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InetAddress.getCanonicalHostName() returns IP instead of Hostname

I looked for how to do IP lookup in Java on Stack Overflow but the answers match what I am already doing and do not resolve my problem.
Here is my code:
public void printHostname( String ip ) {
System.out.println( InetAddresses.forString( ip ).getCanonicalHostName( ) );
}
InetAddresses is just a utility class from guava library to get a InetAdress.
The problem: This code works as expected with some IP adresses and not with some others.
A working example
For example, for IP 157.55.39.29, the output is:
msnbot-157-55-39-29.search.msn.com
This result seems correct according to Linux host command:
> host 157.55.39.29
29.39.55.157.in-addr.arpa domain name pointer msnbot-157-55-39-29.search.msn.com.
A not working example
For IP 123.125.71.75, the host command returns:
> host 123.125.71.75
75.71.125.123.in-addr.arpa domain name pointer baiduspider-123-125-71-75.crawl.baidu.com.
But the output of my Java code is:
123.125.71.75
whereas the expected output should be
baiduspider-123-125-71-75.crawl.baidu.com
The javadoc of getCanonicalHostName method says:
Returns:
the fully qualified domain name for this IP address, or if the operation is not allowed by the security check, the textual representation of the IP address.
but I’m pretty sure it's not really a problem with a security check... or I don't understand what is wrong.
Have you any suggestion to explain this behaviour? Do you have a workaround?
EDIT #1
When looking for a solution, I tried to step debug the implementation in JDK:
// first lookup the hostname
host = nameService.getHostByAddr(addr.getAddress());
/* check to see if calling code is allowed to know
* the hostname for this IP address, ie, connect to the host
*/
if (check) {
SecurityManager sec = System.getSecurityManager();
if (sec != null) {
sec.checkConnect(host, -1);
}
}
/* now get all the IP addresses for this hostname,
* and make sure one of them matches the original IP
* address. We do this to try and prevent spoofing.
*/
InetAddress[] arr = InetAddress.getAllByName0(host, check);
In this code, variable host contains the correct value, but the last statement calling getAllByName0 throws an UnknownHostException which is handled by returning just the requested IP. The exception is thrown by internal method getAddressesFromNameServicewith message:
"java.net.UnknownHostException: baiduspider-123-125-71-75.crawl.baidu.com"
I don't know why.
Can I get the host variable value, bypassing the internal exception?

The problem lies in the fact that the java.net.InetAdress has a certain procedure against ip-spoofing.
It first resolves the name into (an) ip address(es). This works fine.
In your case the result are two IP adresses. InetAdress then checks back if (at least one of) these adresses resolve to the original input name.
If they do not, it just returns the original ip adress. The following picture shows the situation after the check for baiduspider-123-125-71-75.crawl.baidu.com
Note: The ip adresses resolved by getAllByName0 are the same as via nslookup, namely:
nslookup baiduspider-123-125-71-75.crawl.baidu.com
Server: 192.168.2.1
Address: 192.168.2.1#53
Non-authoritative answer:
Name: baiduspider-123-125-71-75.crawl.baidu.com
Address: 62.157.140.133
Name: baiduspider-123-125-71-75.crawl.baidu.com
Address: 80.156.86.78
A solution would be to use the dnsjava library. It skips the spoofing check and therefore works fine.
dnsjava example:
String addr = Address.getHostName(InetAddress.getByName("123.125.71.75"));
outputs just as expected baiduspider-123-125-71-75.crawl.baidu.com
Disclaimer: As i am a Java developer and not a security expert, i am not totally aware of the security implications of using a spoofed ip address.

I did not dig a lot into this so I don't know why it is happening, but some online tools (like this one) that checks the health of DNS servers indicates that they have some issues which may or may not be related.
As #jah said, Java tries to double check to see if the hostname has the ip it said it has. The exception is thrown on the native code while trying to do that. In fact, in my case, trying to verify on the command line, the nslookup fails to get the ip from the name, which indicates some configuration preventing this on the DNS Server (maybe on purpose? I'm no expert in DNS either).
When it works:
$ nslookup msnbot-157-55-39-29.search.msn.com
Server: 192.168.1.1
Address: 192.168.1.1#53
Non-authoritative answer:
Name: msnbot-157-55-39-29.search.msn.com
Address: 157.55.39.29
When it doesnt work:
$ nslookup baiduspider-123-125-71-75.crawl.baidu.com
Server: 192.168.1.1
Address: 192.168.1.1#53
** server can't find baiduspider-123-125-71-75.crawl.baidu.com: NXDOMAIN
When it works:
$ getent hosts msnbot-157-55-39-29.search.msn.com
157.55.39.29 msnbot-157-55-39-29.search.msn.com
When it doesn't:
$ getent hosts baiduspider-123-125-71-75.crawl.baidu.com
$
As an alternative, you can use a DNS Service Provider for JNDI. The documentation has an example, but I'll leave a working snippet for you to test:
String[] octets = "123.125.71.75".split("\\.");
String host = String.join(".", octets[3], octets[2], octets[1], octets[0], "in-addr.arpa");
Properties props = new Properties();
props.put("java.naming.factory.initial", "com.sun.jndi.dns.DnsContextFactory");
DirContext dirContext = new InitialDirContext(props);
Attributes attrs = dirContext.getAttributes(host, new String[] {"PTR"});
System.out.println(attrs.get("PTR").get());

How to programmatically connect internet via datacard with AT commands?

I have a datacard ZTE MF190. I want to use AT commands to register in 2G or 3G and access internet via datacard. Found this article about how to make data call:
AT+cgatt=1
AT+CGDCONT=1,”IP”,”epc.tmobile.com” //I used my operator PDP context
AT+CGACT=1,1
But ping from OS terminal shows 100% package loss.
I've tried on Ubuntu 14 and Windows 7.
How can I connect internet with AT commands using datacard on Ubuntu?
UPDATE
I gave bounty to #tripleee's answer because it's more full than first one and answered all my questions. But I'm not satisfied with answers, so I'll answer my own question in a week.
In my answer I'll show how to handle this process with Java. So, please do not move this question to other Stack Exchange websites.

Creating a connection between the card and your provider is not sufficient. You need some mechanism for creating a network interface out of this connection, and set up your network stack to route packets over this interface.
Traditionally, the pppd daemon has been a popular choice for this task. You would create a "chat script" with the commands for establishing a data call (these days, pppd might come packaged with a suitable canned script) and the daemon would handle the entire process of placing the call, authenticating, setting up a network interface over the circuit, and configuring the system to route packets over it, as well as configuring DNS etc to use it for resolver queries, etc.

I tried to sniff USB port but on this case dashboard can not connect because of busy port
It is certainly possible. See this question
Found this article about how to make data call
What that article is about is how to set up the call, not how to make it.
After you made correct setup, connect to internet with this command:
ATD*99***1#
UPDATE1: After a bit of research I believe that article was written only to promote their software and has no practical use. In reality dialing is made with pppd or wvdial
UPDATE2: We discussed ways to solve the problem in a chat room (in Russian). It turned out cnetworkmanager will be the way to go

As far as I know wvdial uses ppp daemon to connect to the internet using modem. wvdial is preinstalled on desktop version of Ubuntu.
wvdial uses a config file located /etc/wvdial.conf. Let's edit this file. Type in your terminal
sudo nano /etc/wvdial.conf
and you will see something like this
[Dialer Defaults]
Init1 = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2
Stupid Mode = yes
ISDN = 0
Modem Type = Analog Modem
New PPPD = yes
Phone = *99#
Modem = /dev/ttyUSB2
Username = ''
Password = ''
Baud = 9600
Dial Timeout = 30
Dial Attempts = 3
Explanation of all keys you can find in wvdial.conf(5) - Linux man page. If you need to change your provider dial number, username, password or any other information about connection and device you can change file content and save it.
There are 3 serial ports for ZTE MF190. Normally it's ttyUSB0, ttyUSB1 and ttyUSB2. And in my case ttyUSB2 is for internet connection. It would not work on other ports. So you need to find the right serial port for your modem.
There is an automatic configurator which edits wvdial.conf file, sets serial port baud rate etc. Since it is not always configure correctly I would not recommend to use it:
sudo wvdialconf /etc/wvdial.conf
It would be better if you configure wvdial manually.
Now, when your device connected and wvdial configured to work with device, you can execute this line from terminal:
wvdial
You will see a lot of lines. But if you see those lines - you have succeeded.
local IP address XX.XX.XX.XX
remote IP address XX.XX.XX.XX
primary DNS address XX.XX.XX.XX
secondary DNS address XX.XX.XX.XX
Now, how we can use it in programming? I'll provide some code to work with it on Java. You can use this code to dial.
public int dialer() {
// status for debug. If status == 4 then you connected successfully
int status;
// create process of wvdial
ProcessBuilder builder = new ProcessBuilder("wvdial");
try {
// start wvdial
final Process process = builder.start();
// wvdial listener thread
final Thread ioThread = new Thread() {
#Override
public void run() {
try {
final BufferedReader reader = new BufferedReader(
new InputStreamReader(process.getErrorStream()));
// wvdial output line
String line;
while ((line = reader.readLine()) != null) {
// if "local IP address" line detected set status 1
if (line.contains("local IP address")) {
status = 1;
}
if (line.contains("remote IP address")) {
status = 2;
}
if (line.contains("primary DNS address")) {
status = 3;
}
if (line.contains("secondary DNS address")) {
status = 4;
}
}
reader.close();
} catch (final Exception e) {
}
}
};
// start listener
ioThread.start();
// wait 6 secs and return status. Some kind of timeout
Thread.sleep(6000);
} catch (Exception e) {
}
return status;
}
And here is a disconnector method. All you need is to kill wvdial process and thread will be destroyed:
public boolean disconnect() {
ProcessBuilder builder = new ProcessBuilder("pkill", "wvdial");
try {
builder.start();
return true;
} catch (IOException e) {
return false;
}
}

Connect remotely to an H2 Database using a Java Application

I'm having the following problem:
When I try to createTcpServer with my external IP address (the PC's IP and not my local IP = the one we see as an output after running ipconfig in cmd.exe) the following error occurs:
Error with Server: Exception opening port "9092" (port may be in use), cause: "java.net.BindException: Cannot assign requested address: JVM_Bind" [90061-169]
However, the port is not in use. I've checked that using netstat -a -n .
I have enabled my external IP and I have disabled the firewall from the router. My external IP can now be pinged.
Please help me.
Update: Here is my code to start the tcp server.
package businessApp;
import org.h2.tools.Server; //imports the server utility
public class startTcpServerForH2 {
Server server; //the server's instance variable
private static final String SERVER_IP = "192.168.1.101"; //fixed IP of the server
private static final String SERVER_PORT = "9092"; //fixed port the server is listening to
public void tcpServer() { //method responsible to create the tcp server
optionPane optPane = new optionPane(); //option pane for debugging purposes, shows the server's status
try { //catches any server related errors, if the connection is broken etc.
//server uses the IP and port defined earlier, allows other computers in the LAN to connect and implements the secure socket layer (SSL) feature
server = Server.createTcpServer( //create tcp server
new String[] { "-tcpPort" , SERVER_PORT , "-tcpAllowOthers" , "-tcpSSL" }).start();
System.out.println(server.getStatus()); //prints out the server's status
optPane.checkServerStatus(server.getStatus()); //prints out the server's status on the option pane as well
} catch(Exception ex){
System.out.println("Error with Server: " + ex.getMessage());
}
}
public static void main(String[] args){
startTcpServerForH2 tcpServ = new startTcpServerForH2(); //create a new server object
tcpServ.tcpServer(); //starts the tcp server
}
}
Second Update: here is the h2Connection code.
package businessApp;
import java.sql.*; //imports sql features
//Class responsible for connection with H2 Database Engine
public class h2Connection {
Connection conn; //connection variable
DatabaseMetaData dbmd; /** Metadata variable which include methods such as the following:
* 1) Database Product Name
* 2) Database Product Version
* 3) URL where the database files are located (in TCP mode)
*/
Statement stm; //statements variable
ResultSet rst; //result sets variable
private static final String SERVER_IP = "..."; //here I enter my WAN_IP
private static final String SERVER_PORT = "9092";
public Connection connectionToH2(Connection connt) {
optionPane optPane = new optionPane(); //create new option pane object
String outputConn = null; //declare & initialize string which will hold important messages
try {
Class.forName("org.h2.Driver"); //Driver's name
/** The String URL is pertained of the following:
* 1) jdbc which java implements so that it can take advantage of the SQL features
* 2) Which Database Engine will be used
* 3) URL where the files will be stored (as this is a TCP connection)
* 4) Schema: businessApp
* 5) Auto server is true means that other computers can connect with the same databse at any time
* 6) Port number of the server is also defined
*/
String url = "jdbc:h2:tcp://" + SERVER_IP + ":" + SERVER_PORT + "/C:/Databases/businessApp;IFEXISTS=TRUE";
System.out.println(url); //prints out the url the database files are located as well as the h2 features used (SSL)
connt = DriverManager.getConnection(url, "sa", ""); //Driver Manager defines the username & password of the database
System.out.println(connt.getCatalog()); //prints out the database schema
optPane.checkServerStatus(connt.getCatalog()); //prints out the database schema on the option pane as well
connt.setAutoCommit(false); //set AutoCommit to false to control commit actions manually
//outputs H2 version and the URL of the database files which H2 is reading from, for confirmation
dbmd = connt.getMetaData(); //get MetaData to confirm connection
outputConn = "Connection to "+dbmd.getDatabaseProductName()+" "+
dbmd.getDatabaseProductVersion()+ " with the URL " + dbmd.getURL()+" was successful.\n";
System.out.println(outputConn); //outputs the message on the system (NetBeans compiler)
optPane.checkH2Connection(outputConn); //outputs the message on top of the frame
} catch (ClassNotFoundException ex){ //In case there is an error for creating the class for the Driver to be used
System.out.println("Error creating class: " + ex.getMessage());
} catch(SQLException ex){ //Any error associated with the Database Engine
System.out.println("SQL error: " + ex.getMessage());
optPane.checkServerStatus("SQL error: " + ex.getMessage());
}
return connt; //As the method is not void, a connection variable must be returned
}
}
When I want to connect to the h2 database, I make a new h2Connection object and use it to connect. I have followed the H2 manual word by word. What more do you need?

As suggested in the command line help shown below, Protection against Remote Access advises the following:
By default this database does not allow connections from other machines when starting the H2 Console, the TCP server, or the PG server. Remote access can be enabled using the command line options -webAllowOthers, -tcpAllowOthers, -pgAllowOthers.
See the documentation for important caveats regarding these options.
Addendum: Works for me, as long as I restart the Server after opening the firewall; you don't need the setProperty() line at all; the LAN IP to which your WAN_IP forwards port 9092 should be your host IP address; then you can open a shell via your WAN_IP:
java -cp h2.jar org.h2.tools.Shell -url
jdbc:h2:tcp://WAN_IP/~/path/to/test;ifexists=true"
Command line help:
$ java -cp .:/opt/h2/bin/h2.jar org.h2.tools.Shell -?
Interactive command line tool to access a database using JDBC.
Usage: java org.h2.tools.Shell
Options are case sensitive. Supported options are:
[-help] or [-?] Print the list of options
[-url ""] The database URL (jdbc:h2:...)
[-user ] The user name
[-password ] The password
[-driver ] The JDBC driver class to use (not required in most cases)
[-sql ""] Execute the SQL statements and exit
[-properties ""] Load the server properties from this directory
If special characters don't work as expected, you may need to use
-Dfile.encoding=UTF-8 (Mac OS X) or CP850 (Windows).
See also http://h2database.com/javadoc/org/h2/tools/Shell.html
$ java -cp /opt/h2/bin/h2.jar org.h2.tools.Server -?
Starts the H2 Console (web-) server, TCP, and PG server.
Usage: java org.h2.tools.Server
When running without options, -tcp, -web, -browser and -pg are started.
Options are case sensitive. Supported options are:
[-help] or [-?] Print the list of options
[-web] Start the web server with the H2 Console
[-webAllowOthers] Allow other computers to connect - see below
[-webDaemon] Use a daemon thread
[-webPort ] The port (default: 8082)
[-webSSL] Use encrypted (HTTPS) connections
[-browser] Start a browser connecting to the web server
[-tcp] Start the TCP server
[-tcpAllowOthers] Allow other computers to connect - see below
[-tcpDaemon] Use a daemon thread
[-tcpPort ] The port (default: 9092)
[-tcpSSL] Use encrypted (SSL) connections
[-tcpPassword ] The password for shutting down a TCP server
[-tcpShutdown ""] Stop the TCP server; example: tcp://localhost
[-tcpShutdownForce] Do not wait until all connections are closed
[-pg] Start the PG server
[-pgAllowOthers] Allow other computers to connect - see below
[-pgDaemon] Use a daemon thread
[-pgPort ] The port (default: 5435)
[-properties ""] Server properties (default: ~, disable: null)
[-baseDir ] The base directory for H2 databases (all servers)
[-ifExists] Only existing databases may be opened (all servers)
[-trace] Print additional trace information (all servers)
The options -xAllowOthers are potentially risky.
For details, see Advanced Topics / Protection against Remote Access.
See also http://h2database.com/javadoc/org/h2/tools/Server.html

Java RMI connection to localhost at home network can't find correct remote module

I have been working on this project where two modules on different machines need to be in communication through RMI.
I start both client and server modules on my laptop. RMI seems to work correctly when i am at work and connected to work network, but when i am home, connected to my home network it does not work. It says remote object could not be found.
Here is the method i use at CLIENT side to get the reference to remote object
public static MyRMIApp getRemoteApp() throws RemoteException, NotBoundException, AccessException {
Registry registry = LocateRegistry.getRegistry("localhost", 28999); // tried 127.0.0.1 instead of localhost here, still not working
MyRMIApp app = (MyRMIApp) registry.lookup("COM");
return app;
}
Digging up a bit with some debugging, when i check the object value returned from getRemoteApp method, it shows me the end point is 67.215.65.132. Which is openDNS i am using to connect to internet. Shouldn't that be 127.0.0.1 ?
Then i used my mobile internet and tried again. It seems to be working but end-point is not 127.0.0.1 again it is the address assigned to me, which is 192.168.x.x
So can anybody please tell me what is wrong i am doing here ? I really would appreciate the help.
Oh and this is the piece of code at SERVER side
//Somwhere up top
private final static MyRMIApp rmiApp = new RMIServer();
//Down below
MyRMIApp stub = (MyRMIApp) UnicastRemoteObject.exportObject(rmiApp, 0);
Registry registry = LocateRegistry.createRegistry(28999);
registry.rebind("COM", stub);

See item A.1 of the RMI FAQ: specifically, 'The appropriate workaround is to set the system property java.rmi.server.hostname when starting the server.'

Recommended way to get hostname in Java

Which of the following is the best and most portable way to get the hostname of the current computer in Java?
Runtime.getRuntime().exec("hostname")
vs
InetAddress.getLocalHost().getHostName()

Strictly speaking - you have no choice but calling either hostname(1) or - on Unix gethostname(2). This is the name of your computer. Any attempt to determine the hostname by an IP address like this
InetAddress.getLocalHost().getHostName()
is bound to fail in some circumstances:
The IP address might not resolve into any name. Bad DNS setup, bad system setup or bad provider setup may be the reason for this.
A name in DNS can have many aliases called CNAMEs. These can only be resolved in one direction properly: name to address. The reverse direction is ambiguous. Which one is the "official" name?
A host can have many different IP addresses - and each address can have many different names. Two common cases are: One ethernet port has several "logical" IP addresses or the computer has several ethernet ports. It is configurable whether they share an IP or have different IPs. This is called "multihomed".
One Name in DNS can resolve to several IP Addresses. And not all of those addresses must be located on the same computer! (Usecase: A simple form of load-balancing)
Let's not even start talking about dynamic IP addresses.
Also don't confuse the name of an IP-address with the name of the host (hostname). A metaphor might make it clearer:
There is a large city (server) called "London". Inside the city walls much business happens. The city has several gates (IP addresses). Each gate has a name ("North Gate", "River Gate", "Southampton Gate"...) but the name of the gate is not the name of the city. Also you cannot deduce the name of the city by using the name of a gate - "North Gate" would catch half of the bigger cities and not just one city. However - a stranger (IP packet) walks along the river and asks a local: "I have a strange address: 'Rivergate, second left, third house'. Can you help me?" The local says: "Of course, you are on the right road, simply go ahead and you will arrive at your destination within half an hour."
This illustrates it pretty much I think.
The good news is: The real hostname is usually not necessary. In most cases any name which resolves into an IP address on this host will do. (The stranger might enter the city by Northgate, but helpful locals translate the "2nd left" part.)
In the remaining corner cases you must use the definitive source of this configuration setting - which is the C function gethostname(2). That function is also called by the program hostname.

InetAddress.getLocalHost().getHostName() is the more portable way.
exec("hostname") actually calls out to the operating system to execute the hostname command.
Here are a couple other related answers on SO:
Java current machine name and logged in user?
Get DNS name of local machine as seen by a remote machine
EDIT: You should take a look at A.H.'s answer or Arnout Engelen's answer for details on why this might not work as expected, depending on your situation. As an answer for this person who specifically requested portable, I still think getHostName() is fine, but they bring up some good points that should be considered.

As others have noted, getting the hostname based on DNS resolution is unreliable.
Since this question is unfortunately still relevant in 2018, I'd like to share with you my network-independent solution, with some test runs on different systems.
The following code tries to do the following:
On Windows
Read the COMPUTERNAME environment variable through System.getenv().
Execute hostname.exe and read the response
On Linux
Read the HOSTNAME environment variable through System.getenv()
Execute hostname and read the response
Read /etc/hostname (to do this I'm executing cat since the snippet already contains code to execute and read. Simply reading the file would be better, though).
The code:
public static void main(String[] args) throws IOException {
String os = System.getProperty("os.name").toLowerCase();
if (os.contains("win")) {
System.out.println("Windows computer name through env:\"" + System.getenv("COMPUTERNAME") + "\"");
System.out.println("Windows computer name through exec:\"" + execReadToString("hostname") + "\"");
} else if (os.contains("nix") || os.contains("nux") || os.contains("mac os x")) {
System.out.println("Unix-like computer name through env:\"" + System.getenv("HOSTNAME") + "\"");
System.out.println("Unix-like computer name through exec:\"" + execReadToString("hostname") + "\"");
System.out.println("Unix-like computer name through /etc/hostname:\"" + execReadToString("cat /etc/hostname") + "\"");
}
}
public static String execReadToString(String execCommand) throws IOException {
try (Scanner s = new Scanner(Runtime.getRuntime().exec(execCommand).getInputStream()).useDelimiter("\\A")) {
return s.hasNext() ? s.next() : "";
}
}
Results for different operating systems:
macOS 10.13.2
Unix-like computer name through env:"null"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:""
OpenSuse 13.1
Unix-like computer name through env:"machinename"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:""
Ubuntu 14.04 LTS
This one is kinda strange since echo $HOSTNAME returns the correct hostname, but System.getenv("HOSTNAME") does not:
Unix-like computer name through env:"null"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:"machinename
"
EDIT: According to legolas108, System.getenv("HOSTNAME") works on Ubuntu 14.04 if you run export HOSTNAME before executing the Java code.
Windows 7
Windows computer name through env:"MACHINENAME"
Windows computer name through exec:"machinename
"
Windows 10
Windows computer name through env:"MACHINENAME"
Windows computer name through exec:"machinename
"
The machine names have been replaced but I kept the capitalization and structure. Note the extra newline when executing hostname, you might have to take it into account in some cases.

InetAddress.getLocalHost().getHostName() is better (as explained by Nick), but still not very good
One host can be known under many different hostnames. Usually you'll be looking for the hostname your host has in a specific context.
For example, in a web application, you might be looking for the hostname used by whoever issued the request you're currently handling. How to best find that one depends on which framework you're using for your web application.
In some kind of other internet-facing service, you'll want the hostname your service is available through from the 'outside'. Due to proxies, firewalls etc this might not even be a hostname on the machine your service is installed on - you might try to come up with a reasonable default, but you should definitely make this configurable for whoever installs this.

Although this topic has already been answered there's more to say.
First of all: Clearly we need some definitions here. The InetAddress.getLocalHost().getHostName() gives you the name of the host as seen from a network perspective. The problems with this approach are well documented in the other answers: it often requires a DNS lookup, it's ambiguous if the host has multiple network interfaces and it just plain fails sometimes (see below).
But on any OS there's another name as well. A name of the host that gets defined very early in the boot process, long before the network is initialized. Windows refers to this as computername, Linux calls it kernel hostname and Solaris uses the word nodename. I like best the word computername, so I'll use that word from now on.
Finding the computername
On Linux/Unix the computername is what you get from the C function gethostname(), or hostname command from shell or HOSTNAME environment variable in Bash-like shells.
On Windows the computername is what you get from environment variable COMPUTERNAME or Win32 GetComputerName function.
Java has no way of obtaining what I've defined as 'computername'. Sure, there are workarounds as described in other answers, like for Windows calling System.getenv("COMPUTERNAME"), but on Unix/Linux there's no good workaround without resorting to JNI/JNA or Runtime.exec(). If you don't mind a JNI/JNA solution then there's gethostname4j which is dead simple and very easy to use.
Let's move on with two examples, one from Linux and one from Solaris, which demonstrate how you can easily get into a situation where you cannot obtain the computername using standard Java methods.
Linux example
On a newly created system, where the host during installation has been named as 'chicago', we now change the so-called kernel hostname:
$ hostnamectl --static set-hostname dallas
Now the kernel hostname is 'dallas', as evident from the hostname command:
$ hostname
dallas
But we still have
$ cat /etc/hosts
127.0.0.1 localhost
127.0.0.1 chicago
There's no misconfiguration in this. It just means the host's networked name (or rather the name of the loopback interface) is different from the host's computername.
Now, try executing InetAddress.getLocalHost().getHostName() and it will throw java.net.UnknownHostException. You are basically stuck. There's no way to retrieve neither the value 'dallas' nor the value 'chicago'.
Solaris example
The example below is based on Solaris 11.3.
The host has deliberately been configured so that the loopback name <> nodename.
In other words we have:
$ svccfg -s system/identity:node listprop config
...
...
config/loopback astring chicago
config/nodename astring dallas
and the contents of /etc/hosts :
:1 chicago localhost
127.0.0.1 chicago localhost loghost
and the result of the hostname command would be:
$ hostname
dallas
Just like in the Linux example a call to InetAddress.getLocalHost().getHostName() will fail with
java.net.UnknownHostException: dallas: dallas: node name or service name not known
Just like the Linux example you are now stuck. There's no way to retrieve neither the value 'dallas' nor the value 'chicago'.
When will you really struggle with this?
Very often you'll find that InetAddress.getLocalHost().getHostName() will indeed return a value which is equal to the computername. So there's no problem (except for the added overhead of name resolution).
The problem arises typically within PaaS environments where there's a difference between computername and the name of the loopback interface. For example people report problems in Amazon EC2.
Bug/RFE reports
A bit of searching reveals this RFE report : link1, link2. However, judging from the comments on that report the issue seems to have been largely misunderstood by the JDK team, so it is unlikely it will be addressed.
I like the comparison in the RFE to other programming languages.

Just one-liner ... cross platform (Windows-Linux-Unix-Mac(Unix)) [Always works, No DNS required]:
String hostname = new BufferedReader(
new InputStreamReader(Runtime.getRuntime().exec("hostname").getInputStream()))
.readLine();
You're done !!

Environment variables may also provide a useful means -- COMPUTERNAME on Windows, HOSTNAME on most modern Unix/Linux shells.
See: https://stackoverflow.com/a/17956000/768795
I'm using these as "supplementary" methods to InetAddress.getLocalHost().getHostName(), since as several people point out, that function doesn't work in all environments.
Runtime.getRuntime().exec("hostname") is another possible supplement. At this stage, I haven't used it.
import java.net.InetAddress;
import java.net.UnknownHostException;
// try InetAddress.LocalHost first;
// NOTE -- InetAddress.getLocalHost().getHostName() will not work in certain environments.
try {
String result = InetAddress.getLocalHost().getHostName();
if (StringUtils.isNotEmpty( result))
return result;
} catch (UnknownHostException e) {
// failed; try alternate means.
}
// try environment properties.
//
String host = System.getenv("COMPUTERNAME");
if (host != null)
return host;
host = System.getenv("HOSTNAME");
if (host != null)
return host;
// undetermined.
return null;

The most portable way to get the hostname of the current computer in Java is as follows:
import java.net.InetAddress;
import java.net.UnknownHostException;
public class getHostName {
public static void main(String[] args) throws UnknownHostException {
InetAddress iAddress = InetAddress.getLocalHost();
String hostName = iAddress.getHostName();
//To get the Canonical host name
String canonicalHostName = iAddress.getCanonicalHostName();
System.out.println("HostName:" + hostName);
System.out.println("Canonical Host Name:" + canonicalHostName);
}
}

If you're not against using an external dependency from maven central, I wrote gethostname4j to solve this problem for myself. It just uses JNA to call libc's gethostname function (or gets the ComputerName on Windows) and returns it to you as a string.
https://github.com/mattsheppard/gethostname4j

hostName == null;
Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
{
while (interfaces.hasMoreElements()) {
NetworkInterface nic = interfaces.nextElement();
Enumeration<InetAddress> addresses = nic.getInetAddresses();
while (hostName == null && addresses.hasMoreElements()) {
InetAddress address = addresses.nextElement();
if (!address.isLoopbackAddress()) {
hostName = address.getHostName();
}
}
}
}

Building off of Dan Ortega's answer, I created a generic executeCommand(String) method that takes a command as a paramater.
import java.io.*;
public class SystemUtil {
public static void main(String[] args) throws IOException {
System.out.println(retrieveHostName());
}
public static String retrieveHostName() throws IOException {
return executeCommand("hostname");
}
private static String executeCommand(String command) throws IOException {
return new BufferedReader(
new InputStreamReader(Runtime.getRuntime().exec(command).getInputStream()))
.readLine();
}
}

InetAddress.getLocalHost().getHostName() is the best way out of the two as this is the best abstraction at the developer level.