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Dmitri_Ch__1425's avatar
Dmitri_Ch__1425
Icon for Cirrus rankCirrus
Mar 24, 2014
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nPath and firewall rules

Hi   Does nPath need special firewall configuration?   I've configured nPath in LAB, everything seems to be working fine. But I cannot get it to work on production network and suspect firewall...
application delivery
LTM
  • IheartF5_45022's avatar
    IheartF5_45022
    Mar 25, 2014

    What is interesting is in LAB env. when client ARPs for VS_IP one of the node responds with it's MAC address and everything works OK then. That means the F5 will not be involved at all, and the whole purpose of nPath is for the F5 to be involved!!

     

    In prod, when client ARPs for VS_IP, F5 responds with MAC address of self-ip, which causes the problem, b/c node then trying to send back to client's MAC. That's how nPath should work - client sends to F5, F5 forwards to server, and server responds direct to client.

     

    Also in LAB when I run tcpdump on the F5 I don't see any traffic, only ARP.

     

    Yes because the client is sending only to the node as that is what responded.

     

    You need to prevent the servers from responding to the arp requests for the VS_IP (so get rid of that -arp setting), although they do need to have an interface that will respond to unicast traffic to that IP configured. While the F5 and the nodes are all responding to arp it's a race to see who gets there response in first. Once you have the F5 only responding then you'll need to troubleshoot the other problem.

     

IheartF5_45022's avatar
IheartF5_45022
Icon for Nacreous rankNacreous

What is interesting is in LAB env. when client ARPs for VS_IP one of the node responds with it's MAC address and everything works OK then. That means the F5 will not be involved at all, and the whole purpose of nPath is for the F5 to be involved!!

 

In prod, when client ARPs for VS_IP, F5 responds with MAC address of self-ip, which causes the problem, b/c node then trying to send back to client's MAC. That's how nPath should work - client sends to F5, F5 forwards to server, and server responds direct to client.

 

Also in LAB when I run tcpdump on the F5 I don't see any traffic, only ARP.

 

Yes because the client is sending only to the node as that is what responded.

 

You need to prevent the servers from responding to the arp requests for the VS_IP (so get rid of that -arp setting), although they do need to have an interface that will respond to unicast traffic to that IP configured. While the F5 and the nodes are all responding to arp it's a race to see who gets there response in first. Once you have the F5 only responding then you'll need to troubleshoot the other problem.

 

Dmitri_Ch__1425's avatar
Dmitri_Ch__1425
Icon for Cirrus rankCirrus
Mar 26, 2014
It was 'ARP race' problem indeed. After carefully reviewing tcpdump from both node and F5 I can see that both node and F5 responded on ARP request with it's own MAC address. What I don't understand why is loopback adapter involved in ARP resolution in 1st place and why would lo respond to requests on eth0... '-arp' option on ifconfig suppose to stop interface from responding to ARP (from man). 'ip link lo set arp off' worked better in this case. I'm also looking using arp_announce and arp_ignore sysctl options. Thank you very much for all your help.