IPv4 NAT string substitution

natstrings

#!/usr/bin/env python
import sys, re

def s2ip(s):
  a, b, c, d = map(int, s.split('.'))
  return (a << 24) | (b << 16) | (c << 8) | d
def ip2s(n):
  return '%d.%d.%d.%d' % (
   (n >> 24)
  ,(n >> 16)& 255
  ,(n >>  8)& 255
  ,(n >>  0)& 255
  )
def nat(s):
  global srcnet, dstnet, netmask
  x = s.span()
  s = s.string[x[0]:x[1]]
  srcaddr = s2ip(s)
  if (srcaddr & netmask) != srcnet:
    return s
  srcbits = srcaddr & ~netmask
  return ip2s(dstnet | srcbits)

if len(sys.argv) != 3:
  print 'usage: %s <srcnet> <dstnet> # use cidr notation'
  raise SystemExit

srcnet = sys.argv[1].split('/')
nbits = int(srcnet[1])
srcnet = s2ip(srcnet[0])

dstnet = sys.argv[2].split('/')
if nbits != int(dstnet[1]):
  print 'number of bits in source and destination networks do not match'
  raise SystemExit
dstnet = s2ip(dstnet[0])

netmask = ((1 << (32-nbits)) - 1) ^ 0xffffffff

for line in sys.stdin:
  sys.stdout.write(re.sub(
    r'\b[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\b'
  , nat
  , line))