How-To Alpine Wall

Alpine Wall User Guide is the official source for details about the syntax. The purpose of this page is to illustrate Alpine Wall (AWall) by example. This page explains AWall from the viewpoint of a Shorewall user.

Installation

Install the awall package and make sure you are running the latest version by running the following commands:

# apk add iptables awall $ apk version awall

Prerequisites

After installing AWall, you need to load the following iptables modules:

# modprobe ip_tables # modprobe iptable_nat #if NAT is used

This is needed only one time after awall installation.

To make the firewall autostart at boot and autoload the needed modules:

# rc-update add iptables # rc-update add ip6tables

Configuration files

Your Alpine Wall configuration files go in /etc/awall/optional. From version 0.2.12 and later, Awall will look for Policy files in both the former and /usr/share/awall/optional Each such file is called a Policy.

You may have multiple Policy files. It is useful to have separate files for eg. HTTP, FTP, etc.

The Policy(s) can be enabled or disabled by using the command:

# awall [enable|disable]

An AWall Policy can contain definitions of:

• variables (like /etc/shorewall/params)
• zones (like /etc/shorewall/zones)
• interfaces (like /etc/shorewall/interfaces)
• policies (like /etc/shorewall/policy)
• filters and NAT rules (like /etc/shorewall/rules)
• services (like /usr/share/shorewall/macro.HTTP)

Basic home firewall

The below example shows the "Basic home firewall" configuration for both Shorewall and AWall. Based on below example, it can be clearly seen that AWall Policy files are not equivalent to Shorewall's /etc/shorewall/policy files.

Shorewall configuration

Let's suppose you have the following Shorewall configuration:

Contents of /etc/shorewall/zones

inet ipv4 loc ipv4

Contents of /etc/shorewall/interfaces

inet eth0 loc eth1

Contents of /etc/shorewall/policy

fw all ACCEPT loc inet ACCEPT all all DROP

Contents of /etc/shorewall/masq

eth0 0.0.0.0/0

AWall configuration

The equivalent AWall configuration that does the same thing as the above Shorewall example is given below.

Create a new file called /etc/awall/optional/home-policy.json and add the following content

Contents of /etc/awall/optional/home-policy.json

{ "description": "Home firewall", "zone": { "inet": { "iface": "eth0" }, "loc": { "iface": "eth1" } }, "policy": [ { "in": "_fw", "action": "accept" }, { "in": "loc", "out": "inet", "action": "accept" } ], "snat": [ { "out": "inet" } ] }

The above configuration will:

• Create a description of your Policy
• Define zones
• Define policy
• Define snat (to masqurade the outgoing traffic)

snat means "source NAT". It does not mean "static NAT".

Tip: AWall has a built-in zone named "_fw" which is the "firewall itself". This corresponds to the Shorewall "fw" zone.

Activating/Applying a Policy

After saving the Policy you can run the following commands to activate your firewall settings:

awall list # Listing available 'Policy(s)' (This step is optional) awall enable test-policy # Enables the 'Policy' awall activate # Genereates firewall configuration from the 'Policy' files and enables it (starts the firewall)

If you have multiple policies, after enabling or disabling them, you need to always run awall activate in order to update the iptables rules.

Advanced configuration

Assuming you have your /etc/awall/optional/home-policy.json with your "Basic home firewall" settings, you could choose to modify that file to test the below examples.

Tip: You could also create new files in /etc/awall/optional/ for testing some of the below examples

Logging

AWall will (since v0.2.7) automatically log dropped packets.

You could add the following row to the "policy" section in your Policy file in order to see the dropped packets.

{ "in": "inet", "out": "loc", "action": "drop" }

Note: If you are adding the above content to an already existing file, then make sure you add "," signs where they are needed!

Port forwarding

Let's suppose you have a local web server (192.168.1.10) that you want to make accessible from the "inet".
With Shorewall you would have a rule like this in your /etc/shorewall/rules:

#ACTION  SOURCE  DEST               PROTO  DEST    SOURCE    ORIGINAL
#                                          PORT(S) PORT(S)   DEST
DNAT     inet     loc:192.168.1.10  tcp    80

Lets configure our AWall Policy file likewise by adding the following content.

  "variable": {
    "APACHE": "192.168.1.10",
    "STATIC_IP": "1.2.3.4"
    },

  "filter": [
    { "in": "inet", 
      "dest": "$STATIC_IP", 
      "service": "http", 
      "action": "accept", 
      "dnat": "$APACHE" 
      }
    ]

As you can see in the above example, we create a

• "variable" section where we specify some IP-addresses
• "filter" section where we do the actual port-forwarding (using the variables we just created and using some preexisting "services" definitions)

Note: If you are adding the above content to a already existing file, then make sure you add "," signs where they are needed!

Tip: AWall already has a "service" definition list for several services like HTTP, FTP, SNMP, etc. (see /usr/share/awall/mandatory/services.json)

If you need to forward to a different port (e.g. 8080) you can do:

"dnat": [
  {"in": "inet", "dest": "$STATIC_IP", "to-addr": "$APACHE", "service": "http", "to-port": 8080 }
]

Create your own service definitions

Note: You can not override a "service" definition that comes from /usr/share/awall/mandatory/services.json

You can add your own service definitions into your Policy files:

"service": {  
  "openvpn": { "proto": "udp", "port": 1194 }
  }

Note: If you are adding the above content to a already existing file, then make sure you add "," signs where they are needed!

Inherit services or variables

You can import a Policy into other Policy files for inheriting services or variables definitions:

"import": "myfirewall"

Customize policy loading order

By default policies are loaded on alphabetical order. The load order can be changed with the keywords "before" and "after":

"before": "myfirewall"
"after": "someotherpolicy"

Troubleshooting

If you end up in some kind of trouble, you might find some commands useful when debugging:

awall # (With no parameters) Shows some basic help about awall application awall dump # Dump definitions like zones and variables iptables -L -n # Show what's in iptables

See also

• Alpine Wall User Guide
• Zero-To-Awall
• How To Set Up a Firewall with Awall on Alpine Linux