Dual Stack DHCP clobber each other when using /run/dhcp-server/ for pid files

[Joshua Stark]

When installing dual stack IPv4 and IPv6 DHCP I found that DHCPv4 and DHCPv6 clobber each others /run/dhcp-server path when you stop or restart one of the daemons

I found that the config in the systemd unit has the /run path set to the same location for both DHCPv4 and DHCPv6

In /lib/systemd/system/isc-dhcp-server.service the offending line that conflicts is:
  exec dhcpd -user dhcpd -group dhcpd -f -4 -pf /run/dhcp-server/dhcpd.pid -cf $CONFIG_FILE $INTERFACES'

In /lib/systemd/system/isc-dhcp-server6.service the offending line that conflicts is:
  exec dhcpd -user dhcpd -group dhcpd -f -6 -pf /run/dhcp-server/dhcpd6.pid -cf $CONFIG_FILE $INTERFACES'

This means if you stop/restart DHCPv4 or DHCPv6, then it cleans up the path /run/dhcp-server and removed the pid file for the alternate process.

I have found a possible fix is to give DHCPv6 it's own /run path, thus I suggest:

1) update DHCPv6 to use /run/dhcp-server6/

Update /lib/systemd/system/isc-dhcp-server6.service to use /run/dhcp-server6/
ie.
  exec dhcpd -user dhcpd -group dhcpd -f -6 -pf /run/dhcp-server6/dhcpd6.pid -cf $CONFIG_FILE $INTERFACES'

Now we need to give dhcp access to this path in Apparmor:

2) Update /etc/apparmor.d/usr.sbin.dhcpd and add an additional line

38d39
  /var/lib/dhcp/dhcpd{,6}.leases* lrw,
  /var/log/ r,
  /var/log/** rw,
  /{,var/}run/{,dhcp-server/}dhcpd{,6}.pid rw,
+ /{,var/}run/{,dhcp-server6/}dhcpd{,6}.pid rw,

Found in package version:
  4.4.1-2ubuntu5