iptables: difference between NEW, ESTABLISHED and RELATED packets

Consider a NEW packet a telephone call before the receiver has picked up. An ESTABLISHED packet is their, "Hello." And a RELATED packet would be if you were calling to tell them about an e-mail you were about to send them. (The e-mail being RELATED.)

In case my analogy isn't so great, I personlly think the man pages handles it well:

NEW -- meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets in both directions, and

ESTABLISHED -- meaning that the packet is associated with a connection which has seen packets in both directions,

RELATED -- meaning that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error.

iptables(8) - Linux man page

Asumming for both server and client a restrictive INPUT and open OUTPUT, i.e.:

iptables -P INPUT DROP
iptables -P OUTPUT ACCEPT

And from iptables-extensions(8) over the example of FTP in active mode:

1. NEW

NEW The packet has started a new connection or otherwise associated with a connection which has not seen packets in both directions.

The client on port 50000 (any random unprivileged port) connects to FTP server on port 21, the server would need at least this to accept this incoming connection:

iptables -A INPUT --dport 21 -m state --state NEW -j ACCEPT

2. ESTABLISHED

ESTABLISHED The packet is associated with a connection which has seen packets in both directions.

Now on the client side, he opened an outgoing connection to server on port 21 using a local port 50000 and he needs the following iptables to allow the response to arrive from server (21) to client (50000):

sudo iptables -A INPUT -m state --state ESTABLISHED -j ACCEPT

3. RELATED

RELATED The packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer or an ICMP error.

Now after the FTP connection has been established and a data connection is about to be performed, client will open a server socket (yes, with active FTP client becomes a server for the data connection) on port 60000 (to my understanding client will mark this port 60000 as RELATED to the other connection from 50000->21) and will send this port number to server using the FTP PORT command. Then the FTP server will open a new connection from its port 20 to port 60000 on the client, and well, client now requires the following to allow this new connection to succeed:

sudo iptables -A INPUT -m state --state RELATED -j ACCEPT

Finally, for this to work you need to enable ip_conntrack_ftp kernel module to allow the system to mark connections/packages as RELATED (this is my understanding, I haven't digged on this too much):