Groups > linux.debian.user > #196193

Re: Get the external IP address from a Linux box

From	Pascal Hambourg <pascal@plouf.fr.eu.org>
Newsgroups	linux.debian.user
Subject	Re: Get the external IP address from a Linux box
Date	2018-05-29 15:00 +0200
Message-ID	<vUMoh-3X8-5@gated-at.bofh.it> (permalink)
References	<vSS4N-5l7-3@gated-at.bofh.it> <vT9oZ-8oU-1@gated-at.bofh.it> <vTFoR-2C8-3@gated-at.bofh.it> <vUp8m-7mU-15@gated-at.bofh.it> <vUxID-3Tn-43@gated-at.bofh.it>
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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 ?

Thread

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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