Perhaps, untested:
a) change /var/log/frr (directory) to frr:adm 0770 (or 2770, to cope with the case where frr's logging is changed from syslog to file)
b) change logrotate to "create frr:adm 0660"
c) postinst chowns frr:adm, chmod 0660 the log files.
This would allow both ubuntu and debian to keep logging in /var/log/frr via rsyslog, at the cost of changing ownership of log files once again in the history of the package (from frr:frr to frr:adm in debian, and no log to frr:adm in ubuntu)
Perhaps, untested:
a) change /var/log/frr (directory) to frr:adm 0770 (or 2770, to cope with the case where frr's logging is changed from syslog to file)
b) change logrotate to "create frr:adm 0660"
c) postinst chowns frr:adm, chmod 0660 the log files.
This would allow both ubuntu and debian to keep logging in /var/log/frr via rsyslog, at the cost of changing ownership of log files once again in the history of the package (from frr:frr to frr:adm in debian, and no log to frr:adm in ubuntu)
Is the above what you more or less meant?