How to discover web servers on a local network?

Question

Suppose I'm running several servers serving basic requests on my local network (say a home network, where all machines generally have an IP in the form K.K.K.x, where x is variable). Is there an easy way to discover all such servers? I would need to find each IP on the network running a particular java server application.

Answer 1

So you have a class C network... just attempt to open the particular server applications port on each IP address between 1 and 254. If you want to get fancy, you can do some checks to verify that if the port actually opens, it does what you expect for that application.

Answer 2

Since not every webserver listens on a fixed port of 80, you'll need to scan ports. Open (and close!) in a loop a socket on every port, you can use java.net.Socket for this. If success, then fire a simple GET request on / on the IP/port in question, you can use java.net.URLConnection for this. If you receive a valid HTTP response (regardless of the status code), then it's a webserver. You can determine the server type by the Server header in the HTTP response. You however need to take into account that the default response header can be spoofed/removed/changed by the server admin.

Note that some firewalls may recognize the scanning pattern of a port scanner and block this.

Answer 3

I would scan IP range and test Socket connection to port 80 with a code like this:

try {
       s = new Socket(addr,80);
       System.out.println("Webserver found on IP:" + addr );
       s.close();
     }
catch (IOException ex) {
       System.out.println("Webserver NOT found on IP:" + addr );
    }

Answer 4

Are you looking for a solution within your Java application? If you are simply looking for a one-time solution, nmap should work.

If your local network is of the form 192.*.*.* try this (I added sudo because it must be run as root):

sudo nmap -vv -sS -O -n "192.168.1.1/24"

Answer 5

There are 2 basic ways to achieve this, and both require you to send/receive packets to/from each host you want to check.

• The simplest approach is trying to establish the TCP connection to the desired port, as you'd normally do.

• A more complicated and fancy approach, but faster in terms of execution, is sending a SYN packet, and waiting for a response packet. If the response is a SYN/ACK, it means the port is open (waiting for a connection), otherwise, it isn't. However, Java doesn't provide the necessary API (see raw socket) for this. You'd have to find a library/wrapper or write one using C and JNI.

For more insights to the 2nd option, read this.