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firewalld

firewalld VS iptables

  1. Stack layers

    1. firewalld/iptables service -> iptables command -> kernel packet filter (netfilter).
    2. We should know the difference between a service and shell command in terms of iptables.
    3. firewalld still requires iptables command underneath.

    Hence, no matter which service you prefer, iptables command is essential.

    firewall

    At the very top, sits the GUI tool.

  2. Check packages

    ~ # yum list iptables iptables-services firewalld
    
  3. Specially, firewalld introduces zone to defines the level of trust for network connections, which resembles Microsoft Windows firewall.

    Rules are attached to a zone.

mask iptables service

Switch to firewalld.

~ # systemctl stop iptables-services
~ # sysremctl disable iptables-services
~ # systemctl mask iptables-services

Install

~ # yum install iptables firewalld
~ # systemctl enable firewalld
~ # systemctl start firewalld

Don't break SSH connection as the default SSH port in default zone is 22 that is usually changed by administrator, otherwise you could no longer SSH into server.

Status

~ # systemctl status firewalld
~ # firewall-cmd --state
~ # firewall-cmd ----get-services
~ # firewall-cmd --get-zones/--list-all-zones
~ # firewall-cmd --get-default-zone/--get-active-zones
~ # firewall-cmd --zone=public --list-all/list-ports/list-services
~ # firewall-cmd --reload
~ # iptables -S [-t nat]
  1. Default zone is public to which unmatched traffic would be directed.

    For use in public areas. You do not trust the other computers on networks to not harm your computer. Only selected incoming connections are accepted.

  2. We activate a zone (--get-active-zones) by binding a network interface or source IP address range(s) to it. Any firewall rules in the zone then apply to that network interface or IP address range(s).
  3. We can also examine firewalld settings by iptables command.

service and port

  1. Services are pre-defined well-known ports like http, https etc.

    Check /usr/lib/firewalld/services XML definitions. You shouldn't edit those files.

  2. To edit a servce (i.e. change ssh port):

    Copy /usr/lib/firewalld/services/ssh.xml to /etc/firewalld/services/; change port there.

    <?xml version="1.0" encoding="utf-8"?>
    <service>
      <short>SSH</short>
      <description>Secure Shell (SSH) is a protocol for logging into and executing commands on remote machines. It provides secure encrypted communications. If you plan on accessing your machine remotely via SSH over a firewalled interface, enable this option. You need the openssh-server package installed for this option to be useful.</description>
      <port protocol="tcp" port="12345"/>
    </service>
    

    Alternatively, adding port directly to default zone.

Adding ports

~ # firewall-cmd --zone=public --add-port=12345/tcp
~ # firewall-cmd --reload (opt)
~ # firewall-cmd --permanent --zone=public --add-port=12345/tcp
  1. Take effect immediately at rumtine without reload.
  2. As we have the runtime command at first, no --reload is required.
  3. Take effect accross reload.
  4. --zone can be ommited unless you want to change other zones.

Adding services

 ~ # firewall-cmd --permanent --zone=public --add-service={https,http}

Create/modify services

Only a permanent service can be created.

~ # firewall-cmd --permanent --new-service-from-file=/path/to/service.xml --name=myservice (using an existing service)
# or
~ # firewall-cmd --permanent --new-service=myservice (create an empty service)
~ # firewall-cmd --info-service=myservice

Modify it after creation:

~ # firewall-cmd --permanent --service=myservice --set-description=description
~ # firewall-cmd --permanent --service=myservice --set-short=description
~ # firewall-cmd --permanent --service=myservice --add-port=portid[-portid]/protocol
~ # firewall-cmd --permanent --service=myservice --remove-port=portid[-portid]/protocol
~ # firewall-cmd --info-service=myservice

Adding the new service to a zone:

~ # firewall-cmd --permanent --zone=public --add-service=myservice

I think the easiest way is to copy an existing service XML to /etc/firewalld/services/myservice.xml and edit that file directly.

drop a port

~ # firewall-cmd --permanent --zone=drop --add-port=12345/tcp

Then access to port 12345 would be dropped.

Finally

~ # firewall-cmd --reload

Refs

  1. firewalld centos 7
  2. Understanding Firewalld in Multi-Zone Configurations