Security Handbook/User and group limitations

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Security Handbook
Pre-installation concerns
Bootloader security
Logging
Mounting partitions
User and group limitations
File permissions
PAM
TCP wrappers
Kernel security
Network security
Securing services
Chrooting and virtual servers
Firewalls
Intrusion detection
Staying up-to-date

This section provides detail on controlling the system's resource usage.

/etc/security/limits.conf

Controlling resource usage can be very effective when trying to prevent a local Denial of Service (DoS) or restricting the maximum allowed logins for a group or user. However, settings that are too strict will impede the system's behavior, so make sure each setting is sanity checked before implemented.

FILE /etc/security/limits.conf
*    soft core 0
*    hard core 0
*    hard nproc 15
*    hard rss 10000
*    -    maxlogins 2
@dev hard core 100000
@dev soft nproc 20
@dev hard nproc 35
@dev -    maxlogins 10

Considering removing a user before setting nproc or maxlogins to 0. The example above sets the group dev settings for processes, core file and maxlogins. The rest is set to a default value.

Note
/etc/security/limits.conf is part of the PAM package and will only apply to packages that use PAM.

/etc/limits

/etc/limits is very similar to the limit file found at /etc/security/limits.conf. The difference between the files is the format and that it only works on users or wild cards (not groups). Let's have a look at a sample configuration:

FILE /etc/limits
*   L2 C0 U15 R10000
kn L10 C100000 U35

Here we set the default settings and a specific setting for the user kn. Limits are part of the sys-apps/shadow package. It is not necessary to set any limits in this file if the pam USE flag has been enabled in /etc/portage/make.conf.

Quotas

Warning
Make sure the file systems present support quotas. In order to use quotas on ReiserFS, the kernel must be patched with patches available from Namesys. User tools are available from the DiskQuota project. While quotas do work with ReiserFS, other issues may be encountered while trying to use them - consider this a warning!

Putting quotas on a file system restricts disk usage on a per-user or per-group basis. Quotas are enabled in the kernel and added to a mount point in /etc/fstab. The kernel option is enabled in the kernel configuration under File systems → Quota support Apply the following settings, rebuild the kernel and reboot using the new kernel.

Start by installing quotas with emerge sys-fs/quota. Then modify /etc/fstab and add usrquota and grpquota to the partitions to be restricted, like in the example below:

FILE /etc/fstab
/dev/sda1 /boot ext2 noauto,noatime 1 1
/dev/sda2 none swap sw 0 0
/dev/sda3 / reiserfs notail,noatime 0 0
/dev/sda4 /tmp ext3 noatime,nodev,nosuid,noexec,usrquota,grpquota 0 0
/dev/sda5 /var ext3 noatime,nodev,usrquota,grpquota 0 0
/dev/sda6 /home ext3 noatime,nodev,nosuid,usrquota,grpquota 0 0
/dev/sda7 /usr reiserfs notail,noatime,nodev,ro 0 0
/dev/cdroms/cdrom0 /mnt/cdrom iso9660 noauto,ro 0 0
proc /proc proc defaults 0 0

On every partition that have quotas enabled, create the quota files (aquota.user and aquota.group) and place them in the root of the partition:

root #touch /tmp/aquota.user
root #touch /tmp/aquota.group
root #chmod 600 /tmp/aquota.user
root #chmod 600 /tmp/aquota.group

This step has to be done on every partition where quotas are enabled.

On OpenRC systems, after adding and configuring the quota files, be sure to add the quota script to the boot run level.

Important
XFS does all quota checks internally, and does not need the quota script. There may be other filesystems not listed in this document with similar behavior, so please read the manpages for each filesystem to learn more about how it handles quota checks.

Add quota to the boot runlevel:

root #rc-update add quota boot

Quotas can be checked once a week by adding the following line to /etc/crontab:

FILE /etc/crontab
0 3 * * 0 /usr/sbin/quotacheck -avug.

After rebooting, it is time to setup the quotas for users and groups. edquota -u kn will start the editor defined in EDITOR environment variable (default is nano) and allow editing quotas of the user kn. edquota -g will do the same thing for groups.

edquota -u kn
Code Listing 3.5: Setting up quota's for user kn

Quotas for user kn: /dev/sda4: blocks in use: 2594, limits (soft = 5000, hard = 6500)

inodes in use: 356, limits (soft = 1000, hard = 1500)

For more detail read man edquota or the Quota mini howto.

/etc/login.defs

If your security policy states that users should change their password every other week, change the value of PASS_MAX_DAYS variable to 14 and PASS_WARN_AGE variable to 7. It is recommended password aging be implemented since brute force methods can find any password, given enough time. Sysadmins are also encouraged to set LOG_OK_LOGINS variable to yes.

/etc/security/access.conf

The access.conf file is also part of the sys-libs/pam package, which provides a login access control table. This table is used to control who can and cannot login based on user name, group name or host name. By default, all users on the system are allowed to login, so the file consists only of comments and examples. Whether securing a server or workstation, we recommend this file be secured so no one other the sysadmin has access to the console.

Note
These settings apply for root, as well.
FILE /etc/security/access.conf
-:ALL EXCEPT wheel sync:console
-:wheel:ALL EXCEPT LOCAL .gentoo.org
Important
Be careful when configuring these options, since mistakes could leave no access to the machine for any user except root.
Note
These settings do not apply to SSH, since SSH does not execute /bin/login per default. This can be enabled by setting UseLogin yes in /etc/ssh/sshd_config.

This will setup login access so members of the wheel group can login locally or from the gentoo.org domain. Maybe too paranoid, but better to be safe than sorry.