Firewall Ubuntu Servers

This post is in follow up to my post on firewalls on Ubuntu Desktops.

IMO firewalls are often helpful for restricting access to servers. Servers come in two varieties, public and private, and often people wish to limit access or black list IP addresses that misbehave.

Again I will use UFW which is installed by default. In the last section I will introduce iptables. One feature that is nice about ufw, if you understand the ufw rules it is an easy transition to iptables.

The most important thing you need to know to firewall servers is;

1. Who (ip address) you wish to allow or restrict access.
2. What protocol (tcp / udp) and port is used by your server.

A listing of ports is available here.

Enable your firewall

If you are accessing your server remotely be sure NOT to lock yourself out

Assuming you are accessing via ssh, allow ssh (we will restrict ssh access below, for now just do not lock yourself out).

sudo ufw allow 22

Now enable your firewall. Except for ssh, which you allowed with the above rule, this will deny all incoming (udp/tcp) traffic to your server.

sudo ufw enable
sudo default deny

Public servers

Examples of public servers would be Apache (http) or FTP servers (to name a few). Here we wish to allow access to just about everyone.

Simply allow by port

sudo ufw allow 80

Or if you wish, by protocol and port (most servers will be tcp).

sudo ufw allow 80/tcp

You may specify multiple ports (comma separated list):

sudo ufw allow 80,443/tcp

Or a range of ports, low:high:

#Allow ports 6881 – 6999 (torrent)
sudo ufw allow 6881:6999/tcp

You may specify most services by name.

By Name :

sudo ufw allow ssh

Some servers can be specified “by application”, although this is still by port and is not application specific. By that I mean : if you allow “Apache”, you open port 80, which can be used by any client application (firefox, wget, etc).

List applications with -

sudo ufw app list


ufw app list
Available applications:
Apache
Apache Full
Apache Secure
CUPS
OpenSSH

To translate the cryptic output to English,
Apache = http = port 80
Apache Secure = https = port 443
Apache Full = both ports

As you install servers, they will be added to the list.

Now allow by application.

Examples (you do not need to use all 3 rules):

sudo ufw allow Apache

#Note: Quotes are needed with “Apache Full”
sudo ufw allow “Apache Full”

sudo ufw allow from 192.168.0.0/24 app OpenSSH

You may add custom applications or custom ports to /etc/ufw/application.d

As an example, /etc/ufw/applications.d/apache2.2-common contains

[Apache]
title=Web Server
description=Apache v2 is the next generation of the omnipresent Apache web server.
ports=80/tcp

[Apache Secure]
title=Web Server (HTTPS)
description=Apache v2 is the next generation of the omnipresent Apache web server.
ports=443/tcp

[Apache Full]
title=Web Server (HTTP,HTTPS)
description=Apache v2 is the next generation of the omnipresent Apache web server.
ports=80,443/tcp

So if you changed the ssh port to 8822, add a file “ssh-custom”, at /etc/ufw/applications.d/ssh-custom

[SSH Custom]
title= SSH Custom port
description=OpenSSH Server Custom port
ports=8822/tcp

You will now see “SSH Custom” when you list apps and can use it as above.

Private servers

Examples may included NFS, Samba, ssh, VNC, and VPN. I will use ssh and Apache as an examples.

For these examples we will assume your LAN is 192.168.0.0/24 and your server is 192.168.0.10

Here we almost always wish to restrict access to a single ip or perhaps range of IP. For example to restrict access for ssh to a single machine, say 192.168.0.20

sudo ufw allow proto tcp from 192.168.0.20 to 192.168.0.10 port 22

The syntax is protocol from <ip> to <server ip> port

To allow ssh from any client on your your lan use:

sudo ufw allow proto tcp from 192.168.0.0/24 to 192.168.0.10 port 22

Limiting access

Limiting access comes in two flavors, the first is to limit a DOS or brute force attempt, and the other blacklisting.

Brute Force

UFW will rate limit connection attempts:

ufw supports connection rate limiting, which is useful for protecting against brute-force login attacks. ufw will deny connections if an IP address has attempted to initiate 6 or more connections in the last 30 seconds.

Example (using ssh):

sudo ufw limit ssh

“Limit” opens the port, so you do not need a second rule.


ufw status
Status: active

To Action From
-- ------ ----
22 LIMIT Anywhere

This output demonstrates – Port 22 is open and access is limited by ufw.

Blacklist

Keep in mind the order of your rules is critical. As such I like to block first, accept second. So for example let us assume we wish to block a misbehaving client on our LAN, 192.168.0.20:

sudo ufw insert 1 deny from 192.168.0.20

Here “insert 1″ is specifying to ufw to insert the rule first (or near the top) of the chain.

Using UFW in this way blocks only NEW connections.

IMO better to use iptables or an application such as iplist.

Block ping

By default, UFW allows ping requests. In order to block these requests you will need to edit /etc/ufw/before.rules

sudo -e /etc/ufw/before.rules

Change
# ok icmp codes
-A ufw-before-input -p icmp --icmp-type destination-unreachable -j ACCEPT
-A ufw-before-input -p icmp --icmp-type source-quench -j ACCEPT
-A ufw-before-input -p icmp --icmp-type time-exceeded -j ACCEPT
-A ufw-before-input -p icmp --icmp-type parameter-problem -j ACCEPT
-A ufw-before-input -p icmp --icmp-type echo-request -j ACCEPT

to

# ok icmp codes
-A ufw-before-input -p icmp –icmp-type destination-unreachable -j ACCEPT
-A ufw-before-input -p icmp –icmp-type source-quench -j ACCEPT
-A ufw-before-input -p icmp –icmp-type time-exceeded -j ACCEPT
-A ufw-before-input -p icmp –icmp-type parameter-problem -j ACCEPT
-A ufw-before-input -p icmp –icmp-type echo-request -j DROP

Restart UFW

sudo ufw disable
sudo ufw enable

Deleting rules

Deleting a rule is also easy. Use the same syntax you used to add a rule to ufw with the word “delete” added.

For example, using Apache as an example:

# sudo ufw allow Apache
Rule added

# ufw status
Status: active

To Action From
– —— —-
22 LIMIT Anywhere
Apache ALLOW Anywhere

# sudo ufw delete allow Apache
Rule deleted

# ufw status
Status: active

To Action From
– —— —-
22 LIMIT Anywhere

Logs

ufw logs messages to /var/log/messages and logging is enabled / disabled from the command line.

sudo ufw logging on
sudo ufw logging off

The options are on, off, low, medium, high, and full. on = Low.

From the ufw man pages :

LOGGING

ufw supports multiple logging levels. ufw defaults to a loglevel of
’low’ when a loglevel is not specified. Users may specify a loglevel
with:

ufw logging LEVEL

LEVEL may be ’off’, ’low’, ’medium’, ’high’ and full. Log levels are
defined as:

off disables ufw managed logging

low logs all blocked packets not matching the default policy (with
rate limiting), as well as packets matching logged rules

medium log level low, plus all allowed packets not matching the default
policy, all INVALID packets, and all new connections. All
logging is done with rate limiting.

high log level medium (without rate limiting), plus all packets with
rate limiting

full log level high without rate limiting

Loglevels above medium generate a lot of logging output, and may
quickly fill up your disk. Loglevel medium may generate a lot of
logging output on a busy system.

Specifying ’on’ simply enables logging at log level ’low’ if logging is
currently not enabled.

Iptables

Now that you have ufw under your belt, it is easier to understand iptables. If you are wanting to use iptables, best disable UFW first.

sudo ufw disable

#These iptables rules clean up after UFW, deleting the custom tables
sudo iptables -F
sudo iptables -X

To deny all incoming traffic (take care not to lock yourself out form remote servers, allow ssh first !!!):

sudo iptables -A INPUT -j DROP

You can set a Policy with iptables, but doing so makes it easy to lock yourself out if you issue the command “iptables -F”.

To allow ssh

sudo iptables -A INPUT -p tcp –dport 22 -j ACCEPT

To allow ssh only from your LAN:

sudo iptables -A INPUT -s 192.168.0.0/24 -p tcp –dport 22 -j ACCEPT

iptables is much more feature rich the UFW and hopefully this blog will both stimulate your interest and facilitate your learning of the use of iptables.

See also : Bodhi Zazen’s Iptables Primer