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Old 06-30-2016, 01:32 PM   #1
nitinchitts
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ARP behavior and changes between kernel 2.6 and 3.14


Hi

I am using a network with following topology

PHP Code:
                       --------- 
 --------- /
__________ Router 1|________\ --------
Linux   |\            ---------         /| Linux   |
Machine1|                                | Machine2|
|         | 
_________\ --------- /________ |         |
 ---------           /| 
Router 2|\          --------
                       --------- 
I have established a IPsec tunnel between 2 linux machines and configured my routes such that packets from M1 to M2 goes through rounter 2 and M2 to M1 through Router 1. I have TCP session inside the IPsec Tunnel. ARP is resolved, tunnels established and tcp communication is working fine.

ARP resolved in M1 is for Router2
ARP resolved in M2 is for Router1

I read in http://linux.die.net/man/7/arp that is ARP refresh is done as below

base_reachable_time (since Linux 2.2)
Once a neighbor has been found, the entry is considered to be valid for at least a random value between base_reachable_time/2 and 3*base_reachable_time/2. An entry's validity will be extended if it receives positive feedback from higher level protocols. Defaults to 30 seconds. This file is now obsolete in favor of base_reachable_time_ms.

My base_reachable_time is configured to 400sec

In kernel 2.6, Since TCP session is established i see there is no further ARP send out even after expiry of base_reachable_time. Even after 30 mins i didnt see an ARP. I assume it is due to this

An entry's validity will be extended if it receives positive feedback from higher level protocols

In Kernel 3.14, I see ARP packets after refresh time expires even if TCP session is active.

Was there any changes in kernel for the above behavior?
Which all higher layer protocols can give positive feedback to ARP?


Regards
Nitin
 
Old 06-30-2016, 03:58 PM   #2
jefro
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I might be tempted to set a static arp entry for such an issue. Just be warned that static arp may remain hidden if you don't document.
 
Old 06-30-2016, 11:55 PM   #3
nitinchitts
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I am using this in a dynamic environment. Also this case was in a product which is deployed widely. Not sending ARP was creating issues. The issue was solved by some local changes in our software. But i wanted to understand the linux behavior. I am not looking at solution rather i am looking into understanding the kernel behavior and changes between versions to the above mentioned behavior.
 
Old 07-05-2016, 03:14 PM   #4
jefro
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It should not have changed. Not any mention on other sites on this that I've seen.

Use arp -a or other tools to view network and arp resolution.

Last edited by jefro; 07-06-2016 at 03:33 PM.
 
  


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