Apache HTTP Server Version 1.3

Known Problems in Clients

Over time the Apache Group has discovered or been notified of problems with various clients which we have had to work around. This document describes these problems and the workarounds available. It's not arranged in any particular order. Some familiarity with the standards is assumed, but not necessary.

For brevity, Navigator will refer to Netscape's Navigator product, and MSIE will refer to Microsoft's Internet Explorer product. All trademarks and copyrights belong to their respective companies. We welcome input from the various client authors to correct inconsistencies in this paper, or to provide us with exact version numbers where things are broken/fixed.

For reference, RFC1945 defines HTTP/1.0, and RFC2068 defines HTTP/1.1. Apache as of version 1.2 is an HTTP/1.1 server (with an optional HTTP/1.0 proxy).

Various of these workarounds are triggered by environment variables. The admin typically controls which are set, and for which clients, by using mod_browser. Unless otherwise noted all of these workarounds exist in versions 1.2 and later.

Trailing CRLF on POSTs

This is a legacy issue. The CERN webserver required POST data to have an extra CRLF following it. Thus many clients send an extra CRLF that is not included in the Content-Length of the request. Apache works around this problem by eating any empty lines which appear before a request.

Broken keepalive

Various clients have had broken implementations of keepalive (persistent connections). In particular the Windows versions of Navigator 2.0 get very confused when the server times out an idle connection. The workaround is present in the default config files:

BrowserMatch Mozilla/2 nokeepalive

MSIE 4.0b2, which claims to support HTTP/1.1, does not properly support keepalive when it is used on 301 or 302 (redirect) responses. Unfortunately Apache's nokeepalive code prior to 1.2.2 would not work with HTTP/1.1 clients. You must apply this patch to version 1.2.1. Then add this to your config:

BrowserMatch "MSIE 4\.0b2;" nokeepalive

Incorrect interpretation of HTTP/1.1 in response

To quote from section 3.1 of RFC1945:

HTTP uses a "<MAJOR>.<MINOR>" numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather than the features obtained via that communication.

The first major indication of this problem was with AOL's proxy servers. When Apache 1.2 went into beta it was the first wide-spread HTTP/1.1 server. After some discussion, AOL fixed their proxies. In anticipation of similar problems, the force-response-1.0 environment variable was added to Apache. When present Apache will indicate "HTTP/1.0" in response to an HTTP/1.0 client, but will not in any other way change the response.

The pre-1.1 Java Development Kit (JDK) that is used in many clients (including Navigator 3.x and MSIE 3.x) exhibits this problem. As do some of the early pre-releases of the 1.1 JDK. We think it is fixed in the 1.1 JDK release. In any event the workaround:

BrowserMatch Java/1.0 force-response-1.0
BrowserMatch JDK/1.0 force-response-1.0

RealPlayer 4.0 from Progressive Networks also exhibits this problem. However they have fixed it in version 4.01 of the player, but version 4.01 uses the same User-Agent as version 4.0. The workaround is still:

BrowserMatch "RealPlayer 4.0" force-response-1.0

Requests use HTTP/1.1 but responses must be in HTTP/1.0

MSIE 4.0b2 has this problem. Its Java VM makes requests in HTTP/1.1 format but the responses must be in HTTP/1.0 format (in particular, it does not understand chunked responses). The workaround is to fool Apache into believing the request came in HTTP/1.0 format.

BrowserMatch "MSIE 4\.0b2;" downgrade-1.0 force-response-1.0

Boundary problems with header parsing

All versions of Navigator from 2.0 through 4.0b2 (and possibly later) have a problem if the trailing CRLF of the response header starts at offset 256, 257 or 258 of the response. A BrowserMatch for this would match on nearly every hit, so the workaround is enabled automatically on all responses. The workaround is to detect when this condition would occur in a response and add extra padding to the header to push the trailing CRLF past offset 258 of the response.

Multipart responses and Quoted Boundary Strings

On multipart responses some clients will not accept quotes (") around the boundary string. The MIME standard recommends that such quotes be used. But the clients were probably written based on one of the examples in RFC2068, which does not include quotes. Apache does not include quotes on its boundary strings to workaround this problem.

Byterange requests

A byterange request is used when the client wishes to retrieve a portion of an object, not necessarily the entire object. There was a very old draft which included these byteranges in the URL. Old clients such as Navigator 2.0b1 and MSIE 3.0 for the MAC exhibit this behaviour, and it will appear in the servers' access logs as (failed) attempts to retrieve a URL with a trailing ";xxx-yyy". Apache does not attempt to implement this at all.

A subsequent draft of this standard defines a header Request-Range, and a response type multipart/x-byteranges. The HTTP/1.1 standard includes this draft with a few fixes, and it defines the header Range and type multipart/byteranges.

Navigator (versions 2 and 3) sends both Range and Request-Range headers (with the same value), but does not accept a multipart/byteranges response. The response must be multipart/x-byteranges. As a workaround, if Apache receives a Request-Range header it considers it "higher priority" than a Range header and in response uses multipart/x-byteranges.

The Adobe Acrobat Reader plugin makes extensive use of byteranges and prior to version 3.01 supports only the multipart/x-byterange response. Unfortunately there is no clue that it is the plugin making the request. If the plugin is used with Navigator, the above workaround works fine. But if the plugin is used with MSIE 3 (on Windows) the workaround won't work because MSIE 3 doesn't give the Range-Request clue that Navigator does. To workaround this, Apache special cases "MSIE 3" in the User-Agent and serves multipart/x-byteranges. Note that the necessity for this with MSIE 3 is actually due to the Acrobat plugin, not due to the browser.

Netscape Communicator appears to not issue the non-standard Request-Range header. When an Acrobat plugin prior to version 3.01 is used with it, it will not properly understand byteranges. The user must upgrade their Acrobat reader to 3.01.

Set-Cookie header is unmergeable

The HTTP specifications say that it is legal to merge headers with duplicate names into one (separated by semicolon). Some browsers that support Cookies don't like merged headers and prefer that each Set-Cookie header is sent separately. When parsing the headers returned by a CGI, Apache will explicitly avoid merging any Set-Cookie headers.

Expires headers and GIF89A animations

Navigator versions 2 through 4 will erroneously re-request GIF89A animations on each loop of the animation if the first response included an Expires header. This happens regardless of how far in the future the expiry time is set. There is no workaround supplied with Apache, however there are hacks for 1.2 and for 1.3.

POST without Content-Length

In certain situations Navigator 3.01 through 3.03 appear to incorrectly issue a POST without the request body. There is no known workaround. It has been fixed in Navigator 3.04, Netscapes provides some information. There's also some information about the actual problem.

Apache HTTP Server Version 1.3