[pkg-dhcp-devel] Bug#792894: systemd unit files for isc-dhcp-server

[Terry Burton]

Hi Marc, et al.

As you know from discussions on the dhcp-users mailing list I've been
threatening to do some systemd packaging work for ISC DHCP so I'm
taking your contributions as a starting point.

I can't speak for the Debian packaging team so maybe Michael Gilbert
can review this and let us know whether this is on track.

I've taken a look at your unit files and taken the liberty to create
some new ones based on them that I've added to my repo here [2].

I've used the latest Debian sys-v init script from the packaging
repository [3] as my reference for what's sane.

Compared to the originals units:

(1) I've added "Wants" to isc-dhcp-server.target which ensures that
subordinate services are started. Therefore you only need "enable" the
.target file to ensure that the v4 and v6 daemons are started assuming
they are configured.
(2) In the -v4 service file I've replaced references to dhcpd4.conf
with dhcpd.conf since this provides compatibility across upgrades.
(3) I've added an EnvironmentFile option that reads the user provided
settings from /etc/default/isc-dhcp-server. I think only INTERFACES,
INTERFACESv4, INTERFACESv6 and possibly OPTIONS remain relevant for
reasons given in (8) and (9) below.
(4) You use ConditionPathExists=/etc/dhcp/{dhcpd.conf,dhcpd6.conf} to
determine whether to start the units. I like this approach and have
kept it as it provides nice semantics in (5) and (6) below for whether
to start the v4 and v6 subcomponent services.
(5) Since a dhcpd6.conf file isn't currently installed by the package
(and probably shouldn't be, at least not yet) there is no attempt to
start a IPv6 instance - following the principle of least surprise. You
can simply add a dhcpd6.conf file and restart isc-dhcp-server.target
to enable the v6 instance.
(6) You can correspondingly disable the v4 instance by moving
dhcpd.conf out of the way.
(7) I note that this is a different condition for deciding whether or
not to start the daemon than the sys-v script which us whether or not
INTERFACESv{4,6} is defined in /etc/default/isc-dhcp-server. Such a
condition can't be done cleanly in systemd, i.e. you cannot have an
enabled service (or component of a compound target) that simply
decides not to start based on an arbitrary test). Perhaps the sys-v
init script can be aligned to the dhcpd{,6}.conf exists approach?
(8) Since you can't expand environment variables in the
ConditionPathExists and therefore refer directly to the config files
you may as well also hardcode the the values for DHCPDv{4,6}_CONF in
the ExecStart/ExecStartPre lines, i.e. don't read these from
/etc/default/isc-dhcp-server.
(9) I've dropped support for extracting OPTIONS (and OPTIONSv4,
OPTIONSv6)  from /etc/default/isc-dhcp-server since it isn't honoured
by the most recent sysv-init script [3]. If this is accidental then
its trivial to add this back into both systemd and sys-v.
(10) Support for changing the PID file location in
/etc/default/isc-dhcp-server now seems a little odd given that we no
longer take the conf file from here so I've dropped it. This could
alternatively be passed in OPTIONSv{4,6} if we were to enable these.

A general problem that I've noticed which might cause us to modify the
isc-dhcp-server.target approach is that the following commands will
not have the expected effect:

service isc-dhcp-server {start,stop,restart}
systemctl {start,stop,restart} isc-dhcp-server

(1) It doesn't appear as though you can invoke .target unit files
using /usr/sbin/service. This'll drive some purists nuts!
(2) Additionally, these commands as-is will actually invoke LSB
compatibility and trigger the sys-v init script rather than the native
isc-dhcp-server.target unit file.
(3) The packaging would need to do something ensure that the
isc-dhcp-server init script (via LSB) and isc-dhcp-server.target unit
are not both started at boot.

This could spoil an otherwise neat approach. Any suggestions welcome...

I'm not intending to wire up the maintainer scripts until we have an
agreed approach so you'll have to hand-pick the unit files from my
repo [2] into {/etc,/lib}/systemd/system for the time being.

Would you mind taking a look to make sure these work for you, caveats
aside, and let me have any feedback.


[1] https://lists.isc.org/pipermail/dhcp-users/2016-May/020093.html
[2] https://github.com/terryburton/isc-dhcp-debian/commit/26d3a94e00853f17141a6f748169dc2a26366b15
[3] http://anonscm.debian.org/cgit/pkg-dhcp/isc-dhcp.git/tree/debian/isc-dhcp-server.init.d


Thanks.