CODEWITHSUNDEEP
javaip-addresshostname
user10059
user10059

Reputation: 99

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

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.

Upvotes: 4

Views: 4750

Answers (9)

BillS
BillS

Reputation: 85

This code still performs the DNS lookup if a host name is specified, but at least it skips the reverse lookup that may be performed with other approaches:

   ...
   isDottedQuad("1.2.3.4");
   isDottedQuad("google.com");
   ...

boolean isDottedQuad(String hostOrIP) throws UnknownHostException {
   InetAddress inet = InetAddress.getByName(hostOrIP);
   boolean b = inet.toString().startsWith("/");
   System.out.println("Is " + hostOrIP + " dotted quad? " + b + " (" + inet.toString() + ")");
   return b;
}

It generates this output:

Is 1.2.3.4 dotted quad? true (/1.2.3.4)
Is google.com dotted quad? false (google.com/172.217.12.238)

Do you think we can expect the toString() behavior to change anytime soon?

Upvotes: 0

Denis Kalinin
Denis Kalinin

Reputation: 337

Use InetAddress#getAllByName(String hostOrIp) - if hostOrIp is an IP-address the result is an array with single InetAddress and it's .getHostAddress() returns the same string as hostOrIp.

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Arrays;

public class IPvsHostTest {
    private static final org.slf4j.Logger LOG = org.slf4j.LoggerFactory.getLogger(IPvsHostTest.class);

    @org.junit.Test
    public void checkHostValidity() {
        Arrays.asList("10.10.10.10", "google.com").forEach( hostname -> isHost(hostname));
    }
    private void isHost(String ip){
        try {
            InetAddress[] ips = InetAddress.getAllByName(ip);
            LOG.info("IP-addresses for {}", ip);
            Arrays.asList(ips).forEach( ia -> {
                LOG.info(ia.getHostAddress());
            });
        } catch (UnknownHostException e) {
            LOG.error("Invalid hostname", e);
        }
    }
}

The output:

IP-addresses for 10.10.10.10
10.10.10.10
IP-addresses for google.com
64.233.164.100
64.233.164.138
64.233.164.139
64.233.164.113
64.233.164.102
64.233.164.101

Upvotes: 0

Sean F
Sean F

Reputation: 4605

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

Upvotes: 1

davenpcj
davenpcj

Reputation: 12684

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.

Upvotes: 1

zxcv
zxcv

Reputation: 7631

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.

Upvotes: 2

Sam
Sam

Reputation: 2201

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.

Upvotes: 15

jjnguy
jjnguy

Reputation: 138874

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.

Upvotes: 1

davetron5000
davetron5000

Reputation: 24841

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

Upvotes: 0

Joel Coehoorn
Joel Coehoorn

Reputation: 415735

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.

Upvotes: 2

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