Comment 8 for bug 1693900

Actually this is the problem: Users think, their system is up-to-date, but it is not for sure because a site failed to respond. Therefore only if _all_ sites answered the request properly, apt-get should return 0. If not, it should return a specified return code, which lets the callee know, that there was a problem [and imply, that a subsequent apt-get upgrade might bring the system to the latest supported state, or not]. If the exit code for such situations is documented properly, the tool can still decide, whether to run the upgrade or would be bredless art.

BTW: It doesn't really matter, what error (whether temp. DNS, LDAP lookup, network, etc.) prevented a successful update. Fact is, that there was an error and this needs to be communicated. CLI tools do that via exit code.