Matching connection tracking stateful metainformation

nftables conntrack (ct) expressions enable stateful firewalls by matching packets that correspond to connections tracked by netfilter's Connection Tracking System.

Conntrack expressions

The following sections will make use of this sample partial conntrack output:

% conntrack -L -o id,extended
...
ipv4     2 tcp      6 421957 ESTABLISHED src=192.168.0.2 dst=192.168.0.8 sport=34621 dport=22 src=192.168.0.8 dst=192.168.0.2 sport=22 dport=34621 [ASSURED] mark=6 use=1 id=2014938051
...

In addition, it use useful to refer to the conntrack data types.

ct state - conntrack state

The ct state expression is almost certainly the one you will use the most.

The conntrack state may be one of:

conntrack states
State	Description
new	Netfilter has so far seen packets between this pair of hosts in only one direction. At least one of these packets is part of a valid initialization sequence, e.g. SYN packet for a TCP connection.
established	Netfilter has seen valid packets travel in both directions between this pair of hosts. For TCP connections, the three-way-handshake has been successfully completed.
related	This connection was initiated after the main connection, as expected from normal operation of the main connection. A common example is an FTP data channel established at the behest of an FTP control channel.
invalid	Assigned to packets that do not follow the expected behavior of a connection.
untracked	Dummy state assigned to packets that have been explicitly excluded from conntrack. See notrack.

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.

ct status - conntrack status

The conntrack status may be one of:

• expected
• seen-reply
• assured
• confirmed
• snat
• dnat
• dying

ct mark - 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.

ct helper - 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.