I am trying to data from a server. Sometimes my code fails due to an UnknownHostException. Why is that? What is the cause of this problem?
This may occur if a hiccup in DNS server has occurred. Apart from making the DNS server more robust or looking for another one, you can also just use the full IP address instead of the hostname. This way it doesn't need to lookup the IP address based on the hostname. However, I would rather fix the DNS issue and prefer the DNS since IP addresses may change from time to time.
An UnknownHostException indicates the host specified couldn't be translated to an IP address. It could very well be a problem with your DNS server.
If the DNS resolution fails intermittently, catch the exception and try again until you get name resolution. You can only control, what you can control... And if you can't control/fix the DNS server, make your app robust enough to handle the quirky DNS server.
I too am seeing sporadic UnknownHostExceptions in Java for no apparent reason. The solution is just to retry a few times. Here is a wrapper for DocumentBuilder.parse that does this:
static Document DocumentBuilder_parse(DocumentBuilder b, String uri) throws SAXException, IOException {
UnknownHostException lastException = null;
for (int tries = 0; tries < 2; tries++) {
try {
return b.parse(uri);
} catch (UnknownHostException e) {
lastException = e;
System.out.println("Retrying because of: " + e);
continue;
}
}
throw lastException;
}
Related
I am trying to connect to an external server from my Solaris server using SSH in my Java application. Some how we are getting exception while authenticating the user but after 60min. How can we decrease the timing to get the exception to 5min or so.
There was a problem while connecting to IP:PORT
java.io.IOException: There was a problem while connecting to IP:PORT
The time difference between time2 and time3 are around 60min. I want to decrease this time.
Please find below the code snippet that we are using.
try
{
timeout = 1;
if (connection == null)
{
//time1
connection = new ch.ethz.ssh2.Connection(getIpAddress(), Integer.parseInt(getPort()));
if (connection == null)
{
System.out.println("connection is null");
}
else
{
//time2
connection.connect(null, timeout, 0);
}
}
}
catch (Exception t)
{
//time3
System.out.println(t.getLocalizedMessage());
System.out.println(t.toString());
}
Edit1: After checking SSH related configuration files I found KeyRegenerationInterval having a value of 3600s. Is this useful to resolve this issue. What might be the outcome if I decrease its value to some 30min or 5min.
Let me summarize your problem, please correct me if something wrong.
You can not connect to SSH server.
An exception raised after 60 mins.
According to below Throws section of method connect(ServerHostKeyVerifier, int, int), I guess you might be facing issue due to a buggy proxy which doesn't return proper HTTP response. Using direct internet connection to see whether the issue is gone.
Throws:
java.io.IOException - If any problem occurs, e.g., the server's host key is not accepted by the verifier or there is problem during
the initial crypto setup (e.g., the signature sent by the server is
wrong).
In case of a timeout (either connectTimeout or kexTimeout) a SocketTimeoutException is thrown.
An exception may also be thrown if the connection was already successfully connected (no matter if the connection broke in the mean
time) and you invoke connect() again without having called close()
first.
If a HTTP proxy is being used and the proxy refuses the connection, then a HTTPProxyException may be thrown, which contains
the details returned by the proxy. If the proxy is buggy and does not
return a proper HTTP response, then a normal IOException is thrown
instead.
Try setting the "kexTimeout" (Timeout for complete connection establishment (non-negative, in milliseconds). Zero means no timeout.) to non-zero
connection.connect(null, timeout, timeout);
Also, in your catch print the full stacktrace to verify where the timeout occurs
catch (Exception t) {
//time3
t.printStackTrace();
}
I am trying to data from a server. Sometimes my code fails due to an UnknownHostException. Why is that? What is the cause of this problem?
This may occur if a hiccup in DNS server has occurred. Apart from making the DNS server more robust or looking for another one, you can also just use the full IP address instead of the hostname. This way it doesn't need to lookup the IP address based on the hostname. However, I would rather fix the DNS issue and prefer the DNS since IP addresses may change from time to time.
An UnknownHostException indicates the host specified couldn't be translated to an IP address. It could very well be a problem with your DNS server.
If the DNS resolution fails intermittently, catch the exception and try again until you get name resolution. You can only control, what you can control... And if you can't control/fix the DNS server, make your app robust enough to handle the quirky DNS server.
I too am seeing sporadic UnknownHostExceptions in Java for no apparent reason. The solution is just to retry a few times. Here is a wrapper for DocumentBuilder.parse that does this:
static Document DocumentBuilder_parse(DocumentBuilder b, String uri) throws SAXException, IOException {
UnknownHostException lastException = null;
for (int tries = 0; tries < 2; tries++) {
try {
return b.parse(uri);
} catch (UnknownHostException e) {
lastException = e;
System.out.println("Retrying because of: " + e);
continue;
}
}
throw lastException;
}
I am trying to data from a server. Sometimes my code fails due to an UnknownHostException. Why is that? What is the cause of this problem?
This may occur if a hiccup in DNS server has occurred. Apart from making the DNS server more robust or looking for another one, you can also just use the full IP address instead of the hostname. This way it doesn't need to lookup the IP address based on the hostname. However, I would rather fix the DNS issue and prefer the DNS since IP addresses may change from time to time.
An UnknownHostException indicates the host specified couldn't be translated to an IP address. It could very well be a problem with your DNS server.
If the DNS resolution fails intermittently, catch the exception and try again until you get name resolution. You can only control, what you can control... And if you can't control/fix the DNS server, make your app robust enough to handle the quirky DNS server.
I too am seeing sporadic UnknownHostExceptions in Java for no apparent reason. The solution is just to retry a few times. Here is a wrapper for DocumentBuilder.parse that does this:
static Document DocumentBuilder_parse(DocumentBuilder b, String uri) throws SAXException, IOException {
UnknownHostException lastException = null;
for (int tries = 0; tries < 2; tries++) {
try {
return b.parse(uri);
} catch (UnknownHostException e) {
lastException = e;
System.out.println("Retrying because of: " + e);
continue;
}
}
throw lastException;
}
How to write a java program which will tell me whether I have internet access or not. I donot want to ping or create connection with some external url because if that server will be down then my program will not work. I want reliable way to detect which will tell me 100% guarantee that whether I have internet connection or not irrespective of my Operating System. I want the program for the computers who are directly connected to internet.
I have tried with the below program
URL url = new URL("http://www.xyz.com/");
URLConnection conn = url.openConnection();
conn.connect();
I want something more appropriate than this program
Thanks
Sunil Kumar Sahoo
It depends on what you mean by "internet" connection. Many computers are not connected directly to the internet, so even if you could check whether they have a network connection, it doesn't always mean they can access the internet.
The only 100% reliable way to test whether the computer can access some other server is to actually try.
Effective connectivity to the internet (i.e. where you can actually do stuff) depends on lots of things being correct, on your machine, your local net, your router, your modem, your ISP and so on. There are lots of places where a failure or misconfiguration will partly or completely block network access.
It is impossible to test all of these potential failure points with any certainty ... or even to enumerate them. (For example, you typically have no way of knowing what is happening inside your ISP's networking infrastructure.)
As #codeka says: "the only 100% reliable way to test whether the computer can access some other server is to actually try".
I think if you were to open up a HTTP session with all of:
www.google.com
www.microsoft.com
www.ibm.com
www.ford.com
and at least one of them came back with a valid response, you would have internet connectivity. Don't keep testing once you get a valid response since that would be a waste.
Feel free to expand on that list with some more mega-corporations in case you fear that all four of them may be down at the same time :-)
Of course, even that method can be tricked if someone has taken control of your DNS servers but it's probably about as reliable as you're going to get.
Just put a try/catch block around the code you mentioned. If an exception is thrown/caught then you don't have connectivity.
boolean connectivity;
try {
URL url = new URL("http://www.xyz.com/");
URLConnection conn = url.openConnection();
conn.connect();
connectivity = true;
} catch (Exception e) {
connectivity = false;
}
For better results investigate what kind of exceptions can be thrown and handle each individually.
You can check the connectivity by ask the Internet Protocol from InetAddress class. If you get an exception, or for example you use getLocalHost() -- which is returns the address of the local host -- give you the following output:
localhost/127.0.0.1 instead of fully qualified name like for example jaf-stephen-lenovoG40-80/152.6.44.13 then you're not connected to the Internet.
public static void main(String[] args) {
try {
InetAddress address = InetAddress.getByName("www.facebook.com");
System.out.println(address);
} catch (UnknownHostException e) {
System.err.println("Couldn't find www.facebook.com");
}
}
if you're connected to the Internet, you'll get the following output:
www.facebook.com/31.13.78.35
Enumaration<NetworkInterface> networkInterface = null;
networkInterface = NetworkInterface.getNetworkInterfaces();
for(NetworkInterface interface : Collections.list(networkInterface)){
System.out.println("Internet Available status is :"+ interface.isUp());
}
I am trying to get hostname/computer name using this method. Unfortunately i only can get localhost but not other computer.
private String getHostName(String _strIP) {
try {
InetAddress inetAddress = InetAddress.getByName(_strIP);
System.out.println("getHostAddress : " + inetAddress.getHostAddress());
System.out.println("getHostName : " + inetAddress.getHostName());
System.out.println("getCanonicalHostName : " + inetAddress.getCanonicalHostName());
return inetAddress.getHostName();
} catch (UnknownHostException e) {
e.printStackTrace();
}
return strDefaultHostName;
}
the result (not localhost)
getHostAddress : 192.168.2.139
getHostName : 192.168.2.139
getCanonicalHostName : 192.168.2.139
the result (localhost)
getHostAddress : 127.0.0.1
getHostName : localhost
getCanonicalHostName : localhost
Thank you
We've established roughly what the problem is in tangens' answer.
I think you can fix the problem pretty simply by putting host names into your hosts file.
%SystemRoot%\system32\drivers\etc\hosts
is the file you're looking for; localhost is defined here. You want to put a name and address line in it for every host you want to resolve.
I've never tried this. If it doesn't work, you get your money back.
Update
The above is the "quick hack" solution. This essentially entails that whenever someone manually changes the IP address of a host you're interested in, someone must at the same time change the hosts files on any machines that want to access those hosts.
The other alternative is to operate your own DNS server. You still need to update IP addresses when a host's address changes, but you only need to do so in one place, and you get both forward and reverse name resolution throughout your network. This takes more setting up but is easier to maintain in the long run.
Here is a very useful reference: http://www.dns.net/dnsrd/servers/windows.html
They mention that the "built in" Microsoft DNS server is a terrible solution (up until the one in Windows 2003 Server) but mention at least two alternatives, one commercial and one free. BIND is what is currently holding much of the Internet together, DNS-wise, and it's great that they have a Windows port too.
Looking at the source for InetAddress.getHostName() (Sun JDK8)...
The method performs the following logic:
Loops through the available sun.net.spi.nameservice.NameService's
Performs a reverse DNS lookup - e.g. 192.168.0.23 -> frodo.baggins.com.au
*Checks with the java.lang.SecurityManager, to see if "we have permission to connect" to hostname
*Performs a forward DNS lookup on the hostname, to prevent spoofing - e.g. frodo.baggins.com.au -> 192.168.0.99
If forward lookup result matches the original address (e.g. 192.168.0.23 == 192.168.0.99?), return hostname, otherwise return getHostAddress()
*If step 3 or 4 throws a SecurityException/UnknownHostException, return getHostAddress()
For me, step #2 successfully resolved the hostname, but failed at step #4 with an UnknownHostException.
TLDR; you must fulfill ALL of the following requirements:
the SecurityManager must provide permission to access the host
you must be able to forward AND reverse DNS lookup your InetAddress
the forward lookup details MUST match the reverse lookup details
Only then will Java give you the hostname.
OR, you could bypass these steps with the following method, and just get the hostname.
#SuppressWarnings("unchecked")
public static String getHostName(InetAddress addr) {
String host = null;
List<NameService> nameServicesImpl = new ArrayList<>();
try {
// do naughty things...
Field nameServices = InetAddress.class.getDeclaredField("nameServices");
nameServices.setAccessible(true);
nameServicesImpl = (List<NameService>) nameServices.get(null);
} catch (Throwable t) {
throw new RuntimeException("Got caught doing naughty things.", t);
}
for (NameService nameService : nameServicesImpl) {
try {
// lookup the hostname...
host = nameService.getHostByAddr(addr.getAddress());
} catch (Throwable t) {
// NOOP: problem getting hostname from this name service, continue looping...
}
}
return host != null ? host : addr.getHostAddress();
}
Your DNS is broken. Then IP-numbers are returned instead.
The javadoc of InetAddress.getCanonicalHostName() says:
Gets the fully qualified domain name for this IP address. Best effort method, meaning we may not be able to return the FQDN depending on the underlying system configuration.
If there is a security manager, this method first calls its checkConnect method with the hostname and -1 as its arguments to see if the calling code is allowed to know the hostname for this IP address, i.e., to connect to the host. If the operation is not allowed, it will return the textual representation of the IP address.
I looks like your system configuration isn't correct. Are you running from within an applet?
Reply Feedback for Carl Smotricz
Great answer, but we still don't know if the host name has been updated or not...
This is something like we hardcode.
Anyway thank you so much
# Copyright (c) 1993-1999 Microsoft Corp.
#
# This is a sample HOSTS file used by Microsoft TCP/IP for Windows.
#
# This file contains the mappings of IP addresses to host names. Each
# entry should be kept on an individual line. The IP address should
# be placed in the first column followed by the corresponding host name.
# The IP address and the host name should be separated by at least one
# space.
#
# Additionally, comments (such as these) may be inserted on individual
# lines or following the machine name denoted by a '#' symbol.
#
# For example:
#
# 102.54.94.97 rhino.acme.com # source server
# 38.25.63.10 x.acme.com # x client host
127.0.0.1 localhost
192.168.2.139 dev-testing
The problem can be caused by multiple reasons.
Reason 1: the IP address doesn't have a hostname
This is probably the most common reason, and has nothing to do with security managers.
If an IP address doesn't resolve to a hostname, because there is no hostname, then you would expect getHostName() to return null or throw a UnknownHostException, but this doesn't happen. Instead getHostName() simply returns the IP address as a string back again. For reasons unknown to me, this common situation is undocumented.
So if the IP address is the same as the result returned by getHostName(), then the hostname doesn't exist.
Detailed explanation
The following JDK code is the cause of this undocumented problem:
https://github.com/openjdk/jdk/blob/jdk-17+35/src/java.base/share/classes/java/net/InetAddress.java#L697
public class InetAddress implements java.io.Serializable {
private static String getHostFromNameService(InetAddress addr, boolean check) {
String host = null;
try {
// 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) {
#SuppressWarnings("removal")
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);
boolean ok = false;
if(arr != null) {
for(int i = 0; !ok && i < arr.length; i++) {
ok = addr.equals(arr[i]);
}
}
//XXX: if it looks a spoof just return the address?
if (!ok) {
host = addr.getHostAddress();
return host;
}
} catch (SecurityException e) {
host = addr.getHostAddress();
} catch (UnknownHostException e) {
host = addr.getHostAddress();
// let next provider resolve the hostname
}
return host;
}
}
So what happens is that the IP-address is passed to NameService.getHostByAddr() (NameService is a private interface), which has this (private) documentation in the source code:
Lookup the host corresponding to the IP address provided
#param addr byte array representing an IP address
#return {#code String} representing the host name mapping
#throws UnknownHostException if no host found for the specified IP address
So NameService.getHostByAddr() throws an UnknownHostException if the IP doesn't have a hostname, but InetAddress.getHostFromNameService() swallows this exception and instead, it returns the provided IP-address itself!!! IMO it should have let the exception be thrown instead of swallowing it, because swallowing it makes it more difficult for the client to determine whether a hostname exists.
You can check if the IP address has a hostname by using the nslookup commandline tool: nslookup 192.168.2.139. If it returns something like:
** server can't find 139.2.168.192.in-addr.arpa: NXDOMAIN (Linux) or *** can't find 192.168.2.139: Non-existent domain (Windows) then there is no hostname.
Reason 2: a security manager is applied
By default, Java doesn't have a security manager enabled. In that case, this reason doesn't apply.
A security manager is an object that defines a security policy for an application. If you have a security manager and want to find out if it is the cause of your problem, then you should check whether it is allowing you to open a socket to the resolved hostname (if any). To do so, first use nslookup 192.168.2.139 and verify if a hostname is resolved. If no hostname is resolved, then your problem is caused by "Reason 1". If it does resolve to a hostname, for example myhostname, then try this:
SecurityManager sec = System.getSecurityManager();
if (sec != null) {
sec.checkConnect("myhostname", -1);
}
If checkConnect() throws a SecurityException, then a SecurityManager is active and is causing the problem. So then you could look into how you can configure your securityManager to solve the problem.