser has already checked that all the cfg.servers are IP addresses, which Dial will use without a DNS lookup.

	var c Conn
	var err error
	if r != nil && r.Dial != nil {
		c, err = r.Dial(ctx, network, server)
	} else {
		var d Dialer
		c, err = d.DialContext(ctx, network, server)
	}
	if err != nil {
		return nil, mapErr(err)
	}
	return c, nil
}

func (r *Resolver) lookupHost(ctx context.Context, host string) (addrs []string, err error) {
	order := systemConf().hostLookupOrder(r, host)
	if !r.preferGo() && order == hostLookupCgo {
		if addrs, err, ok := cgoLookupHost(ctx, host); ok {
			return addrs, err

cgo not available (or netgo); fall back to Go's DNS resolver

		order = hostLookupFilesDNS
	}
	return r.goLookupHostOrder(ctx, host, order)
}

func (r *Resolver) lookupIP(ctx context.Context, network, host string) (addrs []IPAddr, err error) {
	if r.preferGo() {
		return r.goLookupIP(ctx, host)
	}
	order := systemConf().hostLookupOrder(r, host)
	if order == hostLookupCgo {
		if addrs, err, ok := cgoLookupIP(ctx, network, host); ok {
			return addrs, err

cgo not available (or netgo); fall back to Go's DNS resolver

		order = hostLookupFilesDNS
	}
	ips, _, err := r.goLookupIPCNAMEOrder(ctx, host, order)
	return ips, err
}

func (r *Resolver) lookupPort(ctx context.Context, network, service string) (int, error) {
	if !r.preferGo() && systemConf().canUseCgo() {
		if port, err, ok := cgoLookupPort(ctx, network, service); ok {

Issue 18213: if cgo fails, first check to see whether we have the answer baked-in to the net package.

				if port, err := goLookupPort(network, service); err == nil {
					return port, nil
				}
			}
			return port, err
		}
	}
	return goLookupPort(network, service)
}

func (r *Resolver) lookupCNAME(ctx context.Context, name string) (string, error) {
	if !r.preferGo() && systemConf().canUseCgo() {
		if cname, err, ok := cgoLookupCNAME(ctx, name); ok {
			return cname, err
		}
	}
	return r.goLookupCNAME(ctx, name)
}

func (r *Resolver) lookupSRV(ctx context.Context, service, proto, name string) (string, []*SRV, error) {
	var target string
	if service == "" && proto == "" {
		target = name
	} else {
		target = "_" + service + "._" + proto + "." + name
	}
	p, server, err := r.lookup(ctx, target, dnsmessage.TypeSRV)
	if err != nil {
		return "", nil, err
	}
	var srvs []*SRV
	var cname dnsmessage.Name
	for {
		h, err := p.AnswerHeader()
		if err == dnsmessage.ErrSectionDone {
			break
		}
		if err != nil {
			return "", nil, &DNSError{
				Err:    "cannot unmarshal DNS message",
				Name:   name,
				Server: server,
			}
		}
		if h.Type != dnsmessage.TypeSRV {
			if err := p.SkipAnswer(); err != nil {
				return "", nil, &DNSError{
					Err:    "cannot unmarshal DNS message",
					Name:   name,
					Server: server,
				}
			}
			continue
		}
		if cname.Length == 0 && h.Name.Length != 0 {
			cname = h.Name
		}
		srv, err := p.SRVResource()
		if err != nil {
			return "", nil, &DNSError{
				Err:    "cannot unmarshal DNS message",
				Name:   name,
				Server: server,
			}
		}
		srvs = append(srvs, &SRV{Target: srv.Target.String(), Port: srv.Port, Priority: srv.Priority, Weight: srv.Weight})
	}
	byPriorityWeight(srvs).sort()
	return cname.String(), srvs, nil
}

func (r *Resolver) lookupMX(ctx context.Context, name string) ([]*MX, error) {
	p, server, err := r.lookup(ctx, name, dnsmessage.TypeMX)
	if err != nil {
		return nil, err
	}
	var mxs []*MX
	for {
		h, err := p.AnswerHeader()
		if err == dnsmessage.ErrSectionDone {
			break
		}
		if err != nil {
			return nil, &DNSError{
				Err:    "cannot unmarshal DNS message",
				Name:   name,
				Server: server,
			}
		}
		if h.Type != dnsmessage.TypeMX {
			if err := p.SkipAnswer(); err != nil {
				return nil, &DNSError{
					Err:    "cannot unmarshal DNS message",
					Name:   name,
					Server: server,
				}
			}
			continue
		}
		mx, err := p.MXResource()
		if err != nil {
			return nil, &DNSError{
				Err:    "cannot unmarshal DNS message",
				Name:   name,
				Server: server,
			}
		}
		mxs = append(mxs, &MX{Host: mx.MX.String(), Pref: mx.Pref})

	}
	byPref(mxs).sort()
	return mxs, nil
}

func (r *Resolver) lookupNS(ctx context.Context, name string) ([]*NS, error) {
	p, server, err := r.lookup(ctx, name, dnsmessage.TypeNS)
	if err != nil {
		return nil, err
	}
	var nss []*NS
	for {
		h, err := p.AnswerHeader()
		if err == dnsmessage.ErrSectionDone {
			break
		}
		if err != nil {
			return nil, &DNSError{
				Err:    "cannot unmarshal DNS message",
				Name:   name,
				Server: server,
			}
		}
		if h.Type != dnsmessage.TypeNS {
			if err := p.SkipAnswer(); err != nil {
				return nil, &DNSError{
					Err:    "cannot unmarshal DNS message",
					Name:   name,
					Server: server,
				}
			}
			continue
		}
		ns, err := p.NSResource()
		if err != nil {
			return nil, &DNSError{
				Err:    "cannot unmarshal DNS message",
				Name:   name,
				Server: server,
			}
		}
		nss = append(nss, &NS{Host: ns.NS.String()})
	}
	return nss, nil
}

func (r *Resolver) lookupTXT(ctx context.Context, name string) ([]string, error) {
	p, server, err := r.lookup(ctx, name, dnsmessage.TypeTXT)
	if err != nil {
		return nil, err
	}
	var txts []string
	for {
		h, err := p.AnswerHeader()
		if err == dnsmessage.ErrSectionDone {
			break
		}
		if err != nil {
			return nil, &DNSError{
				Err:    "cannot unmarshal DNS message",
				Name:   name,
				Server: server,
			}
		}
		if h.Type != dnsmessage.TypeTXT {
			if err := p.SkipAnswer(); err != nil {
				return nil, &DNSError{
					Err:    "cannot unmarshal DNS message",
					Name:   name,
					Server: server,
				}
			}
			continue
		}
		txt, err := p.TXTResource()
		if err != nil {
			return nil, &DNSError{
				Err:    "cannot unmarshal DNS message",
				Name:   name,
				Server: server,
			}

Multiple strings in one TXT record need to be concatenated without separator to be consistent with previous Go resolver.

		n := 0
		for _, s := range txt.TXT {
			n += len(s)
		}
		txtJoin := make([]byte, 0, n)
		for _, s := range txt.TXT {
			txtJoin = append(txtJoin, s...)
		}
		if len(txts) == 0 {
			txts = make([]string, 0, 1)
		}
		txts = append(txts, string(txtJoin))
	}
	return txts, nil
}

func (r *Resolver) lookupAddr(ctx context.Context, addr string) ([]string, error) {
	if !r.preferGo() && systemConf().canUseCgo() {
		if ptrs, err, ok := cgoLookupPTR(ctx, addr); ok {
			return ptrs, err
		}
	}
	return r.goLookupPTR(ctx, addr)
}

concurrentThreadsLimit returns the number of threads we permit to run concurrently doing DNS lookups via cgo. A DNS lookup may use a file descriptor so we limit this to less than the number of permitted open files. On some systems, notably Darwin, if getaddrinfo is unable to open a file descriptor it simply returns EAI_NONAME rather than a useful error. Limiting the number of concurrent getaddrinfo calls to less than the permitted number of file descriptors makes that error less likely. We don't bother to apply the same limit to DNS lookups run directly from Go, because there we will return a meaningful "too many open files" error.

func concurrentThreadsLimit() int {
	var rlim syscall.Rlimit
	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rlim); err != nil {
		return 500
	}
	r := int(rlim.Cur)
	if r > 500 {
		r = 500
	} else if r > 30 {
		r -= 30
	}
	return r