can forge ICMP Redirect packets, and if your target host pays attention to them, you can alter the routing tables on the host and possibly subvert the security of the host by causing traffic to flow via a path the network manager didn't intend. ICMP Redirects also may be employed for denial of service attacks, where a host is sent a route that loses it connectivity, or is sent an ICMP Network Unreachable packet telling it that it can no longer access a particular network.

Many firewall builders screen ICMP traffic from their network, since it limits the ability of outsiders to ping hosts, or modify their routing tables.

What about denial of service?

Denial of service is when someone decides to make your network or firewall useless by disrupting it, crashing it, jamming it, or flooding it. The problem with denial of service on the Internet is that it is impossible to prevent. The reason has to do with the distributed nature of the network: every network node is connected via other networks which in turn connect to other networks, etc. A firewall administrator or ISP only has control of a few of the local elements within reach. An attacker can always disrupt a connection "upstream" from where the victim controls it. In other words, if someone wanted to take a network off the air, they could do it either by taking the network off the air, or by taking the networks it connects to off the air, ad infinitum. There are many, many, ways someone can deny service, ranging from the complex to the brute-force. If you are considering using Internet for a service which is absolutely time or mission critical, you should consider your fall-back position in the event that the network is down or damaged.

Glossary of firewall related terms

Abuse of Privilege:
When a user performs an action that they should not have, according to organizational policy or law.
Application-Level Firewall:
A firewall system in which service is provided by processes that maintain complete TCP connection state and sequencing. Application level firewalls often re-address traffic so that outgoing traffic appears to have originated from the firewall, rather than the internal host.
Authentication:
The process of determining the identity of a user that is attempting to access a system.
Authentication Token:
A portable device used for authenticating a user. Authentication tokens operate by challenge/response, time-based code sequences, or other techniques. This may include paper-based lists of one-time passwords.
Authorization:
The process of determining what types of activities are permitted. Usually, authorization is in the context of authentication: once you have authenticated a user, they may be authorized different types of access or activity.
Bastion Host:
A system that has been hardened to resist attack, and which is installed on a network in such a way that it is expected to potentially come under attack. Bastion hosts are often components of firewalls, or may be "outside" Web servers or public access systems. Generally, a bastion host is running some form of general purpose operating system (e.g., UNIX, VMS, WNT, etc.) rather than a ROM-based or firmware operating system.
Challenge/Response:
An authentication technique whereby a server sends an unpredictable challenge to the user, who computes a response using some form of authentication token.
Chroot:
A technique under UNIX whereby a process is permanently restricted to an isolated subset of the filesystem.
Cryptographic Checksum:
A one-way function applied to a file to produce a unique "fingerprint" of the file for later reference. Checksum systems are a primary means of detecting filesystem tampering on UNIX.
Data Driven Attack:
A form of attack in which the attack is encoded in innocuous-seeming data which is executed by a user or other software to implement an attack. In the case of firewalls, a data driven attack is a concern since it may get through the firewall in data form and launch an attack against a system behind the firewall.
Defense in Depth:
The security approach whereby each system on the network is secured to the greatest possible degree. May be used in conjunction with firewalls.
DNS spoofing:
Assuming the DNS name of another system by either corrupting the name service cache of a victim system, or by compromising a domain name server for a valid domain.
Dual Homed Gateway:
A dual homed gateway is a system that has two or more network interfaces, each of which is connected to a different network. In firewall configurations, a dual homed gateway usually acts to block or filter some or all of the traffic trying to pass between the networks.
Encrypting Router:
see Tunneling Router and Virtual Network Perimeter.
Firewall:
A system or combination of systems that enforces a boundary between two or more networks.
Host-based Security:
The technique of securing an individual system from attack. Host based security is operating system and version dependent.
Insider Attack:
An attack originating from inside a protected network.
Intrusion Detection:
Detection of break-ins or break-in attempts either manually or via software expert systems that operate on logs or other information available on the network.
IP Spoofing:
An attack whereby a system attempts to illicitly impersonate another system by using its IP network address.
IP Splicing / Hijacking:
An attack whereby an active, established, session is intercepted and co-opted by the attacker. IP Splicing attacks may occur after an authentication has been made, permitting the attacker to assume the role of an already authorized user. Primary protections against IP Splicing rely on encryption at the session or network layer.
Least Privilege:
Designing operational aspects of a system to operate with a minimum amount of system privilege. This reduces the authorization level at which various actions are performed and decreases the chance that a process or user with high privileges may be caused to perform unauthorized activity resulting in a security breach.
Logging:
The process of storing information about events that occurred on the firewall or network.
Log Retention:
How long audit logs are retained and maintained.
Log Processing:
How audit logs are processed, searched for key events, or summarized.
Network-Level Firewall:
A firewall in which traffic is examined at the network protocol packet level.
Perimeter-based Security:
The technique of securing a network by controlling access to all entry and exit points of the network.
Policy:
Organization-level rules governing acceptable use of computing resources, security practices, and operational procedures.
Proxy:
A software agent that acts on behalf of a user. Typical proxies accept a connection from a user, make a decision as to whether or not the user or client IP address is permitted to use the proxy, perhaps does additional authentication, and then completes a connection on behalf of the user to a remote destination.
Screened Host:
A host on a network behind a screening router. The degree to which a screened host may be accessed depends on the screening rules in the router.
Screened Subnet:
A subnet behind a screening router. The degree to which the subnet may be accessed depends on the screening rules in the router.
Screening Router:
A router configured to permit or deny traffic based on a set of permission rules installed by the administrator.
Session Stealing:
See IP Splicing.
Trojan Horse:
A software entity that appears to do something normal but which, in fact, contains a trapdoor or attack program.
Tunneling Router:
A router or system capable of routing traffic by encrypting it and encapsulating it for transmission across an untrusted network, for eventual de-encapsulation and decryption.
Social Engineering:
An attack based on deceiving users or administrators at the target site. Social engineering attacks are typically carried out by telephoning users or operators and pretending to be an authorized user, to attempt to gain illicit access to systems.
Virtual Network Perimeter:
A network that appears to be a single protected network behind firewalls, which actually encompasses encrypted virtual links over untrusted networks.
Virus:
A self-replicating code segment. Viruses may or may not contain attack programs or trapdoors.

Contributors:

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Copyright(C) 1995 Marcus J. Ranum. All rights reserved. This document may be used, reprinted, and redistributed as is providing this copyright notice and all attributions remain intact.