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What arp -s is good for

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Old Unix and Linux 10-24-2012
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As I recall from my old network engineering days a ping to a broadcast address should be done on the same subnet as the broadcast address, as a general rule. In this case, all the IP addresses on the network will respond for all interfaces configured and operational (and not blocking).
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Corona688 Corona688 is offline Forum Staff  
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It is on the same subnet.

Theoretically yes, broadcast ping should work, but many common operating systems and hardware bridges block or ignore broadcast ping -- or any ping -- as a matter of course now.

I can see how it could get the pinger's MAC into other people's ARP tables, but can't see see how broadcast ping gets other MAC's into my ARP table when they don't reply.
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They may reply with some nasty packet type other than echo response ICMP, like source quench!

If they have ever swapped any sort of IP packet with you, I would think it will be in your arp cache. Does ARP cache hold everything that arrives on your stack, or just arp responses for arp you initiated? Is there an arp cache poisoning attack?
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Corona688 Corona688 is offline Forum Staff  
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Quote:
Originally Posted by DGPickett View Post
If they have ever swapped any sort of IP packet with you, I would think it will be in your arp cache.
Swapped, sure, but that assumes a reply. No reply? No ARP entry.

Quote:
Does ARP cache hold everything that arrives on your stack, or just arp responses for arp you initiated? Is there an arp cache poisoning attack?
I think it tracks all ARP, since it's asynchronous, and it's useful to know things you didn't necessarily ask for anyway. ARP poisoning attacks are possible.
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Swapped, well received, right.

I believe ARP entries are good for about 5 minutes (which fouls up IP failover strategies on very high uptime systems), so the list does not usualy get very long. I suppose a NIC with a promiscuous arp algorythm could collect ARP from packets not for you, if you are not segregated by a switch from all such traffic. Network is a very competitive arena.
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My memory was faulty, only some ARP packets are broadcasts, the requests, not the replies. You are correct.
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Old Unix and Linux 10-24-2012
bakunin bakunin is offline Forum Staff  
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Quote:
Originally Posted by Neo View Post
a ping to a broadcast address should be done on the same subnet as the broadcast address, as a general rule.
True - still, this works only if network (the department, not the device) hasn't decided that every network connection, even one on the same subnet, has to go over a switch with firewall capabilities enabled (that is: each and every port blocked per default).

In my last project i had such a network, which is truly a PITA: you won't even get a ping from your default gateway back - but should manage HA-networks over that crap. Usually it took us 2-3 weeks to set up a cluster - 1 hour for installing and configuring it, the rest for filling out the forms required to get the various necessary ports opened in the firewall (which never worked right the first time).

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Last edited by bakunin; 10-24-2012 at 04:56 PM..
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