/usr/bin/external-ip is in miniupnpc 1.9.20140610-4ubuntu2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 | #!/bin/sh
# $Id: external-ip.sh,v 1.1 2010/08/05 12:57:41 nanard Exp $
# (c) 2010 Reuben Hawkins
upnpc -s | grep ExternalIPAddress | sed 's/[^0-9\.]//g'
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