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// Copyright 2011 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.
//go:build unix
package net
import (
"context"
"internal/bytealg"
"sync"
"syscall"
)
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()
for line, ok := file.readLine(); ok; line, ok = file.readLine() {
// 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)
}
func (r *Resolver) lookupHost(ctx context.Context, host string) (addrs []string, err error) {
order, conf := 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, conf)
}
func (r *Resolver) lookupIP(ctx context.Context, network, host string) (addrs []IPAddr, err error) {
if r.preferGo() {
return r.goLookupIP(ctx, network, host)
}
order, conf := 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, network, host, order, conf)
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 {
if err != nil {
// 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) {
order, conf := systemConf().hostLookupOrder(r, name)
if !r.preferGo() && order == hostLookupCgo {
if cname, err, ok := cgoLookupCNAME(ctx, name); ok {
return cname, err
}
}
return r.goLookupCNAME(ctx, name, order, conf)
}
func (r *Resolver) lookupSRV(ctx context.Context, service, proto, name string) (string, []*SRV, error) {
return r.goLookupSRV(ctx, service, proto, name)
}
func (r *Resolver) lookupMX(ctx context.Context, name string) ([]*MX, error) {
return r.goLookupMX(ctx, name)
}
func (r *Resolver) lookupNS(ctx context.Context, name string) ([]*NS, error) {
return r.goLookupNS(ctx, name)
}
func (r *Resolver) lookupTXT(ctx context.Context, name string) ([]string, error) {
return r.goLookupTXT(ctx, name)
}
func (r *Resolver) lookupAddr(ctx context.Context, addr string) ([]string, error) {
order, conf := systemConf().hostLookupOrder(r, "")
if !r.preferGo() && order == hostLookupCgo {
if ptrs, err, ok := cgoLookupPTR(ctx, addr); ok {
return ptrs, err
}
}
return r.goLookupPTR(ctx, addr, conf)
}
// 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
}
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