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I think I'm using the wrong technique, but not sure of the right one.

Machine: Red Hat release 7.2

firewalld.noarch: 0.3.9-14.el7

I've been asked to close two ports but insure that all other ports are open. The solution needs to be easy to turn on and off. To that end I have done:

  • bring up firewalld

  • set "trusted" as default zone # Trusted opens all ports

  • firewall-cmd --zone=trusted --add-interface=eno16780032 # only Ethernet interface on this server.

  • For testing purposes, executing nc -l port_number to have something answering on that port.

Test by: go to a different machine, execute "telnet machine_name port_number" and observe that I get a response. (Restarting nc after each test.)

Turn off port:

  • firewall-cmd --zone=trusted --remove-port port_number/tcp

Verify:

  • firewall-cmd --zone=trusted --query-port port_number/tcp

Returns "no"

At this point, nc should be listening on port_number, but it should be blocked by firewalld. I shouldn't be able to connect to it.

However, "telnet machine_name port_number" from a different machine still connects.

I'm not even trying to make it persistent at this point, just trying to get the rule to work. What am I doing wrong?

The application: We have a homegrown back end service that runs as a master/slave configuration. The slave is up at all times, to sync data with the master. Only the system designated "master" can be used by the front end. (To make it a true cluster would involve too much work, the developers tell me.)

There's a load balancer in "the cloud" (over which we don't have direct control) that points to both machines. The objective is to block two key ports on the slave so the load balancer always goes to the master. When we fail over, the ports on the "slave" (now master) are unblocked and the ports on the "master" (now slave) are blocked, forcing the load balancer to go to the new master.

This is probably not a good use of the load balancer or of firewalld, but it's an odd application and we're just trying to find something that works that doesn't involve either mucking with the load balancer or shutting down services on the slave.

Any ideas?

Ron Christian
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1 Answers1

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IIRC the philosophy behind firewalld is to close anything and open just the ports you need. So just the inverse you are trying to do. Therefore commands like --add-port will add the port specified to be open.
As an example, adding port 80 will just add another ACCEPT rule to iptables but since the target for the zone trusted is ACCEPT already, this rule just has no meaning.

$ firewall-cmd --zone=trusted --add-port=80/tcp

$ iptables -L -n | grep 80
ACCEPT     tcp  --  0.0.0.0/0            0.0.0.0/0            tcp dpt:80 ctstate NEW

Same if you remove the port with --remove-port, which should give a hint if the corresponding port is not configured.

$ firewall-cmd --zone=trusted --remove-port 80/tcp
success
$ firewall-cmd --zone=trusted --remove-port 80/tcp
Warning: NOT_ENABLED: '80:tcp' not in 'trusted'
success

firewalld also provides rich-rules which can be used for what you want to achieve.

The command as follows would close port 80/tcp.

firewall-cmd --zone=trusted --add-rich-rule='rule family="ipv4" port port="80" protocol="tcp" reject'

If you want to allow a single IP address to connect to that port, you could add a source.

firewall-cmd --zone=trusted --add-rich-rule='rule family="ipv4" source NOT address="192.168.122.1" port port="80" protocol="tcp" reject'firewall-cmd --zone=trusted --add-rich-rule='rule family="ipv4" source NOT address="192.168.122.1" port port="80" protocol="tcp" reject'

The source parameter also accepts subnets in CIDR notation.

firewall-cmd --zone=trusted --add-rich-rule='rule family="ipv4" source NOT address="192.168.122.1" port port="80" protocol="tcp" reject'firewall-cmd --zone=trusted --add-rich-rule='rule family="ipv4" source NOT address="192.168.122.0/24" port port="80" protocol="tcp" reject'
Thomas
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