/etc/modprobe.d/xenomai.conf is in xenomai-system-tools 2.6.4+dfsg-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | install xeno_nucleus /sbin/modprobe --ignore-install xeno_nucleus $CMDLINE_OPTS \
$(/usr/bin/test $(/bin/echo -n '$CMDLINE_OPTS' | /bin/grep xenomai_gid) \
|| /usr/bin/getent group xenomai | /usr/bin/cut -d: -f3 | /bin/sed -e 's/^/xenomai_gid\=/')
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