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Understanding the ICMP options

Before adjusting any of the options in the ICMP window, it is important to understand what impact it could have on your network and on the ability of hackers to learn information about the network/computer the ICF is protecting. For the average home user, there will not be a need to enable any of the ICMP options. However, for the corporate user, the need may arise to allow ICMP while restricting other connection attempts.


Do not enable the ICMP options without a full understanding of the consequences. These options are required when a network technician needs to trouble shoot a connection. Immediately disable any option once the issue has been resolved.

The following will describe each of the ICMP options available to allow, or disable.

Allow incoming echo request: This option controls whether or not a computer can reply to messages sent that ask for a response. PING is a program that would require this option to be enabled. However, if this option remains disabled, hackers would not be able to 'see' the host computer.

Allow incoming timestamp request: The timestamp request is used to signal that a computer is listening to incoming requests by replying with the time/date that the signal was received.

Allow incoming mask request: This option of the ICMP is used to provide its requestor with the subnet mask of the target network. This information is useful for those who wish to connect to a network since this requires two components, namely the subnet mask and the IP address.

Allow incoming router request: This option supports the ability to pass on information to its requestor about any routers to which the host is connected. Since the Internet is a virtual web of routers that are used to control the flow of traffic, each router builds a table of available paths that data can travel. To support this, a router must be able to communicate with other routers located in between itself and the destination.

Allow outgoing destination unreachable: This option supports the ability of a router inside the network to pass information back to the requestor about the status of a nonexistent or unreachable host.

Allow outgoing source quench: This option is available to allow routers and other bridging devices to inform the sender that too much data is being sent and to slow down. Although it is not commonly used since the reply messages slow down traffic even more, it is nevertheless one of the options of the ICMP.

Allow outgoing parameter problem: In case there is a problem with a connection that is not covered by any of the other ICMP reporting features, this option serves as the cover-all. When an error is encountered, the receiving computer replies to the client computer with a generic 'bad header' response. If a hacker sent a malformed packet to a host, they would receive a response back, thus learning that the host does exists.

Allow outgoing time exceeded: If the TTL (time-to-live) part of a packet's datagram is exceeded due to an excessively long data transfer time, the received information will be dumped and a message will be sent back to the original computer. Using this automated utility, a hacker could determine who was present on a network without raising alarms by using PING to query the existence of computers on a network.

Allow redirect: The redirect function of ICMP allows routers to correctly point data in the right direction. If a router sends information via the wrong path, the next intercepting router will send a 'redirect' signal back to the originating router to correct its path table. This keeps data flowing efficiently on the Internet, but hackers could abuse it.

As you can see, the ICMP is responsible for many aspects of maintaining a healthy network. Routers, host and network technicians all use the ICMP to diagnose problems, make adjustments to router tables, and keep a watchful eye on where resources are located. However, hackers have been known to abuse the advantages offered by the ICMP; therefore, it is important to understand what can happen if it is re-enabled on a protected connection.

Internet Connection Sharing

Although not directly related to the ICF, the indirect relationship the Internet Connection Sharing (ICF) has with the ICF warrants a segment in this chapter. This is because the ICF cannot be enabled on connections between client and host computers that are using the ICS. Although this is usually only on the internal part of a network, users who are not aware of how the ICS works may find that their connection to the Internet is wide open for attack or that their ICS will not work correctly.

The ICS is a two-part program that allows a network to require only one connection to the Internet, which is then shared to the rest of the computers on the internal network. The host part of the program runs on the computer that has the Internet connection, and the client part is installed on each computer that wishes to use the shared connection. In other words, the ICS acts as a proxy server that relays Internet requests from the network through one central computer. It also fields incoming requests for services on the network and passes them along to the appropriate destination. However, these services must be set up and enabled in the ICF in order for the connection to be successful.

Microsoft has many help files available for the proper setup and use of the ICS. For the most part, it is a fairly simple program without many options for the user. However, it is very important that a user understand the ramifications that setting up ICS can have on her network. For this reason, we have included the basic enable/disable instruction as a list (with explanation) of how the ICS affects a network.

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