The FE server passes on the request to the BE server with a slight
modification that enables the response to be sent back to the FE server
instead of the client. Other than that, the BE server receives the request
as if it were coming from a client.
So here's how it flows. IIS receives OWA client requests as a proxy for
message traffic between a Web browser client and an Exchange server. IIS
receives the client requests, looks at the namespace, and passes the client
to the appropriate OWA scripts registered for that URL. If the server
contains an Exchange 2000 database, OWA uses a high-speed channel to access
the mailbox store. If the server is a front-end server, OWA proxies the
request to a back-end server by using HTTP.
Here is some additional information from an FE/BE Topology White Paper by
"Whether generated by a browser or a specialized client like the Windows
2000 Web Folders feature, HTTP requests from the client computer are sent to
the front-end server. The server uses Active Directory to determine the
back-end server to which the request should be proxied. The precise
mechanics of the determination vary depending on whether user mailboxes or
public folders are being accessed, but the effect is that the requested data
is found wherever it is in the Exchange organization, assuming appropriate
permissions have been granted to the user who attempts to access the data.
After the correct back-end server is determined, the front-end server
forwards a request to the back-end server. The request is identical to the
original request sent from the client. In particular, the HTTP host header
(which is generated by the client and matches the name of the front-end
server to which the request was sent) remains unchanged, although the new
request is not sent to a server with that name. The host header setting
ensures that the request is handled by the Exchange virtual server, which
must be configured on each back-end server to handle front-end server
The front-end server always contacts the back-end server over TCP port 80
(the default HTTP port), regardless of the port the client used to contact
it. This means that SSL encryption is never used between the front-end and
back-end servers, although the client might use it in communication with the
front-end server. This also means that HTTP virtual servers that
differentiate themselves from others only by port number are not supported
in a front-end and back-end topology. For example, if a back-end server has
an HTTP virtual server listening at port 8080, the client could access it
only if it was pointed directly to the back-end server (for example,
http://backend:8080/data). The server would not be accessible through the
The request from the front-end server is processed normally by the back-end
server, and the response is sent unchanged through the front-end server back
to the client. In most cases, the back-end server handles the front-end
server as if it were simply another HTTP client.
The client, therefore, never receives the information that the request was
not handled on the front-end server."
I'm checking on documentation from Microsoft regarding HTTP as a hard-coded
requirement and I will post back here when I get more info.
MCT MCSE MCSA MCP+I
Exchange 2000 Server: The Complete Reference - ISBN 0072127392
TNT Software - www.tntsoftware.com
Microsoft Cluster Server Center - www.nwnetworks.com/cluster.html
Internet Explorer Security Center - www.nwnetworks.com/iesc.html
> Thanks for the reply, Scott.
> When you say that FE proxies HTTP, does that mean that the FE actually
> fetches the user's mail info from the BE server and presents it via
> HTML/HTTP, or does it just pass HTTP traffic through itself to the
> appropriate server, and the BE server is actually processing the
> request and presentation?
> Also, do you have a KB article number handy that actually states that
> HTTP in IIS for Exch 2000 is an absolute requirement?
> Many Thanks!
> > Hi,
> > Yes, HTTP is required for all Exchange servers. This is a hard-coded
> > requirement for Exchange 2000. This is not to say that you cannot
> > HTTP protocol for some users - you can. But you absolutely need HTTP on
> > your backend servers.
> > Note that the FE server proxies the HTTP request to the BE server and
> > so via HTTP (TCP port 80).
> > Hope this helps.
> > --
> > Regards,
> > Scott Schnoll
> > MCT MCSE MCSA MCP+I
> > Microsoft MVP
> > Exchange 2000 Server: The Complete Reference - ISBN 0072127392
> > TNT Software - www.tntsoftware.com
> > Microsoft Cluster Server Center - www.nwnetworks.com/cluster.html
> > Internet Explorer Security Center - www.nwnetworks.com/iesc.html
> > > We are deploying Exchange 2000. OWA will be provided via front-end
> > > servers.
> > > I understand the need for NNTP and SMTP to be provided by IIS on the
> > > backend (mailbox) servers, but are HTTP services absolutely necessary?
> > > Our preference would be to have HTTP not running on the backend
> > > servers.
> > > Thanks and regards,
> > > - e