This article contains configuration information and tips on how to protect your machine. The article is applicable to all popular Linux distributions.
General
The firewall is the front-line defense against remote attacks. It's highly recommended that you enable and configure it. Linux firewall infrastructure is called netfilter/iptables. To configure it, check this Iptables howto, or use a configuration frontend such asm0n0wall (CLI), shorewall (CLI), or FireStarter (GUI).
I suggest that you drop all incoming connections, and then open the ports you need, like SSH or BitTorrent.
Avoid Easy-to-compromise User Accounts
Your machine must not have user accounts with easy-to-guess passwords, especially accounts like test/test or guest/guest. Many Linux worms try to exploit such accounts over SSH. open
/etc/passwd and make sure there are no such accounts, if you do have an account like this, delete it:
$ sudo userdel <username>
If you really need such an account for some odd reason, change its shell to
/bin/false, so that an attacker cannot login using it:
$ sudo chsh -s /bin/false <username>
Mount /tmp as noexec
Many script kiddies rely on downloading scripts to
/tmp and executing them. By mounting
/tmp as
noexec, scripts located in
/tmp will not be executable. This effectively disables exploits that rely on
/tmp. Here is the
/tmp config line from my
/etc/fstab:
/dev/hda5 /tmp ext2 noatime,noexec 0 0
Protect against Fork Bombs
Fork bombs are programs that keep creating child processes until system resources are all used. They actually aren't remote exploits because they require a local user to execute the bomb; however, users may be tricked into running a fork bomb. For example, the following example may look innocent, but running it on an unprotected system may take the whole system down:
Do not run the above code on an unprotected system!
The above shell script will actually keep forking at an exponential rate until system resources are exhausted.
To protect a system against such attacks, there is a file for limiting the number of processes per user. It is
/etc/security/limits.conf. Add the following two lines to it:
@users soft nproc 100
@users hard nproc 150
These lines prevent anyone in the
users group from having more than 150 processes, and issue a warning at 100 processes.
Your system may not have a
users group, so you may want to edit the lines to match your needs.
Limit Usage of su/sudo
su lets normal users switch to the root account, and
sudo enables granting more privileges to users. It's always better to grant only the absolutely necessary privileges to specific users, and limit the usage of
su to a specific group.
When the usage of
su/sudo is limited, even if the system is compromised through a dummy account (like test as username and password), the attacker will have less options to play with.
Linux Daemons
OpenSSH
Users of machines with broadband connections usually need to remotely connect to their machines through SSH. So even if the workstation is protected by a firewall, the port of SSH needs to be open for inbound connections. Therefore, SSH is a common target for remote attacks.
Here is a list of OpenSSH configuration settings that make it more secure against attacks. SSH settings are usually located in
/etc/ssh/sshd_config:
SSH default port is 22, change it through the line below. This will stop many automated attacks.
(change the port number)
Notice that when remotely connecting to your machine, the new port number needs to be specified to the SSH client, for example:
$ ssh -p <new-port> <username>@<host>
Make sure the protocol is set to 2. Version 1 is not secure.
The following config lines protect against brute force attacks.
LoginGraceTime 2m
MaxAuthTries 6
Random attackers will usually try random usernames when trying to break through SSH, and since the root account exists on every machine, it will be on the attack list. The following config line disables root login over SSH and stops such attackers. If root access to a remote machine is needed, login using your regular account and use
sudo:
A username with blank password may be added accidentally. The following config line disables SSH for such accounts.
In addition to the configuration lines I listed, and in case you login to your machine from the same IP address or range, limit IP addresses that can connect to SSH using
/etc/hosts.allow. Use the following format:
sshd : 127.0.0.1 : allow
sshd : IP address here : allow
sshd : IP address here : allow
sshd : ALL : deny
MySQL
if you use MySQL for local development, then it's safer to limit its connections to localhost (among the other things). To do so, run the
mysql_secure_installation script, and it will take care of things for you.
Samba
Many need Samba for sharing files over the local network. Here is a list of config lines to secure it. The configuration file is usually located at
/etc/samba/smb.conf or
/etc/smb.conf:
hosts allow = 127.0.0.1 192.168.0.0/24
hosts deny = 0.0.0.0/0
These config lines limit the hosts that can connect to Samba to localhost and local IP ranges; modify to suit your needs.
Set security to user. In this case, users connecting to Samba will need to login before continuing. To add Samba user accounts, use the following command:
$ sudo smbpasswd -a <username>
You will be asked to provide a password for the new account.
If you login to your Samba share from a MS Windows machine, you may set your Samba username/password to match those on Windows, and avoid having to enter them every time you connect to the share.
As a final Samba tip, do not share your home directory. If you do so, you are just asking for trouble. Create a directory for sharing, and drop files there when necessary.
General Tips
- Keep your system up-to-date, especially when security vulnerabilities appear in packages you use. All major Linux distros have security mailing lists, subscribe to your distro's.
- Disable services you don't need. Every open service makes your system more open to attacks.
- Regularly monitor the output of the following command for odd entries:
$ sudo less /var/log/messages # System log.
$ sudo ps aux # Running processes.
$ sudo netstat -anp # Active connections.
If you have your domain pointed to an external server and the MX record is resolving to one of the SiteGround servers use the server name as the Incoming/Outgoing mail server.