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Time and Resource Limits

You may also wish to impose access time restrictions and resource limitations on your users. PAM affords the opportunity to impose such limitations through the modules pam_time and pam_limits, respectively.

Using pam_time This module is used with module type account only. Although it accepts no arguments, it does expect a configuration file, /etc/security/ time.conf (Red Hat 5.2/6.0; other distributions may vary), to supply it with login location and time limitations. The absence of this file has the effect of not restricting access in any way. The limitations apply to all users, including root.

Suppose that you'd like to impose some limits on user access to a particular system. Take a look at the account module type entries in Example 5-10. The pam_time and the pam_pwdb entries both use the control flag required. This has the effect of causing the account verification step to proceed. For example, if Mary's password has expired, when she attempts to log in, she will be so informed and refused access regardless of the limitations imposed by pam_time. Let's assume that Mary's password has expired and that she is attempting to connect to the system livfreeordie outside the limits imposed by /etc/security/time.conf (see The /etc/security/time.conf File on page 96). Example 5-11 shows what happens if Mary attempts to log in under these assumptions and /etc/pam.d/login is configured as shown in Example 5-10. Even though Mary is logging in outside of her approved time, the only information she gets is that her password has expired.

Example 5-10 Partial Configuration File Using pam_time required

Account  required  /lib/security/pam_pwdb.so
Account  required  /lib/security/pam_time.so

Example 5-11 Login Attempt with Expired Password and Outside of Permitted Times 1

$telnet livfreeordie
Trying 10.1.1.1...
Escape character is '^]'.
Red Hat Linux release 5.2 (Apollo)
Kernel 2.0.36 on an i686
login:mary
Password:
Your account has expired;please contact your system administrator
User account has expired
Connection closed by foreign host.
$

If you reverse the entries and set the control flag to requisite for pam_time, the behavior is quite different. Example 5-12 shows the new configuration. Example 5-13 shows what happens with the new configuration when Mary attempts to log in.

Example 5-12 Partial Configuration File Using pam_time.so requisite

Account  requisite  /lib/security/pam_time.so 
account  required  /lib/security/pam_pwdb.so 

Example 5-13 Login Attempt with Expired Password and Outside of Permitted Times 2

$ telnet livfreeordie
Trying 10.1.1.1...
Escape character is '^]'.
Red Hat Linux release 5.2 (Apollo)
Kernel 2.0.36 on an i686
login:mary
Password:
Permission denied
Connection closed by foreign host.
$

The moral of these examples is: watch your order and control flag settings. Experiment before you implement! Make sure the configuration settings provide the functionality you expect.

Next we turn our attention to the time.conf file.

The /etc/security/time.conf File. This file controls access time and login location by device when using the pam_time module. Each line in the file is a record, called a rule, except lines beginning with # which are comments. Each record has the following syntax:

services;ttys;users;times

This syntax is further detailed in Table 5.6.

The phrase logical list referenced in Table 5.6 means that the special characters, described in Table 5.7, are used as conditional operators

Logical operators may be mixed. For example, tty* & !ttyp* means that any serial device is allowed for this rule, but all pseudo-devices are not.

The syntax used to specify days in timed.conf is summarized in Table 5.8. These codes are then used with time ranges, all times being specified by the 24-hour clock. For example, Wd0800-1600 means weekends between the hours of 8 A.M. and 4 P.M.. Notice that there are no spaces between the day code and the time.

Table 5.6 Description of /etc/security/time.conf Entries

Parameter

Description

Services

A logical list of the services associated with PAM. Multiple records with the same service are acceptable. Examples include login, rsh, and su.

Ttys

A logical list of device(s). The login device is stored in PAM_TTY. Normally this includes such devices as tty1 and tty2 for the console, ttyS0 and ttyS1 for serial ports, and ttyp1 and ttyp2 for pseudo-devices normally associated with network and X Window connections.

users

A logical list of (valid) users. May include root.

times

A logical list of times at which this rule applies.


Table 5.7 Conditional Operators Used in /etc/security/time.conf

Operator

Function

Examples

&

logical AND

user1 & user2—means this rule applies to both user1 and user2.

|

logical OR

tty1 | tty2—means this rule applies to either tty1 or tty2.

!

logical NOT

! login—this rule does not apply to the login service.

*

wildcard

Matches any value, its meaning depending on its location in a field.


Table 5.8 Day Codes in /etc/security/timed.conf

Day Code

Description

Mo, Tu, We, Th, Fr, Sa, Su

Each code individually applies to the day of the week it indicates. These codes may be concatenated; for example, MoTuWe means Monday, Tuesday, and Wednesday.

Wk and Wd

Wk means weekdays while Wd means weekends (Saturday and Sun-day). Note that, for example, MoWk means all weekdays except Monday.

Al

All seven days. Note that, for example, AlSu means all days except Sunday.


Example 5-14 shows a series of entries in a sample /etc/security/ time.conf file. In this example, the root user has access to all services all of the time so long as root logs in from tty1. The users—joe, bill, and jane—have access every day of the week between 8 A.M. and 6 P.M. from any device so long as they connect via login or rsh. The user guest may log in from anywhere, Monday through Friday between the hours of 9 A.M. and 4 P.M., except between 12 noon and 1 P.M. Finally, any user may access the system using ftp from any source Monday through Friday between the hours of 9 A.M. and 4 P.M. All other users are unrestricted.

Example 5-14 Sample /etc/security/time.conf File

*;tty1;root;Al0000-2400
login ↱*;joe|bill|jane;Al0800-1800
login;*;guest;Wk0900-1600&!Wk1200-1300
ftp;*;*;Wk0900-1600

This file is not order dependent. If there are entries that overlap, the least permissive (actually, the intersection of all entries) is used. Remember, you must place a pam_time entry in each file in /etc/pam.d for those services for which you want to restrict access. If you are going to limit access using pam_time, consider placing entries in at least the following files (and hence the associated services will be limited) in /etc/pam.d: ftp, login, ppp, rexec, rlogin, rsh, su, and xdm.

WARNING

If you use an entry of the form *;*;*;!AL0000-24000 in the /etc/security/time.conf file, you will lock out all users, including root, from the system! Should you find yourself locked out of the system, see the section Recovering a Corrupt System on page 24 in Chapter 3 for recovery procedures.

Using pam_limits You may additionally restrict user accounts by imposing limits on the system resources available to each user login session. This may be useful in limiting system-based DoS attacks. The pam_limits module operates as a session module type only. It supports two arguments—debug and conf=/path/to/config_file (the default configuration file is /etc/security/ limits.conf). It does not impose any limits on the root account.

The /etc/security/limits.conf file is used to impose limits on a per-user or per-group basis. All limits specified apply to a single session. Each line in the file is a record, except for those beginning with #. The syntax of a record is

username|@groupname type resource limit

where username|@groupname specifies that either a username or a groupname preceded with @ may be used. The wildcard character * is acceptable and represents all users (hence all groups). The type field is either the hard or soft parameter; hard imposes a fixed limit and soft specifies a default limit. The resource parameter is one of the items described in Table 5.9. The limit parameter is the limit itself on the associated resource.

It is important to note that the limits imposed are on a per-session basis. The total limitation may be controlled with the maxlogins parameter. Limits may be completely disabled for particular users with the special character, -, an instance of which is shown in Example 5-15 . In this example, all users are limited to a resident set size of approximately 10 megabytes (MB) per session. All users may also have a maximum of only four simultaneous logins. It is this value that sets the overall maximum per user. The user, bin, has all limits disabled, including the previous entries in the file. The remaining limitations in the file are additional limitations for the users and groups indicated. The user ftp is allowed only 10 logins (this is an excellent limit to impose on anonymous ftp accounts since it limits the number of simultaneous logins). All users in the group managers have a process limitation of 40 and all users in the group developers have a memlock limit of approximately 64 MB.

Example 5-15 Sample /etc/security/limits.conf File

*      hard  rss     10000
*      hard  maxlogins  4
bin     -	
ftp     hard  maxlogins  10
@managers  hard  nproc    40
@developers hard  memlock   64000

Table 5.9 Limitable Resources in /etc/security/limits.conf

Resource

Description

core

Limits the size of a core file (KB*)

data

Maximum data size (KB)

fsize

Maximum file size (KB)

memlock

Maximum locked-in memory address space (KB)

nofile

Maximum number of open files

rss

Maximum resident set size (KB)

stack

Maximum stack size (KB)

cpu

Maximum CPU time in minutes

nproc

Maximum number of processes

as

Address space limit

maxlogins

Maximum number of logins allowed for this user


Of course, you must determine the limits necessary for each system at your site. Make sure you place the entry session required /lib/security/pam_limits.so in each appropriate file in /etc/pam.d.

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