How do you tell whether a string is an IP or a hostname

Question

So you have a String that is retrieved from an admin web UI (so it is definitely a String). How can you find out whether this string is an IP address or a hostname in Java?

Update: I think I didn't make myself clear, I was more asking if there is anything in the Java SDK that I can use to distinguish between IPs and hostnames? Sorry for the confusion and thanks for everybody who took/will take the time to answer this.

Answer 1

You can use a regular expression with this pattern:

\b(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b

That will tell you if it's an IPv4 address.

Answer 2

Do we get to make the assumption that it is one or the other, and not something completely different? If so, I'd probably use a regex to see if it matched the "dotted quad" format.

Answer 3

You can see if the string matches the number.number.number.number format, for example:

\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

will match anything from 0 - 999.

Anything else you can have it default to hostname.

Answer 4

URI validator = new URI(yourString);

That code will validate the IP address or Hostname. (It throws a malformed URI Exception if the string is invalid)

If you are trying to distinguish the two..then I miss read your question.

Answer 5

Couldn't you just to a regexp match on it?

Answer 6

You can use a security manager with the InetAddress.getByName(addr) call.

If the addr is not a dotted quad, getByName will attempt to perform a connect to do the name lookup, which the security manager can capture as a checkConnect(addr, -1) call, resulting in a thrown SecurityException that you can catch.

You can use System.setSecurityManager() if you're running fully privileged to insert your custom security manager before the getByName call is made.

Answer 7

It is not as simple as it may appear, there are some ambiguities around characters like hyphens, underscore, and square brackets '-', '_', '[]'.

The Java SDK is has some limitations in this area. When using InetAddress.getByName it will go out onto the network to do a DNS name resolution and resolve the address, which is expensive and unnecessary if all you want is to detect host vs address. Also, if an address is written in a slightly different but valid format (common in IPv6) doing a string comparison on the results of InetAddress.getByName will not work.

The IPAddress Java library will do it. The javadoc is available at the link. Disclaimer: I am the project manager.

static void check(HostName host) {
    try {
        host.validate();
        if(host.isAddress()) {
            System.out.println("address: " + host.asAddress());
        } else {
            System.out.println("host name: " + host);
        }
    } catch(HostNameException e) {
        System.out.println(e.getMessage());
    }
}

public static void main(String[] args) {
    HostName host = new HostName("1.2.3.4");
    check(host);
    host = new HostName("1.2.a.4");
    check(host);
    host = new HostName("::1");
    check(host);
    host = new HostName("[::1]");
    check(host);
    host = new HostName("1.2.?.4");
    check(host);  
}

Output:

address: 1.2.3.4
host name: 1.2.a.4
address: ::1
address: ::1
1.2.?.4 Host error: invalid character at index 4