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Daniel Hilst
33bf641a72
On Debian and Ubuntu, makedhcp writes the list of interfaces it is serving
into /etc/default/isc-dhcp-server using the key set at dhcp.pm:2296:
$dhcpd_key = "INTERFACES";
That key stopped being the one the daemon is started with. The variable the
isc-dhcp-server systemd unit expands onto dhcpd's command line changed with
the package; verified by unpacking the archive's own debs:
trusty 4.2.4-7ubuntu12 sysvinit only INTERFACES
xenial 4.3.3-5ubuntu12 unit $INTERFACES
bionic 4.3.5-3ubuntu7 unit $INTERFACES
focal 4.4.1-2.1ubuntu5 unit $INTERFACES
jammy 4.4.1-2.3ubuntu2 unit $INTERFACESv4
noble 4.4.3-P1-4ubuntu2 unit $INTERFACESv4 (+ a v6 unit
reading $INTERFACESv6)
and every package from bionic onward ships a default file whose only
interface variables are INTERFACESv4 and INTERFACESv6, both seeded empty by
postinst. No INTERFACES line ships at all.
Two things go wrong on jammy and later.
The unit runs `exec dhcpd ... -cf $CONFIG_FILE $INTERFACESv4`. That variable
is never set, so it expands to nothing and dhcpd is launched with no
interface argument. dhcpd does not fail for this: it binds every interface
it can find and merely warns about the ones with no subnet declaration. So
site.dhcpinterfaces and servicenode.dhcpinterfaces are silently inert. The
provisioning NIC is served only because makedhcp also emitted a subnet
stanza for it, not because anything honoured the setting, and any other
interface on that same subnet -- a bridge port, a bond member, a second NIC
-- is served too.
The line match is m/^$dhcpd_key/, which is not anchored on the assignment.
"INTERFACES" is a prefix of both variables the package ships, so the rewrite
claims the INTERFACESv4 and INTERFACESv6 lines and overwrites both. The file
is left holding two INTERFACES lines and nothing the units read, discarding
whatever debconf or the administrator had set there.
Extract the rewrite into _sysconfig_interfaces_content() with no change in
behaviour, and add a unit test that asserts the effect rather than the
spelling: it writes the produced file to a temp path and sources it with sh
exactly as the unit does, then checks what would land on dhcpd's command
line. The test is red against the current key and stays red until the writer
sets the variables the daemon is actually started with.