Firewall

Different ways to handle iptables and nftables.

Using iptables-persistent on Debian

Note: In use on vrijdagmiddagborrel.

It's basically a set of iptables plugins for netfilter-persistent, which itself is a loader for different netfilter configuration. Once installed, it will take care of restoring rules at boot time, and through a small helper, can be used to reload/update/save rules on the fly.

Installation and config

• Installation:

apt install iptables-persistent netfilter-persistent

• Add/change iptables rules located at /etc/iptables/rules.v4 and /etc/iptables/rules.v6

Usage

• Apply new rules after changes made to rules.v* files and check result

netfilter-persistent reload
iptables -L

iptables oneliners

note - for newer setups see nftables below. iptables commands still work but directly using nftables is preferred.

• list all rules from all chains

iptables -L

• block an IP

iptables -I INPUT -s 192.168.111.111 -j DROP
iptables -I OUTPUT -d 192.168.111.111 -j DROP

nftables

nftables is the hip new thing in the kernel. It has nicer, easier to read config syntax and has a bunch of performance improvements. Current Debian (12) comes with it installed (but turned off).

• enable the firewall

systemctl enable nftables

• a basic firewall config you can drop into /etc/nftables.conf

#!/usr/sbin/nft -f

flush ruleset

table firewall {
  chain incoming {
    type filter hook input priority 0; policy drop;

    # established/related connections
    ct state established,related accept

    # loopback interface
    iifname lo accept

    # open vpn traffic
    iifname lurknet accept

    # icmp
    icmp type echo-request accept

    # open tcp ports: sshd (22), httpd (80)
    tcp dport { ssh, http, https, 655, 999 } accept

    # tinc
    udp dport { 655, 60000-61000 } accept
  }
}

table ip6 firewall {
  chain incoming {
    type filter hook input priority 0; policy drop;

    # established/related connections
    ct state established,related accept

    # invalid connections
    ct state invalid drop

    # loopback interface
    iifname lo accept

    # vpn interface
    iifname lurknet accept

    # icmp
    # routers may also want: mld-listener-query, nd-router-solicit
    icmpv6 type { echo-request, nd-neighbor-solicit } accept
    meta l4proto ipv6-icmp accept

    # open tcp ports: sshd (22), httpd (80)
    tcp dport { ssh, http, https, 655, 999 } accept

    # tinc
    udp dport { 655, 60000-61000 } accept
  }
}

• start the firewall

systemctl start nftables

• see how it looks (assuming you have not just accidentally locked yourself out of the server)

nft list ruleset

get rich off your NFT!