FFe: Unrestricted default RESTful interface on 127.0.0.1:8000

This is a feature freeze exception request to address the security vulnerability described below in [Original Description] in the kea-ctrl-agent daemon, shipped in the bin:kea-ctrl-agent package.

Upstream was contacted: https://lists.isc.org/pipermail/kea-dev/2023-March/001001.html. Advice was to not start or not install kea-ctrl-agent.

The way this was addressed is via:
- a "high" debconf prompt giving 3 options (see below)
- a restriction on starting the kea-ctrl-agent daemon only if the /etc/kea/kea-api-password file exists and has content
- default kea-ctrl-agent config file is patched to include an "authentication" section, which hardcodes a "kea-api" username, and a password to be read from the /etc/kea/kea-api-password file.

Users who changed any of the following in the kea-ctrl-agent config file prior to this change will get a dpkg conf prompt unfortunately:
- listen address (default: 127.0.0.1)
- listen port (8000)
- path to control sockets of the other kea daemons (*)
- added/changed hooks libraries
- logging configuration (destination, method, rotation parameters, etc) (*)

(*) These were changed already in prior uploads of this package to lunar.

###
Unattended fresh installs, or upgrades from previous versions (before $now), of this package with a "next/next/next/finish" type of installation, will end up with a non-running kea-ctrl-agent daemon, because no password will be set by default.
###

Upon fresh install, or upgrade from a pre-debconf version of this package, the user will get (subject to debconf prio levels) a "high" prio debconf prompt (screenshot attached) explaining the change and asking to pick one of 3 options:

- unconfigured: do nothing. If the /etc/kea/kea-api-password file isn't in place, the service won't start (but won't fail either). This is the default for unattended installs.
- configured with random password: we generate a random password and put that in place. Service starts.
- configured with user-provided password: we ask for a password and use that. Empty passwords are not allowed. Service starts.

This should:
- fresh install: get a debconf high prio prompt with the 3 choices.
- upgrade from version-before-debconf: same as fresh install, you get the 3 choices
- if user runs dpkg-reconfigure, they get the 3 choices
- upgrade from version that has these debconf questions in place already: nothing gets asked
- if the user has apt listchanges installed, the NEWS file will be shown stating these change

Together with this change we are also adding a DEP8 test to exercise the debconf prompts and behavior.

Diff: https://code.launchpad.net/~ahasenack/ubuntu/+source/isc-kea/+git/isc-kea/+merge/439352

PPA build: https://launchpad.net/~ahasenack/+archive/ubuntu/isc-kea-debconf/+packages

DEP8 run for ppa18 (amd64 only):

https://autopkgtest.ubuntu.com/results/autopkgtest-lunar-ahasenack-isc-kea-debconf/lunar/amd64/i/isc-kea/20230329_221328_f9adf@/log.gz

[Original Description]

The kea-ctrl-agent package, when installed, starts a daemon (kea-ctrl-agent) that by default listens on 127.0.0.1:8000. It responds to commands like "shutdown", "config-get", and many others[1][2].

What's problematic is that these commands are accepted without authentication. Anyone on the localhost system can:

a) shutdown a kea daemon:
ubuntu@j-kea:~$ pidof kea-dhcp4
2884
ubuntu@j-kea:~$ curl -X POST -H "Content-Type: application/json" -d '{ "command": "shutdown", "service": [ "dhcp4" ] }' http://localhost:8000/
[ { "result": 0, "text": "Shutting down." } ]ubuntu@j-kea:~$
ubuntu@j-kea:~$ pidof kea-dhcp4
ubuntu@j-kea:~$

b) read the config file (in this example, I made the config file 0640 root:_kea so the ubuntu user cannot read it):
ubuntu@andreas-isc-kea-server:~$ cat /etc/kea/kea-dhcp4.conf
cat: /etc/kea/kea-dhcp4.conf: Permission denied

ubuntu@andreas-isc-kea-server:~$ curl -X POST -H "Content-Type: application/json" -d '{ "command": "config-get", "service": [ "dhcp4" ] }' http://localhost:8000/| grep secret
  % Total % Received % Xferd Average Speed Time Time Time Current
                                 Dload Upload Total Spent Left Speed
100 4049 100 3998 100 51 134k 1751 --:--:-- --:--:-- --:--:-- 136k
[ { "arguments": { "Dhcp4": { "authoritative": false, "boot-file-name": "", "calculate-tee-times": false, "config-control": { "config-databases": [ { "name": "kea", "password": "keasecret", ....

The same could be done via the unix sockets, but the permissions there are not world writable, so this is avoided:

$ ls -la /tmp/kea*socket
srwxr-xr-x 1 _kea _kea 0 Feb 14 19:13 /tmp/kea-ddns-ctrl-socket
srwxr-xr-x 1 _kea _kea 0 Feb 14 19:14 /tmp/kea4-ctrl-socket
srwxr-xr-x 1 _kea _kea 0 Feb 14 19:13 /tmp/kea6-ctrl-socket

One course of action is to disable listening on 127.0.0.1:8000 via the config file:

/etc/kea/kea-ctrl-agent.conf:
"Control-agent": {
    "http-host": "127.0.0.1",
    // If enabling HA and multi-threading, the 8000 port is used by the HA
    // hook library http listener. When using HA hook library with
    // multi-threading to function, make sure the port used by dedicated
    // listener is different (e.g. 8001) than the one used by CA. Note
    // the commands should still be sent via CA. The dedicated listener
    // is specifically for HA updates only.
    "http-port": 8000,
(...)

Or maybe setup authentication with a user created in postinst for this purpose, with a random password. The documentation[3], in the end of section 7.2, lists a mechanism to include username and password from an external file, so we don't have to adjust the permissions of kea-ctrl.agent.conf because of this.

Finally, there is also a question about what to do on upgrades from systems that have this unprotected open port.

1. https://kea.readthedocs.io/en/kea-2.2.0/arm/ctrl-channel.html#commands-supported-by-both-the-dhcpv4-and-dhcpv6-servers
2. https://kea.readthedocs.io/en/kea-2.2.0/arm/ctrl-channel.html#commands-supported-by-the-d2-server
3. https://kea.readthedocs.io/en/kea-2.2.0/arm/agent.html#configuration