Calibre is used as a server all the time, see calibre-web.

calibre-web is technically not Calibre and is written and maintained by different people, although it does use the Calibre database (and I believe it must be created with desktop Calibre initially). But it’s a good option and I highly recommend it.

you just load your books from Calibre (or right through USB if you’re hardcore for some reason) and you’re basically off to the races.

There’s also an OPDS server option with calibre-web that you can use to load books from if you’re using koreader.

You can also use the Kobo server replacement option with calibre-web although I personally couldn’t get it to work at the time I tried it. But this will give you a sync option that works like the official Kobo server which is quite nice.

You can also set --accept-dns to false with the commandline client although magic DNS etc won’t work.