Detecting if modified since in Classic Asp

[Simon]

Hi

I am trying to read the html header 'if modified since' by using
Request.ServerVariables("HTTP_IF_MODIFIED_SINCE") in classic ASP. But I am
unable to get anything from this, this variable is always empty. It is also
excluded when I try to list all the server variables. We are using IIS6.

So please can someone confirm what is the correct syntax?

Cheers,

Simon.

 

[Bob Barrows [MVP]]

Hi

I am trying to read the html header 'if modified since' by using
Request.ServerVariables("HTTP_IF_MODIFIED_SINCE") in classic ASP. But
I am unable to get anything from this, this variable is always empty.
It is also excluded when I try to list all the server variables. We
are using IIS6.

So please can someone confirm what is the correct syntax?

That is not a standard header so it will not be included when listing
all the server variables (I assume you've done something like this:
http://www.aspfaq.com/2036).

You might want to wade through this thread:
http://www.webmasterworld.com/forum3/6005-2-30.htm

 

[Simon]

Hi Bob,

Already looked on webmasterworld and that's where I found the following
Request.ServerVariables("HTTP_IF_MODIFIED_SINCE")

But this can't be correct

--
Cheers,

Simon.

 

[Simon]

Hi Jon,

Isn't that just the collection of all the headers as one long string. The
page you pointed me to did not appear to show this data anyway.

--
Cheers,

Simon.

 

[Bob Barrows [MVP]]

I believe it was in there when Jon looked because he got there via a
google search for that variable name, causing it to appear in the
HTTP_REFERRER.

Again, this isn't a standard header and, while you are supposed to be
able to use this syntax:
HTTP_<HeaderName>
to get non-standard headers, the header you are requesting has to exist
in RAW for it to be available. I can't offer more assistance because
I've never used this header, which seems to be useful for google
indexing, correct? I just do intranet apps so I've never gone there.

 

[Simon]

Hi Bob,

Yes this task is related to Google indexing, I am surprised there is so
little info out there on this.

--
Cheers,

Simon.

 

[Anthony Jones]

Hi

I am trying to read the html header 'if modified since' by using
Request.ServerVariables("HTTP_IF_MODIFIED_SINCE") in classic ASP. But I am
unable to get anything from this, this variable is always empty. It is also
excluded when I try to list all the server variables. We are using IIS6.

So please can someone confirm what is the correct syntax?

Ok mucho confusion in this thread I clear that up first.

All headers that are included in the request are accessible to code via the
ServerVariables accessor. IIS does some additional things with headers and
creates a set of other named variables such as URL and also adds things that
have nothing to do with headers such as REMOTE_ADDR.

IIS does some tranforms on the name of a header before making it available.
It Prepends HTTP_, makes all the characters upper case and it converts any
hyphens to underscore. Hence if a client sends the header x-pink-elephants
it can be access as HTTP_X_PINK_ELEPHANTS.

The If-Modified-Since header is a standard header but it isn't always
supplied.

A HTTP server when responding to a request may include in the response a
Last-Modified header. When a client such as IE makes a subsequent request
for the _same_ URL it would normally include the If-Modified-Since request
header.

(Side note: a user can suppress the sending of the If-Modified-Since and the
If-None-Match headers by refreshing using Ctrl-F5 in IE).

Active Server Script generated pages would not send a Last-Modified header
in the response unless the developer deliberately created and added it to
the response themselves. Hence you are not seeing a If-Modified-Since
header.

What is it you want to do with this header? Generate a 304 response?

 

[Simon]

Hi Anthony,

Yes I am trying to respond with the 304 header, I am already generating the
last modifed date, which is been detected by the browser, and I can also see
the browser generate the 'if modified since' header. But the webserver does
not see it with Request.ServerVariables("HTTP_IF_MODIFIED_SINCE")

From your comments, it appears that this is the correct way to detect this
header, so maybe I have something blocking this header from reaching the
server, since I know it is been generated.

Regards,

Simon.

 

[Simon]

Hi Anthony,

I have since done some tests on a simple asp page, and found that the I can
after all read the value in Request.ServerVariables("HTTP_IF_MODIFIED_SINCE").

My test is failing on custom 404 pages, where the web page requested does
not actually exist. I use server.transfer method to a template file that gets
its content from a database.

I imagine that the header is been lost somehow in this process. So I need to
investigate down this route I think now.

 

[Simon]

Just to confirm I can read Request.ServerVariables("HTTP_IF_MODIFIED_SINCE"),
on a standard asp page, or an a page that uses transfer.direct.

But I can't read Request.ServerVariables("HTTP_IF_MODIFIED_SINCE"), if I use
a custom 404 page. Yet I can read other variables ok on this same page.

This is driving me nuts!

 

[Anthony Jones]

Just to confirm I can read Request.ServerVariables("HTTP_IF_MODIFIED_SINCE"),
on a standard asp page, or an a page that uses transfer.direct.

But I can't read Request.ServerVariables("HTTP_IF_MODIFIED_SINCE"), if I use
a custom 404 page. Yet I can read other variables ok on this same page.

This is driving me nuts!

When IIS uses the custom 404 page it is effectively a new request. It is a
bit weird.

What other variables are you able to read?

 

[Simon]

Hi Anthony,

I think I can read most other headers including QUERY_STRING, HTTP_HOST. The
last one I tried was USER_AGENT.

 

[Anthony Jones]

Hi Anthony,

I think I can read most other headers including QUERY_STRING, HTTP_HOST. The
last one I tried was USER_AGENT.

QUERY_STRING and USER_AGENT aren't header variables. You'll note that
QUERY_STRING doesn't contain the original query string but begins 404; and
then the URL requested.

What does USER_AGENT and HTTP_HOST header indicate?

Like I said custom error handling is a bit weird, it appears to be a new
request but the response is sent on the same connection that created the
original request.

I suspect that some headers of the original request are copied into this
'new' request but I've never tested which ones.

It doesn't surprise me that If-Modified-Since isn't copied, the resource now
being requested is not the same one as the client originally requested.

 

[Simon]

Hi Anthony.

The headers USER_AGENT and HOST, come through as expected unmodified.

I guess you are saying this could be a dead end for me, if certain headers
are not passed, although it is strange that everything else seems to be. I
don't understand why no one else has come across this problem before.

 

[Anthony Jones]

Hi Anthony.

The headers USER_AGENT and HOST, come through as expected unmodified.

I guess you are saying this could be a dead end for me, if certain headers
are not passed, although it is strange that everything else seems to be. I
don't understand why no one else has come across this problem before.

Mostly likely because you are merging two unusual things.

1) Using the 404 trick is a bit of a hack which works well on its own but a
more robust solution would involve an ISAPI filter.

2) Having an ASP page add a Last-Modified header and potentially responding
with a 304. This also works reasonably well but is even less common than
the 404 trick. It take some effort to acheive especially in when using only
VBScript.

Hence the combination of the two is likely to be very rare.

How complex is the effective URL re-write you are doing? There are I
believe some freebie simple URL re-writering filters that are based on
Regular expressions. If that can work for you then you won't need the 404
trick.