2025-11-11

Python Decorator for tcpdump

python, ai-code

This is a decorator to run tcpdump in the background while running a function. This way I can capture network traffic that my program is generating on the server. This was mostly an AI translation of my javascript decorator.

import subprocess
import time
from functools import wraps

def with_tcpdump(outfile='capture.pcap', iface='lo'):
    def decorator(fn):
        @wraps(fn)
        def wrapper(*args, **kwargs):
            cmd = [
                "/usr/sbin/tcpdump",
                "-i", iface,
                "-n",
                "-A",
                "port", "31438",
                "-w", outfile
            ]
            
            tcpdump = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True)
            
            # Wait until tcpdump is listening
            for line in tcpdump.stderr:
                print(line, end='')
                if "listening on" in line:
                    break
                if "Permission denied" in line:
                    tcpdump.kill()
                    raise PermissionError("tcpdump permission denied")
                    
            time.sleep(0.3)
            
            try:
                fn(*args, **kwargs)
            finally:
                time.sleep(1.5)
                tcpdump.terminate()
                print("tcpdump stopped")
        return wrapper
    return decorator