Note: this post has been updated slightly to correct some minor errors. Also, commands may have been improperly formated due to WordPress’s treatment as such. Converted now to “code” formating.
1. Software Firewall – For security purposes, the software-based firewall that is included in all freshly deployed dedicated server operating system has been enabled and configured to allow on the minimal amount of connectivity required for you to access and configure your server. For Linux/UNIX users, this means that port 22 is permitting SSH connections. Port 80 (HTTP/Web) and port 443 (HTTPS/SSL Web) have been opened to allow all standard web traffic. In addition, the required ports for control panel access have been opened if you have ordered a control panel from ServePath. Finally, ICMP Ping has been permitted to allow our monitoring services the necessary access to aid in managing out network.
2. File Permission – There are certain files whose presence in the Linux file system can present a security risk and should be remedied as soon as possible. When the SUID (set user ID) or SGID (set group ID) bits are set on an executable, that program executes with the UID or GID of owner of the file as opposed to the user executing it. This means that all executables with SUID bit set and are owned by root are executed with the UID of root. This situation is a security risk and should be minimized unless the program is designed for this risk. To find all files on your file system that have the SUID or SGID bit set, execute the command:
# find / -path /proc –prune –o –type f –perm +6000 –ls
It is good practice to generate a list of SUID or SGID files on your server as soon as possible and re-run the above command on a regular basis to ensure new binaries with unsafe permissions are not being added to your server. World-writable files are a security risk as well. World-writable files and directories are dangerous since it allows anyone to modify them. World-writable directories allow anyone to add or delete files. To find all world-writable files and directories, execute the command:
# find / -path /proc –prune –o –perm -2 ! –type 1 –ls
# find / -path /proc –prune –o –perm -2 ! –type l –ls
Another file permission issue is when files are not owned by any user or group. While this is not technically a security vulnerability, an audited system should not contain any unowned files. This is to prevent the situation where a new user is assigned a previous user’s UID so that the previous owner’s files, if any, are all owned by the new user. To find all files that are not owned by any user or group, execute the command:
# find / -path /proc –prune –o –nouser –o –nogroup\
# find / -path /proc –prune –o –nouser –o –nogroup
3. Listening Ports – It is very important to ensure that all listening ports on your server are limited to only those that are necessary for you’re your server and its applications. To get a list of listening network ports, run the following command:
# netstat –tulp
Disable any ports that are not necessary. To do so, kill the PID (process ID) shown by netstat. The only port that your server must be listening on is SSH (port 22/tcp). Other ports that will need to be listening depend upon the specific purpose of your dedicated server. Note that by killing the PID of the process you are not preventing your server from starting the same service again on bootup. In order to see what programs your server is launching on startup, execute the following command:
# chkconfig –list |grep on (Red Hat systems)
# chkconfig –list | grep on (Red Hat systems)
# ls -l /etc/rc2.d/S* | cut -d/ -f6 (Debian systems)
This command will show you which programs are to be executed in which run levels. In Red Hat, full multi-user mode is 3. To disable a service permanently, issue the following command:
# chkconfig <service_name> off where <service_name> equals the name of your service, such as httpd
To disable any service in Debian, simply execute the following command:
# rm –f /etc/rc2.d/S*<service_name>
Please note that the above commands do not actually disable the service, they simply prevent the service from being executed on startup.
4. Unlocked User Accounts – The first thing you should take stock of on a new server are the users with unlocked accounts. Users with unlocked accounts are allowed to login if assigned a valid shell, and should be kept to a minimum. To get a list of unlocked users, execute the following command:
# egrep –v ‘.*:\*|:!' /etc/shadow|awk -F: '{print $1}'
# egrep –v '.*:\* | :!' /etc/shadow | awk -F: '{print $1}'
If you do not recognize any user returned by the above command, check to see if that user owns any files by executing the command:
# find / -path /proc -prune -o -user <user_name> -ls where <username> is the name of the user you do not recognize, such as jdoe
If the user does not own any files, or files that will not hinder the stability of your server, delete the user by executing the command:
# userdel –r <user_name>
5. Enable/Disable Features – All of the following lines and values should be added to the file /etc/sysctl.conf if you want to enable or disable the feature mentioned. You will need to restart your system for these changes to take effect:
| TCP SYN Cookie Protection |
net.ipv4.tcp_syncookies = 1 |
| Disable IP Source Routing |
net.ipv4.conf.all.accept_source_router = 0 |
| Disable ICMP Redirect Acceptance |
net.ipv4.conf.all.accept_redirects = 0 |
| IP Spoofing Protection |
net.ipv4.conf.all.rp_filter = 1 |
| Ignoring Broadcasts Request |
net.ipv4.icmp_echo_ignore_broadcasts=1 |
| Bad Error Message Protection |
net.ipv4.icmp_ignore_bogus_error_response = 1 |
6. Basic Access Control – One of the most important things you can do to protect your server is to implement very basic access control. Access control can eliminate a majority of the risk involved in running out of date services on the Internet. In order to implement an effective access control policy on your dedicated server, you will need the following pieces of information: The IP address or addresses of your Internet connection. For some, this may be one static address, while for others it is a pool of addresses. If you have more than one Internet connection, please be sure to get ALL the IP addresses you could be assigned at any time. You may need to contact your Internet Service Provider for this information.
7. Restrict SSH Connections – While we do not recommend anybody running outdated software, especially something as crucial as SSH, a not insignificant portion of the risks involved in running an outdated SSH server can be mitigated by only allowing certain IP networks to access your SSH server via iptables. Execute the following command to only allow SSH connections from certain IP address:
# $IPTABLES –A INPUT –p tcp –dport 22 –s <X.X.X.X/NN> –j ACCEPT
The above line will allow TCP packets destined for port 22 to be accepted if and only if the source of the packets are within the network denoted in <X.X.X.X/NN>. If you have more than one Internet connection, or have multiple networks, simply add another line, replacing <X.X.X.X/NN> with the proper values.
8. Access Control on Control Panel – If your server is running a control panel, you can also improve your security by implementing an access control policy on the control panel administrative port.
Plesk:
|
$IPTABLES –A INPUT –p tcp –dport 8443 –s X.X.X.X/NN –j ACCEPT
|
Ensim:
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$IPTABLES –A INPUT –p tcp –dport 19638 –s X.X.X.X/NN –j ACCEPT
|
Cpanel:
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$IPTABLES –A INPUT –p tcp –dport 2082 –s X.X.X.X/NN –j ACCEPT
|
9. Access Control on FTP – Another service you may want to implement an access control policy on is FTP. If you or a small handful of people are the only allowed users to FTP into your dedicated server, then you will certainly benefit from employing some iptables rules by entering the commands:
$IPTABLES –A INPUT –p tcp –s X.X.X.X/NN –dport 20 –syn –j ACCEPT
$IPTABLES –A INPUT –p tcp –s X.X.X.X/NN –dport 21 –syn –j ACCEPT
Note that both of the above lines must be executed for each source network.
10. Enable IPTABLES – Lastly, if you do not have a hardware firewall you will want to enable iptables, the software firewall in Linux systems. For a detailed iptables tutorial from ServePath, please visit our Support Center pages at: http://www.servepath.com/support/iptables.htm.
Hope that helps you get your Linux Server get even more secured! For other helpful tips like this, be sure to visit the ServePath Knowledge Base.
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