how to create a URL object using an InetAddress object in Java?

Question

I have a need to create a http or https URL object from an IPv4 address (String or InetAddress objects - either one is ok) in Java. I have been at this for 10 hours now.

Attempts that hit a wall described below:

Attempt #1: I tried to make the URL by assembling a string, and then feeding it to a URL constructor.
- Textbook states that a URL can be "protocol://host", with host being either a host name or IP address. but creating a URL like this: URL a = new URL("http://151.101.65.69"); and opening a stream to this URL (a) gives a HTTP error 500 (Internal Server Error - An unexpected condition occurred that the server does not know how to handle).
- What get me fuming is that URL a = new URL("http://stackoverflow.com"); works.
- At this point I am stuck. I have no Idea what to change, or how to move forward.
Attempt #2: I tried to do a reverse lookup on the IP address using "getHostName()" method in the InetAddress class.
- this should return the host name by doing a reverse DNS lookup. Yet, I keep trying it for 151.101.65.69 (stackoverflow web server IP address), and the look up fails. By fails I mean the IP address is returned as string rather than the host name as a string. I read the Oracle docs http://docs.oracle.com/javase/1.5.0/docs/api/java/net/InetAddress.html#getHostName(), but I don't understand how to overcome the "security manager" the document mentions (or if it is indeed the reason the reverse lookup fails).
- I also tried "getCannonicalHostName()", but that didn't fly either.
I am trying to figure out how to open a website using the IP address. It looks like my browser is running into the same issue as my code. I read up on How to access site through IP address when website is on a shared host? but I do not have any user names, as I want to be able to open any website that a user has an IP address for. Adding a port (such as 80) does not seem to work; neither does leaving the user name blank or using a generic 'user' or 'guest'.

I need is to create a URL object from an IPv4 String or InetAddress object, and I am stuck. I understand that a knowledgeable programmer such as you, may say that making URLs from IP addresses is not what IP addresses are for, or point out that I am not including a file portion of the URL, but that is not the problem at this moment. Could you please help me with my core challenge?

What happens if you load `http://151.101.65.69` in a Web browser? — chrylis -cautiouslyoptimistic-, Sep 18 '16 at 01:45
@chrylis I get an error message that reads: "Fastly error: unknown domain: 151.101.65.69. Please check that this domain has been added to a service." — Mike, Sep 18 '16 at 01:46
I forgot to mention this in the post, but i get this message for other addresses I tried as well, such as Google, and Duckduckgo. — Mike, Sep 18 '16 at 01:48
I realize that somehow IP addresses do not mesh well in URLs. I was hoping someone would suggest some way to figure this out. Perhaps suggest a route to take. — Mike, Sep 18 '16 at 01:51
@chrylis I did try to include a `~` at the end of the string I was feeding to `URL a = new URL("http://151.101.65.69~");` but it did not work either. — Mike, Sep 18 '16 at 01:59
Take a look at the documentation for the HTTP `Host` header. — chrylis -cautiouslyoptimistic-, Sep 18 '16 at 02:25

score 2 · Answer 1 · answered Sep 20 '16 at 02:25

The answer provided by D.B. is good. I had very similar code; but you will find that this code will not work every time. There are IPv4 addresses you pass to the code offered D.B.'s answer which will not be able to open a URL stream (for example the IP address for stackoverflow). I thought the problem was my coding, and that is what I was hoping to get help with on stackoverflow. But I now realize the problem was my lack of understanding when asking this question. What I now understand, is that having an IPv4 address is not sufficient to open every website on the web. Anytime a server hosts multiple websites, the IP address can be used to connect to the server, but not to simultaneously identify the website we want to open/access. This gentleman explains this quite well: http://ask-leo.com/why_doesnt_accessing_a_site_by_its_ip_address_work.html

@D.B. thanks for taking the time to help. Much appreciated!

score 1 · Accepted Answer · answered Sep 18 '16 at 03:10

The following code works for me.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.InetAddress;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.UnknownHostException;

public class InetAddressMain {

    public static void main(String[] args) {
        try {
            InetAddress addr = InetAddress.getByName("172.217.4.110");

            URL url = new URL("http://"+addr.getHostAddress());
            InputStream is = url.openStream();

            InputStreamReader isReader = new InputStreamReader(is);
            BufferedReader reader = new BufferedReader(isReader);
            String line;
            while((line = reader.readLine()) != null){
                System.out.println(line);
            }

        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

Output:

<!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="en"><head><meta content="Search the world's information, including webpages, images, videos and more. Google has many special features to help you find exactly what you're looking for." ... [output shortened for readability]

This answer is great, but this code will not always work. See my answer as well to get a full understanding of the issue. — Mike, Sep 20 '16 at 02:29

how to create a URL object using an InetAddress object in Java?

2 Answers2