On Mon, Sep 17, 2018 at 12:20:51AM +0200, local10 wrote:
;; ANSWER SECTION: old.reddit.com. 241 IN CNAME reddit.map.fastly.net. reddit.map.fastly.net. 8 IN A 151.101.21.140
this number ^^^is the TTL/"time to live" in seconds. It is set by the server distributing the records and is typically set to something fairly long, like a day or more, for domains which don't change much. For domains which do change often, or if a move is planned in the near future, the TTL may be set low so that old records aren't cached long after they're obsolete. In this case the TTL in your cache shows 241 seconds left for old.reddit.com and 8 seconds remaining for reddit.map.fastly.net. By checking the authoritative nameservers directly it seems that the former is never more than 5 minutes while the latter is set to only 30 seconds. So your caching server should be updating fairly often.
Mike Stone