Python Cookbook

Checking the Status of a Unix Network Interface

Credit: Jürgen Hermann

Problem

You need to check the status of a network interface on a Linux or other Unix-compatible platform.

Solution

One approach to system-administration scripts is to dig down into system internals, and Python supports this approach:

#! /usr/bin/env python
import fcntl, struct, sys
from socket import *

# Set some symbolic constants
SIOCGIFFLAGS = 0x8913
null256 = '\0'*256

# Get the interface name from the command line
ifname = sys.argv[1]

# Create a socket so we have a handle to query
s = socket(AF_INET, SOCK_DGRAM)

# Call ioctl(  ) to get the flags for the given interface
result = fcntl.ioctl(s.fileno(  ), SIOCGIFFLAGS, ifname + null256)

# Extract the interface's flags from the return value
flags, = struct.unpack('H', result[16:18])

# Check "UP" bit and print a message
up = flags & 1
print ('DOWN', 'UP')[up]

# Return a value suitable for shell's "if"
sys.exit(not up)

Discussion

This recipe shows how to call some of the low-level modules of Python’s standard library, handling their results with the struct module. To really understand how this recipe works, you need to take a look at the system includes. On Linux, the necessary definitions are located in /usr/include/linux/if.h.

Though this code is certainly more complex than the traditional scripting approach (i.e., running /sbin/ifconfig and parsing its output), you get two positive effects in return. Directly using the system calls avoids the overhead (albeit modest) ...