xtap

xtap is a Rust crate used with LD_PRELOAD to hook bind() and connect() syscalls to reroute v4 and v6 addresses with environment variables.

Features

• Intercepts bind() and connect() syscalls using runtime hooking via dlsym
• Automatically binds sockets to an interface
• Supports IPv4 and IPv6
• Configurable via multiple environment variables
• Minimal runtime overhead

Environment Variables

xtap should only be given one environment variable. If more are given the interface will take precedence over IP and be ignored. When an IP is specified it will use the corresponding interface.

Variable	Example
XTAP_IFACE	eth0
XTAP_INTERFACE	tun0
XTAP_IP	192.168.1.100
BIND_IP	10.0.0.2
BIND_SRC	10.0.0.3
XTAP_ADDR	172.16.0.1
XTAP_BIND	192.168.2.1
XTAP_BIND_ADDR	192.168.3.1

Building

cargo build --release

Usage

1. Set environment variables to specify interface or IP:

   export XTAP_IFACE=eth0
   export XTAP_IP=192.168.1.100
   

2. Run your binary using LD_PRELOAD. Socket operations will be transparently bound as configured. connect() calls will automatically inject a bind() and setsockopt() call.

   LD_PRELOAD=/usr/lib/xtap/libxtap.so \
   XTAP_IFACE=tun0 \
   curl -4 -L "http://cloudflare.com/cdn-cgi/trace"
   

3. If integrating the xtap crate, ensure your binary uses the provided bind() and connect() hooks:

   pub fn bind(sockfd: i32, addr: *const sockaddr, addrlen: socklen_t) -> i32 {
      xtap::hooks::Bind::bind(sockfd, addr, addrlen)
   }
   
   pub fn connect(sockfd: i32, addr: *const sockaddr, addrlen socklen_t) -> i32 {
      xtap::hooks::Connect::connect(sockfd, addr, addrlen)
   }
   

Debugging

Enable debug logging by compiling with debug assertions:

cargo build

Testing

Run each test per-process to prevent race conditions from environment variables and side effects caused by global static mutations:

cargo nextest run --status-level=all

Dependencies

• libc and socket2 for socket handling and syscall hooking
• netdev to discover network interfaces
• scopeguard for scoped cleanup and guard patterns

License

This project is available under multiple licenses. You may choose to use it under one of the following:

• MIT License
• Apache License 2.0
• Apache License 2.0 with LLVM Exception

Prior Art

Inspired by bindhack

Contribution

Contributions are welcome! Open issues or PRs on the repository.