dcalvo
Posts: 4
Joined: Sat Jan 06, 2018 12:14 am

Doubt with ARP

Thu Jan 18, 2018 10:04 am

Hello folks,

I'm using a raspberry pi with raspbian 4.9.35-v7+ (I think its codename is Jessie) and I'm having some little issues with arp. Years ago, I was told that ARP protocol has a cache and after certain time, if a device is not longer on, its entry is deleted from the arp cache.

Well, that's not the behaviour I'm seeing in my raspberry or maybe (probably) I'm doing something wrong. Let's image that the arp cache has some entries and I can check the content by using the arp command. So I clearly can see some entries beloging to my cell phones. Ok,, now imagine that these cell phones are out of my home so they are not online anymore. No matter how much time I wait, if I use the arp command I see these entries in the cache while I was expecting to not see them.

By the way, I see these entries flagged as C so they are not permanent. Is this the expected behaviour? Am I doing something wrong? And finally, where can I take a look to se see what's going on?

Thank you very much.

User avatar
topguy
Posts: 4611
Joined: Tue Oct 09, 2012 11:46 am
Location: Trondheim, Norway

Re: Doubt with ARP

Thu Jan 18, 2018 10:17 am

What you describe is the basics of how an arp cache can work. In the real world the truth is more complex.
https://stackoverflow.com/questions/153 ... ge-timeout


If your Pi is on wired ethernet and your phone is wireless, then I'm wondering if the MAC in the arp-cache could be to your wireless router or not.. hmm

dcalvo
Posts: 4
Joined: Sat Jan 06, 2018 12:14 am

Re: Doubt with ARP

Thu Jan 18, 2018 3:06 pm

Thank you topguy,

I haven't read that web page yet, and I will do it later. Funny thing is that my entries are never deleted, even after some hours!!! A bit strange.

And regarding your second question, it doesn't matter if they are wireless or wired (as long as they belong to the same subnet) and checking the results I'm pretty sure the mac that shows the command is the real phone's mac address and not router's one.

Thank you very much.

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