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

It is useful to refer to the conntrack data types.

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
...

conntrack command output
Column #	Description	Example value	ct Match	Notes
1	L3 protocol	ipv4	ct l3proto
2		2
3	L4 protocol	tcp	ct protocol
4	inet_proto	6	ct protocol	As shown in in.h protocol value 6 indicates TCP.
5	timeout, s	421957	ct expiration	Default TCP connection timeout is 5 days. You can change this via the conntrack sysfs setting nf_conntrack_tcp_timeout_established.
6	conntrack state	ESTABLISHED	ct state
7	L3 source address, original direction	src=192.168.0.2	ct original saddr
8	L3 destination address, original direction	dst=192.168.0.8	ct original daddr
9	L4 protocol source, original direction	sport=34621	ct original proto‑src
10	L4 protocol destination, original direction	dport=22	ct original proto‑dst
11	L3 source address, reply direction	src=192.168.0.8	ct reply saddr
12	L3 destination address, reply direction	dst=192.168.0.2	ct reply daddr
13	L4 protocol source, reply direction	sport=22	ct reply proto‑src
14	L4 protocol destination, reply direction	dport=34621	ct reply proto‑dst
15	conntrack status	[ASSURED]	ct status
16	conntrack mark	mark=6	ct mark
17	reference count	use=1		Mainly used by the garbage collector.
18	conntrack id	id=2014938051	ct id

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 ruleset shows how to deploy an extremely simple stateful firewall with nftables:

table inet stateful_fw_demo {

    chain IN {
        type filter hook input priority filter; policy drop;

        ct state established,related accept
}

The rule in the IN chain accepts packets that are part of an established connection, and related packets. Note the use of a comma-separated list of the conntrack states that you want to match. The default chain policy drops all other incoming 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.

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.