nftables

Nftables is a stateful Linux firewall which uses the netfilter kernel hooks. It is used for stateless, stateful packet filtering and all sorts of NAT.

Nftables is the successor to iptables.

In nftables, rules are organized with chains and tables.

chain: Orders rules. Chains exist in two kinds:
- base chain: Entry point from netfilter hooks (network stack).
- regular chain: Can be used as jump target to group rules for better organization.
table: Groups chains together. Tables are defined by a name and an address family (eg inet, ip, ip6, ..).

Ruleset

nft list ruleset        # List all tables/chains/rules (whole ruleset).
nft flush ruleset       # Clear whole ruleset.

Examples: Save rules to files and re-apply

nft list ruleset > nft.rules
nft flush ruleset
nft -f nft.rules

Example: Fully trace evaluation of nftables rules

table ip traceall {
    chain filter_prerouting {
        # Install chain with higher priority as the RAW standard priority.
        type filter hook prerouting priority raw - 50; policy accept;
        # Trace each and every packet (very verbose).
        #meta nftrace set 1;
        # Trace packet to port 80/81/8081 from localhost.
        tcp dport { 80, 81, 8081 } ip saddr 127.0.0.1 meta nftrace set 1;
    }
}

Use nft monitor trace to get trace output on tty.

Example: IPv4 port forwarding

table ip fwd {
    chain nat_preroute {
        # Register this chain to the PREROUTE:NAT hook (stateful packet tracking via conntrack).
        type nat hook prerouting priority dstnat + 10 ; policy accept;
        meta nfproto ipv4 tcp dport 81 redirect to :8081
    }
}

Example: Base vs regular chain

# Table named 'playground' handling 'ip' (ipv4) address family.
table ip playground {
    # Base chain.
    chain filter_input_base {
        # Register this chain to the INPUT:FILTER hook in the netfilter package flow.
        # Specify a prioirty relative to the inbuilt 'filter' priority (smaller
        # number means higher priority).
        # Set the default policy to ACCEPT, to let every packet pass by default.
        type filter hook input priority filter - 10; policy accept;

        # Create a rule for tcp packets arriving on either port 8000 or 8100.
        tcp dport { 8000, 8100 } jump input_reg_log;
        # Create a rule for tcp packets arriving on port 8200.
        tcp dport 8200 jump input_reg_log_all;
    }

    # Regular chain.
    chain input_reg_log {
        # Log every packet traversing this chain.
        # Message lands in the kernel ring buffer.
        log;
    }

    # Regular chain.
    chain input_reg_log_all {
        # Log every packet with all flags traversing this chain.
        log flags all;
    }
}

# Load the nf rules.
sudo nft -f playground.rules
# Create test servers.
nc -lk 0.0.0.0 8000
nc -lk 0.0.0.0 8100
nc -lk 0.0.0.0 8200
# See the nftables logging in the kernel ring buffer.
sudo dmesg -w
# Make some client connections.
nc localhost 8000
nc localhost 8200