Why golang Lookup*** function can't provide a server parameter?

问题

For nslookup command, it has nslookup somewhere.com some.dns.server.

However, it seems that golang dnsclient only load config from /etc/resolv.conf

code here: https://golang.org/src/net/dnsclient_unix.go#L225

Does the golang standard library provide something like func LookupTXT(name string, dnsServer string) (txt []string, err error) ?

requirement: 1. Don't change the default /etc/resolv.conf.

回答1:

@holys

"github.com/miekg/dns is too heavy for me"

It's not that heavy:

package main

import (
    "log"

    "github.com/miekg/dns"
)

func main() {

    target := "microsoft.com"
    server := "8.8.8.8"

    c := dns.Client{}
    m := dns.Msg{}
    m.SetQuestion(target+".", dns.TypeA)
    r, t, err := c.Exchange(&m, server+":53")
    if err != nil {
        log.Fatal(err)
    }
    log.Printf("Took %v", t)
    if len(r.Answer) == 0 {
        log.Fatal("No results")
    }
    for _, ans := range r.Answer {
        Arecord := ans.(*dns.A)
        log.Printf("%s", Arecord.A)
    }
}

When run, you should see:

$ go run dns.go
2015/07/26 00:24:46 Took 16.138928ms
2015/07/26 00:24:46 134.170.188.221
2015/07/26 00:24:46 134.170.185.46

回答2:

@holys

You can use this simple dns_resolver based on miekg/dns

go get github.com/bogdanovich/dns_resolver

package main

import (
    "log"
    "github.com/bogdanovich/dns_resolver"
)

func main() {
    resolver := dns_resolver.New([]string{"8.8.8.8", "8.8.4.4"})

    // In case of i/o timeout
    resolver.RetryTimes = 5

    ip, err := resolver.LookupHost("google.com")
    if err != nil {
        log.Fatal(err.Error())
    }
    log.Println(ip)
    // Output [216.58.192.46]
}

回答3:

Since a little while you can set the Dial for the Resolver, where you can define your nameserver in the DialContext

var resolver *net.Resolver

if nameserver != "" {
    resolver = &net.Resolver{
        PreferGo: true,
        Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
            d := net.Dialer{}
            return d.DialContext(ctx, "udp", net.JoinHostPort(nameserver, "53"))
        },
    }
} else {
    resolver = net.DefaultResolver
}

After that you can go as you're used to:

ips, err := resolver.LookupIPAddr(context.Background(), "www.example.com")

回答4:

The net.Lookup* functions provide access to the local resolver. While many requests will be answered with information from a DNS server, this is not always the case.

For instance, LookupHost may return a name from the /etc/hosts file. Or it might use mDNS to resolve a .local name.

If you want to talk to an arbitrary DNS server rather than the local resolver, then you should use a general purpose DNS client library. As suggested in the comments, https://github.com/miekg/dns might fit your needs.

回答5:

Since Go 1.9 this is possible by overriding the Dial function on a Resolver. For example, to ignore the request for the local resolver over UDP and connect to 9.9.9.9 via TCP we might do something like this:

r := &net.Resolver{
    Dial: func(ctx context.Context, _, _ string) (net.Conn, error) {
        return net.Dial("tcp", "9.9.9.9")
    },
}
addrs, err := r.LookupIPAddr(context.TODO(), "example.net")

来源：https://stackoverflow.com/questions/30043248/why-golang-lookup-function-cant-provide-a-server-parameter

标签

dns

nslookup