Mdthbs

hostname is used to display the system's DNS name, and to display or set its hostname or NIS domain name.

Does a computer system (Linux) only have one host name?

In virtual hosting, several host names can be resolved to different root directories in a web server. If a computer system (Linux) can only have one host name, how is virtual hosting possible?

Thanks.

Yes, and no. The are two distinct things called hostnames.

The "internal" hostname is basically a string kept by the kernel. This is the one returned by the hostname command (or the gethostname() call) and it's unique within a system^(*).

It's mostly used when a program wants to output some identifier for the system it's running on. E.g. h in Bash's PS1 expands to the hostname. Similarly, syslog-style logfiles also include the hostname on log entries.

^{(* Though as Stephen Kitt comments, namespaces can be used to show different hostnames to processes on the same system. That's mostly used for containers, which try to act like they're distinct systems.)}

Then there's also DNS names that are used by other systems to look up the IP address of another. There might be more than one DNS name that point to the same IP address, and so the same host.

The internal hostname and the DNS names don't need to be the same. Suppose someone has a webserver they've decided to call orange^(*), with the IP address 192.0.2.9 .
It could serve two different domains and the DNS would be set up to have www.example.org and www.example.com both point to 192.0.2.9, while the internal hostname of the system might be orange.example.org or just orange. In that case, the DNS setup would usually also have a reverse lookup on 192.0.2.9 point back to the name orange.example.org, but there's nothing to force that.

_{(* because they like to name their servers after fruit. Someone might use webserver1 or such, but the point is that it doesn't need to be named after one of the actual domains.)}

In addition to that, virtual hosting requires that the browser tell the web server the name of the site it tried to access. Otherwise the server would not know which virtual site the client tried to reach. HTTP has the Host header for that.

What muddies the distinction between a DNS name and the internal hostname is the mDNS protocol
(implemented e.g. by the avahi daemon) and other discovery protocols. mDNS makes it possible for hosts to query all other
hosts on the same network for name information, and to make their own hostnames visible on other hosts
without explicitly setting them up in DNS.

Does a computer system (Linux) only have one host name?

No.

[I'll completely ignore that anybody may give your host any name they like by having a DNS record pointing to its address, or that you can have more than one alias for it in /etc/hosts, or any philosophical considerations about what a name really is]

On a linux system, the hostname is simply a per-process resource (the "UTS namespace") that is inherited by its children by default, but could be disjoined from with unshare(2) or clone(2) by using the CLONE_NEWUTS flag. You should look into the namespaces(7) manpage.

Just like pids, port numbers, mount points, etc the hostname is no longer a global identifier, and it could be virtualized just the same as the address space (virtual memory) or the file descriptor table of a process.

In virtual hosting, several host names can be resolved to different root directories in a web server. If a computer system (Linux) can only have one host name, how is virtual hosting possible?

Virtual hosting is something completely different, and it's usually done by configuring a wildcard DNS record (eg. *.foo.com => 13.13.13.13) with a web server running on 13.13.13.13 and serving different directories / resources based on value of the http Host: header[1] (which is highly configurable; most web servers are able to serve different resources based on any http header, not just Host:). In all this, the OS of the system the web server is running on plays no part.

[1] or on the value of the server name indication in case of https, which allows the same webserver to use different certificates for different virtual hosts.

You usually set a single hostname in /etc/hostname. And, you set many domain names in /etc/hosts.

UTS namespaces allow a single system to appear to have different host and domain names to different processes. For more info: https://en.wikipedia.org/wiki/Linux_namespaces. So, it is absolutely possible to have multiple hostnames and domain names for the same system. E.g., docker container inside the same host has different hostname than host system.