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Transmission methods overview

Wake-on-LAN (or WOL in short) is a feature provided by network adapters and motherboards that allows the computer to be powered up a certain activity in the network is detected. Making use of WOL decreases security risks, since the computer is not left on connected and unattended for prolonged periods of time; it saves power and thus reduces your bills; ultimately, it extends the life of the components.

EMCO Remote Shutdown uses two transmission methods for sending wake-up packets, namely:

Directed Broadcast
Unicast

Both methods are described in detail below.

Directed Broadcast method

Directed Broadcasts, as a transmission method for sending wake-up packets to a Machine in Remote Shutdown, uses the Media Access Control (MAC) address and IP subnet address of the target Machine from the application database. A wake-up packet is sent to the Machine's last known subnet and is then broadcast to all Machines on that subnet. For this method to be successful, all intervening routers must be configured to forward directed broadcasts. During such broadcast transmission, the Machine with the MAC address specified in the wake-up packet will respond.

Since this transmission method uses the subnet address rather than the IP address, it will succeed even if the target Machine has changed its IP addresses but remained in the same subnet. Please note that subnet-directed broadcasts are not supported by IPv6 and are often disabled on routers.

Unicast method

Unicast, as a transmission method for sending wake-up packets to a Machine in Remote Shutdown, uses the IP address of the target computer from the application database to route the packets to the target computer's subnet, and it uses the Media Access Control (MAC) address of the target Machine from the database to generate a wake-up packet. When a wake-up transmission reaches the target Machine's subnet, the wake-up packet is sent directly to the target Machine.

If the target Machine has changed its IP address since it last sent its inventory information, the wake-up packet will reach a wrong Machine but will not wake it up because the MAC address in the wake-up packet transmission will not match the actual one.

Directed Broadcast and Unicast transition method comparison

Transmission Method

Advantages

Disadvantages

Directed Broadcast

Higher success rate than unicast with computers that frequently change their IP addresses within the same subnet.

No switch reconfiguration is required.

High compatibility with computer adapters in all sleep states, because directed broadcast was the original transmission method for sending wake-up packets.

Less secure solution than unicast, because an attacker could send continuous streams of ICMP echo requests from a falsified source address to the directed broadcast address causing all of the hosts to reply to that source address. This type of denial-of-service attack is commonly called a smurf attack and is usually mitigated by not allowing directed broadcasts. If your routers are configured to allow directed broadcasts, the following additional configuration is recommended for security reasons:

The routers should be configured to allow only IP-directed broadcasts from the Remote Shutdown site server using the UDP port number specified on the WOL Configuration preference page.

Reconfiguration of all intervening routers may be required to enable directed broadcasts.

Consumes more network bandwidth than unicast transmissions.

Supported by IPv4 only; IPv6 is not supported.

Unicast

More secure solution than directed broadcasts, because the packet is sent directly to the required computer rather than to all computers in the subnet.

Reconfiguration of routers is not required.

Consumes less network bandwidth than directed broadcast transmissions.

Supported by IPv4 and IPv6.

Wake-up packets will not be able to find the target computers that have changed their subnet address since the last scheduled hardware inventory.

Switches may need to be configured to forward UDP packets.

Some network adapters may not respond to wake-up packets in all sleep states when unicast is used as the transmission method.