Rotating DNS Limitations with cached DNS resolve requests

This section describes some of the limitations related to setting up a rotating DNS system to load balance connections to the Sametime Community Services cluster.

About this task

Ideally, as users connect to the rotating DNS system, consecutive attempts to resolve a cluster name will result in an even distribution of connections to the servers in the cluster. In practice, the DNS caching mechanism can cause Sametime clients to repeatedly attempt connections to the same server in the cluster. If a server fails, and the DNS resolve requests are cached, Sametime Connect might attempt to reconnect to the server that is down instead of failing over to a different server.

The Sametime Connect Client client's Sametime Connectivity settings control whether the client attempts to connect to the Sametime server through a proxy server or attempts a direct connection to the Sametime server. These connectivity settings affect the failover behavior when DNS resolve requests are cached.

Sametime Connect Client for the desktop

When the DNS resolve requests are cached and a server fails, Sametime Connect for the desktop automatically attempts to connect to another server in the cluster. When any of the following settings are selected on the Sametime Connectivity tab, a successful connection to the cluster depends on the client and its settings:
  • Direct connection using standard Sametime protocol.
  • Use SOCKS4 proxy with "Resolve server name locally" checked.
  • Use SOCKS5 proxy with "Resolve server name locally" checked.
  • Direct connection using HTTP protocol.
If Sametime Connect Client cannot reconnect to the cluster when these settings are selected, the user can try any of the following options:
  • On Microsoft™ Windows™, change the registry key that controls the cache time for DNS requests so the DNS requests are cached for only one second:
    1. Start the registry editor and open

      HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Dnscache\Parameters

    2. Change the value of the registry key "MaxCacheEntryTtlLimit" to "1".
  • In the Sametime Connect Client client's Sametime Connectivity settings, change the name in the Host setting from the cluster name to the name of a specific server within the cluster.
When any of the following settings are selected in the Sametime Connectivity tab, a proxy server resolves the cluster name. Resolving the cluster name depends on the settings of the proxy server. The proxy server might return a valid server name in the cluster, or it might return the address of the server that is already down.
  • Use HTTP proxy.
  • Use HTTPS proxy.
  • Use SOCKS4 proxy with "Resolve server name locally" unchecked.
  • Use SOCKS5 proxy with "Resolve server name locally" unchecked.

If Sametime Connect Client cannot reconnect to the cluster when these settings are selected, check the settings on the proxy server to verify the proxy is attempting to connect to the servers within the cluster in rotating order.

When Use my Internet Explorer browser settings is selected in the Sametime Connectivity tab, the behavior of the client depends on the proxy connectivity settings of the Microsoft Internet Explorer web browser.
  • If the browser settings do not specify a proxy server, the client attempts a Direct connection using HTTP protocol. If the client is unable to reconnect following a server failure, the user can try any of the options listed previously for Direct connection using HTTP protocol.
  • If the browser settings specify an HTTP proxy server, the HTTP proxy server resolves the cluster name. If the client cannot reconnect, check the settings on the proxy server to verify the proxy is attempting to connect to the servers in the cluster.