Procedure

1. Open the /etc/ssh/sshd_config configuration in a text editor, for example:

   # vi /etc/ssh/sshd_config

2. Change the PasswordAuthentication option to no:

   PasswordAuthentication no

   On a system other than a new default installation, check that PubkeyAuthentication no has not been set and the ChallengeResponseAuthentication directive is set to no. If you are connected remotely, not using console or out-of-band access, test the key-based login process before disabling password authentication.

3. To use key-based authentication with NFS-mounted home directories, enable the use_nfs_home_dirs SELinux boolean:

   # setsebool -P use_nfs_home_dirs 1

4. Reload the sshd daemon to apply the changes:

   # systemctl reload sshd

Procedure

1. To generate an ECDSA key pair for version 2 of the SSH protocol:

   $ ssh-keygen -t ecdsa
   Generating public/private ecdsa key pair.
   Enter file in which to save the key (/home/joesec/.ssh/id_ecdsa):
   Enter passphrase (empty for no passphrase):
   Enter same passphrase again:
   Your identification has been saved in /home/joesec/.ssh/id_ecdsa.
   Your public key has been saved in /home/joesec/.ssh/id_ecdsa.pub.
   The key fingerprint is:
   SHA256:Q/x+qms4j7PCQ0qFd09iZEFHA+SqwBKRNaU72oZfaCI joesec@localhost.example.com
   The key's randomart image is:
   +---[ECDSA 256]---+
   |.oo..o=++        |
   |.. o .oo .       |
   |. .. o. o        |
   |....o.+...       |
   |o.oo.o +S .      |
   |.=.+.   .o       |
   |E.*+.  .  . .    |
   |.=..+ +..  o     |
   |  .  oo*+o.      |
   +----[SHA256]-----+

   You can also generate an RSA key pair by using the -t rsa option with the ssh-keygen command or an Ed25519 key pair by entering the ssh-keygen -t ed25519 command.

2. To copy the public key to a remote machine:

   $ ssh-copy-id joesec@ssh-server-example.com
   /usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
   ...
   Number of key(s) added: 1
   
   Now try logging into the machine, with: "ssh 'joesec@ssh-server-example.com'" and check to make sure that only the key(s) you wanted were added.

   If you do not use the ssh-agent program in your session, the previous command copies the most recently modified ~/.ssh/id*.pub public key if it is not yet installed. To specify another public-key file or to prioritize keys in files over keys cached in memory by ssh-agent, use the ssh-copy-id command with the -i option.

Verification steps

1. Log in to the OpenSSH server without providing any password:

   $ ssh joesec@ssh-server-example.com
   Welcome message.
   ...
   Last login: Mon Nov 18 18:28:42 2019 from ::1

Procedure

1. To see the status of the service:

   # firewall-cmd --state

2. For more information about the service status, use the systemctl status sub-command:

   # systemctl status firewalld
   firewalld.service - firewalld - dynamic firewall daemon
      Loaded: loaded (/usr/lib/systemd/system/firewalld.service; enabled; vendor pr
      Active: active (running) since Mon 2017-12-18 16:05:15 CET; 50min ago
        Docs: man:firewalld(1)
    Main PID: 705 (firewalld)
       Tasks: 2 (limit: 4915)
      CGroup: /system.slice/firewalld.service
              └─705 /usr/bin/python3 -Es /usr/sbin/firewalld --nofork --nopid

Procedure

1. To immediately disable networking traffic, switch panic mode on:

   # firewall-cmd --panic-on

Procedure

1. Check that the service is not already allowed:

   # firewall-cmd --list-services
   ssh dhcpv6-client

2. List all predefined services:

   # firewall-cmd --get-services
   RH-Satellite-6 amanda-client amanda-k5-client bacula bacula-client bitcoin bitcoin-rpc bitcoin-testnet bitcoin-testnet-rpc ceph ceph-mon cfengine condor-collector ctdb dhcp dhcpv6 dhcpv6-client dns docker-registry ...
   [trimmed for clarity]

3. Add the service to the allowed services:

   # firewall-cmd --add-service=<service-name>

4. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

1. Start the firewall-config tool and select the network zone whose services are to be configured.
2. Select the Services tab.
3. Select the check box for each type of service you want to trust or clear the check box to block a service.

1. Start the firewall-config tool.
2. Select Permanent from the menu labeled Configuration. Additional icons and menu buttons appear at the bottom of the Services window.
3. Select the service you want to configure.

1. Enter the following command to add a new and empty service:

   $ firewall-cmd --new-service=service-name --permanent

2. To add a new service using a local file, use the following command:

   $ firewall-cmd --new-service-from-file=service-name.xml --permanent

   You can change the service name with the additional --name=service-name option.

3. As soon as service settings are changed, an updated copy of the service is placed into /etc/firewalld/services/.

   As root, you can enter the following command to copy a service manually:

   # cp /usr/lib/firewalld/services/service-name.xml /etc/firewalld/services/service-name.xml

1. Start the firewall-config tool and select the network zone whose settings you want to change.
2. Select the Ports tab and click the Add button on the right-hand side. The Port and Protocol window opens.
3. Enter the port number or range of ports to permit.
4. Select tcp or udp from the list.

1. Start the firewall-config tool and select the network zone whose settings you want to change.
2. Select the Protocols tab and click the Add button on the right-hand side. The Protocol window opens.
3. Either select a protocol from the list or select the Other Protocol check box and enter the protocol in the field.

1. Start the firewall-config tool and select the network zone whose settings you want to change.
2. Select the Source Port tab and click the Add button on the right-hand side. The Source Port window opens.
3. Enter the port number or range of ports to permit. Select tcp or udp from the list.

Procedure

1. To see which zones are available on your system:

   # firewall-cmd --get-zones

   The firewall-cmd --get-zones command displays all zones that are available on the system, but it does not show any details for particular zones.

2. To see detailed information for all zones:

   # firewall-cmd --list-all-zones

3. To see detailed information for a specific zone:

   # firewall-cmd --zone=zone-name --list-all

Procedure

1. To work in a different zone, use the --zone=zone-name option. For example, to allow the SSH service in the zone public:

1. Display the current default zone:

   # firewall-cmd --get-default-zone

2. Set the new default zone:

   # firewall-cmd --set-default-zone zone-name

   Note

   Following this procedure, the setting is a permanent setting, even without the --permanent option.

1. List the active zones and the interfaces assigned to them:

   # firewall-cmd --get-active-zones

2. Assign the interface to a different zone:

   # firewall-cmd --zone=zone_name --change-interface=interface_name --permanent

Procedure

1. Assign the zone to the NetworkManager connection profile:

   # nmcli connection profile modify connection.zone zone_name

2. Reload the connection:

   # nmcli connection up profile

Procedure

1. To set a zone for a connection, edit the /etc/sysconfig/network-scripts/ifcfg-connection_name file and add a line that assigns a zone to this connection:

   ZONE=zone_name

1. Create a new zone:

   # firewall-cmd --new-zone=zone-name

2. Check if the new zone is added to your permanent settings:

   # firewall-cmd --get-zones

3. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

1. List the information for the specific zone to see the default target:

   $ firewall-cmd --zone=zone-name --list-all

2. Set a new target in the zone:

   # firewall-cmd --permanent --zone=zone-name --set-target=<default|ACCEPT|REJECT|DROP>

Procedure

1. List all available zones:

   # firewall-cmd --get-zones

2. Add the source IP to the trusted zone in the permanent mode:

   # firewall-cmd --zone=trusted --add-source=192.168.2.15

3. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

Procedure

1. List allowed sources for the required zone:

   # firewall-cmd --zone=zone-name --list-sources

2. Remove the source from the zone permanently:

   # firewall-cmd --zone=zone-name --remove-source=<source>

3. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

Procedure

1. To add a source port:

   # firewall-cmd --zone=zone-name --add-source-port=<port-name>/<tcp|udp|sctp|dccp>

Procedure

1. To remove a source port:

   # firewall-cmd --zone=zone-name --remove-source-port=<port-name>/<tcp|udp|sctp|dccp>

Procedure

1. List all available zones:

   # firewall-cmd --get-zones
   block dmz drop external home internal public trusted work

2. Add the source to the trusted zone to route the traffic originating from the source through the zone:

   # firewall-cmd --zone=trusted --add-source=192.168.1.0/24

3. Add the http service in the trusted zone:

   # firewall-cmd --zone=trusted --add-service=http

4. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

5. Check that the trusted zone is active and that the service is allowed in it:

   # firewall-cmd --zone=trusted --list-all
   trusted (active)
   target: ACCEPT
   sources: 192.168.1.0/24
   services: http

1. Add the port to be forwarded:

   # firewall-cmd --add-forward-port=port=port-number:proto=tcp|udp:toport=port-number:toaddr=IP

2. Enable masquerade:

   # firewall-cmd --add-masquerade

Procedure

1. Redirect the port 80 to port 88 for TCP traffic:

   # firewall-cmd --add-forward-port=port=80:proto=tcp:toport=88

2. Make the new settings persistent:

   # firewall-cmd --runtime-to-permanent

3. Check that the port is redirected:

   # firewall-cmd --list-all

Procedure

1. Remove the forwarded port:

   # firewall-cmd --remove-forward-port=port=port-number:proto=<tcp|udp>:toport=port-number:toaddr=<IP>

2. Disable masquerade:

   # firewall-cmd --remove-masquerade

Procedure

1. List redirected ports:

   ~]# firewall-cmd --list-forward-ports
   port=80:proto=tcp:toport=88:toaddr=

2. Remove the redirected port from the firewall::

   ~]# firewall-cmd  --remove-forward-port=port=80:proto=tcp:toport=88:toaddr=

3. Make the new settings persistent:

   ~]# firewall-cmd --runtime-to-permanent

Procedure

1. Add a rich rule with a very low precedence to log all traffic that has not been matched by other rules:

   # firewall-cmd --add-rich-rule='rule priority=32767 log prefix="UNEXPECTED: " limit value="5/m"'

   The command additionally limits the number of log entries to 5 per minute.

2. Optionally, display the nftables rule that the command in the previous step created:

   # nft list chain inet firewalld filter_IN_public_post
   table inet firewalld {
     chain filter_IN_public_post {
       log prefix "UNEXPECTED: " limit rate 5/minute
     }
   }

Procedure

1. Steps that are required only once:

   1. Optionally, set the owner of the script to root:

      # chown root /etc/nftables/example_firewall.nft

   2. Make the script executable for the owner:

      # chmod u+x /etc/nftables/example_firewall.nft

2. Run the script:

   # /etc/nftables/example_firewall.nft

   If no output is displayed, the system executed the script successfully.

   Important

   Even if nft executes the script successfully, incorrectly placed rules, missing parameters, or other problems in the script can cause that the firewall behaves not as expected.

Procedure

1. Edit the /etc/sysconfig/nftables.conf file.

   • If you enhance *.nft scripts created in /etc/nftables/ when you installed the nftables package, uncomment the include statement for these scripts.

   • If you write scripts from scratch, add include statements to include these scripts. For example, to load the /etc/nftables/example.nft script when the nftables service starts, add:

     include "/etc/nftables/example.nft"

2. Enable the nftables service.

   # systemctl enable nftables

3. Optionally, start the nftables service to load the firewall rules without rebooting the system:

   # systemctl start nftables

Procedure

1. Use the nft add table command to create a new table. For example, to create a table named example_table that processes IPv4 and IPv6 packets:

   # nft add table inet example_table

2. Optionally, list all tables in the rule set:

   # nft list tables
   table inet example_table

Procedure

1. Use the nft add chain command to create a new chain. For example, to create a chain named example_chain in example_table:

   # nft add chain inet example_table example_chain { type filter hook input priority 0 \; policy accept \; }

   Important

   To avoid that the shell interprets the semicolons as the end of the command, you must escape the semicolons with a backslash.

   This chain filters incoming packets. The priority parameter specifies the order in which nftables processes chains with the same hook value. A lower priority value has precedence over higher ones. The policy parameter sets the default action for rules in this chain. Note that if you are logged in to the server remotely and you set the default policy to drop, you are disconnected immediately if no other rule allows the remote access.

2. Optionally, display all chains:

   # nft list chains
   table inet example_table {
     chain example_chain {
       type filter hook input priority filter; policy accept;
     }
   }

Procedure

1. To add a new rule, use the nft add rule command. For example, to add a rule to the example_chain in the example_table that allows TCP traffic on port 22:

   # nft add rule inet example_table example_chain tcp dport 22 accept

   Instead of the port number, you can alternatively specify the name of the service. In the example, you could use ssh instead of the port number 22. Note that a service name is resolved to a port number based on its entry in the /etc/services file.

2. Optionally, display all chains and their rules in example_table:

   # nft list table inet example_table
   table inet example_table {
     chain example_chain {
       type filter hook input priority filter; policy accept;
       ...
       tcp dport ssh accept
     }
   }

Procedure

1. To insert a new rule, use the nft insert rule command. For example, to insert a rule to the example_chain in the example_table that allows TCP traffic on port 22:

   # nft insert rule inet example_table example_chain tcp dport 22 accept

   You can alternatively specify the name of the service instead of the port number. In the example, you could use ssh instead of the port number 22. Note that a service name is resolved to a port number based on its entry in the /etc/services file.

2. Optionally, display all chains and their rules in example_table:

   # nft list table inet example_table
   table inet example_table {
     chain example_chain {
       type filter hook input priority filter; policy accept;
       tcp dport ssh accept
       ...
     }
   }

Procedure

1. Create a table:

   # nft add table nat

2. Add the prerouting and postrouting chains to the table:

   # nft -- add chain nat prerouting { type nat hook prerouting priority -100 \; }
   # nft add chain nat postrouting { type nat hook postrouting priority 100 \; }

   Important

   Even if you do not add a rule to the prerouting chain, the nftables framework requires this chain to match incoming packet replies.

   Note that you must pass the -- option to the nft command to avoid that the shell interprets the negative priority value as an option of the nft command.

3. Add a rule to the postrouting chain that matches outgoing packets on the ens3 interface:

   # nft add rule nat postrouting oifname "ens3" masquerade

Procedure

1. Create a table:

   # nft add table nat

2. Add the prerouting and postrouting chains to the table:

   # nft -- add chain nat prerouting { type nat hook prerouting priority -100 \; }
   # nft add chain nat postrouting { type nat hook postrouting priority 100 \; }

   Important

   Even if you do not add a rule to the postrouting chain, the nftables framework requires this chain to match outgoing packet replies.

   Note that you must pass the -- option to the nft command to avoid that the shell interprets the negative priority value as an option of the nft command.

3. Add a rule to the postrouting chain that replaces the source IP of outgoing packets through ens3 with 192.0.2.1:

   # nft add rule nat postrouting oifname "ens3" snat to 192.0.2.1

Procedure

1. Create a table:

   # nft add table nat

2. Add the prerouting and postrouting chains to the table:

   # nft -- add chain nat prerouting { type nat hook prerouting priority -100 \; }
   # nft add chain nat postrouting { type nat hook postrouting priority 100 \; }

   Important

   Even if you do not add a rule to the postrouting chain, the nftables framework requires this chain to match outgoing packet replies.

   Note that you must pass the -- option to the nft command to avoid that the shell interprets the negative priority value as an option of the nft command.

3. Add a rule to the prerouting chain that redirects incoming traffic on the ens3 interface sent to port 80 and 443 to the host with the 192.0.2.1 IP:

   # nft add rule nat prerouting iifname ens3 tcp dport { 80, 443 } dnat to 192.0.2.1

4. Depending on your environment, add either a SNAT or masquerading rule to change the source address:

   1. If the ens3 interface used dynamic IP addresses, add a masquerading rule:

      # nft add rule nat postrouting oifname "ens3" masquerade

   2. If the ens3 interface uses a static IP address, add a SNAT rule. For example, if the ens3 uses the 198.51.100.1 IP address:

      nft add rule nat postrouting oifname "ens3" snat to 198.51.100.1

Procedure

1. For example, to add a rule to example_chain in example_table that allows incoming traffic to port 22, 80, and 443:

   # nft add rule inet example_table example_chain tcp dport { 22, 80, 443 } accept

2. Optionally, display all chains and their rules in example_table:

   # nft list table inet example_table
   table inet example_table {
     chain example_chain {
       type filter hook input priority filter; policy accept;
       tcp dport { ssh, http, https } accept
     }
   }

Procedure

1. Create an empty set. The following examples create a set for IPv4 addresses:

   • To create a set that can store multiple individual IPv4 addresses:

     # nft add set inet example_table example_set { type ipv4_addr \; }

   • To create a set that can store IPv4 address ranges:

     # nft add set inet example_table example_set { type ipv4_addr \; flags interval \; }

   Important

   To avoid that the shell interprets the semicolons as the end of the command, you must escape the semicolons with a backslash.

2. Optionally, create rules that use the set. For example, the following command adds a rule to the example_chain in the example_table that will drop all packets from IPv4 addresses in example_set.

   # nft add rule inet example_table example_chain ip saddr @example_set drop

   Because example_set is still empty, the rule has currently no effect.

3. Add IPv4 addresses to example_set:

   • If you create a set that stores individual IPv4 addresses, enter:

     # nft add element inet example_table example_set { 192.0.2.1, 192.0.2.2 }

   • If you create a set that stores IPv4 ranges, enter:

     # nft add element inet example_table example_set { 192.0.2.0-192.0.2.255 }

     When you specify an IP address range, you can alternatively use the Classless Inter-Domain Routing (CIDR) notation, such as 192.0.2.0/24 in the above example.

Procedure

1. Create the example_table:

   # nft add table inet example_table

2. Create the tcp_packets chain in example_table:

   # nft add chain inet example_table tcp_packets

3. Add a rule to tcp_packets that counts the traffic in this chain:

   # nft add rule inet example_table tcp_packets counter

4. Create the udp_packets chain in example_table

   # nft add chain inet example_table udp_packets

5. Add a rule to udp_packets that counts the traffic in this chain:

   # nft add rule inet example_table udp_packets counter

6. Create a chain for incoming traffic. For example, to create a chain named incoming_traffic in example_table that filters incoming traffic:

   # nft add chain inet example_table incoming_traffic { type filter hook input priority 0 \; }

7. Add a rule with a literal map to incoming_traffic:

   # nft add rule inet example_table incoming_traffic ip protocol vmap { tcp : jump tcp_packets, udp : jump udp_packets }

   The literal map distinguishes the packets and sends them to the different counter chains based on their protocol.

8. To list the traffic counters, display example_table:

   # nft list table inet example_table
   table inet example_table {
     chain tcp_packets {
       counter packets 36379 bytes 2103816
     }
   
     chain udp_packets {
       counter packets 10 bytes 1559
     }
   
     chain incoming_traffic {
       type filter hook input priority filter; policy accept;
       ip protocol vmap { tcp : jump tcp_packets, udp : jump udp_packets }
     }
   }

   The counters in the tcp_packets and udp_packets chain display both the number of received packets and bytes.

Procedure

1. Create a table. For example, to create a table named example_table that processes IPv4 packets:

   # nft add table ip example_table

2. Create a chain. For example, to create a chain named example_chain in example_table:

   # nft add chain ip example_table example_chain { type filter hook input priority 0 \; }

   Important

   To avoid that the shell interprets the semicolons as the end of the command, you must escape the semicolons with a backslash.

3. Create an empty map. For example, to create a map for IPv4 addresses:

   # nft add map ip example_table example_map { type ipv4_addr : verdict \; }

4. Create rules that use the map. For example, the following command adds a rule to example_chain in example_table that applies actions to IPv4 addresses which are both defined in example_map:

   # nft add rule example_table example_chain ip saddr vmap @example_map

5. Add IPv4 addresses and corresponding actions to example_map:

   # nft add element ip example_table example_map { 192.0.2.1 : accept, 192.0.2.2 : drop }

   This example defines the mappings of IPv4 addresses to actions. In combination with the rule created above, the firewall accepts packet from 192.0.2.1 and drops packets from 192.0.2.2.

6. Optionally, enhance the map by adding another IP address and action statement:

   # nft add element ip example_table example_map { 192.0.2.3 : accept }

7. Optionally, remove an entry from the map:

   # nft delete element ip example_table example_map { 192.0.2.1 }

8. Optionally, display the rule set:

   # nft list ruleset
   table ip example_table {
     map example_map {
       type ipv4_addr : verdict
       elements = { 192.0.2.2 : drop, 192.0.2.3 : accept }
     }
   
     chain example_chain {
       type filter hook input priority filter; policy accept;
       ip saddr vmap @example_map
     }
   }

Procedure

1. Create a table named nat with the ip address family:

   # nft add table ip nat

2. Add the prerouting and postrouting chains to the table:

   # nft -- add chain ip nat prerouting { type nat hook prerouting priority -100 \; }

   Note

   Pass the -- option to the nft command to avoid that the shell interprets the negative priority value as an option of the nft command.

3. Add a rule to the prerouting chain that redirects incoming packets on port 8022 to the local port 22:

   # nft add rule ip nat prerouting tcp dport 8022 redirect to :22

Procedure

1. Create a table named nat with the ip address family:

   # nft add table ip nat

2. Add the prerouting and postrouting chains to the table:

   # nft -- add chain ip nat prerouting { type nat hook prerouting priority -100 \; }
   # nft add chain ip nat postrouting { type nat hook postrouting priority 100 \; }

   Note

   Pass the -- option to the nft command to avoid that the shell interprets the negative priority value as an option of the nft command.

3. Add a rule to the prerouting chain that redirects incoming packets on port 443 to the same port on 192.0.2.1:

   # nft add rule ip nat prerouting tcp dport 443 dnat to 192.0.2.1

4. Add a rule to the postrouting chain to masquerade outgoing traffic:

   # nft add rule ip daddr 192.0.2.1 masquerade

5. Enable packet forwarding:

   # echo "net.ipv4.ip_forward=1" > /etc/sysctl.d/95-IPv4-forwarding.conf
   # sysctl -p /etc/sysctl.d/95-IPv4-forwarding.conf

Procedure

1. Add a rule that allows only two simultaneous connections to the SSH port (22) from an IPv4 address and rejects all further connections from the same IP:

   # nft add rule ip example_table example_chain tcp dport ssh meter example_meter { ip saddr ct count over 2 } counter reject

2. Optionally, display the meter created in the previous step:

   # nft list meter ip example_table example_meter
   table ip example_table {
     meter example_meter {
       type ipv4_addr
       size 65535
       elements = { 192.0.2.1 : ct count over 2 , 192.0.2.2 : ct count over 2  }
     }
   }

   The elements entry displays addresses that currently match the rule. In this example, elements lists IP addresses that have active connections to the SSH port. Note that the output does not display the number of active connections or if connections were rejected.

Procedure

1. Create the filter table with the ip address family:

   # nft add table ip filter

2. Add the input chain to the filter table:

   # nft add chain ip filter input { type filter hook input priority 0 \; }

3. Add a set named blacklist to the filter table:

   # nft add set ip filter blacklist { type ipv4_addr \; flags dynamic, timeout \; timeout 5m \; }

   This command creates a dynamic set for IPv4 addresses. The timeout 5m parameter defines that nftables automatically removes entries after 5 minutes from the set.

4. Add a rule that automatically adds the source IP address of hosts that attempt to establish more than ten new TCP connections within one minute to the blacklist set:

   # nft add rule ip filter input ip protocol tcp ct state new, untracked limit rate over 10/minute add @blacklist { ip saddr }

5. Add a rule that drops all connections from IP addresses in the blacklist set:

   # nft add rule ip filter input ip saddr @blacklist drop

Procedure

1. Add a new rule with the counter parameter to the chain. The following example adds a rule with a counter that allows TCP traffic on port 22 and counts the packets and traffic that match this rule:

   # nft add rule inet example_table example_chain tcp dport 22 counter accept

2. To display the counter values:

   # nft list ruleset
   table inet example_table {
     chain example_chain {
       type filter hook input priority filter; policy accept;
       tcp dport ssh counter packets 6872 bytes 105448565 accept
     }
   }

Procedure

1. Display the rules in the chain including their handles:

   # nft --handle list chain inet example_table example_chain
   table inet example_table {
     chain example_chain { # handle 1
       type filter hook input priority filter; policy accept;
       tcp dport ssh accept # handle 4
     }
   }

2. Add the counter by replacing the rule but with the counter parameter. The following example replaces the rule displayed in the previous step and adds a counter:

   # nft replace rule inet example_table example_chain handle 4 tcp dport 22 counter accept

3. To display the counter values:

   # nft list ruleset
   table inet example_table {
     chain example_chain {
       type filter hook input priority filter; policy accept;
       tcp dport ssh counter packets 6872 bytes 105448565 accept
     }
   }

Procedure

1. Display the rules in the chain including their handles:

   # nft --handle list chain inet example_table example_chain
   table inet example_table {
     chain example_chain { # handle 1
       type filter hook input priority filter; policy accept;
       tcp dport ssh accept # handle 4
     }
   }

2. Add the tracing feature by replacing the rule but with the meta nftrace set 1 parameters. The following example replaces the rule displayed in the previous step and enables tracing:

   # nft replace rule inet example_table example_chain handle 4 tcp dport 22 meta nftrace set 1 accept

3. Use the nft monitor command to display the tracing. The following example filters the output of the command to display only entries that contain inet example_table example_chain:

   # nft monitor | grep "inet example_table example_chain"
   trace id 3c5eb15e inet example_table example_chain packet: iif "enp1s0" ether saddr 52:54:00:17:ff:e4 ether daddr 52:54:00:72:2f:6e ip saddr 192.0.2.1 ip daddr 192.0.2.2 ip dscp cs0 ip ecn not-ect ip ttl 64 ip id 49710 ip protocol tcp ip length 60 tcp sport 56728 tcp dport ssh tcp flags == syn tcp window 64240
   trace id 3c5eb15e inet example_table example_chain rule tcp dport ssh nftrace set 1 accept (verdict accept)
   ...

   Warning

   Depending on the number of rules with tracing enabled and the amount of matching traffic, the nft monitor command can display a lot of output. Use grep or other utilities to filter the output.