There are plenty of ways to mitigate the risk of LDAP server unavailability. Besides, the document said that the bastion server uses it, so why isn't what's good for the bastion good enough for the rest of the fleet?
Adding extra brittle moving parts like LDAP and linux clustering to a greenfield deployment just doesn't seem attractive when a CA is so easy to run, and you should already have one if you're doing config management sanely.
Also you're making an assumption about the need for consistency, when as a practical matter there's rarely a need for it. Caching is effective and practical for this use case and you'd have to make a very strong case that it should be thrown out.
Finally, it is my experience that people grossly misjudge the difficulty of securely and scalably running a CA. Most such comments come from those who have never actually operated one.
mfdutra's comment that you replied to said: If things go south really really bad, we can just get the private key and sign certificates by hand.
That's obviously an incomplete answer - it doesn't explain how you connect to the server if you can't get onto the bastion due to a directory failure - but the basic premise is that the core system is isolated & contains everything it needs to handle the authentication process.
The bastion host is not strictly necessary in the process. Although mfdutra didn't spell it out explicitly, they clearly have a means for accessing their core servers without needing to get on to the bastion - that's where the signing by hand comes in. The benefits of using LDAP in that case justify the cost of having an external dependency - by integrating with the central password store, the bastion provides a path into the system that delivers the desired security features (centralised identity management, auditing back to individual user accounts), but other paths can exist if needed.
That trade-off isn't applicable to core servers. If they use LDAP (exclusively) for authentication and the directory is down then you're hosed. There's no other path into the server that you're trying to admin other than to get onto the server.
You could use LDAP with a fallback to some other scheme (like signed certs, or a small set of locally defined users managed with a password vault, etc) but then you've got 2 different paths onto each server and both paths needs to be maintained, tested, and regularly audited. That's certainly possible, but why would you want to when you can just have 1 authentication mechanism that has zero dependencies.
In any event:
First, the CA itself is a dependency: if it fails, no login certificates will issue, and users won't be able to log in. So they traded one kind of service dependency for another.
Second, it hasn't been established that a backup login path is available. In many environments this may not even be acceptable (e.g. PCI and billing systems, which I know Facebook has).
A CA is just some bytes, not a service. And it has been established that there's a backup login path: use (a copy of) the CA outside of the automated certificate signing service to manually sign the needed certificates.
They'd be screwed if they lost the CA's private key, but it is much easer to keep some data around than to keep a service functioning properly.
In the scheme described, users don't log into servers with the CA's certificate and private key -- the CA's private key is always protected, preferably in an HSM of some sort. Instead, the CA issues a signed certificate to the user with a set of principals; and that latter certificate is the one used to log in.
So, the process that issues certificates (the "Authority", as opposed to the "Authority Certificate") is the one I'm concerned with here.
>If things go south really really bad, we can just get the private key and sign certificates by hand.
Very clearly states that someone has access to the CA cert's private key outside the context of the automated signing service, and can use it to manually sign certs for users if the CA service is down. So the CA service can be bypassed if it goes down,.
My point is that you cannot eliminate all dependencies. And if I must have dependencies, I'd rather put my trust in a well-engineered, time-tested, highly-available system. When properly implemented, LDAP + SSSD is such a system.
At any rate, an even faster and more reliable emergency response system would be to place a static user ID and password in a lockbox (virtual or physical) somewhere and use that to log in. You don't need a complex CA infrastructure to attain that; NSS fallbacks to static /etc files would suffice.