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// Copyright 2009 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.
// DNS client.
// Has to be linked into package net for Dial.
// TODO(rsc):
// Check periodically whether /etc/resolv.conf has changed.
// Could potentially handle many outstanding lookups faster.
// Could have a small cache.
// Random UDP source port (net.Dial should do that for us).
// Random request IDs.
// More substantial error reporting.
// Remove use of fmt?
package net
import (
"fmt";
"io";
"net";
"once";
"os";
"strings";
)
var (
DNS_InternalError = os.NewError("internal dns error");
DNS_MissingConfig = os.NewError("no dns configuration");
DNS_No_Answer = os.NewError("dns got no answer");
DNS_BadRequest = os.NewError("malformed dns request");
DNS_BadReply = os.NewError("malformed dns reply");
DNS_ServerFailure = os.NewError("dns server failure");
DNS_NoServers = os.NewError("no dns servers");
DNS_NameTooLong = os.NewError("dns name too long");
DNS_RedirectLoop = os.NewError("dns redirect loop");
DNS_NameNotFound = os.NewError("dns name not found");
);
// Send a request on the connection and hope for a reply.
// Up to cfg.attempts attempts.
func _Exchange(cfg *DNS_Config, c Conn, name string) (m *DNS_Msg, err *os.Error) {
if len(name) >= 256 {
return nil, DNS_NameTooLong
}
out := new(DNS_Msg);
out.id = 0x1234;
out.question = []DNS_Question{
DNS_Question{ name, DNS_TypeA, DNS_ClassINET }
};
out.recursion_desired = true;
msg, ok := out.Pack();
if !ok {
return nil, DNS_InternalError
}
for attempt := 0; attempt < cfg.attempts; attempt++ {
n, err := c.Write(msg);
if err != nil {
return nil, err
}
// TODO(rsc): set up timeout or call ReadTimeout.
// right now net does not support that.
buf := make([]byte, 2000); // More than enough.
n, err = c.Read(buf);
if err != nil {
// TODO(rsc): only continue if timed out
continue
}
buf = buf[0:n];
in := new(DNS_Msg);
if !in.Unpack(buf) || in.id != out.id {
continue
}
return in, nil
}
return nil, DNS_No_Answer
}
// Find answer for name in dns message.
// On return, if err == nil, addrs != nil.
// TODO(rsc): Maybe return [][]byte (==[]IPAddr) instead?
func _Answer(name string, dns *DNS_Msg) (addrs []string, err *os.Error) {
addrs = make([]string, 0, len(dns.answer));
if dns.rcode == DNS_RcodeNameError && dns.authoritative {
return nil, DNS_NameNotFound // authoritative "no such host"
}
if dns.rcode != DNS_RcodeSuccess {
// None of the error codes make sense
// for the query we sent. If we didn't get
// a name error and we didn't get success,
// the server is behaving incorrectly.
return nil, DNS_ServerFailure
}
// Look for the name.
// Presotto says it's okay to assume that servers listed in
// /etc/resolv.conf are recursive resolvers.
// We asked for recursion, so it should have included
// all the answers we need in this one packet.
Cname:
for cnameloop := 0; cnameloop < 10; cnameloop++ {
addrs = addrs[0:0];
for i := 0; i < len(dns.answer); i++ {
rr := dns.answer[i];
h := rr.Header();
if h.class == DNS_ClassINET && h.name == name {
switch h.rrtype {
case DNS_TypeA:
n := len(addrs);
a := rr.(*DNS_RR_A).a;
addrs = addrs[0:n+1];
addrs[n] = fmt.Sprintf("%d.%d.%d.%d", (a>>24), (a>>16)&0xFF, (a>>8)&0xFF, a&0xFF);
case DNS_TypeCNAME:
// redirect to cname
name = rr.(*DNS_RR_CNAME).cname;
continue Cname
}
}
}
if len(addrs) == 0 {
return nil, DNS_NameNotFound
}
return addrs, nil
}
// Too many redirects
return nil, DNS_RedirectLoop
}
// Do a lookup for a single name, which must be rooted
// (otherwise _Answer will not find the answers).
func _TryOneName(cfg *DNS_Config, name string) (addrs []string, err *os.Error) {
err = DNS_NoServers;
for i := 0; i < len(cfg.servers); i++ {
// 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[i] are IP addresses, which
// Dial will use without a DNS lookup.
c, cerr := Dial("udp", "", cfg.servers[i] + ":53");
if cerr != nil {
err = cerr;
continue;
}
msg, merr := _Exchange(cfg, c, name);
c.Close();
if merr != nil {
err = merr;
continue;
}
addrs, aerr := _Answer(name, msg);
if aerr != nil && aerr != DNS_NameNotFound {
err = aerr;
continue;
}
return addrs, aerr;
}
return;
}
var cfg *DNS_Config
func _LoadConfig() {
cfg = DNS_ReadConfig();
}
func LookupHost(name string) (name1 string, addrs []string, err *os.Error) {
// TODO(rsc): Pick out obvious non-DNS names to avoid
// sending stupid requests to the server?
once.Do(&_LoadConfig);
if cfg == nil {
err = DNS_MissingConfig;
return;
}
// If name is rooted (trailing dot) or has enough dots,
// try it by itself first.
rooted := len(name) > 0 && name[len(name)-1] == '.';
if rooted || strings.Count(name, ".") >= cfg.ndots {
rname := name;
if !rooted {
rname += ".";
}
// Can try as ordinary name.
addrs, aerr := _TryOneName(cfg, rname);
if aerr == nil {
return rname, addrs, nil;
}
err = aerr;
}
if rooted {
return
}
// Otherwise, try suffixes.
for i := 0; i < len(cfg.search); i++ {
newname := name+"."+cfg.search[i];
if newname[len(newname)-1] != '.' {
newname += "."
}
addrs, aerr := _TryOneName(cfg, newname);
if aerr == nil {
return newname, addrs, nil;
}
err = aerr;
}
return
}
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