Matching connection tracking stateful metainformation

As with iptables, nftables can match connection state tracking information (often referred to as conntrack or ct information) maintained by netfilter's Connection Tracking System to deploy stateful firewalls.

nftables provides the ct selector which can be used to match:

• State information: new, established, related and invalid. In this regard, there is no changes with iptables.
• The conntrack mark.
• Status information: expected, seen-reply, assured, confirmed, snat, dnat, dying.

Matching the conntrack state

The following example shows how to deploy an extremely simple stateful firewall with nftables:

nft add rule filter input ct state established,related counter accept #1
nft add rule filter input counter drop #2

Rule #1 accepts packets that are part of an already established communication with the network. Rule #2 drops all other packets. Thus, any attempt from a computer in the network to initiate a new connection to your computer will be blocked. However, traffic that is part of a flow that you have started will be accepted. Note that the example above uses a comma-separated list of the conntrack states that you want to match.

Matching the conntrack mark

The following example shows how to match packets based on the conntrack mark:

nft add rule filter input ct mark 123 counter

To know more about conntrack marks and packet marks, see Setting packet metainformation.

Matching the conntrack helper

The following example shows how to match packets based on the conntrack helper:

nft add rule filter input ct helper "ftp" counter

More on using ct helpers.