The DMZ is a security concept. It comprises the separation of the LAN-side network into at least two networks: the user LAN and the DMZ. Generally the DMZ is imprisoned: only access to certain ports from the Internet are allowed into the DMZ, while the DMZ is not allowed to establish new connections to the WAN-side or LAN-side networks. That way, if a server inside of the DMZ is hacked the potential damage that can be done remains restricted! The whole point of the DMZ is to cleanly create a unique firewall rule set that dramatically restricts access in to, and out of the, DMZ.
Publicly accessible servers remain the most vulnerable part of any network, although they are:
Many of the malicious robots and spiders on the internet scan well-known TCP ports and then try to exploit vulnerabilities. The most used webserver, Apache, has numerous POVs (Point of Vulnerability) that can be exploited. See the CVE database on Apache vulnerabilities or Apache 2.4 Vulnerabilities for information - scary!
For more information on splitting up your OpenWrt LAN-side network (default is a single bridge) into more networks:
It is possible to set up firewall rules based simply on the IP addresses of the public servers (see NAT for LAN-side Public Server), but this is not the most secure topology. If an attack exploits a POV and gains access to the public server all stations behind the firewall could be available to the attacker.
A more secure topology is to set up a separate VLAN for the DMZ, stick it into a new firewall zone and groom the firewall rules for only that zone.
This following switch configuration will assign switch ports specifically to traffic to the stations in the DMZ.
The DMZ station(s) should be attached to a wired ethernet switch port. First for better performance and second for easier VLAN provisioning.
Use the switch interface in /etc/config/network or through the LuCI Network → Switch page.
This example is based on, and tested against, the Reference Network Topology. The
STA-server2 station is a public webserver accessible from the internet.
The switch ports are highly dependent on the switch hardware. To view how the physical ports are configured, use:
swconfig list swconfig dev switch0 show
Confirm the switch ports align with the physical labeling on the router. Sometimes wired ports
1,2,3,4 are labelled as
The ethernet switches used in this example have the following switch port mappings (Atheros/Qualcomm).
So where are the 802.11 WLANs? For the DUT, there is only one, a 2.4GHz 802.11n interface integrated on the CPU chip. Both the CPU and the WLAN use the CPU eth0 switch port.
/etc/config/network shows VLAN1 for the WAN-side and VLAN2 for the LAN-side. VLAN1 has port 0 and port 5: the wired WAN port and eth1 to the CPU. VLAN2 has port 1,2,3,4 and 6: the four wired ports and eth0 for the CPU and WLAN PHY.
config switch option name 'switch0' option reset '1' option enable_vlan '1' config switch_vlan option device 'switch0' option vlan '1' option ports '0 5' config switch_vlan option device 'switch0' option vlan '2' option ports '1 2 3 4 6'
To split off the DMZ on the LAN-side, pick a port (
1) for the DMZ and use it in a vlan. Notice both VLANs have
6t in the port list. In VLAN 102 this is the CPU and WLAN, in VLAN 103 this is just the CPU. It must be a tagged port to differentiate which VLAN the packet is on.
config switch_vlan option device 'switch0' option vlan '102' option ports '2 3 4 6t' list comment 'vlan102: LAN ports' config switch_vlan option device 'switch0' option vlan '103' option ports '1 6t' list comment 'vlan103: LAN-side DMZ'
It is very helpful to add a comment using
list comment and to assign vlans with some sort of pattern. The DUT switch can support up to 4095 VLANs.
vid option defaults to the
vlan index so no need to specify it also. In fact, it is generally just a configuration error source.
On many Realtek and Atheros switches the ARL table (where the switch stores already learned MACs and the corresponding ports), uses only the MAC address for indexing. This has the effect that the switch tries to forward a frame to a port that isn't part of the current VLAN (since it learned that the destination is at that port), notes that the destination isn't part of the current VLAN, and drops the frame. When making changes be sure to flush the router ARL cache when provisioning the new VLANs.
Next, add the interface for the DMZ:
config 'interface' dmz option 'ifname' eth0.103 option 'proto' static option 'ipaddr' 192.168.30.1 option 'netmask' 255.255.255.0
ifname must correspond to the
<port>.<vlan>. The subnet must be different than the other VLANs.
Once you set up the firewall, DNS should automatically be available to the new network, DHCP will not. For DHCP, you need to add a new section to
/etc/config/dhcp. It is very similar to the
config dhcp 'dmz' option interface 'dmz' option start '50' option limit '70' option leasetime '12h'
Additionally, the public server should be very static, so it helps to add a static DHCP lease. First because it makes the firewall rules easier and second to debug future issues:
config host option name 'STA-server2' option dns '1' option mac '24:B6:FD:24:59:B9' option ip '192.168.30.20' option leasetime '12h'
Now the most important thing, the reason why you split up your network: the firewall rules. A typical DMZ can be fully provisioned using the fw3 application.
Each VLAN should be a unique firewall zone. So create one for the DMZ. Nothing is initiated from the DM zone so the policy is to REJECT everything.
config 'zone' option 'name' 'dmz' option 'input' 'REJECT' option 'output' 'REJECT' option 'forward' 'REJECT' option 'network' 'dmz'
Now, the main goal is to redirect HTTP/S from the Internet to the
STA-server2. Next, allow the server to do DNS lookups. Finally, for maintenance, allow SSH from any station in the LAN zone.
# Make STA-server:80 publicly accessible using the public ipv4 of the router config redirect option target 'DNAT' option src 'wan' option src_dport '80 443' option proto 'tcp' option family 'ipv4' option dest 'dmz' option dest_ip '192.168.30.20' option dest_port '80' option name 'DNAT-HTTP-WAN-DNZ' option enabled '1' # Allow the DMZ to access a DNS server config 'rule' option src 'dmz' option proto 'tcpudp' option dest 'wan' option dest_port 53 option target 'ACCEPT' option name 'ACCEPT-DNS-DMZ-WAN' option enabled '1' # Allow all LAN stations to SSH to DMZ stations config rule option src 'lan' option dest 'dmz' option proto 'tcp' option family 'ipv4' option dest_port '22' option target 'ACCEPT' option name 'ACCEPT-SSH-LAN-DMZ' option enabled '1'
What about using LuCI from the WAN zone now that a public webserver is sitting there? All requests coming in the WAN to port 80 are going to public web server so LuCI requests from the WAN-side would do the same.
If the requirement is to have LuCI access from the WAN-side, redirect HTTP to a new TCP port:
config redirect option target 'DNAT' option src 'wan' option src_dport '8080' option proto 'tcp' option family 'ipv4' option dest_port '80' option name 'DNAT-HTTP-WAN-DEVICE' option enabled '1'
http://dut:8080 to access the router's LuCI webserver.