docs: is IP address supported?

[e-kwsm]

Describe the bug

sudo-rs seems to accept an IP address as a host, but it is not mentioned in the man.

To Reproduce

$ echo 'Defaults@192.168.10.10 lecture' | visudo-rs -c -
stdin: parsed OK

sudo-rs/docs/man/sudoers.5.man

Lines 230 to 235 in 91a0784

	Host_List ::= Host |
	Host \[aq],\[aq] Host_List
	Host ::= \[aq]!\[aq]* host name |
	\[aq]!\[aq]* Host_Alias
	.EE

Expected behavior

Man specifies that IP addresses are supported.

Environment (please complete the following information):

• Linux distribution: EndeavourOS
• sudo-rs version: 0.2.12

Additional context

[bjorn3]

I think the 192.168.10.10 is parsed as a hostname, not as ip address. So it would only match a system where /etc/hostname contains 192.168.10.10.

[squell]

Of course it's unlikely that that is a hostname. I would't be opposed to adding a small check in the tokenizer which explicitly complains about hostnames that look like they're an IPv4 address.

I'm not sure what the use case of using IP addresses in /etc/sudoers would be, seems like a fragile way of configuring (it's under influence of the network; a machine can have multiple IP addresses) and so fail the Berlin Criteria. We also don't support fully qualified domain lookups even though that might be slightly more useful.

[squell]

Of course as @rnijveld has pointed out, in default setups it's also easy to change the hostname; but at least that can be bolted down more easily.

[squell]

Relevant here is RFC952 and the way it is modified by RFC1123:

If a dotted-decimal number can be entered without such
identifying delimiters, then a full syntactic check must be
made, because a segment of a host domain name is now allowed
to begin with a digit and could legally be entirely numeric
(see Section 6.1.2.4). However, a valid host name can never
have the dotted-decimal form #.#.#.#, since at least the
highest-level component label will be alphabetic.

This is a bit ambiguous but seems to suggest that we can check if there are three dots, then the final component should not be a number. Seems pretty useful to have an error message for that.