#!/usr/bin/nft -f

flush ruleset

table inet filter {
    chain prerouting-raw {
        type filter hook prerouting priority raw

        # drop packets without a reverse path (strict reverse path filtering)
        fib saddr . iif oif missing counter drop

        iif lo notrack accept

        # drop packets to address not configured on incoming interface (strong host model)
        fib daddr . iif type != { local, broadcast, multicast } counter drop

        tcp dport { 22, 80, 443 } notrack accept
        meta l4proto { icmp, ipv6-icmp } notrack accept
    }

    chain output-raw {
        type filter hook output priority raw

        oif lo notrack accept
        tcp sport { 22, 80, 443 } notrack accept
        meta l4proto { icmp, ipv6-icmp } notrack accept
    }

    chain input {
        type filter hook input priority filter
        policy drop

        iif lo accept
        tcp dport { 22, 80, 443 } accept
        meta l4proto { icmp, ipv6-icmp } accept

        ct state vmap { invalid : drop, established : accept, related : accept }

        meta l4proto udp reject
        meta l4proto tcp reject with tcp reset
        reject
    }

    chain forward {
        type filter hook forward priority filter
        policy drop
    }

    chain output {
        type filter hook output priority filter

        oif lo goto output-internal
        skuid != { root, systemd-network, unbound, chrony, http } counter goto graceful-reject
    }

    chain output-internal {
        skuid unbound meta l4proto { tcp, udp } th sport 53 th dport >= 1024 accept
        skuid chrony meta l4proto { tcp, udp } th sport >= 1024 th dport 53 accept

        skuid != root counter goto graceful-reject
        accept
    }

    chain graceful-reject {
        meta l4proto udp reject
        meta l4proto tcp reject with tcp reset
        reject
    }
}