When you have users depending on Windows Terminal Services for their main desktop, it’s a good idea to have more than one Terminal Server. RDP, however, is not an easy protocol to load balance; sessions are long-lived and need to be persistent to a particular server, and users may connect from different source addresses during one session.

The current development version of HAProxy has made an important step forward in making this possible. Thanks to work by Exceliance, it now supports RDP Cookies, offering a solution to the persistence problem.

We have been testing the latest development release of HAProxy, 1.4-dev4, on a loadbalancer.org Enterprise R16 device. The real servers were two Windows Server 2008 machines, with identical test users set up on both.

We settled upon the following HAProxy configuration:

   defaults
        clitimeout 1h
        srvtimeout 1h
   listen VIP1 192.168.0.10:3389
        mode tcp
        tcp-request inspect-delay 5s
        tcp-request content accept if RDP_COOKIE
        persist rdp-cookie
        balance rdp-cookie
        option tcpka
        option tcplog
        server Win2k8-1 192.168.0.11:3389 weight 1 check   inter 2000 rise 2 fall 3
        server Win2k8-2 192.168.0.12:3389 weight 1 check   inter 2000 rise 2 fall 3
        option redispatch

Note that this is only a fragment of the haproxy.cfg file, showing the relevant options.

The load balancer’s Virtual IP is set to 192.168.0.10, listening on port 3389 for RDP. The two real servers are on 192.168.0.11 and 192.168.0.12, in the same subnet as the Virtual IP.

The two new configuration directives are persist rdp-cookie and balance rdp-cookie. These instruct HAProxy to inspect the incoming RDP connection for a cookie; if one is found, it is used to persistently direct the connection to the correct real server. The two tcp-request lines help to ensure that HAProxy sees the cookie on the initial request.

The only other tweak needed is to increase the clitimeout and srvtimeout values to one hour. In testing, this was found to be necessary to keep idle RDP sessions established.

Testing involved making multiple connections with different usernames, from varying IP addresses, using both Windows XP Professional and Linux clients. Sessions were disconnected and reconnected, and real servers removed from the cluster and re-inserted.

We found that, once a user had established a session with a particular real server, that user consistently reconnected to the correct server if it was available. When we removed and re-inserted servers, existing sessions were unaffected. After a simulated server failure, users could start a session on the remaining server.

When a failed server was brought back on-line, users that had been connected to that server would reconnect to it again - even if they had started a new session on the other server in the meantime. This may not be what you want, and requires further testing.

With client and server time-outs set to one hour, we were able to leave idle sessions running for 16 hours without problems.

For more information on the new configuration options, see the development version of HAProxy’s Configuration Manual.

This entry was posted on Tuesday, October 27th, 2009 at 3:04 pm and is filed under High Availability, Linux, Load Balancing, haproxy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.