如何使用Java从互联网读取文本文件？

Question

I want to read the second line of the text at this URL: " http://vuln2014.picoctf.com:51818/ " (this is a capture-the-flag competition but only asking for flags or direction to flags breaks the competition rules). I am attempting to open an input stream from the URL but I get an Invalid HTTP Response exception. Any help is appreciated, and I recognize that my error is likely quite foolish.

Code:

URL url = new URL("http://vuln2014.picoctf.com:51818");
URLConnection con = url.openConnection();
InputStream is = con.getInputStream()

The error occurs at the third line. java.io.IOException: Invalid Http response at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1342) at name.main(name.java:41)

curl happily gets the text from the page, and it is perfectly accessible from a web browser.

Answer 1

When you do this:

URL url = new URL("http://vuln2014.picoctf.com:51818");
URLConnection con = url.openConnection();

You are entering into a contract that says that this URL uses the http protocol. When you call openConnection it expects to get http responses because you used http:// in the URL as the protocol. The Java Documentation says:

If for the URL's protocol (such as HTTP or JAR), there exists a public, specialized URLConnection subclass belonging to one of the following packages or one of their subpackages: java.lang, java.io, java.util, java.net, the connection returned will be of that subclass. For example, for HTTP an HttpURLConnection will be returned , and for JAR a JarURLConnection will be returned.

The server you are connecting to just returns a couple lines of data. I retrieved them with the command nc vuln2014.picoctf.com 51818 . There is no http response code like HTTP/1.1 200 OK :

Welcome to the Daedalus Corp Spies RSA Key Generation Service. The public modulus you should use to send your updates is below. Remember to use exponent 65537. b4ab920c4772c5247e7d89ec7570af7295f92e3b584fc1a1a5624d19ca07cd72ab4ab9c8ec58a63c09f382aa319fa5a714a46ffafcb6529026bbc058fc49fb1c29ae9f414db4aa609a5cab6ff5c7b4c4cfc7c18844f048e3899934999510b2fe25fcf8c572514dd2e14c6e19c4668d9ad82fe647cf9e700dcf6dc23496be30bb

In this case I would use java.net.Socket to establish a connection and then read the lines. This is a simplistic approach that assumes there are 2 lines of data:

Socket theSocket;
try {
    theSocket = new Socket("vuln2014.picoctf.com", 51818);
    BufferedReader inFile = new BufferedReader(new InputStreamReader(theSocket.getInputStream()));

    String strGreet = inFile.readLine();
    String strData = inFile.readLine();
} catch (IOException e) {
    e.printStackTrace();
}

As for why curl and browsers may render it properly? They are likely more lenient about the data they read and will just dump what is read from the port even if it doesn't conform to the specified protocol (like http )