InetAddress.getCanonicalHostName() returns IP instead of Hostname

Question

I looked for how to do IP lookup in Java on Stack Overflow but the answers match what I am already doing and do not resolve my problem.

Here is my code:

Answer 1

I did not dig a lot into this so I don't know why it is happening, but some online tools (like this one) that checks the health of DNS servers indicates that they have some issues which may or may not be related.

As @jah said, Java tries to double check to see if the hostname has the ip it said it has. The exception is thrown on the native code while trying to do that. In fact, in my case, trying to verify on the command line, the nslookup fails to get the ip from the name, which indicates some configuration preventing this on the DNS Server (maybe on purpose? I'm no expert in DNS either).

When it works:

$ nslookup msnbot-157-55-39-29.search.msn.com
Server:         192.168.1.1
Address:        192.168.1.1#53

Non-authoritative answer:
Name:   msnbot-157-55-39-29.search.msn.com
Address: 157.55.39.29

When it doesnt work:

$ nslookup baiduspider-123-125-71-75.crawl.baidu.com
Server:         192.168.1.1
Address:        192.168.1.1#53

** server can't find baiduspider-123-125-71-75.crawl.baidu.com: NXDOMAIN

When it works:

$ getent hosts msnbot-157-55-39-29.search.msn.com
157.55.39.29    msnbot-157-55-39-29.search.msn.com

When it doesn't:

$ getent hosts baiduspider-123-125-71-75.crawl.baidu.com
$

As an alternative, you can use a DNS Service Provider for JNDI. The documentation has an example, but I'll leave a working snippet for you to test:

String[] octets = "123.125.71.75".split("\\.");
String host = String.join(".", octets[3], octets[2], octets[1], octets[0], "in-addr.arpa");

Properties props = new Properties();
props.put("java.naming.factory.initial", "com.sun.jndi.dns.DnsContextFactory");
DirContext dirContext = new InitialDirContext(props);

Attributes attrs = dirContext.getAttributes(host, new String[] {"PTR"});

System.out.println(attrs.get("PTR").get());

Answer 2

The problem lies in the fact that the java.net.InetAdress has a certain procedure against ip-spoofing.

It first resolves the name into (an) ip address(es). This works fine. In your case the result are two IP adresses. InetAdress then checks back if (at least one of) these adresses resolve to the original input name.

If they do not, it just returns the original ip adress. The following picture shows the situation after the check for baiduspider-123-125-71-75.crawl.baidu.com

Note: The ip adresses resolved by getAllByName0 are the same as via nslookup, namely:

nslookup baiduspider-123-125-71-75.crawl.baidu.com
Server:     192.168.2.1
Address:    192.168.2.1#53

Non-authoritative answer:
Name:   baiduspider-123-125-71-75.crawl.baidu.com
Address: 62.157.140.133
Name:   baiduspider-123-125-71-75.crawl.baidu.com
Address: 80.156.86.78

A solution would be to use the dnsjava library. It skips the spoofing check and therefore works fine.

dnsjava example:

String addr = Address.getHostName(InetAddress.getByName("123.125.71.75")); outputs just as expected baiduspider-123-125-71-75.crawl.baidu.com

Disclaimer: As i am a Java developer and not a security expert, i am not totally aware of the security implications of using a spoofed ip address.