How do I set up SSH firewall rules on a Google VM so that only my office computer can access the VM via SSH?

I understand that you want to create a new virtual machine with a secure firewall SSH and want to restrict and allow access from a specific IP address of your office router.

To do this, you can create firewall rules as described in 1 here . To manage access to a specific instance, I recommend that you use network labels for firewall rule 2 .

Going back to your concern, even if you create a firewall rule for a specific IP address, SSH connections to the VM will indeed be established from other IP addresses. The reasons for this may be:

Every project you create in GCP comes with default firewall rules. So you might need to block a default-allow-ssh rule, which I think might be causing the problem. Note that the default network includes some additional rules that override this, allowing certain types of incoming traffic. See Additional Links [3][4] for more details.

[3] https://cloud.google.com/vpc/docs/firewalls#default_firewall_rules [4] https://cloud.google.com/vpc/docs/firewalls#more_rules_default_vpc

You can also use eg "iptables" to add guest level firewall rules to add another level of security to your VM instance. However, GCP project-level firewall rules are responsible for inspecting network traffic before it enters your VM instance. The OS firewall blocks all internet traffic to any port 22.

To allow specific addresses to be able to connect to your VM instance, you can add a CIDR of /32 to the "IP Range" value of the "default-allow-ssh" GCP firewall rule. For example, 45.56.122.7/32 and 208.43.25.31/32.