We Keep Coding

Recreating Linux traceroute for an Android app

I am extremely new the Android app development and Stack Overflow. I am trying to recreate traceroute in an Android app since Android devices do not come with traceroute by default. I've encountered a couple stack overflow posts talking about solutions to this, but I have still run into challenges.
Traceroute on android - the top post on this thread links an Android Studio project that implements traceroute using ping. If I understand the algorithm correctly, it continually pings the destination IP, incrementing the time-to-live field to obtain information about intermediary routers. I've tried to recreate this behavior, but for certain values of TTL, the ping stalls and doesn't retrieve any router information. I'm not really sure why this happens. Here's a quick demo function I spun up... at some point in the loop the pings stall.
public static void smallTracerouteDemoShowingThatTheProgramStallsAtCertainTTLs() {
try {
String host = "google.com";
int maxTTL = 20;
for (int i = 1; i < maxTTL; i++) {
// Create a process that executes the ping command
Process p = Runtime.getRuntime().exec("ping -c 1 -t " + i + " " + host);
// Get a buffered reader with the information returned by the ping
BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
// Convert the BufferedReader to a string
String dataReturnedByPing = "";
for (String line; (line = br.readLine()) != null; dataReturnedByPing += "\n"+line);
// Print out information about each TTL
System.out.println("TTL = " + i + " out of " + maxTTL);
System.out.println(dataReturnedByPing);
System.out.println("========================================");
}
} catch (Exception e) {
e.printStackTrace();
}
}
how to run traceroute command through your application? - The solution on this thread suggests using BusyBox. I've not used BusyBox as yet, but it seems like I would have to embed BusyBox into my app to get things to work. After doing some research it looks like BusyBox provides numerous Linux commands through one executable. I'm a bit hesitant to explore this option because I really only need the traceroute command. In addition, I know that Android targets a few different CPU architectures, and I'm not sure if one executable will support them all.
I've also run into a github repository that takes another approach to running traceroute:
https://github.com/wangjing53406/traceroute-for-android - In this repository the author embeds the traceroute source code into the project and uses the NDK to build the source code along with the rest of his app. I really like this approach because it feels the most "correct." It uses a built traceroute instead of a Java-based implementation, so you can't find yourself in a situation where the Java implementation gives you one thing and the actual traceroute gives you another. When I open this project to experiment with it, my build fails. The top line says:
org.gradle.initialization.ReportedException: org.gradle.internal.exceptions.LocationAwareException: A problem occurred configuring root project 'traceroute-for-android-master'.
Any help on why this happens or ways to troubleshoot it would be fantastic.
For reference, the minimum SDK I am targeting is API 21 and I am running on Android Studio 3.3.0.
So, at this point I'm stumped. If you were trying to make an app that would let you execute traceroute commands, how would you do it? I really like the NDK approach because it guarantees you're getting true traceroute behavior. If you have any guides to getting that set up for my Android version/SDK, I would appreciate if you would post them. If you'd take another approach I'd to hear about it as well.
Thank you in advance.

ProcessBuilder's inputstream empty depending of OS

I made this simple piece of code to test ProcessBuilder:
#SpringBootApplication
public class TerminalDemoApplication {
public static void main(String[] args) {
SpringApplication.run(TerminalDemoApplication.class, args);
try {
System.out.println("hello");
Process process = new ProcessBuilder("python", "--version").start();
BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line);
}
int exitCode = process.waitFor();
System.out.println("\nExited with error code : " + exitCode);
} catch (IOException | InterruptedException e) {
e.printStackTrace();
}
}
}
It works in Windows (returns python version of my system) but the same code in my macbook returns end of line, so basically empty. ¿This needs further configuration according to the OS? ¿why is this happening?

What error code are you getting?
There are (at least) two explanations; that error code would indicate which one it is.
You're not running python, or running 'the wrong' python
This would mean you are getting an error code of some sort, or an exception.
The likely reason for this is a path issue.
Running python, just like that - as in, no path information at all, is nominally neccessarily broken: That's just not how your OS works, it has no idea what to do with this path.
It's a bashism (as in, the shell does it, not the OS) to interpret such a command as 'oh, actually, go through each listed entry in the $PATH environment variable, and stick that path in front of this name, see if you find an executable there. If you do, run that and stop).
Java mostly doesn't engage in any bashisms. But, in a few bizarre places, it does - it tries to do basic space splitting when you use the single-string version of new ProcessBuilder), which is a shellism, and it does attempt to do basic PATH lookup, but that's about where it ends. It won't do * unpacking, which on windows is an OS-level thing but on posix systems is a shellism.
I strongly, strongly advise you to avoid java's basic shellisms. It's unreliable and highly OS-specific.
So: Always pass arguments explicitly (good, you're doing that), always use ProcessBuilder (good, you're doing that), never use relative paths (that's where you're going wrong).
It's going to the error stream instead
processes on OSes are generally hooked up to 3 pipes, not 2. There's the 'standard in', the 'standard out' and the 'standard err'. Your own java process exposes these as System.out, in, and err.
In linux in particular, it is common to redirect standard out of some process to a file or another process.
This means that standard err naturally has the property that it tends to emit to the console, even if you are redirecting things. In other words, the terms 'standard out' and 'standard err' are really stupid names on posix. The much better naming would be 'standard process output' and 'standard process messages'.
Asking python to print its version is in a bit of a limbo scenario. The string "Python v3.0.1" or whatnot is certainly not an error, but it's a bit dubious if one should consider this as 'the output of the process'. It's likely that the authors of the python tool consider it more 'some information I should print to you, even if you are redirecting things.
Thus, my guess is that this version is heading out to standard err instead.
You can solve this in two ways: Either read from standard err as well, or, use process builder's features: You can ask it to bundle up standard out and standard err into a single stream (.redirectErrorStream(true)).
I would expect the exit code to be 0 if this explanation is the correct one.

Get Wi-Fi name and interface in JAVA [duplicate]

We're doing a project coded in Java (compiled for JRE 1.6) and need some help with
a little but apparently complicated feature:
We want to do a certain action when a specific wireless network is connected e.g. when the connected SSID=="myNetworkAtHome" or similar.
After looking through this site, google and the Java documentation we have come a little closer.
After looking at the code here:
http://download.oracle.com/javase/tutorial/networking/nifs/retrieving.html
It seems we were getting close but it hits a deadend, all the interfaces seems to be connected to "net0" through "net13" (on my laptop that is.)
And we're unable to get the SSID out of any interface at all. I do realise the code in the example is only giving the interface names and not connected networks, but it doesn't seem to offer a way of fetching the connected network information.
Any help on this would be extremely helpfull!

You can't access this low-level details of the network in Java. You can get some details of the network interface with the NetworkInterface class but if you see at the provided methods, no one is related to Wifi networks nor any way to get the SSID is provided. As pointed below, you should use some native functionality through calling a native library with JNI or by calling a OS tool with Runtime.
Java is not designed to do that kind of things, is hard to implement in a platform-independent way and any hardware-level detail can not be managed in Java by principle.
Same applies to other networks like 3G, GPRS... the application should not be aware of the connection type nor its details. Java can only manage things at the Transport (TCP) level, not the network (IP) not Link (3G, Wifi, Ethernet...), so you can only manage sockets.

ArrayList<String>ssids=new ArrayList<String>();
ArrayList<String>signals=new ArrayList<String>();
ProcessBuilder builder = new ProcessBuilder(
"cmd.exe", "/c", "netsh wlan show all");
builder.redirectErrorStream(true);
Process p = builder.start();
BufferedReader r = new BufferedReader(new InputStreamReader(p.getInputStream()));
String line;
while (r.read()!=-1) {
line = r.readLine();
if (line.contains("SSID")||line.contains("Signal")){
if(!line.contains("BSSID"))
if(line.contains("SSID")&&!line.contains("name")&&!line.contains("SSIDs"))
{
line=line.substring(8);
ssids.add(line);
}
if(line.contains("Signal"))
{
line=line.substring(30);
signals.add(line);
}
if(signals.size()==7)
{
break;
}
}
}
for (int i=0;i<ssids.size();i++)
{
System.out.println("SSID name == "+ssids.get(i)+" and its signal == "+signals.get(i) );
}

You'll have to resort to a JNI solution. There's something available at http://sourceforge.net/projects/jwlanscan, but that only works for Windows systems. Or you could do it the ugly way and use Runtime.getRuntime().exec(...) and use the command line tools available for your OS (*nix = iwconfig) and resort to parsing.

Saving btmon output

This is my first post here, although I've been reading stuff here for quite some time :)
I'm currently doing my bachelor thesis in computer science where we're doing an Indoors-positioning thing with iBeacons and Bluetooth. Where the device "being found" is currently a Raspberry Pi due to complications with smartphones.
I've got a bit of a problem with the btmon command on my Raspberry Pi. The full structure of our program is as follows:
Scan for bluetooth devices (iBeacons)
Save the output result <---- Here's the problem
Filter the desired data
Send to server node
As this is a part of a program we're building, the commands are executed through Java code that executes the terminal command, and returns the output as a String. The method looks like this:
private String executeCommand(String command) {
StringBuffer output = new StringBuffer();
Process p;
try {
p = Runtime.getRuntime().exec(command);
p.waitFor();
BufferedReader reader = new BufferedReader(new InputStreamReader(p.getInputStream()));
String line = "";
while ((line = reader.readLine()) != null) {
output.append(line + "\r\n");
}
} catch (Exception e) {
e.printStackTrace();
}
return output.toString();
}
I've got everything working when using another command (such as ifconfig) so the method works for ordinary commands, but I can't get it to work with btmon.
The full command I'm trying to run is this:
sudo btmon & sudo hcitool lescan
It seems to me as if btmon is running as a different process/runtime which would explain why the above Java code doesn't catch its output. I've been trying to "get" the btmon runtime but have failed to do so, and I've been trying to use ProcessBuilder with inheritIO() to change it's output source but couldn't get that to work either...
I've been googling this for a couple of days by now, and I've bumped into a couple of Python scripts that I can't get to work either (probably because I've never used Python before ^^).
So, in short: How do I save the output of sudo btmon & sudo hcitool lescan? Preferably to a String through Java code, but saving it as a file would also work!
Thanks in advance! :)

I got the same problem and spent many days to solve this issue. The key point was Thread and not using &. Here is what I discovered:
& literally runs in the background. Change sudo btmon & to sudo btmon
Use Thread.
Thread 1 - start btmon (You can get result this Thread)
Thread 2 - start hcitool lescan
(btmon should first - because "lescan" occur hci events so btmon can get them) Summary - do not use & and use independent thread.

Is Windows 7 Bug Preventing Java getHostName() from Retrieving the Computer Name? [duplicate]

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.