I am puzzling as to why Facebook reports this when I use Facebook's debugger and when I try to add a site running on an ISPConfig3 server to a Facebook business page. " could not resolve the host name into a valid IP address". For what it is worth, the sites all resolve and work as expected. Some of the sites have dns on different servers and some are lazily set up with both dns pointing to the ispconfig3 server. For example, Greenlink.org has different dns servers, and Hcfa.com has both name servers pointing to the ispconfig3 server. No redirects other than to SSL. Debian 10, but the sites are still using php 7.x. All the sites on the server appear to give same results. Any thoughts?
I doubt that this issue is related to the use of ISPconfig at all, resolving issues are most likely DNS problems and not related to which control panel you are using. The error message talks about the host name, so it might be that Facebook checks your server hostname and even reverse DNS. Run the command: hostname -f on your server, and check if it has valid DNS records. Then check if the IP address(es) of the server have a valid reverse record that points to this hostname. If that's all ok, then your issue might be related to IPv6, maybe Facebook tests the IPv6 records of your domains and can't reach the server using them. Finally, if the domains have been moved recently, then it might be that Facebook was using a dns cache with not up to date dns records.
Till, thank you. All of those suggestions are good ideas. But all of them check out just fine. The ipv6 idea remains a slim possibility. But I have to think that if this were the case that there would be thousands of folks fussing given the deployment of ipv6 to date. I double checked to be certain that there were no ipv6 routes being announced on this network. But host name is good, ptr matches, names resolve without incident. And even more so, Facebook successfully "scrapes" the sites. And I can't believe they are fussing about the Let's Encrypt certificates or, again, there would be thousands of folks fussing. I wonder if there is a graceful way to remove http entirely within the ispconfig environment and have only https. I am uneasy making changes that the control panel may get testy about after the fact. On the other hand, the world is pretty used to just typing domain names and letting the machines handle shifting to https. And finally, there appears to be no asking of Facebook. And if this hasn't come up before, it is not likely here that I should be looking.
