While I was implementing a few more changes on my web server - mostly adding the sorely needed blacklistd configuration for sshd - I noticed that NGINX’s log was showing occasional errors when trying to contact the Isso process. They all had one thing in common, namely that they were all trying to contact ISSO via IPV6 as the server has both stacks enabled. Turns out that isso only listens on an IPV4 socket and I could not find an obvious way to get it to listen on both.