** Description changed:
I don't know of any better way to report this bug. The configuration of
archive.ubuntu.com prevents proxy servers from caching packages. This
may be because of poor configuration of archive.ubuntu.com, buggy
behaviour of that software or an opportunity for the apt client to
ignore one part of web standards to improve things.
To reproduce, configure a Squid proxy server and then use it to upgrade
one machine eg to Intrepid. Now do the same on a second machine. If
you monitor the Squid logs you will see all the files being redownloaded
due to a TCP_REFRESH_MISS. This means the cache had the file but on
checking with the server found the cached file to be stale.
The root cause is a round robin set of ip addresses are returned for
us.archive.ubuntu.com. For example I see 91.189.88.31, 91.189.88.45 and
91.189.88.46.
You can then query each one of these for the same file, looking at the
headers returned replacing the IP address as appropriate.
wget --no-proxy --header="Host: us.archive.ubuntu.com" -O /dev/null -S
http://91.189.88.31/ubuntu/pool/main/libh/libhtml-tagset-perl/libhtml-
tagset-perl_3.20-2_all.deb
The Last-Modified header is identical across all 3 servers, but the ETag
is different. Because the ETag is different the proxy server has to
conclude that the content is stale.
The bad effect is that upgrading N computers at a site that uses a
normal proxy server requires N downloads which for a dist upgrade can be
close to 1GB. That sucks up more of your bandwidth and pointlessly
increases server utilization.
Note that .deb with the same name do not change anyway.
Some suggested fixes:
* Stop sending back ETag from servers and only rely on Last-Modified
/Content-Length to detect cache invalidation
* Don't include inode in ETag calculation:
http://httpd.apache.org/docs/2.2/mod/core.html#fileetag
* Calculate the ETag from the file md5/sha
* Have a really long DNS cache timeout for round robin returned values
(eg two weeks) rather than the very short interval so the same server
will be hit from the same proxy
I haven't been able to work out a way for the apt client to prevent the
Squid proxy from paying attention to ETag. Note that this same issue
will affect any caching/proxy server that obeys web standards. One
other workaround is to install an apt/deb specific proxy (which then
- ignores the web standards) but that then requires me to manage to
+ ignores the web standards) but that then requires me to manage two
proxy/cache servers when just the one would work fine if the ubuntu.com
ones were fixed.
--
archive.ubuntu.com config prevents caching
https://bugs.launchpad.net/bugs/290921
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