A middlebox or network appliance is a computer networking device that transforms, inspects, filters, or otherwise manipulates traffic for purposes other than packet forwarding. Common examples of middleboxes include firewalls, which filter unwanted or malicious traffic, and network address translators, which modify packets' source and destination addresses. Dedicated middlebox hardware is widely deployed in enterprise networks to improve network security and performance, however, even home network routers often have integrated firewall, NAT, or other middlebox functionality. The widespread deployment of middleboxes and other network appliances has resulted in some challenges and criticism due to poor interaction with higher layer protocols.
Types of middleboxes
The following are examples of commonly deployed middleboxes:
- Firewalls filter traffic based on a set of pre-defined security rules defined by a network administrator. IP firewalls reject packets "based purely on fields in the IP and Transport headers (e.g., disallow incoming traffic to certain port numbers, disallow any traffic to certain subnets, etc.)" Other types of firewalls may use more complex rulesets, including those that inspect traffic at the session or application layer.
- Intrusion Detection Systems monitor traffic and collect data for offline analysis for security anomalies. Because IDSes unlike firewalls do not filter packets in real-time, they traditionally are capable of more complex inspection than firewalls, which must make an accept/reject decision about each packet as it arrives.
- Network Address Translators replace the source and/or destination IP addresses of packets that traverse them. Typically, NATs are deployed to allow multiple end hosts to share a single IP address: hosts "behind" the NAT are assigned a private IP address, and their packets destined to the public Internet traverse a NAT which replaces their internal, private address, with a shared public address.
- WAN Optimizers improve bandwidth consumption and perceived latency between dedicated endpoints. Typically deployed in large enterprises, WAN optimizers are deployed near both sending and receiving endpoints of communication; the devices then coordinate to cache and compress traffic that traverses the Internet.
- Load balancers provide one point of entry to a service, but forward traffic flows to one or more hosts that actually provide the service.
Criticism and challenges
Although widely deployed, middleboxes have generated some technical challenges for application development and some controversy regarding their impact.
Some middleboxes interfere with application functionality, restricting or preventing end host applications from performing properly.
Network Address Translators present a challenge in that NAT devices divide traffic destined to a public IP address across several receivers. When connections between a host on the Internet and a host behind the NAT are initiated by the host behind the NAT, the NAT learns that traffic for that connection belongs to the local host. Thus, when traffic coming from the Internet is destined to the public (shared) address on a particular port, the NAT can direct the traffic to the appropriate host. However, connections initiated by a host on the Internet do not present the NAT any opportunity to "learn" which internal host the connection belongs to. Moreover, the internal host itself may not even know its own public IP address to announce to potential clients what address to connect to. To resolve this issue, several new protocols have been proposed.
Internet extensibility and design
Other criticisms of middleboxes include their limiting the extensibility of Internet protocols. Because middleboxes may filter or drop traffic that does not conform to expected behaviors, new protocols or protocol extensions may trigger unwanted filtering in middleboxes. More generally, middleboxes are considered to violate the The End to End Principle of computer system design.
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