Features/1000SystemAccounts

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1000 System Accounts

Summary

Standardize on login.defs as the authoritative definition of UID/GID space allocation, and move the boundary between system and user accounts from 500 to 1000.

Owner

• Name: Miloslav Trmač
• Email: mitr@redhat.com

Current status

• Targeted release: Fedora 16
• Last updated: Sep 29 2011
• Percentage of completion: 100% (6/6 required packages)

Detailed Description

The UID/GID ("ID" from now on) space allocation is for Fedora is not clearly defined, and it is hard-coded in various applications. Historically Fedora has allocated 500 ID values for system users; given that current Fedora packages allocate over 270 accounts, more space would be required for a larger initiative to stop running system processes as root, or for allowing more statically-allocated IDs (whether by Fedora or site-local policy). The value 500 also deviates from the upstream allocation in shadow(-utils).

The intent of this feature is, therefore, to allocate 1000 ID values for system accounts. Quite a few applications have hard-coded the 500 value as a boundary; instead of replacing this with a hard-coded value of 1000, such applications will be modified to read the boundary from /etc/login.defs.

The ID layout will change as follows:

Range	Fedora ≤15	Fedora ≥16
Statically-allocated system accounts (/usr/share/doc/setup/uidgid)	0-?	0-200
Dynamically-allocated system accounts (allocated from higher to lower values)	?-499	201-999
User accounts	500-60,000	1,000-60,000

As shown above, the boundary between statically-allocated and dynamically-allocated system accounts has not been well-defined (it was supposed to be 100, but static UIDs up to 173 have been already allocated in Fedora 15); the boundary is now explicitly defined (using SYS_[UG]ID_MIN) to be 201, and because dynamically-allocated system accounts are allocated from higher values, it will be both easy and possible to expand the space for statically allocated system accounts in the future.

/etc/login.defs is already the de-facto standard for configuring the system/user account boundary (used at least by accountsservice, libsemanage, libuser and of course shadow-utils), so we will formally codify it instead of inventing a new and incompatible configuration file.

Making the boundary configurable also allows some users to stay with the old boundary of 500, if they wish:

• Because /etc/login.defs is %config(noreplace), upgrades will retain the boundary value 500, and nothing should break.
• New installations in setups where the UIDs are centrally allocated (e.g. using LDAP) from 500 could be likewise configured to use the boundary value 500 by creating /etc/login.defs in a kickstart %pre script.

Benefit to Fedora

• More space for system accounts in total, making it possible to try a larger-scale initiative to stop running system processes as root
• Explicitly defined boundary between statically- and dynamically-allocated system accounts that reflects the actual allocation
• Enough space to allow expanding the statically-allocated space in the future
• Using the same ID allocation as upstream shadow(-utils), and some prominent Linux distributions (e.g. Debian, OpenSUSE)

Scope

Overall tracking bug

The allocation has already been changed in shadow-utils; the following packages (out of the "desktop" and "web server" installations available in Anaconda) need to be updated to read /etc/login.defs:

• PackageKit
• authconfig
• firstboot
• kde-settings/kdebase-workspace
• system-config-users

and the following packages might be updated as well:

• audit (documentation only)
• libuser (documentation and unused API only)
• openldap (documentation only)
• pam (documentation only)
• sssd (documentation only)
• system-config-network (unused code)
• systemtap (documentation only)

I (mitr) didn't review the remaining Fedora packages (not in the "desktop" and "web server" installations in Anaconda), and I plan to ask their package maintainers on fedora-devel to review their/fix their own packages if possible.

Extending Anaconda's kickstart facilities to make it easier to override the change for fresh installs is a possibility, but not committed to by anyone at this point.

How To Test

Both on a clean installation and on an upgrade from Fedora 15:

• Verify that after a clean installation, UID_MIN is 1000 and after an upgrade, UID_MIN is 500; review the other values in /etc/login.defs similarly.
• Go through firstboot; review /etc/passwd and /etc/group to verify that the user account created in firstboot, and the system accounts created during installation follow the policy specified in /etc/login.defs.
• Start gdm, verify that all users and no system accounts are offered for login. Verify that users can log in.
• Check kdm likewise.
• Set up httpd with suexec, verify that each user can run their own scripts through suexec. (FIXME: expand this?)
• Start system-config-users, verify that user accounts and no system accounts are visible. Create a new user/group and verify that their ID allocation follow the policy specified in /etc/login.defs.
• Verify other aspects of the system as possible, or other components that will be discovered to hard-code the "500" boundary, if any.

User Experience

For users that don't use a site-wide ID allocation mechanism (e.g. LDAP), no visible impact is expected - neither on fresh installs nor on upgrades.

For users with a site-wide ID allocation mechanism (e.g. LDAP) that allocates user IDs >= 1000, no impact is expected either. If user IDs in the range 500-1000 are allocated, upgrades will work fine, but new installs that follow the site-wide policy will only be possible using kickstart.

Dependencies

See "Scope".

Contingency Plan

Revert the /etc/login.defs settings to match Fedora 15; then all packages (whether unchanged from F15 or ported to use /etc/login.defs) will work as they did in Fedora 15.

Documentation

login.defs(5)

Release Notes

Fedora 16 changes the UID and GID allocation policy: user accounts now start from value 1000 instead of the previous value 500. This policy is now globally set in /etc/login.defs variables GID_MIN and UID_MIN, see login.defs(5) for more details. Upgrades from earlier Fedora releases will keep their configuration, starting user accounts from 500.

If you need to install a new system from scratch, while starting user accounts from 500 (to connect the system to a network with globally-defined UIDs), install using a kickstart script that places /etc/login.defs on the file system before package installation starts.

Comments and Discussion

• See Talk:Features/1000SystemAccounts