如何在Java中解析远程计算机名称以获取IP地址

Question

I'm trying to come up with a program that send packets from computer A to computer B . both computers must have a copy of my java program. this require one to enter the name of the remote computer B in a JTextField object, enter the message in a JTextArea object and then click the button send.

My program should be able to resolve the given computer name to ip address so to include the IP address as a parameter in my DatagramPacket constuctor.

I have tried using the below method to do the resolving but i get a javax.naming.CommunicationException .

String clientname="user";
Hashtable<String,Object> env=new Hashtable<String,Object>();
        env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
        env.put(Context.PROVIDER_URL,"dns://www.google.com");
        DirContext con=new InitialDirContext(env);
       Object obj=con.lookup("clientname");

Exception thrown

javax.naming.CommunicationException: DNS error [Root exception is java.net.SocketTimeoutException: Receive timed out]; remaining name 'user'
at com.sun.jndi.dns.DnsClient.query(DnsClient.java:300)
at com.sun.jndi.dns.Resolver.query(Resolver.java:81)
at com.sun.jndi.dns.DnsContext.c_lookup(DnsContext.java:286)
at com.sun.jndi.toolkit.ctx.ComponentContext.p_lookup(ComponentContext.java:544)
at com.sun.jndi.toolkit.ctx.PartialCompositeContext.lookup(PartialCompositeContext.java:177)
at com.sun.jndi.toolkit.ctx.PartialCompositeContext.lookup(PartialCompositeContext.java:166)
at javax.naming.InitialContext.lookup(InitialContext.java:411)
Caused by: java.net.SocketTimeoutException: Receive timed out
at java.net.DualStackPlainDatagramSocketImpl.socketReceiveOrPeekData(Native Method)
at java.net.DualStackPlainDatagramSocketImpl.receive0(DualStackPlainDatagramSocketImpl.java:121)
at java.net.AbstractPlainDatagramSocketImpl.receive(AbstractPlainDatagramSocketImpl.java:145)
at java.net.DatagramSocket.receive(DatagramSocket.java:786)
at com.sun.jndi.dns.DnsClient.doUdpQuery(DnsClient.java:411)
at com.sun.jndi.dns.DnsClient.query(DnsClient.java:203)
... 7 more

Question

How can i achieve my goal coz it seems to me like java DNS service provider can only resolve domain names and not individual computer names? i've been struggling with this for 3 days.

Any help is appreciated.

Answer 1

Use of JNDI is only useful if you need specific DNS attributes/entries.

Maybe the following is more suitable for you:

final InetAddress inetAddress = InetAddress.getByName("clientname");
final String ipAddress = inetAddress.getHostAddress();

The local DNS infrastructure - like the OS it does - will be use and you don't need to provide an DNS server by yourself.

http://docs.oracle.com/javase/7/docs/api/java/net/InetAddress.html

http://docs.oracle.com/javase/7/docs/api/java/net/InetAddress.html#getByName%28java.lang.String%29

Determines the IP address of a host, given the host's name.

The host name can either be a machine name, such as "java.sun.com", or a textual representation of its IP address. If a literal IP address is supplied, only the validity of the address format is checked.

For host specified in literal IPv6 address, either the form defined in RFC 2732 or the literal IPv6 address format defined in RFC 2373 is accepted. IPv6 scoped addresses are also supported.

Answer 2

Did you read http://docs.oracle.com/javase/7/docs/technotes/guides/jndi/jndi-dns.html ?

It seams like:

Hashtable env = new Hashtable();
env.put("java.naming.factory.initial", "com.sun.jndi.dns.DnsContextFactory");
env.put("java.naming.provider.url",    "dns://google-public-dns-a.google.com");

DirContext ictx = new InitialDirContext(env);
Attributes attrs1 = ictx.getAttributes("clientname", new String[] {"A"});

is what you want.

The timeout values are configurable either:

env.put("com.sun.jndi.dns.timeout.initial", "2000");

env.put("com.sun.jndi.dns.timeout.retries", "3");

Use FQDN (full qualified domain name) if you will use a public DNS server instead of clientname like shown in your Exception. Google doesn't hopefully know anything about clientname ;-).