>> What I'd like to know now is : what prevents apt-get from >> downloading the Packages file from the mirror ? Wget can ! > > Move all the files out of /var/lib/apt/lists/ so that apt-get update > has to download fresh copies. That should get you back on track. > > By all means take the opportunity to compare the old and the new files > to look for causes. In the simplest scenario, which probably is not > the cause but might illustrate the kind of problem, a failed download > could leave a file with a timestamp at the point of failure. That > erroneous timestamp would be more recent than the mirror's (correct) > timestamp and could cause update to think there's nothing to do on > subsequent runs until the original file on the mirror was modified.
The problem is not that apt-get sees the new Packages file and decides that it does not need to be downloaded. If that were the case, the output would be Hit http://mymirror/debian stretch/main amd64 Packages The actual output is Ign:5 http://mymirror/debian stretch/main amd64 Packages which indicates that it tried to download it but failed. At least if the link I gave previously is correct (and it appears to be). The reason why it failed is that it looked for Packages.xz which did not exist on my mirror. The reason why my mirror did not have a Packages.xz file is that it was made with the version of debmirror in Debian 8, which only concerns itself with Packages.gz and Packages.bz2. Those files *were* there and apt-get could have used them but, in its wisdom, it didn't even try. I solved the problem by installing the latest version of debmirror on the Debian 8 machine where the mirror is. Thankfully, there were no dependency problems. There is probably material for a few bug reports in there. Thank you.