Resolving RPC: Procedure unavailable Errors with RPC Programs in C

Discover how to troubleshoot and resolve the issue of "RPC: Procedure unavailable" in your program generated by `rpcgen` for time and date retrieval from a remote server.
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Understanding "RPC: Procedure unavailable" Errors in C Programs

When developing network applications in C, it’s common to use the Remote Procedure Call (RPC) protocol to facilitate communication between a client and server. However, developers often encounter issues, particularly when deploying their programs across different operating systems or versions. One such issue is the error message “call failed: RPC: Procedure unavailable.”

In this guide, we’ll explore a practical example involving a program created using rpcgen to retrieve the time and date from a remote host, its related challenges, and how to effectively resolve them.

The Problem: RPC Procedure Unavailability

You’re attempting to implement a client-server application that uses RPC to fetch the current time from a server. Unfortunately, when running the client against the server, you receive the following error:

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This error often indicates that the client is unable to find the procedure defined in the server, even though the server is up and running. This leads to confusion, especially since similar implementations on older operating systems seemed to work without issues.

Context of the Issue

In the provided scenario, the issue was replicated on Ubuntu 22.04, while it worked perfectly on Ubuntu 21. This disparity hints at potential compatibility issues between different OS versions when handling RPC communication.

Breakdown of the Programs

A closer examination of the code reveals the structure of both the client and server:

Server Code (rpctime_server.c)

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This code aims to provide the current time in seconds since the epoch when invoked.

Client Code (rpctime_client.c)

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The client receives the time from the server. If the function call fails, the error message is printed.

Steps to Troubleshoot the Issue

Here are some potential solutions you can implement to resolve the “RPC: Procedure unavailable” error.

1. Verify Server Process

Ensure that the server is running correctly. You can check this by running:

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Confirm that no other program is conflicting with the designated ports used by your RPC service.

2. Review the RPC Program Versioning

Ensure the client and server are using compatible versions as defined in your .x file. Any misalignment can lead to procedure availability mismatches.

3. Check the RPC Daemon

On newer versions of Ubuntu, verify if there are changes to the RPC daemon configuration affecting how services are registered and discovered. This can be essential, as new security and networking features could alter default behaviors.

4. Compile with Appropriate Flags

Make sure that you compile your RPC client and server with the correct flags. For instance, in your Makefile, ensure that the correct libraries (-ltirpc instead of -lnsl) are included as follows:

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Run the compilation command again.

5. Update System Libraries

Sometimes, the libraries or the RPC stack might need an update. Ensure that your Ubuntu system is fully up to date:

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Conclusion

While the “call failed: RPC: Procedure unavailable” error can be frustrating, following these troubleshooting steps can aid in resolving the issue. By verifying server processes, checking compatibility, reviewing your code configurations, and ensuring your environment is up to date, you should be able to successfully make your RPC program work across different Ubuntu versions.

Should you continue to experience difficulties, c