1. What is a firewall?
A firewall is a security device or software used to protect computer networks and systems. Its main function is to monitor and control network traffic, and decide which data packets are allowed to pass and which should be blocked or rejected according to predefined rules.
Firewalls can be placed in different places on the network, such as:
1. Network border firewall
is located at the boundary between the network and the external Internet. It is used to filter traffic from the external network to prevent unauthorized access and malicious attacks.
2. Host firewall
Located on a single host (computer) to protect that host from attacks from a local area network or public network.
The firewall determines whether to allow the data packet to pass by checking the source, destination, protocol, port and other information. These rules can be configured to allow or deny specific types of traffic. For example, it can prevent unauthorized access to a corporate network, block malware attempts to establish malicious connections, or allow access from specific IP addresses.
2. Working principle of stateful firewall
Stateful firewall is a firewall technology that maintains a state table in network communications to record the status information of network connections for more effective traffic filtering and control. By maintaining connection status, stateful firewalls can intelligently filter data packets in and out of the network, allowing only data related to existing valid connections to pass through, thereby enhancing network security.
working principle:
1. Connection establishment: When a new data packet attempts to establish a connection, the stateful firewall will check the source address, destination address, port number and other information of the data packet, and add the connection status information to the status table. At this point, the connection is considered "established".
2. Connection tracking: Once a connection is established, stateful firewall continues to track the status of this connection. It will know accurately, based on the information in the status table, that the connection is an established, valid connection and not a malicious attempt from an unauthorized source.
3. Packet filtering: When a data packet passes through the firewall, it will check the relevant information of the data packet and compare it with the records in the status table. If the packet belongs to an existing valid connection, it will be allowed through. Otherwise, if this packet is a new connection attempt, it will be processed according to predefined rules and may be blocked or rejected.
4. Connection termination: When a connection ends, the stateful firewall will delete the corresponding connection state information from the state table to release resources.
advantage:
Compared with traditional stateless firewalls, stateful firewalls can provide more efficient and intelligent data packet filtering, reducing unnecessary data packet processing.
It can prevent some network attacks, such as DoS (denial of service) attacks against network connections and some spoofing attacks.
3. How the firewall handles dual-channel protocols
1. Support state tracking: Since the dual-channel protocol involves multiple connections, the firewall needs to support state tracking. In this way, when a new connection is established, the firewall can recognize it as a data connection related to the existing control connection and allow it to pass through. The state tracking capability enables the firewall to intelligently handle the data flow of the dual-channel protocol.
2. Dynamic port range setting: Some dual-channel protocols (such as FTP) use dynamic ports in data connections instead of fixed ports. The firewall needs to allow communication on these dynamic ports, so it needs to be configured to allow a specific range of port numbers through the firewall. This range is usually defined in the specification of the dual-channel protocol.
3. Application layer proxy: For some complex dual-channel protocols, the firewall may need to implement an application layer proxy in order to understand the details of the protocol and process its data flow. Such a proxy can help the firewall better manage the control and data connections of the dual channel protocol.
4. Reasonable security strategy: Dual-channel protocols may involve different security risks. For example, in FTP, data connections may contain sensitive information, and it is necessary to ensure that only authorized users can access these connections. Therefore, the firewall's security policy should allow necessary connections while preventing unauthorized access.
5. Deep packet inspection: Due to the complexity of dual-channel protocols, firewalls may require deep packet inspection to identify potential security threats and malicious data flows. This increases the level of protection against dual channel protocols.