Those are all accidental limitations from the architecture as it exists, not as it could exist. There is no fundamental reason why the IP address is a combination of network and host address. There is no fundamental reason why a host is presumed to have only one IP address. There is no fundamental reason why [email protected] and [email protected] can’t be the same daemon listening on different IP addresses, but anybody connecting to the alice.com interface gets a different certificate and no access to the bob.com resources.
I think the systemd socket activation with declarative configuration files, and the Serverless cloud computing fad, are hints of how it is possible to control exactly what program is running, and even have some custom code, without having to duplicate and maintain all the binaries. Too bad they’re doing it on Linux, so they still have all those accidental limitations.
Hosts are not presumed to have only one IP address. This is a mistake that people make. (They often made it with djbdns, hence https://cr.yp.to/djbdns/ifconfig.html .) But it has never actually been the case. Indeed, one can find discussions of hosts that have multiple IP addresses in RFC 1122 and discussion that IP addresses do not have a 1:1 correspondence with network interfaces in RFC 791.
Details. In practice, few applications do interesting things when binding to multiple IP addresses. It’s like a special case of single IP address.
Perhaps I should phrase it, there is no fundamental reason why IP addresses are associated with hosts rather than users, or even services. There is no fundamental reason why you need to be root to listen to privileged ports, which includes many of the most useful ports.
I think the systemd socket activation with declarative configuration files, and the Serverless cloud computing fad, are hints of how it is possible to control exactly what program is running, and even have some custom code, without having to duplicate and maintain all the binaries. Too bad they’re doing it on Linux, so they still have all those accidental limitations.