graphene-os-server-infrastructure/nftables-discuss.conf

#!/usr/bin/nft -f

flush ruleset

table inet filter {
    define ip-allowlist-ssh = {
        127.0.0.1,
    }

    define ip6-allowlist-ssh = {
        ::1,
    }

    set ip-connlimit-ssh {
        type ipv4_addr
        flags dynamic
    }

    set ip6-connlimit-ssh {
        type ipv6_addr
        flags dynamic
    }

    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

        # IPv6 interacts badly with IP-based spam filtering
        meta nfproto ipv6 tcp dport { 80, 443 } reject with tcp reset

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

    chain output-raw {
        type filter hook output priority raw

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

    chain input {
        type filter hook input priority filter
        policy drop

        iif lo goto input-loopback
        tcp dport { 80, 443 } accept
        meta l4proto { icmp, ipv6-icmp } accept
        ct state vmap { established : accept, related : accept, new : goto graceful-reject }

        tcp dport 22 ip saddr @ip-connlimit-ssh reject with tcp reset
        tcp dport 22 ip6 saddr and ffff:ffff:ffff:ffff:ffff:: @ip6-connlimit-ssh reject with tcp reset
        tcp dport 22 synproxy mss 1460 wscale 7 timestamp sack-perm
    }

    chain input-loopback {
        tcp dport 22 tcp flags syn ip saddr != $ip-allowlist-ssh add @ip-connlimit-ssh { ip saddr ct count over 1 } reject with tcp reset
        tcp dport 22 tcp flags syn ip6 saddr != $ip6-allowlist-ssh add @ip6-connlimit-ssh { ip6 saddr and ffff:ffff:ffff:ffff:ffff:: ct count over 1 } reject with tcp reset
        accept
    }

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

    chain output {
        type filter hook output priority filter

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

    chain output-loopback {
        skuid unbound meta l4proto { tcp, udp } th sport 53 th dport >= 1024 accept
        skuid { chrony, http, flarum, flarum-admin, geoipupdate } 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
    }
}