I don't have a strong opinion either, but given that scripts would ignore the warnings and the resulting numeric users are going to face random, seemingly unrelated issues thanks to the interaction with systemd, I think I prefer the failure.
FWIW, I've prepared a test version in a PPA[1] which keeps the rules from Debian[2] but prevents the fully numeric names. This is what it looks like: $ useradd 0 useradd: invalid user name '0' $ echo $? 3 $ sudo useradd 0c0 $ sudo useradd 0 --badnames $ cat /etc/passwd | grep ^0 0c0:x:1001:1001::/home/0c0:/bin/sh 0:x:1002:1002::/home/0:/bin/sh [1] https://launchpad.net/~vtapia/+archive/ubuntu/sf305373 [2] https://salsa.debian.org/debian/shadow/-/blob/master/debian/patches/506_relaxed_usernames -- You received this bug notification because you are a member of Ubuntu Touch seeded packages, which is subscribed to shadow in Ubuntu. https://bugs.launchpad.net/bugs/1927078 Title: Don't allow useradd to use fully numeric names Status in shadow package in Ubuntu: New Status in shadow source package in Focal: New Status in shadow source package in Groovy: New Status in shadow source package in Hirsute: New Status in shadow source package in Impish: New Bug description: [Description] Fully numeric names support in Ubuntu is inconsistent in Focal onwards because systemd does not like them[1] but are still allowed by default by useradd, leaving the session behavior in hands of the running applications. Two examples: 1. After creating a user named "0", the user can log in via ssh or console but loginctl won't create a session for it: root@focal:/home/ubuntu# useradd -m 0 root@focal:/home/ubuntu# id 0 uid=1005(0) gid=1005(0) groups=1005(0) .. [email protected]'s password: Welcome to Ubuntu 20.04.2 LTS (GNU/Linux 5.8.0-48-generic x86_64) Last login: Thu Apr 8 16:17:06 2021 from 192.168.122.1 $ loginctl No sessions. $ w 16:20:09 up 4 min, 1 user, load average: 0.03, 0.14, 0.08 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT 0 pts/0 192.168.122.1 16:17 0.00s 0.00s 0.00s w And pam-systemd shows the following message: Apr 08 16:17:06 focal sshd[1584]: pam_unix(sshd:session): session opened for user 0 by (uid=0) Apr 08 16:17:06 focal sshd[1584]: pam_systemd(sshd:session): pam-systemd initializing Apr 08 16:17:06 focal sshd[1584]: pam_systemd(sshd:session): Failed to get user record: Invalid argument 2. With that same username, every successful authentication in gdm will loop back to gdm again instead of starting gnome, making the user unable to login. Making useradd fail (unless --badnames is set) when a fully numeric name is used will make the default OS behavior consistent. [Other info] - Upstream does not support fully numeric usernames - useradd has a --badnames parameter that would still allow the use of these type of names To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/1927078/+subscriptions -- Mailing list: https://launchpad.net/~touch-packages Post to : [email protected] Unsubscribe : https://launchpad.net/~touch-packages More help : https://help.launchpad.net/ListHelp
