Mailing List Archive: VIP reallocation and opened TCP connection

I want to know if this case can occur:

We have two hosts (A and B )running wackamole and managing three Virtual
IP Ip1 and Ip2 and Ip3. These VIP are exposed via DNS for the
myserver.example.com.
On each host, we have a web server. We use a DNS (round robin ) to reach
servers.

In the first state, A and B are running well with wackamole.
We want to add a new host C ( it's the same case when restarting a host
that has failed).

Let's consider this use case:

1- A client (web browser )establish a TCP socket connection with web
server in host A via Ip1. (it may bee a long request )
2- An administrator start a web server, spread and wackamole on host C.
3- wackamole reallocate Ip1 to host C before the end of client request.

Questions :
what happen to the socket opened by the client with Ip1 on host A?
Is there any check for TCP connection opened before reallocation ?

Thanks for help

The reallocation will only happen if you have balancing turned on, or if server C prefers the address. However, the concern you raise is a valid one; clients connected to IP1 on A (or B) will have their service fail when it is moved to C. The packets destined for IP1 will now be routed to C, who will discard them (and maybe send a RST, I don't remember the TCP state machine off the top of my head) because it has no state for the connection. Wackamole doesn't attempt to interact with the TCP protocol stack to detect open connections and avoid breaking them (which is a reason to consider not using balance/preferred IPs if address load balancing is not a desired goal).

--Ryan

----- Original Message -----
From: ahmed maouche <ahmed.maouche@nextenso.com>
Date: Tuesday, May 27, 2003 8:57 am
Subject: [Wackamole-users] VIP reallocation and opened TCP connection

> I want to know if this case can occur:
>
> We have two hosts (A and B )running wackamole and managing three
> Virtual
> IP Ip1 and Ip2 and Ip3. These VIP are exposed via DNS for the
> myserver.example.com.
> On each host, we have a web server. We use a DNS (round robin ) to
> reach
> servers.
>
> In the first state, A and B are running well with wackamole.
> We want to add a new host C ( it's the same case when restarting a
> host
> that has failed).
>
> Let's consider this use case:
>
> 1- A client (web browser )establish a TCP socket connection with
> web
> server in host A via Ip1. (it may bee a long request )
> 2- An administrator start a web server, spread and wackamole on
> host C.
> 3- wackamole reallocate Ip1 to host C before the end of client
> request.
> Questions :
> what happen to the socket opened by the client with Ip1 on host A?
> Is there any check for TCP connection opened before reallocation ?
>
> Thanks for help
>
>
>
>
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>