linuxexpert-practice-labs/labs/lfcs/5-3-user-resource-limits

LFCS 5.3: User resource limits

Limiting users' processes, open files and logins with pam_limits, changing the limits of a running process, and capping a user's memory with systemd.

Before you start

The lab creates group students with members student1 and student2, and user alice (password Lab-Pass-1, shell bash). Service lab-worker runs a program of the same name as alice. So that le-lab check can test ssh logins, the lab puts a key in student2's ~/.ssh/authorized_keys.

Tasks

1. Limit each member of group students to 50 processes. Test it with sudo -iu student1 bash -c 'ulimit -u'.
2. Give user alice an open-file limit of 10000 soft and 20000 hard. Test it with sudo -iu alice bash -c 'ulimit -Sn; ulimit -Hn'.
3. Allow members of students at most two simultaneous logins. Test it with three ssh student1@localhost sessions at once.
4. Find the open-file limits the running lab-worker process has, and write them to /root/lab-answers/4 as two numbers, soft then hard. Then raise its open-file limit to 4096, soft and hard, without restarting it.
5. Cap all of alice's processes together at 1 GiB of memory, and only hers.

Hints

• /etc/security/limits.d/*.conf: @group, soft, hard, -, and the items nproc, nofile and maxlogins. They apply at the next login.
• pgrep, /proc/PID/limits, prlimit --pid.
• systemctl set-property user-UID.slice; systemctl show -p MemoryMax.