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Date: Wed, 6 Jun 2012 09:11:02 -0500 (CDT)
From: Robert Bonomi <bonomi@mail.r-bonomi.com>
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To: freebsd-questions@freebsd.org
In-Reply-To: <20120606134611.GA2185@tiny>
Subject: Re: IP -> e-mail
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Matthias Apitz <guru@unixarea.de> opined:
> El dia Wednesday, June 06, 2012 a las 09:17:47AM -0400, Robert Huff escribio:
> > Matthias Apitz <guru@unixarea.de> opined:
> > >  
> > >  lynx -dump myip.nl | fgrep 'WAN IP'
> > >  
> > >  strore the result in a file and when it changes, trigger a mail;
> > 
> >   Or, using only tools in the base system:
> > 
> > ifconfig | head | grep "inet " | awk '{print $2}'
>
> This will not work if your host has some private addr which is NAT'ed by a
> router; 

FALSE TO FACT.  Given the OP's actual request.

>         the real test is ask some remote side "how I do apear to you?"
> ofc you could do this as well by SSH'ing to some side and asking with
> netstat(1) there (which may be shows another NAT'ed addr too :-))

Matthias, your lynx-based 'solution' does *NOT* solve the OP's question.

He wants to know -when- his DHCP assigned address changes.  Consider
what happens if both the expired address and the new address are behind
the _same_ NAT translation.  The internal addrress changes, but the 
external one does not.

To do what the OP _asked_, parsing the 'ifconfig' output *is* the correct
approach.

_IF_, on the other hand, he wants to know when the 'externally visible'
address (a _very_ different question) for that host changes, then your 
approach is the correct one.