Add support for FIB rule messages

[alexsikomas]

This PR adds support for managing the routing policy database through FIB rule messages, enabling the creation, deletion, and querying of ip rules.

Changes:

• Fibmsg struct: A message struct that corresponds to the fib_rule_hdr in the Linux kernel.
• Frattr enum: Netlink attributes for a FIB rule.
• FrAct enum: Possible actions for a FIB rule.
• Frf flags: Possible flags associated with a FIB rule
• fwmark.rs example: An example is included to demonstrate how to create and delete a firewall mark-based routing rule. It shows how to build a Fibmsg, send it to the kernel, handle the acknowledgment, and then send a corresponding delete request.

The new functionality can be tested by running the example:

# Requires network capabilities or root privileges
sudo cargo run --example fwmark

or

cargo build --example fwmark
sudo ./target/debug/examples/fwmark

When run, the example will:

1. Create a new rule: [PRIORITY]: not from all fwmark 0xca6c lookup 246813579
2. Print a success message.
3. Delete the rule.

You can comment out the deletion part in examples/fwmark.rs and verify the rule's creation with ip rule show.

[jbaublitz]

Hi @alexsikomas, thanks for the contribution. Can you provide more information on where these constants are defined?

[alexsikomas]

Hi, the definitions for these constants can be found here: include/uapi/linux/fib_rules.h

[jbaublitz]

@alexsikomas Thanks for providing the link to the constants. I wanted to check if it's covered in linux-sys-raw. It appears that it is not currently. If you could submit a PR to linux-sys-raw or libc to have the constants automatically generated from there, I would be willing to accept this PR. As it stands, hardcoding constants often causes breakage, so I'd prefer to have it generated from C headers by another crate.

[alexsikomas]

Sounds good, @jbaublitz I'll make a PR to libc.

[jbaublitz]

@alexsikomas Just be aware that if libc doesn't want to add it, linux-raw-sys may be willing to. There have been some constants that libc has avoided adding related to netlink.

[alexsikomas]

I've changed the hardcoded constants to rely on linux-raw-sys, which has support for fib-rules.h as of 0.12.1.

[jbaublitz]

Just one request and then I'll merge this. Also please rebase onto the current branch.

[jbaublitz]

For this file, I think it would be better to use the namespace in the path for the constants to avoid collisions as we're ultimately using two separate libraries now to supply constants.

[alexsikomas]

I've made the requested changes, removing the wildcard import, and rebased onto the current branch.

[jbaublitz]

@alexsikomas I've fully reviewed the code and it all looks good and consistent with the style of neli. Thanks for that! The one concern is that I can't find the definition of fibmsg anywhere in the kernel or kernel headers based on a search. Can you link me to the place you're seeing this data structure defined?

[jbaublitz]

Okay I found the definition. The naming was a little bit off so it was not easy to immediately find but I believe this is fine. I'm going to merge this now, but if you can follow up and explain your usage of libc::u_char instead of u8 as it is specified in the definition, that would be helpful. If I don't hear from you, I may change the type, but based on the C spec, they're equivalent anyway so it shouldn't be a problem.

[alexsikomas]

You're right, u8 is the better type, I originally used u_char because I was just starting out with FFI and thought that it was the 'more correct' type to use.