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What is Circuit-Level Gateway?

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A firewall is a device that secures our network by monitoring traffic that can be incoming or outgoing by allowing or blocking it on the basis of security rules. It is also called the first defense device for network security.

The main purpose of a firewall is to filter the packets it receives or sends. When a packet is transferred between computers through a network, it transfers in small packets, the firewall keeps track of it. Usually, we use firewalls that are helpful in blocking unauthorized requests which are passing through designated traffic.

A circuit-level gateway firewall helps in providing the security between UDP and TCP using the connection. It also acts as a handshaking device between trusted clients or servers to untrusted hosts and vice versa.

Circuit-level Gateway

Circuit-level Gateway

Generally, these circuit-level gateways work at the session layer of the OSI model. To determine whether the session request is confirmed or not by the circuit-level gateway is with the help of handshaking between packets.

The information that passes to a remote computer with the help of a circuit-level gateway appears as it is initiated from the gateway. This is everything because of information hiding in protected networks. Circuit Level gateways are not expensive.

For defining a valid session in Circuit Level Gateway the component used are:-

  1. The Destination addresses, Source addresses, and Ports.
  2. The time of delay.
  3. The protocol is being utilized.
  4. The user and the password.


This circuit-level gateway setup can be done with the help of two TCP connections, establishing the connection between the gateway and inner host TCP and outer host of TCP users.

After the connection establishment of the inner and outer host, the Gateway transmits the TCP segments from one to another without bothering about the contents.

The circuit-level gateway maintains a table that helps in validating the connections and checks which network packets contain data to pass, whenever a network packet information matches an entry in the virtual circuit table. When the firewall terminates the connection, it tries to remove that entry in the table and the virtual-circuit connection between two nodes is closed.

In the Circuit Level Gateway, once a session is permitted no further checks at the level of individual packets are executed.


  1. It consists of security functions that determine which connections have to be allowed.
  2. It works at the session layer of the OSI model or in between the application and transport layer of TCP/IP.
  3. It hides the information about the private network they protect.
  4. It is a stand-alone system.
  5. An example of Circuit level gateway is SOCKS packages

Circuit-Level Proxy Firewall

Circuit-level proxy firewall

Circuit-level proxy firewall


  1. A circuit-level gateway acts as a proxy for hiding the internal host from the serving host.
  2. It avoids the filtering of individual packets.
  3. These gateways are inexpensive.
  4. Address schemes can easily develop.
  5. Simple to implement.
  6. Every application does not require a separate proxy server.


  1. Circuit-level Gateway does not filter the individual packets
  2. Frequent updates are required
  3. Within the firewall, it does not offer protection against data leakage from devices.
  4. For using Circuit level gateways the TCP/IP stacks are mandatory to be modified by the vendor.
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Last Updated : 03 Dec, 2021
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