Some of my experience and solution to those issues
1) UptimeRobot [0] - use to monitor various client websites. The free plan checks every 5 minutes, which should be enough. Notifications can be sent to email, slack, sms and many others. If you think there may be a problem only from some locations make a fast check with [1]. If you suspect DNS issues [2] or [3].
2) Again use UptimeRobot for monitoring device publicly accessible from your network. Moreover, if you are in control of your office network, using pfSense [4] notifications when a network gateway goes down works well (still, that works only if you have 2 or more ISPs). Or use a dedicated monitoring device/service like Zabbix.
3) Using to Twitter to Slack notification, subscribe for updates from both services that you use and major services responsible for Internet backbone. An example is, that using GitLab, comes with multiple time when the service dies (even that they are improving) - seeing the message in Slack that something is WIP currently by all team members (in a dedicated channel), helps to skip unnecessary debugging [5] :)
Not affiliate with any of the service. Still - met the UptimeRobot guys some ago - they are a small startup based in Malta, are very cool and have very stable service :)
Pro-active monitoring rather than reactive diagnosis.
Zabbix is an example piece of software, probably overkill for most but I haven't used anything else in the last 5 or so years so don't have any better suggestions.
External monitoring from AWS or a colo.
Simple icmp checks and tcp connects + possible up to
app layer checks allowed from these failsafes. Obfuscate
as needed.
uhh, monitoring those things? you can monitor your internet provider a number of ways. something like smokeping allows for an exceedingly simplistic test: ping stuff on the internet.
logs will tell you if a client server or individual service has died. that are literally hundreds of solutions for these.
1) did we break our client server
2) did our internet provider die
3) did the service die
What are recommended ways of finding out fast and reliably in these cases where the fault is.