1.  /etc/security/limits.conf

Controlling resource usage can be very effective when trying to prevent a local Denial of Service or restricting the maximum allowed logins for a group or user. However, too strict settings will impede on your system's behavior and will result in program failures so make sure that you check each setting first.

*    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

If you find yourself trying to set nproc or maxlogins to 0, maybe you should delete the user instead. The example above sets the group dev settings for processes, core file and maxlogins. The rest is set to a default value.

1.  /etc/limits

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

*   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 you have enabled pam in /etc/make.conf.

1.  Quotas

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 quota. Then modify your /etc/fstab and add usrquota and grpquota to the partitions that you want to restrict disk usage on, like in the example below.

/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,usrjquota=aquota.user,grpjquota=aquota.group,jqfmt=vfsv=1 0 0
/dev/sda5 /var ext3 noatime,nodev,usrjquota=aquota.user,grpjquota=aquota.group,jqfmt=vfsv1 0 0
/dev/sda6 /home ext3 noatime,nodev,nosuid,usrjquota=aquota.user,grpjquota=aquota.group,jqfmt=vfsv1 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 you have enabled quotas, create the quota files (aquota.user and aquota.group) and place them in the root of the partition.

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

This step has to be done on every partition where quotas are enabled. After adding and configuring the quota files, we need to add the quota script to the boot run level.

# rc-update add quota boot

The Linux kernel will track the quota usage as the system works. If for any reason the quota files become corrupt or you think the data is wrong, you will need to bring the system in single-user mode (or at least ensure that the file systems are not being actively written to) and then call quotacheck -avugm.

After rebooting the machine, it is time to setup the quotas for users and groups. edquota -u kn will start the editor defined in $EDITOR (default is nano) and let you edit the quotas of the user kn. edquota -g will do the same thing for groups.

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.

1.  /etc/login.defs

If your security policy states that users should change their password every other week, change the value PASS_MAX_DAYS to 14 and PASS_WARN_AGE to 7. It is recommended that you use password aging since brute force methods can find any password, given enough time. We also encourage you to set LOG_OK_LOGINS to yes.

1.  /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 you are securing your server or workstation, we recommend that you setup this file so no one other than yourself (the admin) has access to the console.

-:ALL EXCEPT wheel sync:console
-:wheel:ALL EXCEPT LOCAL .gentoo.org

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.