Discovering public IP programmatically

This may be the easiest way. Parse the output of the following commands:

1. run a traceroute to find a router that is less than 3 hops out from your machine.
2. run ping with the option to record the source route and parse the output. The first IP address in the recorded route is your public one.

For example, I am on a Windows machine, but the same idea should work from unix too.

> tracert -d www.yahoo.com

Tracing route to www-real.wa1.b.yahoo.com [69.147.76.15]
over a maximum of 30 hops:

  1    <1 ms    <1 ms    <1 ms  192.168.14.203
  2     *        *        *     Request timed out.
  3     8 ms     8 ms     9 ms  68.85.228.121
  4     8 ms     8 ms     9 ms  68.86.165.234
  5    10 ms     9 ms     9 ms  68.86.165.237
  6    11 ms    10 ms    10 ms  68.86.165.242

The 68.85.228.121 is a Comcast (my provider) router. We can ping that:

> ping -r 9 68.85.228.121 -n 1

Pinging 68.85.228.121 with 32 bytes of data:

Reply from 68.85.228.121: bytes=32 time=10ms TTL=253
    Route: 66.176.38.51 ->
           68.85.228.121 ->
           68.85.228.121 ->
           192.168.14.203

Voila! The 66.176.38.51 is my public IP.

Python code to do this (hopefully works for py2 or py3):

#!/usr/bin/env python

def natIpAddr():
  # Find next visible host out from us to the internet
  hostList = []
  resp, rc = execute("tracert -w 100 -h 3 -d 8.8.8.8") # Remove '-w 100 -h d' if this fails

  for ln in resp.split('\n'):
    if len(ln)>0 and ln[-1]=='\r': ln = ln[:-1]  # Remove trailing CR
    if len(ln)==0: continue
    tok = ln.strip().split(' ')[-1].split('.') # Does last token look like a dotted IP address?
    if len(tok)!=4: continue
    hostList.append('.'.join(tok))
    if len(hostList)>1: break  # If we found a second host, bail
    
  if len(hostList)<2:
    print("!!tracert didn't work, try removing '-w 100 -h 3' options")
    # Those options were to speed up tracert results

  else:
    resp, rc = execute("ping -r 9 "+hostList[1]+" -n 1")
    ii = resp.find("Route: ")
    if ii>0: return resp[ii+7:].split(' ')[0]
  return none     


def execute(cmd, showErr=True, returnStr=True):
  import subprocess
  if type(cmd)==str:
    cmd = cmd.split(' ')
  # Remove ' ' tokens caused by multiple spaces in str             
  cmd = [xx for xx in cmd if xx!='']
  proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
  out, err = proc.communicate()
  if type(out)==bytes:  # Needed for python 3 (stupid python)
    out = out.decode()
    try:
      err = err.decode()
    except Exception as ex: 
      err = "!!--"+str(type(ex))+"--!!"
  
  if showErr and len(err)>0:
    out += err
  if returnStr and str(type(out))=="<type 'unicode'>":
    # Trying to make 'out' be an ASCII string whether in py2 or py3, sigh.
    out = out.encode()  # Convert UNICODE (u'xxx') to string
  return out, proc.returncode


if __name__ == "__main__":
  print("(This could take 30 sec)")
  print(natIpAddr())

Use it from the command line (on Windows) or from a python program:

import natIpAddr
myip = natIpAddr.natIpAddr()
print(myip)