6.8. Debugging nftables rules
6.8. Debugging nftables rules The nftables framework provides different options for administrators to debug rules and if packets match them. This section describes these options. 6.8.1. Creating a rule with a counter To identify if a rule is matched, you can use a counter. This section describes how to create a new rule with a counter. For a procedure that adds a counter to an existing rule, see Section 6.8.2, "Adding a counter to an existing rule" . Prerequisites The chain to which you want to add the rule exists. Procedure 6.21. Creating a rule with a counter 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: To display the counter values: 6.8.2. Adding a counter to an existing rule To identify if a rule is matched, you can use a counter. This section describes how to add a counter to an existing rule. For a procedure to add a new rule with a counter, see Section 6.8.1, "Creating a rule with a counter" . Prerequisites The rule to which you want to add the counter exists. Procedure 6.22. Adding a counter to an existing rule Display the rules in the chain including their handles: Add the counter by replacing the rule but with the counter parameter. The following example replaces the rule displayed in the step and adds a counter: To display the counter values: 6.8.3. Monitoring packets that match an existing rule The tracing feature in nftables in combination with the nft monitor command enables administrators to display packets that match a rule. The procedure describes how to enable tracing for a rule as well as monitoring packets that match this rule. Prerequisites The rule to which you want to add the counter exists. Procedure 6.23. Monitoring packets that match an existing rule Display the rules in the chain including their handles: 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 step and enables tracing: 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 : 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.
nft add rule inet example_table example_chain tcp dport 22 counter accept 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 } } 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 } } nft replace rule inet example_table example_chain handle 4 tcp dport 22 counter accept 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 } } 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 } } nft replace rule inet example_table example_chain handle 4 tcp dport 22 meta nftrace set 1 accept 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)