Doc: Distributed Monitoring: add section "External CA/PKI"

[Al2Klimov]

The following already works:

• Custom key sizes, e.g. 2048 bits
• Custom key types, e.g. ECC
• Multiple trusted root CAs in /var/lib/icinga2/certs/ca.crt
• Different root CAs per cluster subtree, as long as each node trusts the issuers of the certificates of all nodes it's directly connected to
• Any number of intermediate CAs

refs #9798
refs #7323

[Al2Klimov]

FYI @bobapple @widhalmt @tbauriedel

[yhabteab]

It might also be worth mentioning how several certificates should be ordered in this file, rather than just "starting with the leaf", i.e leaf certificate -> intermediate certificates -> root CA.

Quote from https://www.rfc-editor.org/rfc/rfc4346#section-7.4.2:

certificate_list
This is a sequence (chain) of X.509v3 certificates. The sender's
certificate must come first in the list. Each following
certificate must directly certify the one preceding it. Because
certificate validation requires that root keys be distributed
independently, the self-signed certificate that specifies the root
certificate authority may optionally be omitted from the chain,
under the assumption that the remote end must already possess it
in order to validate it in any case.

[yhabteab]

I would express this in a slightly different, more end-user friendly way:

Suggested change

	Neither the above commands, nor their automatic counterparts in the Icinga
	cluster do anything special during certificate issuance. I.e. Icinga
	isn't the only possible source of the certificates it uses. E.g.
	`openssl req/x509 ...` may be used as well as long as the leaf certificates' CN
	and SAN match the endpoint names. Pretty much everything else is limited only by
	your imagination and the oldest OpenSSL version of two Icinga nodes connected to
	each other. E.g. the following works:
	There is nothing special about the above commands or their automatic counterparts in the Icinga cluster when it comes to certificate issuance. It should be noted that Icinga is not the sole source of the certificates it uses. For instance, openssl `req/x509 ...` can be used as well, as long as the **CN** and **SAN** of the leaf certificates match the endpoint names. Otherwise, the only restrictions are your own imagination and the oldest OpenSSL version of two connected Icinga nodes. You can find a working example below:

Even I wouldn't consider the steps listed below to be a working example of this. IMHO they're just too abstract to offer as a working example for the end user who has no knowledge of how openssl req/x509 works. One shouldn't use external CAs if doesn't know what to do though. So 🤷‍♂️!

[Al2Klimov]

One shouldn't use external CAs if doesn't know what to do though.

Actually a reason not to make this more user friendly. 😅

[yhabteab]

I was referring to the examples below and not this opening text. This is still a technical documentation, so you have to comply with certain standards (avoiding the use of i.e or e.g).

[Al2Klimov]

@julianbrost What's your opinion?

[julianbrost]

In general, it's a good idea to write down what Icinga 2 expects from certificates and what it issues by itself. However, I think I'd rather place this somewhere into the technical details documentation instead of here and telling users so be "limited only by [their] imagination". It should be more of a "only do this if you know what you're doing and don't expect much help" thing not an invitation to generate special certificates and then be surprised that they weren't (properly) renewed by Icinga 2.

[Al2Klimov]

I'd even agree with the second half of your comment, but isn't Manual Certificate Creation exactly where users would expect such? Users already can't find something in our docs w/o a search engine!

[julianbrost]

Actually a reason not to make this more user friendly. 😅

exactly where users would expect such

Which of these two now? Currently it's a "be limited only by your imagination and good luck with that" for end users. I'd be happy with not directly presenting it to most standard users but moving it to the more technical documentation that's more for developers and expert users.

If this is supposed to become standard user documentation, this at the very least needs a warning that custom certificates may not be renewed by Icinga 2 and that one has to take care of this externally. By the way, did you test what happens when Icinga 2 would attempt to renew an externally issued certificate that does not closely resemble the ones issued by Icinga 2?

[oxzi]

As I was asked to put my two euro cents in, I am generally in favor of this PR, as it both (loosely) describes Icinga 2's own requirements for certificates as well as how to realize a "SSL added and removed here! :-)"-setup required by some corporations.

However, I would advise adding more details and moving it to "12-icinga2-api" or another later section, to not confuse or overload first time users. Furthermore, a hint how to monitor the certificate expiration - link to ITL, check_ssl_cert, for example - would be good, as otherwise people tend to forget those things, even if noted that Icinga 2 itself does not do a thing.

[Al2Klimov]

However, I would advise adding more details and moving it to "12-icinga2-api" or another later section, to not confuse or overload first time users.

I'm afraid this naturally belongs to Manual Certificate Creation. That section can be seen as a bit advanced by itself and its parent section is even called Advanced Hints, so... 🤷‍♂️ (Have I already mentioned there's a big warning now in here?)