How to Reliably Determine a Machine's External IP Address in Java?

Determining the IP Address of the Current Machine in Java

Context:

In a distributed system with multiple nodes, each node requires a unique IP address to communicate effectively. This IP address must be registered with a central bootstrap node for seamless connectivity. However, obtaining the correct IP address can be challenging due to potential complications with network configurations.

Problem:

The default approach using InetAddress.getLocalHost().getHostAddress() often returns the LAN IP address instead of the external PPP IP address. This mismatch creates difficulties when nodes attempt to connect to the bootstrap node or other nodes.

Solution 1: Network Interface Enumeration

To retrieve all available IP addresses associated with the machine, use the following code:

This approach provides a comprehensive list of all IP addresses. However, distinguishing the correct IP address remains challenging.

Solution 2: IP Address Type Differentiation

Determine the type of IP address based on its value range:

• Loopback: 127.xxx.xxx.xxx
• Private (LAN): 192.168.xxx.xxx
• Link local: 169.254.xxx.xxx
• Multicast: 224.xxx.xxx.xxx - 239.xxx.xxx.xxx
• Broadcast: 255.255.255.255

The InetAddress API provides methods to test for each type of address to identify the most appropriate one.

Solution 3: External FQDN Retrieval

Obtain the externally advertised fully qualified domain name (FQDN) and use it to look up the primary IP address:

This method relies on DNS resolution and may not be feasible in all scenarios, particularly in the presence of load balancers.

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