[lvs-users] Sorry, it's pretty unusable!

Ulrich Windl Ulrich.Windl at rz.uni-regensburg.de
Thu Oct 17 07:48:23 BST 2013


Hi!

I'm not subscribed to the list, so I hope someone will receive it anyway:

I could pretty well use LVS for a load-balance, high-availability scenario like distributing SMTP requests to different servers, but the setup seems so complicated that I won't do. Reading the documentation, I felt that the NAT (masq) mechanism would be the most elegant for my requirements. However as it tuned out it did not work (as for many others). The reason is simple: LVS rewrites the destination TSAP (IP address and port), but it leaves the source TSAP unchanged. So any replies from a real server go to the original sender, instead of the LVS host.

The proposed solution is to set the LVS host as default gateway on any real server. This has several problems:
1) You create a SPoF on the LVS host
2) You create a network bottleneck on the LVS host (_all_ traffic from a real goes to the LVS host which must be a router)
3) If LVS host and real server are not in the same subnet, you cannot route from the real server to the LVS directly
4) You cannot have two different LVS hosts that use different services on the same real host

I reall wonder why you don't rewrite the source TSAP (in addition to the destination TSAP) as well so that the sender of the packet seems to be the LVS host. On a second rewrite the LVS destination TSAP would be rewritten to the original requester. I feel this would work like a charm:
1) The real server will reply to the LVS host automatically
2) Only LVS traffic needs to go through LVS host
3) LVS host does not need to be a router (after rewriting the destination, I think)
4) LVS host and real server can be in different subnets
5) You can use one real server from different LVS hosts

Did I overlook something that makes this impossible or impractical?

Regards,
Ulrich





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