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Troubleshooting slow virtual Web servers

I am load balancing five virtual Web servers running Apache 2.2 on Ubuntu Linux 7.04 on VMware ESX. Users are complaining that the Website is extremely slow to respond, and I can't figure out what is going on. When I hit the servers directly, they respond very quickly, but when I go through the load balancer, they are very slow.

It seems that the load balancer is at fault, but when I attempt to access other services that go through the load balancer, but are not on the ESX servers, everything works fine. The networking department tells me that I have managed to create something called a "bridging loop" because of the way I have configured the ESX servers to do VLANs. I have searched the VMware forums but no one seems to have run into this issue before. Do you have any idea what could be going on?

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Actually, I do. I've seen this problem first hand, and you are not the first person who has run into it.

First of all, you can go tell your networking guys that the problem is not a bridging loop. Actually, since you indicated that you are not familiar with bridging loops you should read up on them. Cisco has a good reference document that explains what a bridging loop is.

Basically, when a bridging loop occurs, two or more bridges essentially perform a denial of service (DOS) attack on the network segments involved in the loop. This is bad mojo. Transparent bridging is actually a good thing, and the spanning tree algorithm was invented to help cure the problem of bridging loops by finding non-loop pathways in a network topology.

I spoke with VMware earlier this year (2007) in March about the network internals of ESX. Patrick Lin and Jacob Jensen related that ESX does not possess loop detection or implement the spanning tree algorithm because ESX does not allow you to interconnect multiple virtual switches, which would create a brewing ground for a bridging loop. There you have it, from the mouths of folks at VMware: it is not possible to create a bridging loop with ESX.

So what is going on in your situation then? Here is the kicker: your problem has nothing to do with ESX whatsoever. It just happens that you, and many, many, other IT administrations like yourself are for the first time able to cost effectively create server farms - be it Web servers, database servers, DNS servers, whatever, thanks to virtualization.

Of course, when there is a problem, ESX and its mysterious networking capabilities get blamed. In all fairness to your networking department, I have tried to talk to VMware many times and get them to be far more open about the internal workings of ESX with regards to networking. The fact of the matter is that network administrators are very suspicious people - and for a good reason - they are expected to have 100% uptime. They do not want any device on their network that could potentially chew through bandwidth faster than I can make my way through a 300-count bucket of Super Bubble bubble- gum (23 minutes is my record). Hence these poor chaps are quick to blame ESX: novus scelorum and all that.

The fact of that matter is that this same problem would have occurred regardless of the type of servers you were load balancing: virtual or physical. You just have not tried this particular configuration on your load-balancer before because you had no reason to without the plethora of severs you can create with ESX.

The problem is occurring because your load-balancer is configured to forward incoming Ethernet frames based on their media access control (MAC)-address, and not their internet protocol (IP) address for purposes of creating a network design so that the server bypasses the load-balancer when sending frames back to the client.

This setting by itself would not be a problem except that you have either mistakenly or intentionally also configured the load-balancer, or a particular service on the load-balancer, to not use the source IP address of the client when forwarding frames. Let me guess: you are trying to configure a service on the load-balancer to forward Ethernet frames such that when they arrive at their destination it appears as if they originated from the original client and not the load-balancer itself? Yep, been there. The issue is that if you do not set the configuration option so that the source IP address is rewritten as well, you will create what appears to be a bridging loop. Please allow me to illustrate:

Values exist as such:

Client IP: 111.111.11.111
Client MAC: 00:00:00:00:00:11

Load-Balancer IP: 222.222.22.222
Load-Balancer MAC: 00:00:00:00:00:22

Server IP: 333:333:33:333
Server MAC: 00:00:00:00:00:33

Phase 1: A client builds an Ethernet frame to send to a server. First it has to travel through a gateway and load-balancer. The client sends the frame to the gateway.

Phase 2: The gateway forwards the frame to the load-balancer.

Phase 3: The load-balancer alters the frame's source MAC address so that it is the MAC address of the load-balancer itself. However, the load-balancer is incorrectly configured and does not change the source IP address of the frame to be that of the client's before forwarding it to the server.

Phase 4: The server receives the frame and builds a response. Because the server thinks the frame came from the load-balancer it sets the response frame's destination IP address to the IP address of the load- balancer, the same for the destination MAC address. This is where things fail. In accordance with most load-balancer systems that implement something akin to this (see the Citrix Netscaler documentation on Direct Server Return), you must also create a loopback network interface card (NIC) on the server.

The loopback NIC is configured so that its IP address is that of the load-balancer and its gateway is that of the router that sits in front of the load-balancer. This is to explicitly keep the server from sending anything back to the load-balancer since in this configuration to do so would flood the load-balancer. However, because the load-balancer failed to re-write the source IP address of the original frame, the server's response puts the IP address of the load- balancer in the destination field of the new Ethernet frame. The new frame gets sent to the gateway which then sends it right back to the load-balancer. Even if the loopback adapter on the server is improperly configured, the frames will still get sent back to the load- balancer. This is where I am a little hazy on the inner-workings of specific models of load-balancers, but for some reason the frames then get looped right back at the server, which throws them right back at the load-balancer: hence a DOS attack that causes long queues for other, properly configured, services that use the load-balancer.

I've personally caused this on a Netscaler series from Citrix, and two other IT administrators have seen this as well on a Netscaler. I would hazard a guess you are using a Netscaler? Even if you are not, this configuration is possible on other load-balancing devices. The moral of the story is this: if you are going to set up a type of Direct Server Return architecture, be sure to use the source IP address, otherwise you can spam yourself into oblivion!

Hope this helps!

This was first published in November 2007

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