原始连接：
http://www.puschitz.com/SecuringLinux.shtml信息来源：邪恶八进制信息安全团队（
www.eviloctal.com）
This article is a practical step-by-step guide for securing Linux production systems. It discusses basic Linux security requirements for systems that need to pass various audits in an enterprise environment. If you work on a corporate Linux Security Standard, or if you do
Sarbanes-Oxley Act (SOX) or
Statement on Auditing Standards No. 70 (SAS 70) related work, then you will find very useful and practical information here.
Focus of this ArticleThis Linux Security HOWTO is intended for a technical audience, Linux system administrators, and security people in corporations and organizations that have to use commercial Linux distributions in production. The main objective of this Linux Security guide is to discuss basic Linux security requirements for production systems that are being audited. This document covers various system services like SSH which is usually enabled and needed on all Linux production servers. Services like Apache, Samba etc. are not covered here since these services are applications which are usually not needed on all Linux systems and therefore should not be installed on all Linux servers. Also, this article does not cover security features which require kernel patching. This is not an option for most companies due to vendor support issues.
This Linux Security Cookbook has been tested on Red Hat Linux but should also be applicable to many other Linux distributions like Novell SUSE.
FeedbackThis document comes without warranty of any kind. But every effort has been made to provide the information as accurate as possible. I welcome emails from any readers with comments, suggestions, and corrections at webmaster_at_puschitz.com. If you think that I should address another basic and important Linux security topic, please drop me an email.
Contents* General
* Limiting Software Packages (RPMs)
* Patching Linux Systems
* Disabling System Services
* Checking Accounts
* Enabling Password Aging
* Enforcing Stronger Passwords
* Restricting Use of Previous Passwords
* Locking User Accounts After Too Many Login Failures
* Restricting Direct Login Access for System and Shared Accounts
* Restricting su Access to System and Shared Accounts
* Restricting System Access from Servers and Networks
* Preventing Accidental Denial of Service
* Checking File Permissions and Ownership
* Securing NFS
* Securing SSH
* Securing Sendmail
* Securing Postfix
* Secure Remote Logging
* Kernel Tunable Parameters
* Copying Files Using SSH Without Providing a Login Prompt
* Displaying Login Banners
* Miscellaneous
* Bibliography and References
Verifying Security Action ItemsIt is strongly recommended to have scripts available that check whether all security action items have been carried out. Even the best sysadmins can make mistakes and forget things. If you have a larger Linux environment, it would be a good investment to write scripts to have all Linux security actions items executed automatically.
Physical SecurityPhysical security should be of the utmost concern. Linux production servers should be in locked datacenters where only people with passed security checks have access. Since this article assumes that all Linux production systems are physically secured, this guide will not cover the configuration of a boot loader password. This could actually pose a problem for rebooting servers remotely.
Retiring Linux Servers with Sensitive DataTo retire servers with sensitive data, it is important to ensure that data cannot be recovered from the hard disks. To ensure that all traces of data are removed, the
DiskSanitizer tool can be used. This tool can be operated from a floppy disk and it removes data according with the U.S. Department of Defense (DoD) standards.
DiskSanitizer is available at
http://freshmeat.net/projects/disksanitizer.
Disk PartitionsServers should have separate partitions for at least
/,
/boot,
/usr,
/var,
/tmp, and
/home. You don't want that e.g. logging and temporary space under
/var and
/tmp fill up the root partition. Third party applications should be on separate filesystems as well, e.g. under
/opt.
Firewall (iptables)I will not cover
iptables in this paper. Most companies use hardware based firewalls to protect their servers in a production network which is strongly recommended for such environments. If you are interested in a Linux Stateful Firewall using
iptables, you may want to check out my instructions for
Stateful Firewall and Masquerading on Linux.
SELinuxSELinux is an advanced technology for securing Linux systems. Hardening Linux using SELinux technology requires a whole other article and is out of scope for this guide.
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 hard time to understand what needs to be secured and you will get off to a bad start for securing Linux systems proactively. Therefore, it is very critical to look at the default packages and remove unneeded packages or packages that don't comply with your security policy. For example, you should not have Apache or Samba installed on your system if you don't use it. Also, it is a good practice not to have development packages, desktop software packages (e.g. X Server) etc. installed on production servers.
One of the first action items should be to create an image that only contains RPMs needed by applications and for maintenance and troubleshooting purposes. A good approach is to start with a minimum list of RPMs and then add packages as needed. It may be time-consuming but it's 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
To check for and report potential conflicts and dependencies for deleting a RPM, run:
rpm -e --test
For information on performing Kickstart installations and how to build an image, see
Kickstart Installations for more information.
Building an infrastructure for patch management is another very important step to proactively secure Linux production environments. It is recommended to have a written security policy and procedure to handle Linux security updates and issues.
For example, a security policy should detail the timeframe for assessment, testing, and rollout of patches. Network related security vulnerabilities should get the highest priority and should be addressed immediately within a short timeframe.
For example, a security procedure should detail the process for assesment, testing, and rollout of patches. The assessment phase should occur within a testing lab, and initial rollout should occur on development systems first.
A separate security log should detail what Linux security notices have been received, when patches have been researched and assessed, when patches have been applied etc.
For Red Hat systems I recommend
Red Hat Network (RHN) for patch management. In fact, for secure environments you may have to consider Red Hat's Satellite solution. For more information, see
Red Hat Network Architectural Overview.
One of the most important tasks is to remove any network services from the system startup process that are not needed.
Various network system services which are usually needed on most Linux systems are being addressed later in this article. Finding Listening Network PortsTo get a list of listening network ports (TCP and UDP sockets), you can run the following command:
# netstat -tulp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 *:auth *:* LISTEN 2328/xinetd
tcp 0 0 localhost.localdomain:smtp *:* LISTEN 2360/sendmail: acce
tcp 0 0 *:ssh *:* LISTEN 2317/sshd
From the output you can see that
xinetd,
sendmail, and
sshd are listening.
On all newer Red Hat Linux distributions
sendmail is configured to listen for local connections only. Sendmail should not listen for incoming network connections unless the server is a mail or relay server. Running a port scan from another server will confirm that:
# nmap -sTU
Starting nmap 3.70 ( http://www.insecure.org/nmap/ ) at 2004-12-10 22:51 CST
Interesting ports on jupitor (172.16.0.1):
(The 3131 ports scanned but not shown below are in state: closed)
PORT STATE SERVICE
22/tcp open ssh
113/tcp open auth
Nmap run completed -- 1 IP address (1 host up) scanned in 221.669 seconds
#
Note that the above
nmap command can take a while. If you remove the UDP port scan (without the option "
-U"), then
nmap will finish the port scan immediately. If you run it on the local machine it will also complete very fast. Also note that
nmap might not show all listening network sockets if a firewall is being used to block ports.
From the output above you can see that the
xinetd daemon is listening on port
auth (port 113). You can also see that
sendmail is not listening for remote incoming network connections, see also
Securing Sendmail.
Another method to list all of the TCP and UDP sockets to which programs are listening is
lsof:
# lsof -i -n | egrep 'COMMAND|LISTEN|UDP'
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
sshd 2317 root 3u IPv6 6579 TCP *:ssh (LISTEN)
xinetd 2328 root 5u IPv4 6698 TCP *:auth (LISTEN)
sendmail 2360 root 3u IPv4 6729 TCP 127.0.0.1:smtp (LISTEN)
#
Closing Network Ports and Disabling ServicesOn Red Hat systems you can 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 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. Ensure not to disable services that are needed by your system to run smoothly.
To disable e.g. the
telnet service permanently on a Red Hat system, run:
chkconfig telnet off
To disable the
telnet service immediately on a Red Hat system, run:
service telnet stop
Tighting up /etc/inittab and /etc/rc.d/rc.localThe inittab file
/etc/inittab also describes which processes are started at bootup and during normal operation. For example, Oracle uses it to start cluster services at bootup. Therefore, it is recommended to ensure that all entries in
/etc/inittab are legitimate in your environment.
I would remove at least the CTRL-ALT-DELETE trap entry to prevent accidental reboots:
# sed -i 's/ca::ctrlaltdel:/#ca::ctrlaltdel:/g'
The default runlevel should be set to 3 since in my humble opinion X11 (X Windows System) should not be running on production servers:
# grep ':initdefault' /etc/inittab
id:3:initdefault:
#
And depending on your environment you might want to comment out the UPS entries as well.
To have changes in
/etc/inittab become effective immediately, you can run:
# init q
And the
/etc/rc.local script is used for commands or startup scripts which are pertinent only to a specific server. (
/etc/rc.local is a link to
/etc/rc.d/rc.local).
Ensure that all startup scripts in
/etc/rc.d/rc.local are legitimate.
Various System ServicesIt is not recommended to use FTP or Telnet. Use SSH instead of Telnet, and use sftp, scp or commercial products like MQSeries to replace FTP. Or you might want to check out
Copying Files Using SSH Without Providing a Login Prompt.
Checking for Unlocked AccountsIt 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 AccountsAll 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 -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
Without the "
-r" option
userdel will not delete the user's home directory and mail spool (
/var/spool/mail/). Note that many system accounts have no home directory.
Single User Mode Password for rootSome 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 table are recorded in the
/etc/shadow file as follows:
:::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
To get password expiration information:
# chage -l
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 ConsiderationsOn 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 PasswordsThe
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.
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 have to make changes to too many configuration files. Changing
system-auth is usually the preferred method. You might want to disable all execution bits from the
/usr/bin/authconfig binary to prevent authconfig from clobbering your changes.
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/$ISA/pam_unix.so
NOTE:
If the
/etc/security/opasswd doesn't exist, create the file.
# ls -l /etc/security/opasswd
-rw------- 1 root root 0 Dec 8 06:54 /etc/security/opasswd
I do not recommend that the system automatically locks system and shared accounts after too many failed login or su attempts. This could lead to outages if the application's account gets locked due to too many login failures like in this example:
# su oracle -c id
su: incorrect password
#
This could be an easy target for a denial of service attack.
At Restricting Direct Login Access for System and Shared Accounts I will show how to disable direct logins for system or shared accounts. In the following example I will show how to lock only individual user accounts after too many failed su or login attempts.
Add the following two lines highlighted in blue to the
/etc/pam.d/system-auth file as shown below:
auth required /lib/security/$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 -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
# usermod -L
# passwd -u
# usermod -U
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 only can think of setting the SUID bits for these programs.
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 system 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 this can be accomplished 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.
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 come up with another group name.
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.
In this chapter I will show how to restrict people from su-ing to system and shared accounts even if they know the password.
Users are less hesitant to share the passwords of shared accounts but they usually don't share the passwords of their own accounts. This chapter will help to address this problem.
In the following example I will show how to specifially 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 AccountsCreate a new group for each set of users that are allowed to
su to the
root,
oracle, and
postgres account:
# groupadd rootmembers
# groupadd oraclemembers
# groupadd postgresmembers
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 postgresuser1
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 withdraw 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 in 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
The file
/etc/pam.d/su-postgres-members referenced in
/etc/pam.d/su-other-members should read like:
auth required /lib/security/pam_wheel.so use_uid group=postgresmembers
auth required /lib/security/pam_listfile.so item=user sense=allow onerr=fail file=/etc/security/su-postgresmembers-access
The file
/etc/security/su-postgresmembers-access referenced in
/etc/pam.d/su-postgres-members should read like:
postgres
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
postgresuser1 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.
Usually a firewall is used to protect a server from other servers and networks. However, in some cases you may also want to protect a server within a network by using a TCP Wrapper.
The Xinetd super server that comes with most Linux distributions includes a built-in TCP wrapper. It can be used to explicitly define network services to accept incoming connections from specified servers and networks. The TCP wrappers implements access control through the use of two files,
/etc/hosts.allow and
/etc/hosts.deny. Note that the
hosts.allow file takes precedence over the
hosts.deny file. And you may want to change the permissions on the two configuration files since they are both world readable.
A recommended security-strategy is to block all incoming requests by default, but allow specific hosts or networks to connect. This is the strategy I will describe here.
To deny everything by default, add the following line to
/etc/hosts.deny:
ALL: ALL
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.
GeneralLinux allows you to set limits on the amount of system resources that users and groups can use. This is also very handy if bugs in programs accidentally use up too much resources, slow down the machine, or even render the system unusable. I've seen systems where incorrect settings have allowed programs to use up too much resources which made the server unresponsible for new connections or local logins (e.g. a program uses up all file handles on the system). This could become a security issue if someone is allowed to use up all resources and causes a denial of service attack. Depending on your environment you may want to review resource limits for user accounts and groups.
Example for Restricting System ResourcesThe following example shows a practical use of setting or restricting system resources for an Oracle user account. For a list of system resource settings, see
/etc/security/limits.conf.
Most shells like Bash provide control over various resources like the maximum allowable number of open file descriptors or the maximum number of processes available to a user. To see all shell limits, run:
ulimit -a
For more information on
ulimit for the Bash shell, see
man bash and search for
ulimit.
Important Note:Setting "hard" and "soft" limits in the following examples might not work properly when you login as
oracle via an SSH session. It might work if you log in as
root and
su to
oracle. If you have this problem, you should try to set
UsePrivilegeSeparation in
/etc/ssh/sshd_config to "
no" and restart the SSH daemon by executing
service sshd restart. The privilege separation does not work properly with PAM on some Linux distributions.
Make sure to talk to your Unix and/or security people before disabling the SSH security feature "Privilege Separation". For example, to change the number of file handles or open files that the Oracle user can use, you have to edit the file
/etc/security/limits.conf as root and make the following changes or add the following lines, respectively:
oracle soft nofile 4096
oracle hard nofile 63536
The "soft limit" in the first line defines the number of file handles or open files that the Oracle user will have after login. If the Oracle user gets error messages about running out of file handles, then the Oracle user can increase the number of file handles like in this example up to 63536 ("hard limit") by running the following command:
ulimit -n 63536
You can set the "soft" and "hard" limits higher if necessary.
Note that I do not recommend to set the "hard" limit for nofile for the oracle user equal to /proc/sys/fs/file-max. If you do that and the oracle user uses up all the file handles, then the whole system will be out of file handles. This could mean that you won't be able to initiate new remote logins any more since the system won't be able to open any PAM modules which are required for performing a login. You also need to ensure that
pam_limits is configured in the file
/etc/pam.d/system-auth, or in
/etc/pam.d/sshd (for SSH),
/etc/pam.d/su (for su), or
/etc/pam.d/login (local logins and telnet) if you don't want to enable it for all logins, or if
/etc/pam.d/system-auth does not exist like on SUSE. This is the PAM module that will read the
/etc/security/limits.conf file. The entry should read like:
session required /lib/security/pam_limits.so
Here are the two "session" entries I have in my
/etc/pam.d/system-auth file:
session required /lib/security/pam_limits.so
session required /lib/security/pam_unix.so
Now login to the oracle account again since the changes will become effective for new login sessions only.
$ su - oracle
$ ulimit -n
4096
$
Note that the ulimit options are different for other shells. The default limit for oracle is now 4096 and the oracle user can increase the number of file handles up to 63536:
$ su - oracle
$ ulimit -n
4096
$ ulimit -n 63536
$ ulimit -n
63536
$
To make this change permanent, add "
ulimit -n 63536" (for Bash) to the
~oracle/.bash_profile file which is the user startup file for the Bash shell on Red Hat Linux (to verify your shell run:
echo $SHELL). To do this you could simply copy/paste the following commands for the
oracle's Bash shell:
su - oracle
cat >> ~oracle/.bash_profile << EOF
ulimit -n 63536
EOF
Default umaskThe
umask (user file-creation mode mask) command is a shell built-in command which determines the
default file permissions for newly created files. This can be overwritten by system calls but many programs and utilities make use of umask.
By default, Red Hat sets
umask to 022 or 002 which is fine. If the name of the user account and the group account is the same and the UID is 100 or larger, then umask is set to 002, otherwise it's set to 022.
$ id
uid=509(test) gid=510(test) groups=100(users),510(test) context=user_u:system_r:unconfined_t
$ umask
0002
$
# id
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel) context=root:system_r:unconfined_t
# umask
0022
#
Here is an example how
umask works:
$ umask 000
$ touch file1
$ ls -l file1
-rw-rw-rw- 1 oracle oinstall 0 Dec 26 19:24 file1
$ umask 002
$ touch file2
$ ls -l file2
-rw-rw-r-- 1 oracle oinstall 0 Dec 26 19:24 file2
$ umask 022
$ touch file3
$ ls -l file3
-rw-r--r-- 1 oracle oinstall 0 Dec 26 19:25 file3
$
For the
bash shell you can find the setting of
umask in
/etc/bashrc. The
/etc/bashrc file is for system-wide aliases and functions and is invoked by
~/.bashrc.
SUID/SGID FilesWhen the SUID (set user ID) or SGID (set group ID) bits are set on an executable, it executes with the UID or GID of the owner of the executable rather than that of the person executing it. This means that e.g. all executables that have the SUID bit set and are owned by root are executed with the UID of root. A good example is the
passwd command that allows ordinary users to update the password field in the
/etc/shadow file which is owned by root.
But SUID/SGID bits can be misused when the SUID/SGID executable has a security hole. Therefore, you might want to search the entire system for SUID/SGID executables and document it. For example, ensure that code developers don't set SUID/SGID bits on their programs if it's not an absolute requirement. Very often you can use workarounds like removing just the executable bit for world/others. However, a better approach is to change the design of the software if possible.
To search the entire system for SUID or SGID files, you can run the following command:
find / -path /proc -prune -o -type f -perm +6000 -ls
The
-prune option in this example is used to skip the
/proc filesystem.
World-Writable FilesWorld-writable files are a security risk since it allows anyone to modify them. Additionally, world-writable directories allow anyone to add or delete files.
To locate world-writable files and directories, you can use the following command:
find / -path /proc -prune -o -perm -2 ! -type l -ls
The "
! -type l" parameter skips all symbolic links since symbolic links are always world-writable. However, this is not a problem as long as the target of the link is not world-writable, which is checked by the above
find command.
World-Writable directories with sticky bit such as the
/tmp directory do not allow anyone to delete or modify files in this directory. The sticky bit makes files stick to the user who created it and it prevents other users from deleting and renaming the files. Therefore, depending on the purpose of the directory world-writable directories with sticky are usually not an issue. An example is the
/tmp directory:
$ ls -ld /tmp
drwxrwxrwt 18 root root 16384 Dec 23 22:20 /tmp
The "
t" mode, which denotes the sticky bit, allows files to be deleted and renamed only if the user is the owner of this file or the owner of the directory.
Unowned FilesFiles not owned by any user or group might not be a security problem in itself. However, on an audited system unowned files should not exist. For example, if a new user is created and he happens to get the same UID that the unowned files have, then he automatically becomes the owner of these files.
To locate files on your system not owned by any user or group, use the following command:
find / -path /proc -prune -o -nouser -o -nogroup
GeneralNFS (Network File System) allows servers to share files over a network. But like all network services using NFS involves risk.
Here are some basic rules:
- NFS should not be enabled if not needed.
- If you must use NFS, use TCP wrapper to restrict remote access.
- Make sure you export to only those machines that you really need to.
- Use fully qualified domain names to diminish spoofing attempts.
- Export only directories you need to export.
- Export read-only wherever possible.
- Use NFS over TCP.
If you don't have shared directories to export, ensure that the NFS service is NOT enabled and running:
# service nfs status
rpc.mountd is stopped
nfsd is stopped
rpc.rquotad is stopped
# chkconfig --list nfs
nfs 0:off 1:off 2:off 3:off 4:off 5:off 6:off
#
You probably don't need the
portmap service as well which is used by NFS (the portmap daemon registers rpc-based services for services like NFS, NIS, etc.):
# service portmap status
portmap is stopped
# chkconfig --list portmap
portmap 0:off 1:off 2:off 3:off 4:off 5:off 6:off
#
Enabling and Starting NFS ServerIf you must use NFS, it can be activated using the following commands on Red Hat Linux:
chkconfig portmap on
chkconfig nfs on
service portmap start
service nfs start
The "
portmap" service starts the
portmap daemon.
Depending on the Linux OS distribution and version, the "
nfs" service starts the
rpc.rquotad,
nfsd,
lockd,
rpciod,
rpc.mountd and
rpc.idmapd daemons on Red Hat Fedora Core 3 which is the OS that Red Hat Advanced Server 4 is based on. On Red Hat Advanced Server 3, the NFS service starts
rpc.mountd,
nfsd and
rpc.rquotad.
To probe the portmapper for all registered NFS related RPC programs, you can run
rpcinfo. On Red Hat Advanced Server 3, the output will look like this:
# rpcinfo -p
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100011 1 udp 607 rquotad
100011 2 udp 607 rquotad
100011 1 tcp 610 rquotad
100011 2 tcp 610 rquotad
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
100005 1 udp 623 mountd
100005 1 tcp 626 mountd
100005 2 udp 623 mountd
100005 2 tcp 626 mountd
100005 3 udp 623 mountd
100005 3 tcp 626 mountd
#
Restricting Incoming NFS RequestsAs I showed at
Restricting System Access from Servers and Networks a recommended security-strategy is to block all incoming requests by default, but allow specific hosts or networks to connect. This is the strategy I will use here.
The
portmap program and some of the NFS programs include a built-in TCP wrapper. To verify if a program includes a TCP wrapper, you can run the following commands:
# strings /sbin/portmap | egrep "hosts.deny|hosts.allow|libwrap"
hosts_allow_table
hosts_deny_table
/etc/hosts.allow
/etc/hosts.deny
# strings /usr/sbin/rpc.rquotad | egrep "hosts.deny|hosts.allow|libwrap"
libwrap.so.0
# ldd /usr/sbin/rpc.rquotad | grep libwrap
libwrap.so.0 => /usr/lib/libwrap.so.0 (0x00874000)
#
If
hosts.deny and
hosts.allow are displayed, or if
libwrap is displayed, then the program includes a built-in TCP wrapper. If none of these strings are displayed, then adding the program name to
/etc/hosts.deny and
/etc/hosts.allow will most probably have no effect.
To block all incoming requests by default, add the following line to
/etc/hosts.deny if you have not done so yet:
ALL: ALL
Verify from a remote server that portmapper does not list any registered RPC programs:
# rpcinfo -p
No remote programs registered.
#
To allow NFS mounts from trusted servers and networks only, you have to configure the servers and networks for portmap and various NFS related programs in
/etc/hosts.allow. To find out which NFS programs are invoked when you start the NFS server, you can check the
/etc/init.d/nfs service script. Then run the "
strings | egrep "hosts.deny|hosts.allow|libwrap"" command I described above.
On Red Hat Advanced Server 3, the
/usr/sbin/rpc.rquotad program includes a built-in TCP wrapper. On Red Hat Fedora Core 3,
/usr/sbin/rpc.mountd now also includes a built-in TCP wrapper.
To allow NFS requests from e.g. servers rac1pub.example.com, rac2pub.example.com, rac3pub.example.com and from the .subnet.puschitz.com network, the configuration in
/etc/hosts.allow would look like as follows:
portmap: rac1pub.example.com rac2pub.example.com rac3pub.example.com .subnet.puschitz.com
rpc.mountd: rac1pub.example.com rac2pub.example.com rac3pub.example.com .subnet.puschitz.com
rpc.rquotad: rac1pub.example.com rac2pub.example.com rac3pub.example.com .subnet.puschitz.com
Make sure to test NFS access thoroughly when using TCP wrappers. If a TCP wrapper has been added to another NFS related program, please drop me a note.
For portmapper you can now test access from trusted servers or networks using the
rpcinfo command:
# rpcinfo -p
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100011 1 udp 607 rquotad
100011 2 udp 607 rquotad
100011 1 tcp 610 rquotad
100011 2 tcp 610 rquotad
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
100005 1 udp 623 mountd
100005 1 tcp 626 mountd
100005 2 udp 623 mountd
100005 2 tcp 626 mountd
100005 3 udp 623 mountd
100005 3 tcp 626 mountd
#
If you run it from an "untrusted" server or network, you should get the following output:
# rpcinfo -p
No remote programs registered.
#
Exporting NFS File SystemsTo allow a client access to a filesystem or directory, the
/etc/exports serves as the access control list.
To give the network "subnet.example.com" read-only access to /pub, the entries in
/etc/exports would look like as follows:
/pub *.subnet.example.com(ro,sync)
It is very important NOT to give write access to NFS clients if not absolutely needed! Entries in
/etc/exports are exported read-only ("
ro" option) by default.
To allow servers rac1pub, rac2pub and rac3pub read-write access to the /data/OracleArch directory, the entries in
/etc/exports would look like as follows:
/data/OracleArch rac1pub.example.com(rw,sync) rac2pub.example.com(rw,sync) rac3pub.example.com(rw,sync)
Note that options MUST NOT be separated from hostnames or networks with whitespace(s).
And use fully qualified domain names to diminish spoofing attempts. All entries in
/etc/exports are exported with the
root_squash option ('root squashing') by default. This means that a root user on a client machine does not have root privileges (root access) to root-owned files on exported NFS filesystems/directories. It is not recommended to turn 'root squashing" off using the
no_root_squash option!
After you've made all your entries in
/etc/exports, you can export all filesystems/directories using the following command:
# exportfs -a
To unexport all shared filesystems/directories, run:
# exportfs -ua
To see all shared filesystems/directories, run:
# showmount -e localhost
Export list for localhost:
/pub *.subnet.example.com
/data/OracleArch rac3pub.example.com,rac2pub.example.com,rac1pub.example.com
Using NFS over TCPIf you need NFS, it is recommended to use NFS over TCP since NFS over UDP is not very insecure. All 2.4 and 2.6 kernels support NFS over TCP on the client side. Server support for TCP appears in later 2.4 kernels, and in all 2.6 kernels.
To verify whether your server supports NFS over TCP, use the
wire-test command (
/usr/sbin/wire-test is part of the
am-utils package). If your server supports NFS over TCP, the output looks like this:
# wire-test localhost
Network 1: wire="172.16.1.0" (netnumber=172.16.1).
Network 2: wire="172.16.1.1" (netnumber=172.16.1).
My IP address is 0xac100101.
NFS Version and protocol tests to host "localhost"...
testing vers=2, proto="udp" -> found version 2.
testing vers=3, proto="udp" -> found version 3.
testing vers=2, proto="tcp" -> found version 2.
testing vers=3, proto="tcp" -> found version 3.
#
If the server does not support NFS over TCP, the output will look like this:
# wire-test localhost
Network 1: wire="172.16.1.0" (netnumber=172.16.1).
Network 2: wire="172.16.1.1" (netnumber=172.16.1).
My IP address is 0xac100101.
NFS Version and protocol tests to host "localhost"...
testing vers=2, proto="udp" -> found version 2.
testing vers=3, proto="udp" -> found version 3.
testing vers=2, proto="tcp" -> failed!
testing vers=3, proto="tcp" -> failed!
#
To mount a shared directory using NFS over TCP, use the "
proto=tcp" mount option:
# mount -o proto=tcp :/pub /usr/local/pub
Make sure the target directory, in this example /usr/local/pub, exists on the client.
You can verify the NFS over TCP mount using the
mount command:
# mount
...
nfsserver:/pub on /usr/local/pub type nfs (rw,proto=tcp,addr=172.16.10.8)
...
To have the shared directory mounted on the client at boot time, use the
/etc/fstab file.
For the above example, the
/etc/fstab entry could look like this:
nfsserver:/pub /usr/local/pub nfs rsize=8192,wsize=8192,timeo=14,intr,tcp 0 0
Many network services like
telnet,
rlogin, and
rsh are vulnerable to eavedropping which is one of several reasons why SSH should be used instead.
Red Hat's default configuration for SSH meets the security requirements for most environments. However, I would recommend to take a look at a few parameters.
/etc/ssh/sshd_config:
At
Restricting System Access from Servers and Networks I showed how direct logins can be disabled for all shared and system accounts including
root. Nevertheless, it would be prudent to disable direct root login at the SSH layer as well.
PermitRootLogin no
You probably also want to disable TCP forwarding and
sftp if you don't use it:
AllowTcpForwarding no
#Subsystem sftp /usr/lib/misc/sftp-server
/etc/ssh/ssh_config:
Since SSH protocol version 1 is not as secure as Protocol 2, you may want to limit the protocol to version 2 only by setting the following parameter:
Protocol 2
After changing any parameters make sure to restart
sshd:
/etc/init.d/sshd restart
If you think that I should cover other parameter(s) or if you think that other parameter(s) should be explicitly set/changed, please drop me an email.
This article focuses on security issues that pertain to most Linux servers in a production environment. Therefore, securing a mail or relay server is out of scope for this article since not all Linux servers in a production environment are mail or relay servers. However, Sendmail or Postfix is usually required for local mail delivery. Note that it is recommended to use Postfix over Sendmail for various security reasons, see
Securing Postfix for more information.
On newer Linux systems Sendmail is configured to run in the background for local mail delivery and not to accept incoming network connections. If your server is not a mail or relay server, then it is important that Sendmail is not accepting incoming network connections from any host other than the local server.
The default
sendmail.cf configuration file on RedHat does not allow Sendmail to accept incoming network connections. The following setting in
/etc/mail/sendmail.cf tells Sendmail not to accept incoming network connections from servers other than the local node:
O DaemonPortOptions=Port=smtp,Addr=127.0.0.1, Name=MTA
If that's not the case on your system, you can change it by setting or uncommenting the
DAEMON_OPTIONS parameter in the
/etc/mail/sendmail.mc file. Uncomment the
DAEMON_OPTIONS line in
/etc/mail/sendmail.mc to read:
DAEMON_OPTIONS(`Port=smtp,Addr=127.0.0.1, Name=MTA')dnl
Then run:
# mv /etc/mail/sendmail.cf /etc/mail/sendmail.cf.old
# m4 /etc/mail/sendmail.mc > /etc/mail/sendmail.cf
# /etc/init.d/sendmail restart
To verify whether Sendmail is still listening for incoming network request, you can run one of the following commands from another node:
# nmap -sT -p 25
# telnet 25
Don't run these commands on the local host since Sendmail is supposed to accept connections from the local node.
Postfix is a replacement for Sendmail which has several security advantages over Sendmail. Postfix consists of several small programs that perform their own small task. And almost all programs run in a chroot jail. These are just a few examples why Postfix is recommended over Sendmail. For more information on chroot jail, see
Using Chroot Securely.
Linux servers that are not dedicated mail or relay servers should not accept external emails. However, it is important for production servers to send local emails to a relay server.
Before you continue on a Red Hat system, make sure Postfix is activated using the following command:
# alternatives --set mta /usr/sbin/sendmail.postfix
The following parameters in
/etc/postfix/main.cf should be set to ensure that Postfix accepts only local emails for delivery:
mydestination = $myhostname, localhost.$mydomain, localhost
inet_interfaces = localhost
The parameter
mydestination lists all domains to receive emails for.
The parameter
inet_interfaces specifies the network to liston on.
Once you've configured Postfix, restart the mail system with the following command:
# /etc/init.d/postfix restart
To verify whether Postfix is still listening for incoming network request, you can run one of the following commands from another node:
# nmap -sT -p 25
# telnet 25
Don't run these commands on the local host since Postfix is supposed to accept connections from the local node.
If you think that I should cover other parameter(s) or if you think that other parameter(s) should explicitly be set/changed for local mail delivery, please drop me an email.
Instructions for secure remote logging with
syslog-ng and
stunnel is coming soon!
The following list shows tunable kernel parameters you can use to secure your Linux server against attacks.
For each tunable kernel parameters I will show the entry that needs to be added to the
/etc/sysctl.conf configuration file to make the change permanent after reboots. To activate the configured kernel parameters immediately at runtime, use:
# sysctl -p
Enable TCP SYN Cookie ProtectionA "SYN Attack" is a denial of service attack that consumes all the resources on a machine. Any server that is connected to a network is potentially subject to this attack.
To enable TCP SYN Cookie Protection, edit the
/etc/sysctl.conf file and add the following line:
net.ipv4.tcp_syncookies = 1
Disable IP Source RoutingSource Routing is used to specify a path or route through the network from source to destination. This feature can be used by network people for diagnosing problems. However, if an intruder was able to send a source routed packet into the network, then he could intercept the replies and your server might not know that it's not communicating with a trusted server.
To enable Source Route Verification, edit the
/etc/sysctl.conf file and add the following line:
net.ipv4.conf.all.accept_source_route = 0
Disable ICMP Redirect AcceptanceICMP redirects are used by routers to tell the server that there is a better path to other networks than the one chosen by the server. However, an intruder could potentially use ICMP redirect packets to alter the hosts's routing table by causing traffic to use a path you didn't intend.
To disable ICMP Redirect Acceptance, edit the
/etc/sysctl.conf file and add the following line:
net.ipv4.conf.all.accept_redirects = 0
Enable IP Spoofing ProtectionIP spoofing is a technique where an intruder sends out packets which claim to be from another host by manipulating the source address. IP spoofing is very often used for denial of service attacks. For more information on IP Spoofing, I recommend the article
IP Spoofing: Understanding the basics.
To enable IP Spoofing Protection, turn on Source Address Verification. Edit the
/etc/sysctl.conf file and add the following line:
net.ipv4.conf.all.rp_filter = 1
Enable Ignoring to ICMP RequestsIf you want or need Linux to ignore ping requests, edit the
/etc/sysctl.conf file and add the following line:
net.ipv4.icmp_echo_ignore_all = 1 = 1
This cannot be done in many environments.
Enable Ignoring Broadcasts RequestIf you want or need Linux to ignore broadcast requests, edit the
/etc/sysctl.conf file and add the following line:
net.ipv4.icmp_echo_ignore_broadcasts = 1
Enable Bad Error Message ProtectionTo alert you about bad error messages in the network, edit the
/etc/sysctl.conf file and add the following line:
net.ipv4.icmp_ignore_bogus_error_responses = 1
Enable Logging of Spoofed Packets, Source Routed Packets, Redirect PacketsTo turn on logging for Spoofed Packets, Source Routed Packets, and Redirect Packets, edit the
/etc/sysctl.conf file and add the following line:
net.ipv4.conf.all.log_martians = 1
References for Kernel Tunable ParametersNetwork Security with /proc/sys/net/ipv4 IP Spoofing: Understanding the basics
The following example is a suggestion you may want to use in some cases. It shows how files can be copied over the network using
ssh without providing an interactive login prompt.
SSH allows you to do a forced command using the "command" option. When you use this option you can disable
scp and enforce every passed
ssh command to be ignored.
On the server side where you want to retrieve the file from, add the following entry to the beginning of the SSH key in the
~/.ssh/authorized_keys2 file:
command="/bin/cat ~/" ssh-dss AAABB33Nza...OpenSSH key
To copy now the file from the remote server, you can run the following command:
ssh @ >
Since
/bin/cat is executed on the server side, the output has to be redirected to the local file.
Another approach is to replace
/bin/cat with your own script that checks the passed SSH commands by reading the environment variable
$SSH_ORIGINAL_COMMAND. For example:
#!/bin/ksh
if [[ $SSH_ORIGINAL_COMMAND = "File1" ||
$SSH_ORIGINAL_COMMAND = "File2" ]]
then
/bin/cat $SSH_ORIGINAL_COMMAND
else
echo "Invalid file name!"
exit 1
fi
So you replace
/bin/cat with the script name in
~/.ssh/authorized_keys2, and run the following command to copy "File1":
ssh @ File1 >
To copy "File 2", run:
ssh @ File2 >
Every other passed parameter will return an error.
It is prudent to place a legal banner on login screens on all servers for legal reasons and to potentially deter intruders among other things. Consult legal counsel for the content of the banner.
If you want to print a legal banner
after a user logs in using ssh, local console etc., you can use the
/etc/motd file. Create the file if it doesn't exist and type in the banner that applies to your organization.
# cat /etc/motd
This system is classified...
Use of this system constitutes consent to official monitoring.
#
For SSH you can edit the
Banner parameter in the
/etc/ssh/sshd_config file which will display the banner before the login prompt.
For local console logins you can edit the
/etc/issue which will display the banner before the login prompt.
For GDM you could make the following changes to require a user to acknowledge the legal banner by selecting 'Yes' or 'No'. Edit the
/etc/X11/gdm/PreSession/Default file and add the following lines at the beginning of the script:
if ! gdialog --yesno '\nThis system is classified...\n' 10 10; then
sleep 10
exit 1;
fi
You have to add a sleep of 10 seconds, otherwise GDM will believe that X crashed if the session lasted less than 10 seconds. Unfortunately, at the time of this writing the 10 seconds timeout is hardcoded and there is no configuration parameter to change it. I checked the source code myself.
Host-Based Linux Monitoring and Intrusion DetectionBefore you put a server into production or better, before you put a server on the network, you should have an integrity checker installed on your system that lets you check if unauthorized changes have been made. In this way, if an intruder compromises your system you will know what changed on your server. You should also have an Intrusion Detection Software (IDS) solution in place that alarms you about intrusions, as well as Intrusion Prevention software.
It is outside the scope of this article to cover Linux Monitoring and Intrusion Detection solutions. There are lots of interesting articles out there to read and there are several good products available on the market including open source solutions. For a powerful open source network-intrusion prevention and detection system, see
Securing your system with Snort.
Connect Accounting UtilitiesHere is a list of commands you can use to get data about user logins:
who Shows a listing of currently logged-in users.
w Shows who is logged on and what they are doing.
last Shows a list of last logged-in users, including login time, logout time etc.
lastb Same as last, except that by default it shows a log of the file
/var/log/btmp, which contains all the bad login attempts.
lastlog This command reports data maintained in
/var/log/lastlog, which is a record of the last time a user logged in.
ac Prints out the connect time in hours on a per-user basis or daily basis etc. This command reads
/var/log/wtmp.
dump-utmp Converts raw data from
/var/run/utmp or
/var/log/wtmp into ASCII-parsable format.
Also check the
/var/log/messages file.
OtherThe following items may not necessarily be Linux security related but should be configured correctly on all audited Linux systems:
- Resolver (/etc/hosts, /etc/resolv.conf, /etc/nsswitch.conf)
- NTP (/etc/ntp.conf)
Linux PAM Documentation Linux System Security Nine principles of security architecture Securing & Optimizing Linux Smarter Password Management Network Intrusion Detection SELinux - NSA's Open Source Security Enhanced Linux 
