Dropping Packets

In the previous chapter our XDP program just logged traffic. In this chapter we're going to extend it to allow the dropping of traffic.

Source Code

Full code for the example in this chapter is available here

Design

In order for our program to drop packets, we're going to need a list of IP addresses to drop. Since we want to be able to lookup them up efficiently, we're going to use a HashMap to hold them.

We're going to:

Create a HashMap in our eBPF program that will act as a blocklist
Check the IP address from the packet against the HashMap to make a policy decision (pass or drop)
Add entries to the blocklist from userspace

Dropping packets in eBPF

We will create a new map called BLOCKLIST in our eBPF code. In order to make the policy decision, we will need to lookup the source IP address in our HashMap. If it exists we drop the packet, if it does not, we allow it. We'll keep this logic in a function called block_ip.

Here's what the code looks like now:

```rust linenums="1" title="xdp-drop-ebpf/src/main.rs"
#![no_std]
#![no_main]
#![allow(nonstandard_style, dead_code)]

use aya_ebpf::{
    bindings::xdp_action,
    macros::{map, xdp},
    maps::HashMap,
    programs::XdpContext,
};
use aya_log_ebpf::info;

use core::mem;
use network_types::{
    eth::{EthHdr, EtherType},
    ip::Ipv4Hdr,
};

#[cfg(not(test))]
#[panic_handler]
fn panic(_info: &core::panic::PanicInfo) -> ! {
    loop {}
}

#[map] // (1)
static BLOCKLIST: HashMap<u32, u32> =
    HashMap::<u32, u32>::with_max_entries(1024, 0);

#[xdp]
pub fn xdp_firewall(ctx: XdpContext) -> u32 {
    match try_xdp_firewall(ctx) {
        Ok(ret) => ret,
        Err(_) => xdp_action::XDP_ABORTED,
    }
}

#[inline(always)]
unsafe fn ptr_at<T>(ctx: &XdpContext, offset: usize) -> Result<*const T, ()> {
    let start = ctx.data();
    let end = ctx.data_end();
    let len = mem::size_of::<T>();

    if start + offset + len > end {
        return Err(());
    }

    let ptr = (start + offset) as *const T;
    Ok(&*ptr)
}

// (2)
fn block_ip(address: u32) -> bool {
    unsafe { BLOCKLIST.get(&address).is_some() }
}

fn try_xdp_firewall(ctx: XdpContext) -> Result<u32, ()> {
    let ethhdr: *const EthHdr = unsafe { ptr_at(&ctx, 0)? };
    match unsafe { (*ethhdr).ether_type } {
        EtherType::Ipv4 => {}
        _ => return Ok(xdp_action::XDP_PASS),
    }

    let ipv4hdr: *const Ipv4Hdr = unsafe { ptr_at(&ctx, EthHdr::LEN)? };
    let source = u32::from_be(unsafe { (*ipv4hdr).src_addr });

    // (3)
    let action = if block_ip(source) {
        xdp_action::XDP_DROP
    } else {
        xdp_action::XDP_PASS
    };
    info!(&ctx, "SRC: {:i}, ACTION: {}", source, action);

    Ok(action)
}
```

Create our map
Check if we should allow or deny our packet
Return the correct action

Populating our map from userspace

In order to add the addresses to block, we first need to get a reference to the BLOCKLIST map. Once we have it, it's simply a case of calling blocklist.insert(). We'll use the IPv4Addr type to represent our IP address as it's human-readable and can be easily converted to a u32. We'll block all traffic originating from 1.1.1.1 in this example.

Endianness

IP addresses are always encoded in network byte order (big endian) within packets. In our eBPF program, before checking the blocklist, we convert them to host endian using u32::from_be. Therefore it's correct to write our IP addresses in host endian format from userspace.

The other approach would work too: we could convert IPs to network endian when inserting from userspace, and then we wouldn't need to convert when indexing from the eBPF program.

Here's how the userspace code looks:

```rust linenums="1" title="xdp-drop/src/main.rs"
use anyhow::Context;
use aya::{
    include_bytes_aligned,
    maps::HashMap,
    programs::{Xdp, XdpFlags},
    Bpf,
};
use aya_log::BpfLogger;
use clap::Parser;
use log::{info, warn};
use std::net::Ipv4Addr;
use tokio::signal;

#[derive(Debug, Parser)]
struct Opt {
    #[clap(short, long, default_value = "eth0")]
    iface: String,
}

#[tokio::main]
async fn main() -> Result<(), anyhow::Error> {
    let opt = Opt::parse();

    env_logger::init();

    // This will include your eBPF object file as raw bytes at compile-time and load it at
    // runtime. This approach is recommended for most real-world use cases. If you would
    // like to specify the eBPF program at runtime rather than at compile-time, you can
    // reach for `Bpf::load_file` instead.
    #[cfg(debug_assertions)]
    let mut bpf = Bpf::load(include_bytes_aligned!(
        "../../target/bpfel-unknown-none/debug/xdp-drop"
    ))?;
    #[cfg(not(debug_assertions))]
    let mut bpf = Bpf::load(include_bytes_aligned!(
        "../../target/bpfel-unknown-none/release/xdp-drop"
    ))?;
    if let Err(e) = BpfLogger::init(&mut bpf) {
        // This can happen if you remove all log statements from your eBPF program.
        warn!("failed to initialize eBPF logger: {}", e);
    }
    let program: &mut Xdp =
        bpf.program_mut("xdp_firewall").unwrap().try_into()?;
    program.load()?;
    program.attach(&opt.iface, XdpFlags::default())
        .context("failed to attach the XDP program with default flags - try changing XdpFlags::default() to XdpFlags::SKB_MODE")?;

    // (1)
    let mut blocklist: HashMap<_, u32, u32> =
        HashMap::try_from(bpf.map_mut("BLOCKLIST").unwrap())?;

    // (2)
    let block_addr: u32 = Ipv4Addr::new(1, 1, 1, 1).try_into()?;

    // (3)
    blocklist.insert(block_addr, 0, 0)?;

    info!("Waiting for Ctrl-C...");
    signal::ctrl_c().await?;
    info!("Exiting...");

    Ok(())
}
```

Get a reference to the map
Create an IPv4Addr
Write this to our map

Running the program

```console
$ RUST_LOG=info cargo xtask run
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 1.1.1.1, ACTION: 1
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 192.168.1.21, ACTION: 2
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 192.168.1.21, ACTION: 2
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 18.168.253.132, ACTION: 2
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 1.1.1.1, ACTION: 1
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 18.168.253.132, ACTION: 2
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 18.168.253.132, ACTION: 2
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 1.1.1.1, ACTION: 1
[2022-10-04T12:46:05Z INFO  xdp_drop] SRC: 140.82.121.6, ACTION: 2
```