Proxy ARP question?

[me19562]

How can I know which MAC address is the bad one in a Proxy ARP table?

 

[Garion]

Talk to Spidey on this one. Proxy ARP is his favorite subject.

- G

 

[spidey07]

What are you trying to do or is there a problem you're trying to solve?

All hosts and routers maintain an ARP table which maps layer3 IP address to their corresponding layer2 MAC address. Proxy-arp is the process where a router will arp on behalf of remote host. messy stuff sometimes. 😉

 

[lowtech1]

What do you want to do with Proxy ARP?

Proxy arp is usually use in a situation where the net work have real IPs that you want to work with behind a firewall. Intead of the usual NAT/masquerade you have to Routes or Peer your network.

 

[Poontos]

<< How can I know which MAC address is the bad one in a Proxy ARP table? >>


Probably won`t resolve your question, but for reference:

arp -a

At the command prompt of a Windaz boxen will show the PC's arp table.

Have an ARP of a time!

 

[me19562]

Spidey I don't have any problem at all, just want to know how to troubleshoot it if someday I get any related problem.

 

[spidey07]

I'll try to give just a little more detail...

Let's take a router with multiple interfaces. One ethernet interface runs IP with an address of 172.16.0.1/24 and mac address 0000.0000.aaaa. His routing table looks like this-

172.16.0.0/24 = connected
172.17.0.0/16 = 172.18.0.2 (another router)
no default

Host A is 172.16.0.10/24. Host A does not have a default gateway or default route configured. So when HOST A wants to send frames to HOST B (172.17.0.10) he will broadcast an ARP request asking for the MAC address of 172.17.0.10.

router receives the arp request (broadcast ya know) and realize there is a corresponding route to that network in his table and respond in kind (proxy) with the mac address of his ethernet interface.

Host A now has a Layer2 address for ip address 172.17.0.10 and can begin communicating. Host A's arp table looks like this
172.17.0.10 = 0000.0000.aaaa

Host A believes his is communicating on a single layer2 network and doesn't know or care that a router is involved. How do you troubleshoot? look at arp tables on hosts and make sure the mac address are correct. A sure sign of a problem is when you have many IP addresses on the same IP network with the same MAC -

172.16.0.10 = 0000.0000.cccc
172.16.0.11 = 0000.0000.cccc
172.16.0.233 = 0000.0000.cccc

 

[me19562]

Ok, now I understand the concept, thank u very much