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How to put in an entry into dnsjava cache to override DNS lookup

In trying to avoid having a hostname looked up from the public DNS for testing purposes, I need to essentially set /etc/hosts file, but I don't always know what hostnames I'll need to override the IP address for, so I'm trying to use dnsjava since the default Java DNS resolving doesn't allow for inserting directly into the cache.

Basically, you need to grab the correct DNS Type cache for dnsjava (A, AAAA, etc). Most likely A (for IPv4) or AAAA (for IPv6) is what you should use, although all of the other DNS entry types are also supported. You'll need to create a Name instance, and from that an ARecord which will be inserted into the Cache. Example is as below:
public void addHostToCacheAs(String hostname, String ipAddress) throws UnknownHostException, TextParseException {
//add an ending period assuming the hostname is truly an absolute hostname
Name host = new Name(hostname + ".");
//putting in a good long TTL, and using an A record, but AAAA might be desired as well for IPv6
Record aRec = new ARecord(host, Type.A, 9999999, getInetAddressFromString(ipAddress));
Lookup.getDefaultCache(Type.A).addRecord(aRec, Credibility.NORMAL,this);
}
public InetAddress getInetAddressFromString(String ip) throws UnknownHostException {
//Assume we are using IPv4
byte[] bytes = new byte[4];
String[] ipParts = ip.split("\\.");
InetAddress addr = null;
//if we only have one part, it must actually be a hostname, rather than a real IP
if (ipParts.length <= 1) {
addr = InetAddress.getByName(ip);
} else {
for (int i = 0; i < ipParts.length; i++) {
bytes[i] = Byte.parseByte(ipParts[i]);
}
addr = InetAddress.getByAddress(bytes);
}
return addr
}

how to get the ip address of the connected interface

the way how we get Ip address using java code is clear and many questions about this subject were answered.
but assuming that you have Vmware or VirtualBox on your machine,so you will have extra virtual network cards each one have its own Ip Address.
when executing a little program the result was like 192.168.x.x which belongs to one of my virtual network adapters.
but using "What Is My IP" the result was like 197.x.x.x
so how can i get the ip adress of the connected interface ?

this should do the trick:
import java.io.*;
import java.util.*;
public class ExecTest {
public static void main(String[] args) throws IOException {
Process result = Runtime.getRuntime().exec("traceroute -m 1 www.amazon.com");
BufferedReader output = new BufferedReader(new InputStreamReader(result.getInputStream()));
String thisLine = output.readLine();
StringTokenizer st = new StringTokenizer(thisLine);
st.nextToken();
String gateway = st.nextToken();
System.out.printf("The gateway is %s\n", gateway);
}
}
Courtesy of Chris Bunch from: How can I determine the IP of my router/gateway in Java?
Greets!

Your interface IP 192.168.x.x is your local adapter interface. The prefix 192.168 indicates that this is a private non-routable address that is valid only behind a NAT firewall.
The IP returned by WhatIsMyIP is the external address of the firewall/gateway through which you access the Internet. Unless your computer is connected directly to the Internet these two addresses will never be the same.

Retrieving a MAC address from a PING sweep

i have developed a code which enables me to ping a range of IP addresses. The results from my ping sweep identifies what local machines are reachable, if reachable there hostname and also if they are not reachable.
I am having trouble at the moment retrieving the MAC address of the reachable IP addresses. Has anyone got a solution for this?
package networkping;
import java.io.IOException;
import java.net.InetAddress;
/**
*
* #author Learner
*/
public class Networkping {
public static void main(String[] args) throws IOException {
InetAddress localhost = InetAddress.getLocalHost();
// this code assumes IPv4 is used
byte[] ip = localhost.getAddress();
for (int i = 1; i <= 254; i++)
{
ip[3] = (byte)i;
InetAddress address = InetAddress.getByAddress(ip);
if (address.isReachable(1000))
{
System.out.println(address + " Address is reachable" );
}
else if (!address.getHostAddress().equals(address.getHostName()))
{
System.out.println(address + " Address is known in a DNS lookup and is reachable ");
}
else
{
System.out.println(address + " Address is unreachable");
}
}
}
Thanks

You cannot do this just using java.
There are two options:
Execute another process through java and communicate through standard output with your original java application. Such a process is ARP for example.
use JNI as suggested in the comments.
Have a look also at the following resources/answers:
https://stackoverflow.com/a/14650693
https://stackoverflow.com/a/14857412/1688441
The second resource even has an implemented method calling ARP named private String getMac(String ip)

Client computer name in java

I want to find client computer name in java. My applciation runs in intranet. so i am using below code
public String findClientComputerName(HttpServletRequest request) {
String computerName = null;
String remoteAddress = request.getRemoteAddr();
System.out.println("remoteAddress: " + remoteAddress);
try {
InetAddress inetAddress = InetAddress.getByName(remoteAddress);
System.out.println("inetAddress: " + inetAddress);
computerName = inetAddress.getHostName();
System.out.println("computerName: " + computerName);
if (computerName.equalsIgnoreCase("localhost")) {
computerName = java.net.InetAddress.getLocalHost().getCanonicalHostName();
}
} catch (UnknownHostException e) {
log.error("UnknownHostException detected in StartAction. ", e);
}
if(StringUtils.trim(computerName).length()>0) computerName = computerName.toUpperCase();
System.out.println("computerName: " + computerName);
return computerName;
}
but sometimes i am getting host name properly but some time not. I am getting Correct IP. What may be the reason for this? Why inetAddress.getHostName(); is failing to give host name some time? Your help is very appriciated.

private String getHostName (InetAddress inaHost) throws UnknownHostException
{
try
{
Class clazz = Class.forName("java.net.InetAddress");
Constructor[] constructors = clazz.getDeclaredConstructors();
constructors[0].setAccessible(true);
InetAddress ina = (InetAddress) constructors[0].newInstance();
Field[] fields = ina.getClass().getDeclaredFields();
for (Field field: fields)
{
if (field.getName().equals("nameService"))
{
field.setAccessible(true);
Method[] methods = field.get(null).getClass().getDeclaredMethods();
for (Method method: methods)
{
if (method.getName().equals("getHostByAddr"))
{
method.setAccessible(true);
return (String) method.invoke(field.get (null), inaHost.getAddress());
}
}
}
}
} catch (ClassNotFoundException cnfe) {
} catch (IllegalAccessException iae) {
} catch (InstantiationException ie) {
} catch (InvocationTargetException ite) {
throw (UnknownHostException) ite.getCause();
}
return null;
}
above function returning host name correctly in Intranet. for local it will return localhost.
To get the name for local host we use computerName = java.net.InetAddress.getLocalHost().getCanonicalHostName();

HttpServletRequest will return the IP address (either v4 or v6) of whoever is hitting your servlet. That address may or may not resolve to a valid hostname. InetAddress.getHostName() does a reverse DNS resolution of the IP address. It is not required that each IP address allocated maps back to a valid DNS entry. There are, in fact, a large percent of IP addresses in the world that will not resolve to a hostname.
You can see the same thing using the 'host' command on a linux box to look up the reverse DNS entry (if any) for a given IP address.

The InetAddress.getHostName() function will return the host name if the InetAddress object was initialized with a host name. Otherwise, it'll do a reverse DNS lookup to get the host name.
To get this reverse DNS lookup to work, you'll need to make sure all of the clients on your intranet are configured with host names and that your DNS provider (e.g. your router) properly matches up the host names with its records. (Some routers can do this automatically.)

In order to get the hostname of windows machines, you will need to perform a reverse NetBIOS lookup on the IP address. Windows uses a system called WINS to provide hostnames to its computers. This system is based off NetBIOS.
If you don't want to try to find a specification of the protocol and implement it yourself, then you will want to execute the command nbtstat -A [ip address] if you are on Windows, or nmblookup -A [ip address] if you are on a Linux machine. If you are on a Linux machine, the Samba package will have to be installed as the nmblookup executable is installed on all Linux machines. You will then have to parse the output of that command to get the host name.
The alternative is, as stated before, try to find a specification of the protocol, and implement the part that you need to implement.

How do you tell whether a string is an IP or a hostname

So you have a String that is retrieved from an admin web UI (so it is definitely a String). How can you find out whether this string is an IP address or a hostname in Java?
Update: I think I didn't make myself clear, I was more asking if there is anything in the Java SDK that I can use to distinguish between IPs and hostnames? Sorry for the confusion and thanks for everybody who took/will take the time to answer this.

You can use a regular expression with this pattern:
\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b
That will tell you if it's an IPv4 address.

Do we get to make the assumption that it is one or the other, and not something completely different? If so, I'd probably use a regex to see if it matched the "dotted quad" format.

You can see if the string matches the number.number.number.number format, for example:
\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b
will match anything from 0 - 999.
Anything else you can have it default to hostname.

URI validator = new URI(yourString);
That code will validate the IP address or Hostname. (It throws a malformed URI Exception if the string is invalid)
If you are trying to distinguish the two..then I miss read your question.

You can use a security manager with the InetAddress.getByName(addr) call.
If the addr is not a dotted quad, getByName will attempt to perform a connect to do the name lookup, which the security manager can capture as a checkConnect(addr, -1) call, resulting in a thrown SecurityException that you can catch.
You can use System.setSecurityManager() if you're running fully privileged to insert your custom security manager before the getByName call is made.

It is not as simple as it may appear, there are some ambiguities around characters like hyphens, underscore, and square brackets '-', '_', '[]'.
The Java SDK is has some limitations in this area. When using InetAddress.getByName it will go out onto the network to do a DNS name resolution and resolve the address, which is expensive and unnecessary if all you want is to detect host vs address. Also, if an address is written in a slightly different but valid format (common in IPv6) doing a string comparison on the results of InetAddress.getByName will not work.
The IPAddress Java library will do it. The javadoc is available at the link. Disclaimer: I am the project manager.
static void check(HostName host) {
try {
host.validate();
if(host.isAddress()) {
System.out.println("address: " + host.asAddress());
} else {
System.out.println("host name: " + host);
}
} catch(HostNameException e) {
System.out.println(e.getMessage());
}
}
public static void main(String[] args) {
HostName host = new HostName("1.2.3.4");
check(host);
host = new HostName("1.2.a.4");
check(host);
host = new HostName("::1");
check(host);
host = new HostName("[::1]");
check(host);
host = new HostName("1.2.?.4");
check(host);
}
Output:
address: 1.2.3.4
host name: 1.2.a.4
address: ::1
address: ::1
1.2.?.4 Host error: invalid character at index 4

Couldn't you just to a regexp match on it?

Use InetAddress#getAllByName(String hostOrIp) - if hostOrIp is an IP-address the result is an array with single InetAddress and it's .getHostAddress() returns the same string as hostOrIp.
import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Arrays;
public class IPvsHostTest {
private static final org.slf4j.Logger LOG = org.slf4j.LoggerFactory.getLogger(IPvsHostTest.class);
#org.junit.Test
public void checkHostValidity() {
Arrays.asList("10.10.10.10", "google.com").forEach( hostname -> isHost(hostname));
}
private void isHost(String ip){
try {
InetAddress[] ips = InetAddress.getAllByName(ip);
LOG.info("IP-addresses for {}", ip);
Arrays.asList(ips).forEach( ia -> {
LOG.info(ia.getHostAddress());
});
} catch (UnknownHostException e) {
LOG.error("Invalid hostname", e);
}
}
}
The output:
IP-addresses for 10.10.10.10
10.10.10.10
IP-addresses for google.com
64.233.164.100
64.233.164.138
64.233.164.139
64.233.164.113
64.233.164.102
64.233.164.101

This code still performs the DNS lookup if a host name is specified, but at least it skips the reverse lookup that may be performed with other approaches:
...
isDottedQuad("1.2.3.4");
isDottedQuad("google.com");
...
boolean isDottedQuad(String hostOrIP) throws UnknownHostException {
InetAddress inet = InetAddress.getByName(hostOrIP);
boolean b = inet.toString().startsWith("/");
System.out.println("Is " + hostOrIP + " dotted quad? " + b + " (" + inet.toString() + ")");
return b;
}
It generates this output:
Is 1.2.3.4 dotted quad? true (/1.2.3.4)
Is google.com dotted quad? false (google.com/172.217.12.238)
Do you think we can expect the toString() behavior to change anytime soon?