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Groups > linux.debian.user > #196348

Re: Get the external IP address from a Linux box

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On Tue 29 May 2018 at 14:57:22 (+0200), Pascal Hambourg wrote:
> Le 28/05/2018 à 23:14, Pascal Hambourg a écrit :
> >Le 28/05/2018 à 13:54, Alan Greenberger a écrit :
> >>
> >>You are mostly correct.  However, I have one machine on which the
> >>response to
> >>/usr/sbin/arp -n
> >>shows two lines with the HWaddress of the router, one with the internal
> >>address as you said and the other with the external address.  I have no
> >>idea what made arp see the external address.
> >
> >Thinking of it, a router following the "weak host" model (like
> >Linux does) can advertise any local address on any interface. It
> >can be tested with arping. However I am failing to imagine any
> >plausible scenario which could lead a host on the internal LAN to
> >have the router's external IP address in its ARP cache. It means
> >that either :
> >- the host sends an ARP query for the router's external IP address
> 
> I guess this could happen if the host has a direct default route (no
> gateway) and the router acts as an ARP proxy. Quite an unusual
> setup.
> 
> >- the router sends an ARP query to the host from its external IP address
> 
> I checked that the latter can happen when the router must send a
> packet to the host with its external address as source and must
> resolve the host's address. Then the ARP query source IP address is
> the external address, and the destination host stores it in its ARP
> cache.
> 
> Any idea why the router would send packets to this host from its
> external address ?

I think your analysis starts from an assumption that everything is,
and always was, set up correctly from the start, and is working with
a sane and correct configuration, which might not be completely true.

Cheers,
David.

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Get the external IP address from a Linux box André Rodier <andre@rodier.me> - 2018-05-24 08:40 +0200
  Re: Get the external IP address from a Linux box conover@rahul.net (John Conover) - 2018-05-24 08:50 +0200
    Re: Get the external IP address from a Linux box likcoras <likcoras@riseup.net> - 2018-05-24 09:00 +0200
      Re: Get the external IP address from a Linux box conover@rahul.net (John Conover) - 2018-05-24 09:30 +0200
        Re: Get the external IP address from a Linux box André Rodier <andre@rodier.me> - 2018-05-24 10:00 +0200
  Re: Get the external IP address from a Linux box <tomas@tuxteam.de> - 2018-05-24 09:00 +0200
  Re: Get the external IP address from a Linux box Joe <joe@jretrading.com> - 2018-05-24 09:10 +0200
    Re: Get the external IP address from a Linux box Alberto Luaces <aluaces@udc.es> - 2018-05-24 09:10 +0200
      Re: Get the external IP address from a Linux box André Rodier <andre@rodier.me> - 2018-05-24 09:20 +0200
        Re: Get the external IP address from a Linux box <tomas@tuxteam.de> - 2018-05-24 09:40 +0200
        Re: Get the external IP address from a Linux box Joe <joe@jretrading.com> - 2018-05-24 11:30 +0200
          Re: Get the external IP address from a Linux box Joel Rees <joel.rees@gmail.com> - 2018-05-31 02:10 +0200
      Re: Get the external IP address from a Linux box Stefan Monnier <monnier@iro.umontreal.ca> - 2018-05-24 22:20 +0200
        Get the external IP address from a Linux box Erwan David <erwan@rail.eu.org> - 2018-05-24 22:40 +0200
          Re: Get the external IP address from a Linux box Miles Fidelman <mfidelman@meetinghouse.net> - 2018-05-24 23:30 +0200
    Re: Get the external IP address from a Linux box Richard Hector <richard@walnut.gen.nz> - 2018-05-26 08:20 +0200
  Re: Get the external IP address from a Linux box Abdullah Ramazanoğlu <ar018@yahoo.com> - 2018-05-24 09:40 +0200
    Re: Get the external IP address from a Linux box <tomas@tuxteam.de> - 2018-05-24 09:50 +0200
    Re: Get the external IP address from a Linux box Dan Purgert <dan@djph.net> - 2018-05-24 13:20 +0200
      Re: Get the external IP address from a Linux box Abdullah Ramazanoğlu <ar018@yahoo.com> - 2018-05-25 03:30 +0200
        Re: Get the external IP address from a Linux box Dan Purgert <dan@djph.net> - 2018-05-25 20:40 +0200
  Re: Get the external IP address from a Linux box Peter Ludikovsky <peter@ludikovsky.name> - 2018-05-24 13:20 +0200
  Re: Get the external IP address from a Linux box Greg Wooledge <wooledg@eeg.ccf.org> - 2018-05-24 14:10 +0200
    Re: Get the external IP address from a Linux box Dan Purgert <dan@djph.net> - 2018-05-24 15:00 +0200
  Re: Get the external IP address from a Linux box Alan Greenberger <alanjg@ptd.net> - 2018-05-25 03:10 +0200
    Re: Get the external IP address from a Linux box Pascal Hambourg <pascal@plouf.fr.eu.org> - 2018-05-26 13:20 +0200
      Re: Get the external IP address from a Linux box Alan Greenberger <alanjg@ptd.net> - 2018-05-28 14:10 +0200
        Re: Get the external IP address from a Linux box David Wright <deblis@lionunicorn.co.uk> - 2018-05-28 19:50 +0200
          Re: Get the external IP address from a Linux box Alan Greenberger <alanjg@ptd.net> - 2018-05-29 14:10 +0200
            Re: Get the external IP address from a Linux box David Wright <deblis@lionunicorn.co.uk> - 2018-06-02 16:40 +0200
        Re: Get the external IP address from a Linux box Pascal Hambourg <pascal@plouf.fr.eu.org> - 2018-05-28 23:20 +0200
          Re: Get the external IP address from a Linux box Pascal Hambourg <pascal@plouf.fr.eu.org> - 2018-05-29 15:00 +0200
            Re: Get the external IP address from a Linux box David Wright <deblis@lionunicorn.co.uk> - 2018-06-02 16:50 +0200
  Re: Get the external IP address from a Linux box Michael Stone <mstone@debian.org> - 2018-05-26 03:10 +0200
    Re: Get the external IP address from a Linux box Kenneth Parker <sea7kenp@gmail.com> - 2018-05-26 03:20 +0200
      Re: Get the external IP address from a Linux box Greg Wooledge <wooledg@eeg.ccf.org> - 2018-05-29 15:10 +0200
  Re: Get the external IP address from a Linux box Kenneth Parker <sea7kenp@gmail.com> - 2018-05-26 03:10 +0200
  Re: Get the external IP address from a Linux box André Rodier <andre@rodier.me> - 2018-05-26 13:00 +0200
    Get the external IP address from a Linux box Andrew McGlashan <andrew.mcglashan@affinityvision.com.au> - 2018-05-27 13:20 +0200
      Re: Get the external IP address from a Linux box André Rodier <andre@rodier.me> - 2018-05-27 14:20 +0200
        Re: Get the external IP address from a Linux box Andrew McGlashan <andrew.mcglashan@affinityvision.com.au> - 2018-05-27 15:50 +0200
          Re: Get the external IP address from a Linux box André Rodier <andre@rodier.me> - 2018-05-27 16:20 +0200

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