DNS Resolution

public interface DnsResolver {
    /**
     * Resolves the IP Address (A record) to the specified host name.
     * @param hostname The hostname for which you need the IP Address
     * @return IP Address as a String
     * @throws DnsEntryNotFoundException No record found
     * @throws DnsLookupException Unknown DNS error
     */
    String resolveIpAddress(String hostname) throws DnsEntryNotFoundException, DnsLookupException;

    /**
     * Resolves the host name for the specified service in the specified domain.
     * Uses SRV records to find services, returning the record with highest priority
     * (lowest number) and highest weight.
     * @param serviceType The service type (e.g. ldap, kerberos)
     * @param domain The domain in which to search for the service
     * @return The hostname of the service
     * @throws DnsEntryNotFoundException No record found
     * @throws DnsLookupException Unknown DNS error
     */
    String resolveServiceEntry(String serviceType, String domain) 
        throws DnsEntryNotFoundException, DnsLookupException;

    /**
     * Resolves the host name for the specified service and then the IP Address 
     * for this host in one call.
     * @param serviceType The service type (e.g. ldap, kerberos)
     * @param domain The domain in which to search for the service
     * @return IP Address of the service
     * @throws DnsEntryNotFoundException No record found
     * @throws DnsLookupException Unknown DNS error
     */
    String resolveServiceIpAddress(String serviceType, String domain)
        throws DnsEntryNotFoundException, DnsLookupException;
}

public class JndiDnsResolver implements DnsResolver {
    /**
     * Allows injection of custom JNDI context factory.
     * @param ctxFactory factory to use when a DirContext is needed
     */
    public void setCtxFactory(InitialContextFactory ctxFactory);

    /**
     * Resolves the host name for the specified service and then the IP Address 
     * and port for this host in one call.
     * @param serviceType The service type (e.g. ldap, kerberos)
     * @param domain The domain in which to search for the service
     * @return IP address and port formatted as [ip_address]:[port]
     * @throws DnsEntryNotFoundException No record found
     * @throws DnsLookupException Unknown DNS error
     * @since 5.6
     */
    public String resolveServiceIpAddressAndPort(String serviceType, String domain);
}

public interface InitialContextFactory {
    /**
     * Must return a DirContext which can be used for DNS queries.
     * @return JNDI DirContext
     */
    DirContext getCtx();
}

import org.springframework.security.remoting.dns.JndiDnsResolver;
import org.springframework.security.remoting.dns.DnsEntryNotFoundException;
import org.springframework.security.remoting.dns.DnsLookupException;

JndiDnsResolver resolver = new JndiDnsResolver();

try {
    String ipAddress = resolver.resolveIpAddress("example.com");
    System.out.println("IP Address: " + ipAddress);
} catch (DnsEntryNotFoundException e) {
    System.err.println("Host not found: " + e.getMessage());
} catch (DnsLookupException e) {
    System.err.println("DNS lookup failed: " + e.getMessage());
}

import org.springframework.security.remoting.dns.JndiDnsResolver;

JndiDnsResolver resolver = new JndiDnsResolver();

try {
    // Find LDAP service in domain
    String ldapHost = resolver.resolveServiceEntry("ldap", "example.com");
    System.out.println("LDAP Server: " + ldapHost);
    
    // Get both IP and port in one call (Spring Security 5.6+)
    String ldapEndpoint = resolver.resolveServiceIpAddressAndPort("ldap", "example.com");
    System.out.println("LDAP Endpoint: " + ldapEndpoint); // e.g., "192.168.1.10:389"
    
} catch (DnsEntryNotFoundException e) {
    System.err.println("Service not found: " + e.getMessage());
} catch (DnsLookupException e) {
    System.err.println("DNS lookup failed: " + e.getMessage());
}

import org.springframework.security.remoting.dns.JndiDnsResolver;
import org.springframework.security.remoting.dns.InitialContextFactory;
import javax.naming.Context;
import javax.naming.NamingException;
import javax.naming.directory.DirContext;
import javax.naming.directory.InitialDirContext;
import java.util.Hashtable;

// Custom context factory with specific DNS server
InitialContextFactory customFactory = new InitialContextFactory() {
    @Override
    public DirContext getCtx() {
        Hashtable<String, String> env = new Hashtable<>();
        env.put(Context.INITIAL_CONTEXT_FACTORY, "com.sun.jndi.dns.DnsContextFactory");
        env.put(Context.PROVIDER_URL, "dns://8.8.8.8"); // Use Google DNS
        try {
            return new InitialDirContext(env);
        } catch (NamingException ex) {
            throw new DnsLookupException("Cannot create InitialDirContext", ex);
        }
    }
};

JndiDnsResolver resolver = new JndiDnsResolver();
resolver.setCtxFactory(customFactory);

String ipAddress = resolver.resolveIpAddress("example.com");

_service._tcp.domain IN SRV priority weight port target

_ldap._tcp.example.com IN SRV 10 0 389 ldap.example.com.

public class DnsLookupException extends RuntimeException {
    public DnsLookupException(String msg);
    public DnsLookupException(String msg, Throwable cause);
}

public class DnsEntryNotFoundException extends DnsLookupException {
    public DnsEntryNotFoundException(String msg);
    public DnsEntryNotFoundException(String msg, Throwable cause);
}