Comment 3 for bug 568704

The issue still exist in Ubuntu 14.04 (currently in development), as a side effect if a user decides to use:

mirror/http/proxy=http://domain.tld:3142 in the APPEND line in a pxe environment, eg.

APPEND priority=critical locale=en_US interface=auto mirror/http/proxy=http://domain.tld:3142 url=http://another-domain.tld/ubuntu.preseed initrd=ubuntu/1404/i386/initrd.gz

The installer will fail to request the remote preseed file (as it'll try to get it through the mirror url). A work around if you still want to define the mirror in the append line (and not in the remote preseed file) is to configure it at preseed/run or at preseed/early_command.

http://serverfault.com/questions/528582/debian-ubuntu-set-preseed-mirror-variable-via-early-run-command

Configuring the mirror at the APPEND line is useful when you want to use a master preseed file from every machine but want to have control when to use cache.

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