have been told the migration is complete, but now we just have to wait for dns to propagate completely for tug2.com. I am not sure why that process still takes so long but it is what it is at this point.
Well, since you asked, here is a simplified explanation:
Internet messages (including web requests) have to be sent to numeric addresses on the WorldWide Web. But we bookmark sites as domain names, e.g., tugbbs.com. Domain Name Service (DNS) translates tugbbs.com to 69.16.236.219.
When you ask the web browser to visit tugbbs.com, it first needs to determine the Internet Protocol Address (IP address) for tugbbs.com. Your web browser or computer couldn't possibly store and keep up with all this information. So it asks a DNS Server. (This is something you specify in your configuration.) If that server doesn't have the info, it asks an upstream server, closer to the "root DNS server." This process repeats until a correspondence is found. (Side note: When we were on slow 1200 bps dialup modems, we would pick a local (nearby) DNS server to get a peppier response. These days everyone picks Google 8.8.8.8 or 8.8.4.4, Cloudflare 1.1.1.1, or Quad9 9.9.9.9 which are closer to the root.)
Suppose you are browsing several successive web pages on TUGBBS. It doesn't make sense to communicate with a DNS server for each one. Your browser and computer each keep a "cache" of recently used Domain Names and corresponding IP addresses. In this case your browser directly makes the request, skipping the DNS lookup. The browser and computer can't store this information indefinitely; the cache would grow without bound, and the computer/browser would not update when the DNS information changed, just as it is now changing for TUG2. So the cache is periodically cleared. The user is unaware of the few extra milliseconds it takes to do a DNS lookup. The cache clearing schemes for computers and browsers is unclear--possibly the DNS caches are cleared when the computer and or browser is restarted. (This is why the Help Desk has you restart your computer when you can't reach a website. If you want to simply clear a Windows computer's DNS cache, you can open a command prompt window and enter
IPCONFIG /FLUSHDNS in upper or lower case. For MacOS,
sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder and enter password when requested.
Besides the caches in your browser and computer, there are caches in the DNS Servers all the way upstream to the root server. Each one of those also has a scheme for cache management. One common scheme keeps three pieces of information for each domain instead of two: domain name, IP address, and time/date last used. If the DNS cache entry is more than an hour old, it is discarded.
And now you can see why it is difficult to project how long it will take for a DNS update to propagate. You can't tell how long it will take for the servers in the computer and upstream chain to refresh. If you've having trouble connecting to a server and it's urgent, you can flush local caches (reboot is simple), and switch DNS servers looking for a fresher response.
