Nftables rules for hardening Discourse installation

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Thanks @OrkoGrayskull !

Are there any updates on this now that /etc/docker/daemon.json can have firewall-backend set to nftables and I guess you are now running Debian Trixie or Bookworm — I’m experimenting with this…

I have found that in /etc/sysctl.conf it is needed to set net.ipv4.ip_forward=1.

The Docker with nftables documentation has:

Do not modify Docker’s tables directly as the modifications are likely to be lost, Docker expects to have full ownership of its tables.

So I’m hoping not to touch the Docker rules…

This is what I currently have, some rules I have added via Ansible (this server has Postfix forwarding SMTP to Discourse, that is why port 25 is needed):

table inet ansible_firewall {
        chain inbound_ipv4 {
                icmp type echo-request limit rate 5/second accept comment "Accept IPv4 rate limited ICMP pings"
        }

        chain inbound_ipv6 {
                icmpv6 type { nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert } accept comment "Accept IPv6 neighbour discovery"
                icmpv6 type echo-request limit rate 5/second accept comment "Accept IPv6 rate limited ICMP pings"
        }

        chain inbound {
                type filter hook input priority filter; policy drop;
                ct state vmap { invalid : drop, established : accept, related : accept } comment "Allow traffic from established and related packets, drop invalid"
                iifname "lo" accept comment "Allow loopback traffic"
                meta protocol vmap { ip : jump inbound_ipv4, ip6 : jump inbound_ipv6 } comment "Jump to chain according to layer 3 protocol using a verdict map"
                tcp dport 22 accept comment "Allow SSH on port TCP/22"
                tcp dport { 80, 443 } accept comment "Allow HTTP on port TCP/80 and HTTPS on port TCP/443"
        }

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

        chain output {
                type filter hook output priority filter; policy accept;
                oifname "lo" accept comment "Allow outgoing from loopback interface"
        }
}

And the rules from Docker / Discourse (note this server doesn’t have a IPv6 address):

table ip docker-bridges {
        map filter-forward-in-jumps {
                type ifname : verdict
                elements = { "docker0" : jump filter-forward-in__docker0 }
        }

        map filter-forward-out-jumps {
                type ifname : verdict
                elements = { "docker0" : jump filter-forward-out__docker0 }
        }

        map nat-postrouting-in-jumps {
                type ifname : verdict
                elements = { "docker0" : jump nat-postrouting-in__docker0 }
        }

        map nat-postrouting-out-jumps {
                type ifname : verdict
                elements = { "docker0" : jump nat-postrouting-out__docker0 }
        }

        chain filter-FORWARD {
                type filter hook forward priority filter; policy accept;
                oifname vmap @filter-forward-in-jumps
                iifname vmap @filter-forward-out-jumps
        }

        chain nat-OUTPUT {
                type nat hook output priority -100; policy accept;
                ip daddr != 127.0.0.0/8 fib daddr type local counter jump nat-prerouting-and-output
        }

        chain nat-POSTROUTING {
                type nat hook postrouting priority srcnat; policy accept;
                iifname vmap @nat-postrouting-out-jumps
                oifname vmap @nat-postrouting-in-jumps
        }

        chain nat-PREROUTING {
                type nat hook prerouting priority dstnat; policy accept;
                fib daddr type local counter jump nat-prerouting-and-output
        }

        chain nat-prerouting-and-output {
                iifname != "docker0" tcp dport 80 counter dnat to 172.17.0.2:80 comment "DNAT"
                iifname != "docker0" tcp dport 443 counter dnat to 172.17.0.2:443 comment "DNAT"
        }

        chain raw-PREROUTING {
                type filter hook prerouting priority raw; policy accept;
                ip daddr 172.17.0.2 iifname != "docker0" counter drop comment "DROP DIRECT ACCESS"
        }

        chain filter-forward-in__docker0 {
                ct state established,related counter accept
                iifname "docker0" counter accept comment "ICC"
                ip daddr 172.17.0.2 tcp dport 80 counter accept
                ip daddr 172.17.0.2 tcp dport 443 counter accept
                counter drop comment "UNPUBLISHED PORT DROP"
        }

        chain filter-forward-out__docker0 {
                ct state established,related counter accept
                counter accept comment "OUTGOING"
        }

        chain nat-postrouting-in__docker0 {
        }

        chain nat-postrouting-out__docker0 {
                oifname != "docker0" ip saddr 172.17.0.0/16 counter masquerade comment "MASQUERADE"
        }
}
table ip6 docker-bridges {
        map filter-forward-in-jumps {
                type ifname : verdict
        }

        map filter-forward-out-jumps {
                type ifname : verdict
        }

        map nat-postrouting-in-jumps {
                type ifname : verdict
        }

        map nat-postrouting-out-jumps {
                type ifname : verdict
        }

        chain filter-FORWARD {
                type filter hook forward priority filter; policy accept;
                oifname vmap @filter-forward-in-jumps
                iifname vmap @filter-forward-out-jumps
        }

        chain nat-OUTPUT {
                type nat hook output priority -100; policy accept;
                ip6 daddr != ::1 fib daddr type local counter jump nat-prerouting-and-output
        }

        chain nat-POSTROUTING {
                type nat hook postrouting priority srcnat; policy accept;
                iifname vmap @nat-postrouting-out-jumps
                oifname vmap @nat-postrouting-in-jumps
        }

        chain nat-PREROUTING {
                type nat hook prerouting priority dstnat; policy accept;
                fib daddr type local counter jump nat-prerouting-and-output
        }

        chain nat-prerouting-and-output {
        }

        chain raw-PREROUTING {
                type filter hook prerouting priority raw; policy accept;
        }
}

And everything apart from ports 80 and 443 seems fine… :roll_eyes: