Re: connection tracking state in rules

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Tue, Nov 12, 2024 at 06:56:30PM +0100, Thomas Köller wrote:
> Hi,
> 
> tried to create a rule using the conntrack state of a packet:
> 
> [root@sarkovy nftables]# cat ruleset-own.txt
> create table netdev world
> add set netdev world blacklist { type ipv4_addr; flags dynamic,timeout;
> timeout 1h; gc-interval 6h; size 256; }
> create chain netdev world ingress { type filter hook ingress device
> "eth_cable" priority filter; policy drop; }
> add rule netdev world ingress ip protocol tcp ip saddr @blacklist counter
> drop
> add rule netdev world ingress ct state established,related counter accept
> [root@sarkovy nftables]# nft -cf ruleset-own.txt
> ruleset-own.txt:5:31-38: Error: Could not process rule: Protocol error
> add rule netdev world ingress ct state established,related counter accept
>                               ^^^^^^^^
> This is rejected, but why?

Because there is currently no support for connection tracking at
ingress.

> And, btw., which one is correct, 'ct state established,related' or 'ct state
> { established,related }'?

Use of { } implies a set instance.

# nft describe ct state
ct expression, datatype ct_state (conntrack state) (basetype bitmask, integer), 32 bits

pre-defined symbolic constants (in hexadecimal):
        invalid                         0x00000001
        new                             0x00000008
        established                     0x00000002
        related                         0x00000004
        untracked                       0x00000040

Use of:

        ct state established,related

(just like in iptables --m --state) is perfectly fine for this
particular case.