How to resolve a host name from IP using JNDI/DNS service provider

Question

Hi
I got two questions on JNDI/DNS service provider:
1) How to resolve a host name when i got an IP
I understand How I do it inverse.
Hashtable env = new Hashtable();
env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
env.put(Context.PROVIDER_URL, "dns://"+server+"/"+domain);
DirContext ictx = new InitialDirContext(env);
Attributes attrs1 = ictx.getAttributes(host, new String[] {"A"});
2) This example above works when I specify the domain in the provider url.
If I am not specifing a domain but only the dns server I got an NameNotFoundException.
What should I do if I don't got the domain?
get the availible domains and loop on them?

Answer 1

Hi,
if your DNS server supports that, you can do a reverse DNS lookup. This works as follows:
String server = "your.dns.server";
String domain = "in-addr.arpa";
String ip = "4.3.2.1";
Hashtable env = new Hashtable();
env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
env.put(Context.PROVIDER_URL, "dns://" + server + "/" + domain);
DirContext ictx = new InitialDirContext(env);
Attributes attrs1 = ictx.getAttributes(ip, new String[] {"PTR"});1: Use the pseudo-domain in-addr.arpa
2: Use the reverse IP address for lookup, i.e. if your host has 1.2.3.4, use 4.3.2.1!!
3: Request the PTR attribute
see also [http://en.wikipedia.org/wiki/Reverse_DNS_lookup|http://en.wikipedia.org/wiki/Reverse_DNS_lookup]
Martin