Migrating from iptables to nftables

nftables has been enabled by default in latest Ubuntu and Debian, but not fully supported by Docker.

I've been hestitating about migrating from iptables to nftables, but managed to do it today.

Here are my thoughts.

Scripting nftables

The syntax of iptables and nftables are different, but not that different, both are more or less human readable. However, nftables is clearly more friendly for scripting.

I spent quite some time in a python script to generate a iptables rule set, and I was worried that I need lots of time migrating the script. Aftering studying the syntax of nftables, I realized that I could just write /etc/nftables.conf directly. 

In the conf file I can manage tables and chains in a structured way. I'm free to use indentations and new lines, and I no longer need to write "-I CHAIN" for every rule.

Besides, I can group similar rules (e.g. same rule for different tcp ports) easily, and I can define variables and reuse them. 

Eventually I was able to write a nice nftables rule set quickly with basic scripting syntax. It was not as powerful as my custom python script, but it is definitely easier to write. Further, I think it might be worth learning mapping in the future.

Tables & Chains in nftables

Unlike iptables, nftables is decentralized. Instead of pre-defined tables (e.g. filter) and chains (e.g. INPUT), nftables uses hooks and priorities. It sounds like event listeners in JavaScript.

One big difference is: a packet is dropped if it is dropped any matching rule, and a packet is accepted only if all relevant chains accept the packet. Again, this is similar to event listeners. On the other hand, in iptables, a packet is accepted if it is accepted by any rule. It sounds a bit confusing at the beginning, but I think nftables is more flexible, especially in my cases, see below.

Docker & nftables

Docker does not support nftables, but it add rules via iptables-nft. It was painful to managed iptables rules with Docker:

• Docker creates its own DOCKER and DOCKER-USER chains, which may accept some rules.
• If I need to control the traffic from/to containers, I need to make sure that the rules are defined before or in DOCKER-USER.
• Docker may or may not be started at boot. And Docker adds DOCKER to INPUT, so I need to make sure that my rules are in effect in all cases.

Well all the mess is because: in iptables, a packet is accepted if it is accepted by any rule. That means I must insert my REJECT rules before DOCKER/DOCKER-UESR, which might accept the packet.

This is no longer an issue in nftables! I can simply define my own tables and reject some packets as I like.

Finally, I don't need to touch the tables created by Docker via iptables-nft, instead I can create my own nft tables.

Conclusions

I had lots of worries about nftables, about scripting and working with Docker. As it turned out, none was actually an issue thanks to the new design of nftables!