Bug 42497 - 304 response should consistently include ETag header
Summary: 304 response should consistently include ETag header
Alias: None
Product: Tomcat 5
Classification: Unclassified
Component: Catalina (show other bugs)
Version: 5.5.23
Hardware: PC Windows XP
: P2 normal (vote)
Target Milestone: ---
Assignee: Tomcat Developers Mailing List
Depends on:
Reported: 2007-05-23 09:00 UTC by Len Popp
Modified: 2007-06-15 15:50 UTC (History)
0 users

Request and response headers showing the problem (1.63 KB, text/plain)
2007-05-23 09:03 UTC, Len Popp

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Description Len Popp 2007-05-23 09:00:19 UTC
According to the HTTP spec, if a server includes an ETag header when it sends a
file, it must also include the ETag when it sends a 304 (not-modified) response
for that file. Tomcat does not do this for static files - if you request a
static file and get a 200 response, the response has an ETag header; but if you
get a 304 resopnse, the ETag is omitted.

To reproduce:
- In a browser, request a static file from Tomcat (e.g. http://localhost/tomcat.gif)
- Make sure you get a 200 response (force reload or clear browser cache)
- Examine the response headers (using a browser plugin or whatever) - note that
there is an ETag header
- Request the same file again, getting a 304 (not-modified) response from Tomcat
- Examine the response headers - note there is no ETag
The 304 response should include an ETag header, because the 200 response had one.

Spec reference: RFC 2616 section 10.3.5 says:
"304 Not Modified
The response MUST include the following header fields:
      - ETag and/or Content-Location, if the header would have been sent
        in a 200 response to the same request"
Comment 1 Len Popp 2007-05-23 09:03:45 UTC
Created attachment 20251 [details]
Request and response headers showing the problem

I have attached a log file from Firefix LiveHTTPHeaders showing the problem.
A static file is requested twice. The first response is a 200 with an ETag
the second response is a 304 without an ETag header.
Comment 2 Len Popp 2007-06-01 07:43:20 UTC
I see how to fix this, but I'm not set up to compile Tomcat so someone else will
have to make the change and test it.

In org.apache.catalina.servlets.DefaultServlet, the serveResource method sets an
ETag header when it serves a file with a 200 or 206 response. To get it to add
the ETag to 304 responses, this needs to be added:
 response.setHeader("ETag", getETag(resourceAttributes));
in two places where a a status of 304 (SC_NOT_MODIFIED) is set, in the methods
checkIfModifiedSince and checkIfNoneMatch.
Comment 3 Len Popp 2007-06-01 07:45:44 UTC
(I've changed the Component to "Catalina" because none of the more specific
Components seem appropriate. If that's wrong, please correct it.)
Comment 4 Mark Thomas 2007-06-01 18:01:51 UTC
Thanks for the patch. It has been applied to svn and will be in 5.5.24 and
6.0.14 onwards.
Comment 5 Len Popp 2007-06-15 15:50:45 UTC
Tested OK in 5.5.24 release candidate.