Printing MX records with Go DNS, take 3

December 7, 2012

dns

I’m starting to get really happy about the Go DNS API, so invasive API changes are less and less likely.

We want to create a little program that prints out the MX records of domains, like so:

% mx miek.nl
miek.nl.        86400   IN      MX      10 elektron.atoom.net.

Or

% mx microsoft.com
microsoft.com.  3600    IN      MX      10 mail.messaging.microsoft.com.

We are using my Go DNS package. First the normal header of a Go program, with a bunch of imports. We need the dns package:

package main

import (
    "github.com/miekg/dns"
    "os"
    "net"
    "fmt"
    "log"
)

Next we need to get the local nameserver to use:

config, _ := dns.ClientConfigFromFile("/etc/resolv.conf")

Then we create a dns.Client to perform the queries for us. In Go:

c := new(dns.Client)

We skip some error handling and assume a zone name is given. So we prepare our question. For that to work, we need:

  1. a new packet (dns.Msg);
  2. setting some header bits;
  3. define a question section;
  4. fill out the question section: os.Args[1] contains the zone name.

Which translates into:

m := new(dns.Msg)
m.SetQuestion(dns.Fqdn(os.Args[1]), dns.TypeMX)
m.RecursionDesired = true

Then we need to finally ‘ask’ the question. We do this by calling the Exchange() function. The unused return value is the rtt (round trip time).

r, _, err := c.Exchange(m, net.JoinHostPort(config.Servers[0], config.Port))

Check if we got something sane. The following code snippet prints the answer section of the received packet:

Bail out on an error:

if r == nil {
    log.Fatalf("*** error: %s\n", err.Error())
}

if r.Rcode != dns.RcodeSuccess {
        log.Fatalf(" *** invalid answer name %s after MX query for %s\n", os.Args[1], os.Args[1])
}

// Stuff must be in the answer section
for _, a := range r.Answer {
        fmt.Printf("%v\n", a)
}

And we are done.

Full Source

package main

import (
    "github.com/miekg/dns"
    "net"
    "os"
    "log"
    "fmt"
)

func main() {
    config, _ := dns.ClientConfigFromFile("/etc/resolv.conf")
    c := new(dns.Client)

    m := new(dns.Msg)
    m.SetQuestion(dns.Fqdn(os.Args[1]), dns.TypeMX)
    m.RecursionDesired = true

    r, _, err := c.Exchange(m, net.JoinHostPort(config.Servers[0], config.Port))
    if r == nil {
        log.Fatalf("*** error: %s\n", err.Error())
    }

    if r.Rcode != dns.RcodeSuccess {
            log.Fatalf(" *** invalid answer name %s after MX query for %s\n", os.Args[1], os.Args[1])
    }
    // Stuff must be in the answer section
    for _, a := range r.Answer {
            fmt.Printf("%v\n", a)
    }
}
None