Defending against Internet attacks for your Ubuntu Server (14.04 LTS)

Theodotos Andreou — Tue, 14 Apr 2015 14:40:01 +0000

The Ubuntu Server Edition LTS is a highly reliable server system and comes with reasonable security defaults. Still there are additional steps to take if we want to enhance its security.

Note
These steps will only help make your server more secure but they will not make it bulletproof! Security is an evergoing process and you should always be alert for new security issues.

Prerequisites

Install a fresh Ubuntu Server 14.04 (Preferable 64 bit).
Use the following command to install SSH, if not already installed:
```
$ sudo apt-get -y install openssh-server
```

Make sure you have a sudo enabled user:

$ id | grep sudo
uid=1000(theoadm) gid=1000(theoadm) groups=1000(theoadm),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),117(sambashare),1006(gitusers)

If the above is not true, you will have to login as root with su – and execute all the commands as the root user.

Secure SSH

Block remote logins as root. Set the value of the PermitRootLogin keyword, in /etc/ssh/sshd_config, to without-password or no. This will disable password based authentication for the user root and only allow Public Key Authentication.

First check what is the value of the PermitRootLogin keyword:

$  grep PermitRootLogin /etc/ssh/sshd_config
PermitRootLogin without-password

If the value is yes, it is considered a very bad practice, especially on a public server. Use your favorite editor or the following command to change it:
```
$ sudo sed -i 's/^$PermitRootLogin\s$[yY][eE][sS]/\1without-password/' /etc/ssh/sshd_config
```
Don’t forget to restart SSH:
```
$ sudo service ssh restart
```

Change the SSH listening port from 22 to something else. This is not the ultimate security measure but, since most ssh attack bots target the default port, it will largely minimize the attacks.

First select a port not used by a well known service. Let’s assume that we decided to use port 4547:
```
$  grep 4547 /etc/services ; echo $?
  1
```
A return value of 1 indicates that no well known service is using that port.
Then change the value of the Port keyword to 4547. Use your favorite editor or the following sed command to do so:
```
  $  sudo sed -i 's/^$Port\s$22/\14547/' /etc/ssh/sshd_config
```
Restart your SSH server:
```
$ sudo service ssh restart
```

Verify that the port has been changed:

$ sudo netstat -lnpt | grep ssh
  tcp     0    0 0.0.0.0:4547   0.0.0.0:*   LISTEN   11979/sshd
  tcp6    0    0 :::4547        :::*        LISTEN   11979/sshd

Enable Filtering

Enable the firewall functionality. We will be using the pre-installed Uncomplicated Firewall (ufw) which is just a front-end to the, more complicated, iptables.

First allow port 4547. Make sure you type the correct port or you will be locked out of your server! Use the following command to allow traffic to our chosen port:
```
$ sudo ufw allow 4547/tcp
```
Then enable the firewall:
```
$ sudo ufw enable
```

Verify that is working:

$ sudo ufw status
Status: active

To                         Action      From
--                         ------      ----
4547/tcp                   ALLOW       Anywhere
4547/tcp (v6)              ALLOW       Anywhere (v6)

Setup the Fail2ban intrusion prevention software. [Fail2ban](http://www.fail2ban.org/wiki/index.php/Main_Page "Fail2ban") is an excellent tool to block attacks against SSH and many other services.

Install fail2ban:
```
$ sudo apt-get -y install fail2ban
```
SSH protection is enabled by default but we need to reconfigure the ssh port to 4547. In the file /etc/fail2ban/jail.conf change the port = ssh value to 4547:
```
[ssh]

enabled  = true
port     = 4547
filter   = sshd
logpath  = /var/log/auth.log
maxretry = 6
```

Enable protections against distributed attacks. Edit the [ssh-ddos] section in /etc/fail2ban/jail.conf:

[ssh-ddos]

enabled  = yes
port     = 4547
filter   = sshd-ddos
logpath  = /var/log/auth.log
maxretry = 6

Unattended upgrades

Enabling unattended upgrades may not be a very good idea on mission critical servers. On such scenarios you may want to test the upgrades on a test server before you apply them on the production. Nevertheless it may be a good practice to enable this functionality on machines that are expected to run unattended for long periods of time. This will help to automatically patch vulnerabilities of your machine. Note, however, that patches do not automatically apply on the Linux kernel or the glibc library, because a restart is needed in these cases. So even on mostly unattended scenarios, you still need to check occasionally whether your machine needs a restart.

Make sure that the unattended-upgrades package is installed. It is usually pre-installed but if not, you can use the following command to install it:
```
$ sudo apt-get -y install unattended-upgrades
```
Then we must configure automatic upgrades. Answer Yes when asked in the following command:
```
$ sudo dpkg-reconfigure updates unattended-upgrades
```
Alternatively you can edit the /etc/apt/apt.conf.d/20auto-upgrades configuration file as follows:
```
APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Unattended-Upgrade "1";
```

ssh – Mouflons and Penguins

Defending against Internet attacks for your Ubuntu Server (14.04 LTS)

Prerequisites

Secure SSH

Enable Filtering

Unattended upgrades

Further Reading