install AT&K versiion
ksh-93s-23.x86_64.html
| Korn Shell |
OpenRHEL Factory for x86_64 |
ksh-93s-23.x86_64.rpm
Symbolically link /usr/bin/ksh to /usr/bin/ksh93
___ Install mc RPM package
___ Verify that screen RPM package is
installed (should be installed by default).
___ Modify crontab as necessary
Anything else that I missed.
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- ___ Install RHELtar
- ___ Install standard NTI/RU, operator accounts via script
- Note: Use bash as the default shell
for all human users
- Anything else that I missed.
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- ___ Create via Gui the group wheel and enroll into it all active
system administrators
- ___ Group ntiru should contains only staff members of NTI/RU
- ___ Group staff should contain application owners
- ___ Group operators should contain operators
- Anything else that I missed.
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- ___ Check if ZMD package is missing and install it, if necessary
- ___ Verify that proxy server is correctly configured by accesssing Novell.com
via FireFox
- ___ Register the server using registration code for the server provided
by Novell.
- You do not need to put your credentials as a part of proxy configuration
- If registration fails open ticket with Novel and let them do the job.
- ___ Install all the patches:
- might require several iterations
- ___ Important for SAP servers: verify that new LVM kernel patch is applied
- ___ [Cancelled] This step is cancelled until further notice because it
looks like kdump is a dangerous daemon to have.
- Verify that kdump works. Update of kdump package if necessary (this
does not affect SAP)
- For 64G memory you need 70G partition
- For 32G of memory 36G partition
- Anything else that I missed.
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- ___ Create application specific users and directories
- ___ Create NFS mounts
- Anything else that I missed.
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- ___ ref notes 958253, 171356, 941735 (sapBC1cfg.sh)
- ___ For NFS mounts ensure that <new host> is in
/etc/exports
- ___ Make SAP dirs/mount points; mod fstab, mount
- ___ chown….
- ___ Install saplocales rpm pkg (linuxlab-locales-2.4-1) “rpm –Uvh
--force linuxlab-locales-2.4-1.x86_64.rpm”
- ___ modify kern parms: /etc/sysctl.conf.sap
- Anything else that I missed.
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- ___ Remove Unnecessary Software Packages (RPMs). A very important
step in securing a Linux system is to determine the primary function or role
of the Linux server. You should have a detailed knowledge of what is on your
system. Otherwise you will have a difficult time to understand what needs to
be secured and hence securing your Linux systems proactively won't be that effective.
Therefore, it is very critical to look at the default list of software packages
and remove unneeded packages or packages that don't comply with your security
policy. If you do that you will have less packages to update and to maintain
when security alerts and patches are released. For example, you should not have
Apache or Samba installed on your system if you don't use them. Also, it is
a good practice not to have development packages installed on production servers.
FTP and Telnet daemons should be installed if there is a justified business
reason for it (SSH/SCP/SFTP should be used instead).
The idea is to create a Linux image that only contains RPMs needed by the
applications, and needed for maintenance and troubleshooting purposes. It may
be time-consuming but worth the efforts.
To get a list of all installed RPMs you can use the following command:
rpm -qa
If you want to know more about a particular RPM, run:
rpm -qi <package_name>
To check for and report potential conflicts and dependencies for deleting a
RPM, run:
rpm -e --test <package_name>
For information on performing Kickstart installations and how to build an image,
see
Kickstart Installations for more information.
Remove unnnessesary services
To list all services which are started at bootup using the following command:
chkconfig --list |grep on
You will notice that there are quite a lot of services enabled on your system.
But many runlevel services (Stand-Alone Services) are not network related services
like kudzu which is responsible for detecting and configuring new and/or
changed hardware on your system. This service is only run during the boot process.
Here are examples of Red Hat Runlevel System Services which need to be removed:
apmd Advanced Power Management (APM) daemon is used for laptops and some desktops
isdn needed if ISDN is being used
iptables needed if Netfilter (iptables) Firewall is being used
ip6tables needed if ip6tables Firewall is being used
xinetd needed if xinetd services are being used, see /etc/xinetd.d/ for list of services
cups needed if CUPS is used for the printing system
sysstat needed to reset system statistics logs
audit needed only if Linux Audit Subsystem (LAuS) should run for collecting system call audit records
psacct needed only if kernel process accounting information is needed
netdump important if kernel oops data and memory dumps should be sent to a Netdump server for server crashes
- ___ Modify /etc/issue & motd
- Configure syslog and change in /etc/syslog.conf
to point to log server
- All linux servers should forward syslog to NTI253.
- Verify log rotation (/etc/logrotate.conf)
- ___ Install Tivoli endpoint.
- See separate document as for details(/Tivoli/Scott_Notes.doc)
- ___ SecureID installation.
- See separate document as for details.
- Install writable file check script into cron (consult Nikolai about the
latest and greatest version)
- Configure VNC to allow remote administration
To accept incoming SSH connections from e.g. nodes rac1cluster, rac2cluster and
rac3cluster, add the following line to /etc/hosts.allow:
sshd: rac1cluster rac2cluster rac3cluster
To accept incoming SSH connections from all servers from a specific network, add
the name of the subnet to /etc/hosts.allow.
For example:
sshd: rac1cluster rac2cluster rac3cluster .subnet.example.com
To accept incoming portmap connections from IP address 192.168.0.1 and subnet 192.168.5,
add the following line to /etc/hosts.allow:
portmap: 192.168.0.1 192.168.5.
To accept connections from all servers on subnet .subnet.example.com but
not from server cracker.subnet.example.com, you could add the following
line to /etc/hosts.allow:
ALL: .subnet.example.com EXCEPT cracker.subnet.example.com
Here are other examples that show some features of TCP wrapper:
If you just want to restrict ssh connections without configuring or using /etc/hosts.deny,
you can add the following entries to /etc/hosts.allow:
sshd: rac1cluster rac2cluster rac3cluster
sshd: ALL: DENY
The version of TCP wrapper that comes with Red Hat also supports the extended options
documented in the hosts_options(5) man page. Here is an example how an additional
program can be spawned in e.g. the /etc/hosts.allow
file:
sshd: ALL : spawn echo "Login from %c to %s" | mail -s "Login Info for %s" log@loghost
For information on the % expansions, see "man 5 hosts_access".
The TCP wrapper is quite flexible. And xinetd provides its own set of host-based
and time-based access control functions. You can even tell xinetd to limit
the rate of incoming connections. I recommend reading various documentations about
the Xinetd super daemon on the Internet.
Many network services like telnet, rlogin, and rsh are
vulnerable to eavesdropping which is one of several reasons why SSH should be used
instead. Red Hat's default configuration for SSH meets the security requirements
for many environments. However, there are a few parameters in
/etc/ssh/sshd_config that you may want to change
on RHEL and other Linux systems.
The chapter
Restricting System Access from Servers and Networks shows how direct logins
can be disabled for shared and system accounts including root. But it's
prudent to disable direct root logins at the SSH level as well.
PermitRootLogin yes
Also ensure to have privilege separation enabled where the daemon is split into
two parts. With privilege separation a small part of the code runs as root and the
rest of the code runs in a chroot jail environment. Note that on older RHEL systems
this feature can break some functionality, for example see
Preventing Accidental Denial of Service.
UsePrivilegeSeparation yes
Since SSH protocol version 1 is not as secure you may want to limit the protocol
to version 2 only:
Protocol 2
You may also want to prevent SSH from setting up TCP port and X11 forwarding if
you don't need it:
AllowTcpForwarding no
X11Forwarding no
Ensure the StrictModes directive is enabled which checks file permissions
and ownerships of some important files in the user's home directory like ~/.ssh,
~/.ssh/authorized_keys etc. If any checks fail, the user won't be able
to login.
StrictModes yes
Ensure that all host-based authentications are disabled. These methods should be
avoided as primary authentication.
IgnoreRhosts yes
HostbasedAuthentication no
RhostsRSAAuthentication no
Disable sftp if it's not needed:
#Subsystem sftp /usr/lib/misc/sftp-server
After changing any directives make sure to restart the sshd daemon:
/etc/init.d/sshd restart
- Configure SSH for applications, if necessary (SSH is enabled out of the
box in RHEL)
- Disable SSH1
- Verify correctness of home directories permissions
Configure Wheel group -- active adminitrators
the wheel group and enroll everybody who is authorized to become root
into wheel group, no matter what is their primary group.
Note: this is Linux method of controlling access as linux does not
have RBAC.
- Configure Sudo to allow members of the wheel
group to became root
- Populate all home directories and /root
directory with .profile and
.kshrc files (for bash users this is
.bash_profile and
.bashrc)
- ___ Install DataProtector – (reallocate service port 5555 for D.P.-reboot;
RHEL9-install “compress” s/w pkg.)
- ___ Install Sarcheck
- ___ Creation of the baseline of key config files for the server and possibly
burning it to CD.
- We can adopt one of the scripts used in troubleshooting and run it after
the installation just before giving the server to application people.
- Copy all the critical config file to /root/baseline/date
directory the way JASS does this on Solaris.
- ___ Verify the /boot/grub/menu.lst is
configured correctly.
- ___ Check is all online updates are installed:
- use SPident -v and see "Unknown"
percentage
Summary (using 893 packages)
Product/ServicePack conflict match update (shipped)
RHEL-10-x86_64-current 0 0% 751 84.1% 141 (2099 35.8%)
Unknown 142 15.9%
CONCLUSION: System is up-to-date!
found RHEL-10-x86_64-current + "online updates"
- If unknown percentage is large try
SPident -vv and SPident -vvvv to see actual packages claimed
unknown (might be a bug in RHEL 5 before SP 1).
- ___ Check and if necessary disable test1,
guest and any other unused accounts if any
were created during the installation.
Unowned Files
Files not owned by any user or group might not necessarily be a security problem
in itself. However, unowned files could pose a security problem in the future. For
example, if a new user is created and the new users happens to get the same UID
as the unowned files have, then this new user will automatically become the owner
of these files.
To locate files not owned by any user or group, use the following command:
find / -path /proc -prune -o -nouser -o -nogroup
Checking for Unlocked Accounts
It is important that all system and vendor accounts that are not used for logins
are locked.
To get a list of unlocked accounts on your system, you can check for accounts that
do NOT have an encrypted password string starting with "!" or "*" in the /etc/shadow
file. If you lock an account using passwd -l, it will put a '!!' in front
of the encrypted password, effectively disabling the password. If you lock an account
using usermod -L, it will put a '!' in front of the encrypted password.
Many system and shared accounts are usually locked by default by having a '*' or
'!!' in the password field which renders the encrypted password into an invalid
string.
Hence, to get a list of all unlocked (encryptable) accounts, run:
# egrep -v '.*:\*|:\!' /etc/shadow | awk -F: '{print $1}'
Also make sure all accounts have a 'x' in the password field in /etc/passwd.
The following command lists all accounts that do not have a 'x' in the password
field:
# grep -v ':x:' /etc/passwd
A 'x' in the password fields means that the password has been shadowed, i.e. the
encrypted password has to be looked up in the /etc/shadow file. If the
password field in /etc/passwd is empty, then the system will not lookup
the shadow file and it will not prompt the user for a password at the login prompt.
Checking for Unused Accounts
All system or vendor accounts that are not being used by users, applications, by
the system or by daemons should be removed from the system. You can use the following
command to find out if there are any files owned by a specific account:
# find / -path /proc -prune -o -user <account> -ls
The -prune option in this example is used to skip the /proc filesystem.
If you are sure that an account can be deleted, you can remove the account using
the following command:
# userdel -r <account>
Without the "-r" option userdel will not delete the user's home
directory and mail spool (/var/spool/mail/<user>). Note that many system
accounts have no home directory.
Single User Mode Password for root
Some admins suggest to add the following line to the /etc/inittab file
to ensure that a root password is required for Single User Mode logons:
~~:S:wait:/sbin/sulogin
This works but can be circumvented easily! At the GRUB or LILO prompt you
can tell the boot loader to alternate the init program by using the boot params
"init=/bin/bash". This will place you at a root shell prompt without a
password.
In general I do not recommend that the system enforces password expiration for system
and shared accounts. This could lead to outages if an application's account expires:
# su oracle -c id
You are required to change your password immediately (password aged)
Changing password for test
(current) UNIX password:
Rather a corporate policy should govern password changes for system and shared accounts.
But for individual user accounts the system should expire the passwords automatically.
The following example shows how password expiration can be setup for individual
user accounts.
The following files and parameters in the table are used when a new account is created
with the useradd command. These settings are
recorded for each user account in the /etc/shadow
file. Therefore, make sure to configure the following parameters before you create
any user accounts using the useradd command:
| /etc/login.defs |
PASS_MAX_DAYS |
60 |
Maximum number of days a password is valid. |
| /etc/login.defs |
PASS_MIN_DAYS |
7 |
Minimum number of days before a user can change the password
since the last change. |
| /etc/login.defs |
PASS_MIN_LEN |
n/a |
This parameter does not work. It
is superseded by the PAM module "pam_cracklib". See
Enforcing Stronger Passwords for more information. |
| /etc/login.defs |
PASS_WARN_AGE |
7 |
Number of days when the password change reminder starts.
|
| /etc/default/useradd |
INACTIVE |
14 |
Number of days after password expiration that account is
disabled. |
| /etc/default/useradd |
EXPIRE |
|
Account expiration date in the format YYYY-MM-DD. |
Ensure that the above parameters are changed in the /etc/login.defs
and /etc/default/useradd files.
When a user account is created using the useradd
command, the parameters listed in the above table are recorded in the
/etc/shadow file in the following fields:
<username>:<password>:<date>:PASS_MIN_DAYS:PASS_MAX_DAYS:PASS_WARN_AGE:INACTIVE:EXPIRE:
To create a new user account you can execute the following command:
useradd -c "Test User" -g users test
The -g option specifies the primary group for this account:
# id test
uid=509(test) gid=100(users) groups=100(users)
The settings in /etc/login.defs and /etc/default/useradd are recorded
for the test user in the /etc/shadow file as follows:
# grep test /etc/shadow
test:!!:12742:7:60:7:14::
You can change the password aging any time using the chage
command.
To disable password aging for system and shared accounts, you can run the following
chage command:
# chage -M 99999 <system_account_name>
To get password expiration information:
# chage -l <system_account_name>
For example:
# chage -l test
Minimum: 7
Maximum: 60
Warning: 7
Inactive: 14
Last Change: Jan 11, 2005
Password Expires: Mar 12, 2005
Password Inactive: Mar 26, 2005
Account Expires: Never
Practical Considerations
On an audited system it is important to restrict people from using simple passwords
that can be cracked too easily. However, if the passwords being enforced are too
strong, people will write them down. Strong passwords that are written down are
not much safer than weak passwords. Some will argue that strong passwords protect
you against e.g. Dictionary Attacks and you can defeat it by locking the accounts
after a few failed attempts. However, this is not always an option. As I will show
at
Locking User Accounts After Too Many Login Failures, locked system accounts
could bring down your applications and systems which would be nothing short of a
denial of service attack.
Undoubtedly, it is important to practise safe password management. In my opinion,
a password should have at least one digit number, one other character, and one upper
case letter. But keep in mind not to make it overly complicated.
How to Enforce Stronger Passwords
The pam_cracklib module checks the password against dictionary words and
other constraints. Unfortunately, however, the original Linux PAM module pam_cracklib
uses a credit mechanism. E.g. if you define password length minlen=10,
then you will get 1 credit for e.g. using a single digit number in your password
if you defined dredit=1. This means that pam_cracklib will accept
a password of the length of minlen-credit. If you don't use a digit number,
then the minimum length of the password would be minlen. There was no way
to tell the module that a password _must_ include a digit number.
Back in 2000 I wrote a patch for the pam_cracklib module where you can
assign negative values to the pam_cracklib parameters lcredit, ucredit,
dcredit, and ocredit. Using negative values will disable the credit mechanism.
For example, if you define dredit=-1, then the module will only accept
a password if it includes at least one digit number and if the password has a length
of at least minlen.
Red Hat has finally applied my pam_cracklib patch and you don't have to
patch the pam_cracklib module any more. The new pam_cracklib feature
works in Red Hat Enterprise Linux 4 and Red Hat Fedora Core 3. This feature is now
also included with the Red Hat Enterprise Linux 3 Update 4 and Red Hat Enterprise
Linux 2.1 Update 6 release. If the Linux distribution you are using does not use
the patched pam_cracklib module yet, you can find the procedure for patching
pam_cracklib
here.
In the following example I'll assume that you are using the new pam_cracklib
module, or that you patched the module if your Linux distribution doesn't include
the patched version yet.
The following example shows how to enforce the following password rules:
- Minimum length of password must be 8
- Minimum number of lower case letters must be 1
- Minimum number of upper case letters must be 1
- Minimum number of digits must be 1
- Minimum number of other characters must be 1
pam_cracklib.so
|
minlen=8
|
Minimum length of password is 8
|
pam_cracklib.so
|
lcredit=-1
|
Minimum number of lower case letters is 1
|
pam_cracklib.so
|
ucredit=-1
|
Minimum number of upper case letters is 1
|
pam_cracklib.so
|
dcredit=-1
|
Minimum number of digits is 1
|
pam_cracklib.so
|
ocredit=-1
|
Minimum number of other characters is 1
|
To setup these password restrictions, edit the /etc/pam.d/system-auth
file and add/change the following pam_cracklib arguments highlighted in
blue:
auth required /lib/security/$ISA/pam_env.so
auth sufficient /lib/security/$ISA/pam_unix.so likeauth nullok
auth required /lib/security/$ISA/pam_deny.so
account required /lib/security/$ISA/pam_unix.so
account sufficient /lib/security/$ISA/pam_succeed_if.so uid < 100 quiet
account required /lib/security/$ISA/pam_permit.so
password requisite /lib/security/$ISA/pam_cracklib.so retry=3 minlen=8 lcredit=-1 ucredit=-1 dcredit=-1 ocredit=-1
password sufficient /lib/security/$ISA/pam_unix.so nullok use_authtok md5 shadow
password required /lib/security/$ISA/pam_deny.so
session required /lib/security/$ISA/pam_limits.so
session required /lib/security/$ISA/pam_unix.so
Now verify that the new password restrictions work for new passwords. Simply login
to a non-root account and change the password using the passwd command.
Note that the above requirements are not enforced if you run the passwd
command under root.
NOTE: The /etc/pam.d/system-auth PAM configuration file is auto-generated
and contains records which dictate a generic authentication scheme. Keep in mind
that authconfig might clobber some changes you made. Since I never run
authconfig I usually make changes to this file because it's used by many
PAM aware applications. Otherwise I'd have to make changes to many configuration
files. Changing system-auth is usually the preferred method. You might
even want to disable all execution bits from the /usr/bin/authconfig binary
to prevent authconfig from clobbering your changes.
I do not recommend that the system automatically locks system and shared accounts
after too many failed login or su attempts. This could lead to outages if the application's
account gets locked due to too many login failures like in this example:
# su oracle -c id
su: incorrect password
#
This could be an easy target for a denial of service attack. At
Restricting Direct Login Access for System and Shared Accounts I will show how
to disable direct logins for system or shared accounts.
In the following example I will show how to lock only individual user accounts after
too many failed su or login attempts.
Add the following two lines highlighted in blue to the
/etc/pam.d/system-auth file as shown below:
auth required /lib/security/$ISA/pam_env.so
auth required /lib/security/$ISA/pam_tally.so onerr=fail no_magic_root
auth sufficient /lib/security/$ISA/pam_unix.so likeauth nullok
auth required /lib/security/$ISA/pam_deny.so
account required /lib/security/$ISA/pam_unix.so
account required /lib/security/$ISA/pam_tally.so per_user deny=5 no_magic_root reset
account sufficient /lib/security/$ISA/pam_succeed_if.so uid < 100 quiet
account required /lib/security/$ISA/pam_permit.so
password requisite /lib/security/$ISA/pam_cracklib.so retry=3
password sufficient /lib/security/$ISA/pam_unix.so nullok use_authtok md5 shadow
password required /lib/security/$ISA/pam_deny.so
session required /lib/security/$ISA/pam_limits.so
session required /lib/security/$ISA/pam_unix.so
The first added line counts failed login and failed su attempts for each user. The
default location for attempted accesses is recorded in /var/log/faillog.
The second added line specifies to lock accounts automatically after 5 failed login
or su attempts (deny=5). The counter will be reset to 0 (reset) on successful entry
if deny=n was not exceeded. But you don't want system or shared accounts
to be locked after too many login failures (denial of service attack). To exempt
system and shared accounts from the deny=n parameter, I added the per_user
parameter to the module. The per_user parameter instructs the module NOT
to use the deny=n limit for accounts where the maximum number of login
failures is set explicitly. For example:
# faillog -u oracle -m -1
# faillog -u oracle
Username Failures Maximum Latest
oracle 0 -1 Fri Dec 10 23:57:55 -0600 2005 on unknown
The faillog command with the option "-m -1" has the effect of
not placing a limit on the number of failed logins. To instruct the module to activate
the deny=n limit for this account again, run:
# faillog -u oracle -m 0
By default, the maximum number of login failures for each account is set to 0 which
instructs pam_tally to use the deny=n parameter. The faillog
manual page on my Red Hat system says that selecting maximum number of login failures
of 0 will deactivate deny=n for the account. The PAM documentation, however,
says that per_user will only work if the .fail_max field contains
a non-zero value. After testing both values setting it to -1 worked. Maybe
because it's read as a high unsigned value?
To see failed login attempts, run:
# faillog
To unlock an account after too many login failures, run:
# faillog -u <user> -r
Make sure to test these changes thoroughly on your system using e.g.
ssh and su,
and make sure root does not get locked!
To lock/unlock accounts manually, you can run one of the following commands:
# passwd -l <user>
# usermod -L <user>
# passwd -u <user>
# usermod -U <user>
NOTE:
Since the /var/log/faillog is owned by root and only root can write to
the /var/log/faillog file, xscreensaver and vlock won't
work correctly. Each time xscreensaver or vlock is executed as
a non-root user, you won't be able to do an unlock since these programs can't write
to /var/log/faillog. I don't have a good solution for that. I can only
think of setting the SUID bits on these programs.
The pam_unix module parameter remember can be used to configure
the number of previous passwords that cannot be reused. And the pam_cracklib
module parameter difok can be used to specify the number of characters
hat must be different between the old and the new password.
In the following example I will show how to tell the system that a password cannot
be reused for at least 6 months and that at least 3 characters must be different
between the old and new password.
Remember that in the chapter
Enabling Password Aging we set PASS_MIN_DAYS to 7, which specifies
the minimum number of days allowed between password changes. Hence, if we tell
pam_unix to remember 26 passwords, then the previously used passwords cannot
be reused for at least 6 months (26*7 days).
Here is an example. Edit the /etc/pam.d/system-auth
file and add/change the following pam_cracklib and pam_unix arguments:
auth required /lib/security/$ISA/pam_env.so
auth sufficient /lib/security/$ISA/pam_unix.so likeauth nullok
auth required /lib/security/$ISA/pam_deny.so
account required /lib/security/$ISA/pam_unix.so
account sufficient /lib/security/$ISA/pam_succeed_if.so uid < 100 quiet
account required /lib/security/$ISA/pam_permit.so
password requisite /lib/security/$ISA/pam_cracklib.so retry=3 minlen=8 lcredit=-1 ucredit=-1 dcredit=-1 ocredit=-1 difok=3
password sufficient /lib/security/$ISA/pam_unix.so nullok use_authtok md5 shadow remember=26
password required /lib/security/$ISA/pam_deny.so
session required /lib/security/$ISA/pam_limits.so
session required /lib/security/$ISA/pam_unix.so
On an audited production system it is very important to know who switched to which
system or shared account. Therefore it is prudent to restrict direct logins for
all system and shared account where more than one individual knows the password.
All users should do a direct login using their own account and then switch to the
system or shared account. (If you are just interested in restricting direct
root SSH logins, see
Securing SSH.)
However, there are situations where you have to allow direct logins for system or
shared accounts. For example, within an Oracle RAC cluster you have to enable direct
ssh logins for oracle. But in such an environment you have to protect the
whole cluster as a single entity against incoming ssh connection, i.e. direct
oracle logins should not work if you come from a node that is not part
of the cluster. In the following example I will show how to achieve this goal as
well.
Usually all system and shared accounts have one thing in common, that is they are
not in the "users" group. The following example assumes that all individual
user accounts are in the "users" group but system and shared accounts like
root and oracle are not. If you want to go a step further, a good
solution would be to implement a new 'logingroup' users group which would require
users to be given explicit access.
In this example I will show how to restrict direct logins for:
- SSH (/etc/pam.d/sshd)
- Console Login (/etc/pam.d/login)
- Graphical Gnome Login (/etc/pam.d/gdm
- or for all logins (/etc/pam.d/system-auth)
To accomplish this goal I will add the pam_access
module to the PAM configuration files listed above. This module provides logdaemon-style
login access control based on login names, host names, IP addresses, etc. The PAM
module type that has to be used in the configuration files is
account. This module type does the authorization,
i.e. is the user allowed to login (e.g. time, day)? Don't confuse the PAM module
type account with auth which does the authentication, for example
checking the password. And the control flag I will use is
required. It specifies that Success is required,
Failure means that it will still call the remaining modules, but the result is already
determined.
For SSH Logins add the pam_access module
to /etc/pam.d/sshd as follows:
auth required pam_stack.so service=system-auth
auth required pam_nologin.so
account required pam_access.so
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
session required pam_stack.so service=system-auth
For Console Logins add the pam_access
module to /etc/pam.d/login as follows:
auth required pam_securetty.so
auth required pam_stack.so service=system-auth
auth required pam_nologin.so
account required pam_access.so
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
session required pam_selinux.so close
session required pam_stack.so service=system-auth
session optional pam_console.so
session required pam_selinux.so multiple open
For Graphical Gnome Logins add the pam_access
module to /etc/pam.d/gdm as follows:
auth required pam_env.so
auth required pam_stack.so service=system-auth
auth required pam_nologin.so
account required pam_access.so
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
session required pam_stack.so service=system-auth
session optional pam_console.so
Now add the following line to the /etc/security/access.conf
configuration file:
-:ALL EXCEPT users :ALL
The /etc/security/access.conf configuration file is read by the pam_access
module. This entry specifies that no users are accepted except users that are in
the "users" group. Since the pam_access module has been configured
for "Authorization" (account) in the above PAM configuration files, it
denies direct logins for all accounts except the ones that are in the "users"
group.
Now on some systems like Oracle RAC clusters you have to enable direct ssh logins
for oracle within the cluster. On such systems you can enable direct ssh
logins for oracle within the cluster by adding/changing the following lines
in /etc/security/access.conf:
-:ALL EXCEPT users oracle:ALL
-:oracle:ALL EXCEPT rac1cluster.example.com rac2cluster.example.com rac3cluster.example.com
The first line has been edited to include the oracle account which will
allow general direct logins. However, the second line specifies that direct logins
for oracle are only allowed from Oracle RAC nodes (rac1cluster, rac2cluster,
and rac3cluster) that are part of the cluster.
NOTE:
In RHEL4 pam_access is already configured for crond:
# grep pam_access /etc/pam.d/*
/etc/pam.d/crond:account required pam_access.so accessfile=/etc/security/access-cron.conf
#
This means that the above entries in /etc/security/access.conf will stop
cron from working. Note that it is very prudent to always check whether pam_access
is configured for any other service on the system!
To ensure that all users on the system can still run cron jobs you can add the following
argument to pam_access in /etc/pam.d/crond:
account required pam_access.so accessfile=/etc/security/access-cron.conf
This ensures that the /etc/security/access.conf configuration file is not
invoked by crond. Since pam_cracklib does not grant permissions
if the configuration file does not exist, execute the following command to create
an empty file:
# touch /etc/security/access-cron.conf
Now verify that cron jobs can be launched by any user on the system.
NOTE:
The above example will only work if there exists no "users" account in the /etc/passwd
file on the system, which is usually the case. Otherwise you have to either delete
the "users" account or you have to designate or create another group name.
This chapter shows how to restrict people from su-ing to system and shared accounts
even if they know the passwords.
Users usually don't share the passwords of their own accounts but are less hesitant
to share it for shared accounts. This chapter helps to mitigate this problem.
The following example shows how to restrict su access to the root,
oracle, and postgres account to a specific set of users.
NOTE: The documentation about the pam_wheel module included in many
Linux distributions is wrong. For instance, in Red Hat Advanced Server 2.1 the
pam_wheel module does not only restrict people from su-ing to the root
account like it used to be. It restricts people from su-ing to any account.
Example
for Restricting su Access to root, oracle, and postgres Accounts
Create a new group for each set of users that are allowed to su to the
root, oracle, and postgres account:
# groupadd rootmembers
# groupadd oracledba
Add all users who are allowed to su to the root, oracle,
and postgres account to the new member groups created above.
The following requirement will be configured:
- Only admin1 should be able to su to root, oracle,
and postgres.
- Only oracledba1 should be able to su to oracle.
- Only postgresdba1 should be able to su to postgres.
- No one else on the system should be able to su to any account.
# usermod -G rootmembers adminuser1
# usermod -G oraclemembers oracleuser1
# usermod -G postgresmembers postgreRHELr1
As you probably noted, I did not add adminuser1 to the other member groups.
Instead, I will show how to give people in the rootmembers group automatically
su access to the oracle and postgres account without
adding them to the oraclemembers and postgresmembers groups. I
consider root admins an exception. They should not be added to all member groups
on the system.
Next add the three authentication lines highlighted in blue to the
/etc/pam.d/su file as shown below:
auth sufficient /lib/security/$ISA/pam_rootok.so
auth required /lib/security/$ISA/pam_stack.so service=system-auth
auth sufficient /lib/security/$ISA/pam_stack.so service=su-root-members
auth sufficient /lib/security/$ISA/pam_stack.so service=su-other-members
auth required /lib/security/$ISA/pam_deny.so
account required /lib/security/$ISA/pam_stack.so service=system-auth
password required /lib/security/$ISA/pam_stack.so service=system-auth
session required /lib/security/$ISA/pam_selinux.so close
session required /lib/security/$ISA/pam_stack.so service=system-auth
session required /lib/security/$ISA/pam_selinux.so open multiple
session optional /lib/security/$ISA/pam_xauth.so
These additional authentication lines specify that nobody should be able
to su to any account unless at least one of the PAM services,
su-root-members or su-other-members, returns Success. The control
flag sufficient means that a Success will bypass the remaining authentication
modules and overall Success is returned for the authentication part. Failure means
that the failed authentication PAM service is ignored. If both authentication PAM
services fail, then the last authentication module pam_deny is invoked
which will deny all requests for any available authentication module. This will
cause the authentication part to fail for the su command.
Next the new authentication PAM service configuration files /etc/pam.d/su-root-members
and /etc/pam.d/su-other-members need to be created.
The file /etc/pam.d/su-root-members referenced
in /etc/pam.d/su should read like:
auth required /lib/security/pam_wheel.so use_uid group=rootmembers
auth required /lib/security/pam_listfile.so item=user sense=allow onerr=fail file=/etc/security/su-rootmembers-access
The file /etc/security/su-rootmembers-access
referenced in /etc/pam.d/su-root-members should read like:
root
oracle
postgres
The control flag required which is specified for both modules means that
both modules have to return Success. Otherwise this PAM service will return Failure
to the "su" PAM service configured in /etc/pam.d/su. The first line returns
Success only if the user is in the rootmembers groups. The second line
allows only access (sense=allow) to those users specified in /etc/security/rootusername,
which is root, oracle, and postgres - these are the only users that will be accepted
as a user argument to su. The item=user argument instructs
pam_listfile that the entries in /etc/security/rootusername are usernames.
If an error occurs, such as an unreadable configuration file, access is denied (onerr=fail).
NOTE: Once su access to root is working for users in the rootmembers,
I recommend to avoid making any changes to the /etc/pam.d/su-root-members
file in the future. Making a mistake in this file could revoke access to root for
all users on the system. That's the reason why I created two PAM service files,
/etc/pam.d/su-root-members for people in the rootmembers group,
and /etc/pam.d/su-other-members (see below) for all other member groups
since you will most probably add more member groups to this file in the future.
Next the file /etc/pam.d/su-other-members
referenced in /etc/pam.d/su should be created and read like:
auth sufficient /lib/security/pam_stack.so service=su-oracle-members
auth sufficient /lib/security/pam_stack.so service=su-postgres-members
auth required /lib/security/pam_deny.so
If one of the two PAM services returns Success, it will return Success to the "su"
PAM service configured in /etc/pam.d/su. Otherwise the last module will
be invoked which will deny all further requests and the authentication fails.
Next the PAM services "su-oracle-members" and "su-postgres-members" have to be created.
The file /etc/pam.d/su-oracle-members referenced
in /etc/pam.d/su-other-members should read like:
auth required /lib/security/pam_wheel.so use_uid group=oraclemembers
auth required /lib/security/pam_listfile.so item=user sense=allow onerr=fail file=/etc/security/su-oraclemembers-access
The file /etc/security/su-oraclemembers-access
referenced in /etc/pam.d/su-oracle-members should read like:
oracle
Now verify that adminuser1 can su to root, oracle, and
postgres. No one else should be able to su to root.
oracleuser1 should be able to su to oracle only, and postgreRHELr1
should be able to su to postgres only. No one else on the system should
be able su to any of these accounts even if they know the password.
- Anything else that I missed.
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Webliography
Linux Security
Securing and Hardening Linux Production Systems (Linux Security Cookbook - HOWTO
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