When dealing with Server Side Includes (SSI) on Apache, some developers encounter issues when the server automatically applies Transfer-Encoding: chunked for HTTP/1.1 clients. This typically manifests as corrupted output when:

• Using mod_include with dynamic content
• Running on Solaris systems with sendfile() enabled
• Serving to HTTP/1.1 clients while working fine for HTTP/1.0

The core issue stems from Apache's default behavior of chunking responses when:

Content-Length header is absent
AND
HTTP version is 1.1
AND
sendfile optimization is enabled

Here are three approaches to resolve this:

1. Force Content-Length Calculation

Add this to your Apache configuration:

<FilesMatch "\.shtml$">
    SetOutputFilter INCLUDES
    ContentDigest On
</FilesMatch>

2. Disable sendfile() Specifically

For Solaris systems, disable sendfile in httpd.conf:

EnableSendfile off

3. Override Chunked Encoding

Create a custom output filter:

<Location "/ssi-content/">
    SetOutputFilter INCLUDES;DEFLATE
    RequestHeader unset Accept-Encoding
</Location>

After implementing changes, verify with:

curl -I -H "Host: yourdomain.com" http://localhost/path/to/ssi.shtml

Look for either Content-Length header appearing or Transfer-Encoding: chunked disappearing from the response headers.

While these solutions work, be aware that:

• Disabling sendfile() may impact performance for large static files
• Calculating Content-Length adds minor CPU overhead
• The best approach depends on your specific content mix and traffic patterns

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When working with Server Side Includes (SSI) on Apache servers, you might encounter output corruption when the server automatically applies Transfer-Encoding: chunked for HTTP/1.1 clients. This particularly manifests on Solaris 10 SPARC systems with sendfile() enabled.

The key symptoms include:

# Correct output with HTTP/1.0:
curl -0 http://example.com/ssi_page.shtml

# Corrupted output with HTTP/1.1:
curl http://example.com/ssi_page.shtml

Option 1: Disable Chunked Encoding for Specific Files

<FilesMatch "\.shtml$">
    SetEnv no-chunked-encoding 1
</FilesMatch>

Option 2: Force Content-Length Headers

# In httpd.conf or virtual host
SetEnv proxy-sendcl 1
SetEnv proxy-sendchunked 0

For Solaris systems where sendfile() causes the issue:

# Disable sendfile completely
EnableSendfile off

# Or disable just for SSI files
<FilesMatch "\.shtml$">
    EnableSendfile Off
</FilesMatch>

Verify the solution with:

curl -v http://example.com/test.shtml | grep -i "Transfer-Encoding"

The output should not show "chunked" in the response headers.

While disabling chunked encoding solves the SSI issue, be aware that:

• This prevents HTTP/1.1 pipelining for affected resources
• Large files will buffer completely before sending
• Consider applying the fix only to SSI files, not globally