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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.

// +build darwin freebsd linux netbsd openbsd

// DNS client: see RFC 1035.
// 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.

package net

import (
        "math/rand"
        "sync"
        "time"
)

// Send a request on the connection and hope for a reply.
// Up to cfg.attempts attempts.
func exchange(cfg *dnsConfig, c Conn, name string, qtype uint16) (*dnsMsg, error) {
        if len(name) >= 256 {
                return nil, &DNSError{Err: "name too long", Name: name}
        }
        out := new(dnsMsg)
        out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
        out.question = []dnsQuestion{
                {name, qtype, dnsClassINET},
        }
        out.recursion_desired = true
        msg, ok := out.Pack()
        if !ok {
                return nil, &DNSError{Err: "internal error - cannot pack message", Name: name}
        }

        for attempt := 0; attempt < cfg.attempts; attempt++ {
                n, err := c.Write(msg)
                if err != nil {
                        return nil, err
                }

                if cfg.timeout == 0 {
                        c.SetReadDeadline(time.Time{})
                } else {
                        c.SetReadDeadline(time.Now().Add(time.Duration(cfg.timeout) * time.Second))
                }

                buf := make([]byte, 2000) // More than enough.
                n, err = c.Read(buf)
                if err != nil {
                        if e, ok := err.(Error); ok && e.Timeout() {
                                continue
                        }
                        return nil, err
                }
                buf = buf[0:n]
                in := new(dnsMsg)
                if !in.Unpack(buf) || in.id != out.id {
                        continue
                }
                return in, nil
        }
        var server string
        if a := c.RemoteAddr(); a != nil {
                server = a.String()
        }
        return nil, &DNSError{Err: "no answer from server", Name: name, Server: server, IsTimeout: true}
}

// Do a lookup for a single name, which must be rooted
// (otherwise answer will not find the answers).
func tryOneName(cfg *dnsConfig, name string, qtype uint16) (cname string, addrs []dnsRR, err error) {
        if len(cfg.servers) == 0 {
                return "", nil, &DNSError{Err: "no DNS servers", Name: name}
        }
        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.
                server := cfg.servers[i] + ":53"
                c, cerr := Dial("udp", server)
                if cerr != nil {
                        err = cerr
                        continue
                }
                msg, merr := exchange(cfg, c, name, qtype)
                c.Close()
                if merr != nil {
                        err = merr
                        continue
                }
                cname, addrs, err = answer(name, server, msg, qtype)
                if err == nil || err.(*DNSError).Err == noSuchHost {
                        break
                }
        }
        return
}

func convertRR_A(records []dnsRR) []IP {
        addrs := make([]IP, len(records))
        for i, rr := range records {
                a := rr.(*dnsRR_A).A
                addrs[i] = IPv4(byte(a>>24), byte(a>>16), byte(a>>8), byte(a))
        }
        return addrs
}

func convertRR_AAAA(records []dnsRR) []IP {
        addrs := make([]IP, len(records))
        for i, rr := range records {
                a := make(IP, IPv6len)
                copy(a, rr.(*dnsRR_AAAA).AAAA[:])
                addrs[i] = a
        }
        return addrs
}

var cfg *dnsConfig
var dnserr error

func loadConfig() { cfg, dnserr = dnsReadConfig() }

var onceLoadConfig sync.Once

func lookup(name string, qtype uint16) (cname string, addrs []dnsRR, err error) {
        if !isDomainName(name) {
                return name, nil, &DNSError{Err: "invalid domain name", Name: name}
        }
        onceLoadConfig.Do(loadConfig)
        if dnserr != nil || cfg == nil {
                err = dnserr
                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 || count(name, '.') >= cfg.ndots {
                rname := name
                if !rooted {
                        rname += "."
                }
                // Can try as ordinary name.
                cname, addrs, err = tryOneName(cfg, rname, qtype)
                if err == nil {
                        return
                }
        }
        if rooted {
                return
        }

        // Otherwise, try suffixes.
        for i := 0; i < len(cfg.search); i++ {
                rname := name + "." + cfg.search[i]
                if rname[len(rname)-1] != '.' {
                        rname += "."
                }
                cname, addrs, err = tryOneName(cfg, rname, qtype)
                if err == nil {
                        return
                }
        }

        // Last ditch effort: try unsuffixed.
        rname := name
        if !rooted {
                rname += "."
        }
        cname, addrs, err = tryOneName(cfg, rname, qtype)
        if err == nil {
                return
        }
        return
}

// goLookupHost is the native Go implementation of LookupHost.
// Used only if cgoLookupHost refuses to handle the request
// (that is, only if cgoLookupHost is the stub in cgo_stub.go).
// Normally we let cgo use the C library resolver instead of
// depending on our lookup code, so that Go and C get the same
// answers.
func goLookupHost(name string) (addrs []string, err error) {
        // Use entries from /etc/hosts if they match.
        addrs = lookupStaticHost(name)
        if len(addrs) > 0 {
                return
        }
        onceLoadConfig.Do(loadConfig)
        if dnserr != nil || cfg == nil {
                err = dnserr
                return
        }
        ips, err := goLookupIP(name)
        if err != nil {
                return
        }
        addrs = make([]string, 0, len(ips))
        for _, ip := range ips {
                addrs = append(addrs, ip.String())
        }
        return
}

// goLookupIP is the native Go implementation of LookupIP.
// Used only if cgoLookupIP refuses to handle the request
// (that is, only if cgoLookupIP is the stub in cgo_stub.go).
// Normally we let cgo use the C library resolver instead of
// depending on our lookup code, so that Go and C get the same
// answers.
func goLookupIP(name string) (addrs []IP, err error) {
        // Use entries from /etc/hosts if possible.
        haddrs := lookupStaticHost(name)
        if len(haddrs) > 0 {
                for _, haddr := range haddrs {
                        if ip := ParseIP(haddr); ip != nil {
                                addrs = append(addrs, ip)
                        }
                }
                if len(addrs) > 0 {
                        return
                }
        }
        onceLoadConfig.Do(loadConfig)
        if dnserr != nil || cfg == nil {
                err = dnserr
                return
        }
        var records []dnsRR
        var cname string
        cname, records, err = lookup(name, dnsTypeA)
        if err != nil {
                return
        }
        addrs = convertRR_A(records)
        if cname != "" {
                name = cname
        }
        _, records, err = lookup(name, dnsTypeAAAA)
        if err != nil && len(addrs) > 0 {
                // Ignore error because A lookup succeeded.
                err = nil
        }
        if err != nil {
                return
        }
        addrs = append(addrs, convertRR_AAAA(records)...)
        return
}

// goLookupCNAME is the native Go implementation of LookupCNAME.
// Used only if cgoLookupCNAME refuses to handle the request
// (that is, only if cgoLookupCNAME is the stub in cgo_stub.go).
// Normally we let cgo use the C library resolver instead of
// depending on our lookup code, so that Go and C get the same
// answers.
func goLookupCNAME(name string) (cname string, err error) {
        onceLoadConfig.Do(loadConfig)
        if dnserr != nil || cfg == nil {
                err = dnserr
                return
        }
        _, rr, err := lookup(name, dnsTypeCNAME)
        if err != nil {
                return
        }
        cname = rr[0].(*dnsRR_CNAME).Cname
        return
}