Iptables has become a synonym of a firewall in Linux world. Although there is a
nftables which is supposed to replace iptables, iptables will exist for
decades more because of its popularity and ubiquity.
With the growing widespread use of BPF technology and its benefits there is a
temptation to apply the technology for the firewalling purposes.
Despite its advantages iptables is also known for its dark side - performance
and security related issues. What if it's possible to keep the iptables' ABI
and replace its implementation with something more performant and secure by
Such an approach will keep the existing solutions to work and remove an
overhead of switching to a new technology.
There was a RFC patchset back in 2018 which proposed a BPF based firewall -
bpfilter. From a bird's eye view bpfilter is a compiler implemented as a user
mode helper kernel module. bpfilter analyses an iptables' ruleset and
synthesizes an equivalent BPF program. When bpfilter kernel module is loaded
it starts a userspace process that has an IPC with its kernelspace part. Most
of the bpfilter functionaility is implemented in the userspace process what
significantly simplifies its development and improves security. The kernel part
hooks into the kernel iptables ABI and transparently for the userspace consumer
passes control to the userspace process. bpfilter userspace process "compiles"
iptables' ruleset into a BPF program and passes control back to the kernel.
This approach allows to transparently replace iptables' implementation without
breaking its consumers and gain all the benefits of BPF ecosystem.
While the initial patchset was abandoned in 2021 there was an attempt to
ressurect the patchset. Two versions of the updated patchset were submitted to
the bpf@ mailing list and the third iteration is in the process of preparation.
Currently bpfilter is able to process basic rules in INPUT and OUTPUT chains
and translate them into equivalent XDP and TC programs. bpfilter has an easy
way to add new matches and extensions in terms of iptables.
The idea to treat a firewall as a compiler is seductive - as such an approach
provides more opportunities for performance optimisations due to a more precise
context. Combining it with the existing BPF performance and security features
and putting on top of it its userspace nature - this might sound as the next
firewall for Linux.
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