How can I convert an IPv4 address from bytes to a string without using a loop?

Question

I have a simple python script, that creates a socket AF_PACKET, which parses all IPv4 packets and retrieves the source and destination IP addresses:

import socket
import struct

def get_ip(s):
    return '.'.join([str(ord(symbol)) for symbol in s])

def main():
    conn = socket.socket(socket.AF_PACKET, socket.SOCK_RAW, socket.ntohs(3))

    while True:
    pkt, addr = conn.recvfrom(65536)

    proto = struct.unpack('! H', pkt[12:14])
    eth_proto = socket.htons(proto[0])

    print('eth_proto = ', eth_proto)
    if eth_proto == 8:
        src, target = struct.unpack('! 4s 4s', pkt[26:34])
        source_ip = get_ip(src)
        destination_ip = get_ip(target)

        print('Source IP = ', source_ip)
        print('Destination IP = ', destination_ip)

main()

Is it possible to refactor getting the IP address, so it will look better and doesn't use this loop:

'.'.join([str(ord(symbol)) for symbol in s])

Format characters is described here: https://docs.python.org/2/library/struct.html

Please show example input for the get_ip function and the desired output for that. — ruohola
– ruohola, Commented Jan 9, 2019 at 14:28
@ruohola output for the function get_ip is the string with ip address (e.g. 172.15.75.12) input is array of bytes. maybe I can read in different ways this bytes ? — Sfdfsg
– Sfdfsg, Commented Jan 9, 2019 at 15:07

snakecharmerb · Accepted Answer · 2019-01-10 07:09:39Z

1

If you are using Python 2 (as you have linked to the Python 2 docs), you can use a bytearray and a format string to remove the explicit loop.

>>> s = '\n\x0b\xfa\x01'
>>> '{}.{}.{}.{}'.format(*bytearray(s))
'10.11.250.1'

If you are using Python 3.3+, you can use the standard library's ipaddress module.

>> ipa2 = ipaddress.ip_address(b'\n\x0b\xfa\x01')
>>> ipa2
IPv4Address('10.11.250.1')
>>> str(ipa2)
'10.11.250.1'

answered Jan 10, 2019 at 7:09

snakecharmerb

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