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

#!/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
        tcp dport {22, 25, 80, 443, 465, 993} 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, 25, 80, 443, 465, 993} 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, 25, 80, 443, 465, 993} ip daddr {{ipv4_address}} accept
        tcp dport {22, 25, 80, 443, 465, 993} ip6 daddr {{ipv6_address}} 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, postfix, dovecot, dovenull, http} counter goto output-reject
    }

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

        skuid != root counter goto output-reject
        accept
    }

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