Yes, firewalls can send TCP RST (reset) packets.
Firewalls often use TCP RST packets as a way to terminate connections that they have determined should no longer be active or are in violation of configured rules. This is a standard mechanism within TCP communication and is essential for various aspects of network security and management.
How Firewalls Use TCP RST Packets
Here are some common scenarios where a firewall might send a TCP RST packet:
- Connection Termination After Timeout: When a firewall detects a connection that has been idle for a prolonged period, it may send a TCP RST packet to both ends of the connection to close it properly.
- Policy Violations: If traffic violates firewall rules, such as attempts to connect to blocked ports or addresses, the firewall can send a TCP RST to immediately close down the attempted connection.
- Scale Down Scenarios: As described in the provided reference, when a virtual machine scale set (VMSS) scales down, after a delay, the firewall starts rejecting existing connections by sending TCP RST packets. This is to terminate lingering connections that may not be valid anymore.
The reference provided specifically states: "The TCP Reset (TCP RST packets) are sent bidirectional to both client and server endpoints." This clarifies that when a firewall sends a TCP RST, it sends it to both the source and destination of the communication.
Practical Insight
Understanding that firewalls send TCP RST packets is important for troubleshooting network connectivity issues. If an application or device suddenly appears to lose connection, the cause might be a firewall sending RST packets. Examining the firewall logs or using network analysis tools could reveal if RST packets are being sent and help identify the reason.
Example of RST Packet Usage During Scale Down
Imagine a VMSS scaling down. Here's how it works:
- VM instances are terminated.
- After a short delay (e.g., 45 seconds), the firewall notices that connections previously going to the terminated VMs are now invalid.
- The firewall starts to send TCP RST packets to both the client and server endpoints of these old connections.
- The applications or clients receive these RST packets, properly terminating their sessions.
This ensures that the old connections are closed, and the system does not try to keep using them.
In summary, firewalls actively use TCP RST packets to enforce network policies, close inactive connections, and manage scaling scenarios. This is a core functionality for both security and operational efficiency.
