libnftables JSON

libnftables JSON

A future firewalld feature release (0.8.0) will contain a significant change to the nftables backend. In the past firewalld has always called the nft binary. As of git commit 1582c5dd736a firewalld will instead use the libnftables library. This is the same library that nft uses internally. All the communication with the library is a through a thin python wrapper and uses structured JSON. This means firewalld never calls the nft binary.

Why it’s good

Outside of using structured JSON there are a couple other advantages:

  • faster rule application
    firewalld used to make a call to nft for every rule. With libnftables it’s now passing large blobs of JSON that can contain hundreds of rules. This reduces the numbers of round trips to the kernel and yields a significant performance improvement.
  • no fork()ing
    Each call to nft would be a matter of forking a new process and waiting on the result. With libnftables there is no more forking.
  • atomic transactions
    Internally firewalld builds a set of changes and attempts to apply them all at once. The old nft backend still had to call nft once for every rule so this transaction wasn’t atomic and there were small periods of time between the rules being applied. This time frame is small, but can cause real issues in some scenarios. With libnftables the rules are applied in an “all or nothing” manner. Either they all take effect at once or not at all.

What it looks like

From the user’s perspective nothing has changed. With the exception that firewalld now has a new dependency.

From a developer’s point of view rule generation looks quite different. Instead of building strings that are identical to the nft CLI firewalld needs to generate a python dictionary containing other dictionaries and lists. This will then be converted to JSON before being passed to libnftables.

For example the following is an example rule from before libnftables.

"add rule inet %s filter_%s ct state established,related accept" % (TABLE_NAME, "INPUT"))

And here is the equivalent python dictionary.

{"add": {"rule":  {"family": "inet",
                   "table": TABLE_NAME,
                   "chain": "filter_%s" % "INPUT",
                   "expr": [{"match": {"left": {"ct": {"key": "state"}},
                                       "op": "in",
                                       "right": {"set": ["established", "related"]}}},
                            {"accept": None}]}}}

It’s a definitely more verbose, but the nested dictionaries are easier to work with.

You can learn more about libnftables JSON by reading the man page libnftables-json(5). It covers the basics and the full syntax.

python-nftables

As briefly mentioned above, firewalld uses a thin python wrapper around libnftables. This is shipped with the nftables package and is being referred to as python-nftables by the firewalld developers.

Requirements

Short answer: nftables >= 0.9.3

Long answer:

With this change firewalld has new dependencies; libnftables, and python-nftables. Currently both libnftables and python-nftables are shipped with the standard nftables package. Distribution packagers shouldn’t have to change much. It really amounts to calling the nftables configure script with --with-python --with-json.

libnftables has been around for awhile, but due to bug fixes and other changes the firewalld developers are claiming the libnftables backend only works with nftables >= 0.9.3.