Coyote Point Systems E350GX, Installation And Administration Manual

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Chapter 6: Administering Virtual Clusters
134 Equalizer Installation and Administration Guide
properly on the client. If, for example, a server sends an HTTP redirect using the
Location:
header, this URL most
likely will include the
http://
protocol. Equalizer rewrites this response so that the URL uses
https:
//.
For server connections that contain multiple server responses, the setting of the
once only
flag determines whether
Location: headers in all server responses are rewritten. This is shown in the table below.
Note that the Administrative Interface does not permit you to enable
once only
and disable
no header rewrite
--
this option combination would rewrite the
Location:
header in only the first response in the connection, and not
rewrite the headers in subsequent responses in the same connection. Doing so would produce errors on the client.
Of course, you can also direct Equalizer to pass responses from the server without rewriting URLs by enabling the
no header rewrite
flag on the cluster.
Enabling Once Only and Compression
Enabling both the
once only
and
compress
options is not allowed by the Administrative Interface. These two
options are not compatible, since setting them both would mean that only the first response in a connection would be
compressed and not the remainder of the responses, which would likely cause client errors.
Using Active Content Verification (ACV)
Active Content Verification (ACV) is a mechanism for checking the validity of a server. When you enable ACV for
a cluster, Equalizer requests data from each server in the cluster and verifies that the returned data contains a
character string that indicates that the data is valid. You can use ACV with most network services that support a text-
based request/response protocol, such as HTTP. Note, however, that you cannot use ACV with Layer 4 UDP
clusters.
Using ACV
ACV checking is performed as part of the high-level TCP probes that Equalizer sends to every server by default. To
enable ACV, you specify an
ACV response
string for a cluster. Equalizer will then search for the
ACV response
string in the first 1024 characters of the server’s response to the high-level TCP probes. If the ACV response string
is not found, the server is marked down. An ACV probe (see above) can be specified if the service running on the
server’s
probe port
requires input in order to respond.
How ACV works is best explained using a simple example. The HTTP protocol enables you to establish a
connection to a server, request a file, and read the result. Figure 28 illustrates the connection process when a user
requests a telnet connection to an HTTP server and requests an HTML page.
once only
enabled
once only
disabled
no header rewrite
disabled
Not supported.
The Location: headers of every
response in a connection are
rewritten.
no header rewrite
enabled
No headers are rewritten. No headers are rewritten.
The compress
option is not
available on
E250, E350, or
E450 model
Equalizers