A Java program to print a sorted list of MX (Mail Exchange) servers for a domain, using a DNS (Domain Name System) lookup

References

Sample Runs

::] javac MailHostsLookup.java
::] java MailHostsLookup cs.fsu.edu
mail.cs.fsu.edu

::] java MailHostsLookup yahoo.com
mx2.mail.yahoo.com
mx3.mail.yahoo.com
mx1.mail.yahoo.com
mx4.mail.yahoo.com

::] java MailHostsLookup yahoo3253.com
ERROR: No DNS record for 'yahoo3253.com'

::] java MailHostsLookup gcc.org
mail2.gcc.org
mail.gcc.org

::] java MailHostsLookup mit.edu
FORT-POINT-STATION.mit.edu
PACIFIC-CARRIER-ANNEX.mit.edu

::] java MailHostsLookup msn.com
mx1.hotmail.com
mx2.hotmail.com
mx3.hotmail.com
mx4.hotmail.com

::] java MailHostsLookup google.com
smtp1.google.com
smtp2.google.com
smtp4.google.com
smtp3.google.com

::] 

Code

MailHostsLookup.java

// Print out a sorted list of mail exchange servers for a network domain name
import javax.naming.directory.Attribute;
import javax.naming.directory.Attributes;
import javax.naming.directory.InitialDirContext;
import javax.naming.NamingException;
import java.util.Arrays;
import java.util.Comparator;

public class MailHostsLookup 
{
    public static void main(String args[]) 
    {
        // explain what program does and how to use it 
        if (args.length != 1)
        {
            System.err.println("Print out a sorted list of mail exchange servers ");
            System.err.println("    for a network domain name");
            System.err.println("USAGE: java MailHostsLookup domainName");
            System.exit(-1);
        } 
        try
        {   // print the sorted mail exhchange servers
            for (String mailHost: lookupMailHosts(args[0]))
            {
                System.out.println(mailHost);            
            }
        }
        catch (NamingException e)
        {
            System.err.println("ERROR: No DNS record for '" + args[0] + "'");
            System.exit(-2);
        }
     }

    // returns a String array of mail exchange servers (mail hosts) 
    //     sorted from most preferred to least preferred
    static String[] lookupMailHosts(String domainName) throws NamingException
    {
        // see: RFC 974 - Mail routing and the domain system
        // see: RFC 1034 - Domain names - concepts and facilities
        // see: http://java.sun.com/j2se/1.5.0/docs/guide/jndi/jndi-dns.html
        //    - DNS Service Provider for the Java Naming Directory Interface (JNDI)

        // get the default initial Directory Context
        InitialDirContext iDirC = new InitialDirContext();
        // get the MX records from the default DNS directory service provider
        //    NamingException thrown if no DNS record found for domainName
        Attributes attributes = iDirC.getAttributes("dns:/" + domainName, new String[] {"MX"});
        // attributeMX is an attribute ('list') of the Mail Exchange(MX) Resource Records(RR)
        Attribute attributeMX = attributes.get("MX");

        // if there are no MX RRs then default to domainName (see: RFC 974)
        if (attributeMX == null)
        {
            return (new String[] {domainName});
        }

        // split MX RRs into Preference Values(pvhn[0]) and Host Names(pvhn[1])
        String[][] pvhn = new String[attributeMX.size()][2];
        for (int i = 0; i < attributeMX.size(); i++)
        {
            pvhn[i] = ("" + attributeMX.get(i)).split("\\s+");
        }

        // sort the MX RRs by RR value (lower is preferred)
        Arrays.sort(pvhn, new Comparator<String[]>()
            {
                public int compare(String[] o1, String[] o2)
                {
                    return (Integer.parseInt(o1[0]) - Integer.parseInt(o2[0]));
                }
            });

        // put sorted host names in an array, get rid of any trailing '.' 
        String[] sortedHostNames = new String[pvhn.length];
        for (int i = 0; i < pvhn.length; i++)
        {
            sortedHostNames[i] = pvhn[i][1].endsWith(".") ? 
                pvhn[i][1].substring(0, pvhn[i][1].length() - 1) : pvhn[i][1];
        }
        return sortedHostNames;
    }
}