Using CRLs in Icinga 2
Icinga 2.x offers a cluster mode which (from an administrator's point of view) is one of the most important features introduces with the 2.x release. Using the cluster feature, check commands can be executed on satellite nodes or even the complete scheduling of checks can be delegated to other nodes, while still keeping the configuration in a single place.
In order to enable secure communication within the cluster, Icinga 2 uses a public key infrastructure (PKI). This PKI can be managed with the icinga2 pki commands. However, there is no command for generating a CRL. For this reason, it is necessary to use the openssl ca command for generating a CRL. I have documented the steps necessary for generating a CRL in my wiki.
Funnily, it seems like no one has used a CRL in Icinga 2 so far. I know this, because up to today, Icinga 2 has a bug that makes it impossible to load a CRL. Luckily, yours truly already fixed this bug and this bugfix is going to be included in the next Icinga 2 release.
I find it strange that obviously no one is using CRLs, because Icinga 2 uses a very long validity period when generating certificates (15 years), so it is quite likely that at some point a node is decommissioned and thus the corresponding certificate shall be removed.