How to get the client's real IP in golang in net/rpc – Go

Maya Patel

Quick Fix: The core limitation of the proposed solution is that it assumes a unique mapping between the IP address seen by the server(NAT IP) and client’s connection. However, in NAT scenario, this assumption could break down. Consider using a unique client identifier instead, or establish a separate connection specifically for reporting the real IP.

The Problem:

How to obtain a client’s real IP address and the current network connection net.Conn using Go’s net/rpc package? The RemoteAddr method may not always provide the genuine IP, especially in cases like NAT forwarding. Conversely, accessing the current net.Conn within an RPC function is challenging. This information is crucial for enabling two-way communication and issuing RPC commands to particular clients. Can you suggest a solution that retrieves both the real IP and the net.Conn of the client in a Golang net/rpc application?

The Solutions:

Solution 1: Use a unique session identifier:

Instead of relying on IP addresses for mapping, use a unique session identifier generated by the client or the server at the beginning of each connection. That identifier should be included in every RPC call, allowing the server to correctly map the session to the `net.Conn` object. The client sends this identifier in the `ReportRealIP` RPC call, and the server uses it to update the mapping.

Solution 2: Establish a short-lived connection for real IP reporting:

Have the client establish a separate, short-lived connection to the server specifically for reporting its real IP. That ensures the `net.Conn` object used for reporting is directly associated with the client's real IP. The client opens a new connection, reports its real IP, and then closes the connection. The server uses this separate connection to update the mapping.

Solution 3: Use a combination of IP and port for initial mapping:

Use a combination of the external IP and port as a key for the initial mapping. When the client reports its real IP, include the port number as well, allowing the server to find the correct `net.Conn` object. Store connections in the map using "externalIP:port" as the key. The client reports its real IP and the port it is connected to, allowing the server to find the matching `net.Conn`.


How to get the client’s real IP in net/rpc

Use the initial IP-to-connection mapping and update it when the client reports its real IP.

How to handle multiple clients with the same external IP?

Use a unique session identifier generated by the client or the server.

How to deal with private vs public IP discrepancy?

Use a separate short-lived connection to report the real IP.

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