knftables: a golang nftables library

This is a library for using nftables from Go.

It is not intended to support arbitrary use cases, but instead specifically focuses on supporing Kubernetes components which are using nftables in the way that nftables is supposed to be used (as opposed to using nftables in a naively-translated-from-iptables way, or using nftables to do totally valid things that aren't the sorts of things Kubernetes components are likely to need to do).

It is still under development and is not API stable.

Usage

Create an Interface object to manage operations on a single nftables table:

nft, err := knftables.New(knftables.IPv4Family, "my-table")
if err != nil {
        return fmt.Errorf("no nftables support: %v", err)
}

You can use the List, ListRules, and ListElements methods on the Interface to check if objects exist. List returns the names of "chains", "sets", or "maps" in the table, while ListElements returns Element objects and ListRules returns partial Rule objects.

chains, err := nft.List(ctx, "chains")
if err != nil {
        return fmt.Errorf("could not list chains: %v", err)
}

FIXME

elements, err := nft.ListElements(ctx, "map", "mymap")
if err != nil {
        return fmt.Errorf("could not list map elements: %v", err)
}

FIXME

To make changes, create a Transaction, add the appropriate operations to the transaction, and then call nft.Run on it:

tx := nft.NewTransaction()

tx.Add(&knftables.Chain{
        Name:    "mychain",
        Comment: knftables.PtrTo("this is my chain"),
})
tx.Flush(&knftables.Chain{
        Name: "mychain",
})

var destIP net.IP
var destPort uint16
...
tx.Add(&knftables.Rule{
        Chain: "mychain",
        Rule: knftables.Concat(
                "ip daddr", destIP,
                "ip protocol", "tcp",
                "th port", destPort,
                "jump", destChain,
        )
})

err := nft.Run(context, tx)

If any operation in the transaction would fail, then Run() will return an error and the entire transaction will be ignored. You can use the knftables.IsNotFound() and knftables.IsAlreadyExists() methods to check for those well-known error types. In a large transaction, there is no supported way to determine exactly which operation failed.

`knftables.Transaction` operations

knftables.Transaction operations correspond to the top-level commands in the nft binary. Currently-supported operations are:

tx.Add(): adds an object, which may already exist, as with nft add
tx.Create(): creates an object, which must not already exist, as with nft create
tx.Flush(): flushes the contents of a table/chain/set/map, as with nft flush
tx.Delete(): deletes an object, as with nft delete
tx.Insert(): inserts a rule before another rule, as with nft insert rule
tx.Replace(): replaces a rule, as with nft replace rule

Objects

The Transaction methods take arguments of type knftables.Object. The currently-supported objects are:

Table
Chain
Rule
Set
Map
Element

Optional fields in objects can be filled in with the help of the PtrTo() function, which just returns a pointer to its argument.

Concat() can be used to concatenate a series of strings, []string arrays, and other arguments (including numbers, net.IPs / net.IPNets, and anything else that can be formatted usefully via fmt.Sprintf("%s")) together into a single string. This is often useful when constructing Rules.

`knftables.Fake`

There is a fake (in-memory) implementation of knftables.Interface for use in unit tests. Use knftables.NewFake() instead of knftables.New() to create it, and then it should work mostly the same. See fake.go for more details of the public APIs for examining the current state of the fake nftables database.

Note that at the present time, fake.Run() is not actually transactional, so unit tests that rely on things not being changed if a transaction fails partway through will not work as expected.

Missing APIs

Various top-level object types are not yet supported (notably the "stateful objects" like counter).

Most IPTables libraries have an API for "add this rule only if it doesn't already exist", but that does not seem as useful in nftables (or at least "in nftables as used by Kubernetes-ish components that aren't just blindly copying over old iptables APIs"), because chains tend to have static rules and dynamic sets/maps, rather than having dynamic rules. If you aren't sure if a chain has the correct rules, you can just Flush it and recreate all of the rules.

I've considered changing the semantics of tx.Add(obj) so that obj.Handle is filled in with the new object's handle on return from Run(), for ease of deleting later. (This would be implemented by using the --handle (-a) and --echo (-e) flags to nft add.) However, this would require potentially difficult parsing of the nft output. ListRules fills in the handles of the rules it returns, so it's possible to find out a rule's handle after the fact that way. For other supported object types, either handles don't exist (Element) or you don't really need to know their handles because it's possible to delete by name instead (Table, Chain, Set, Map).

The "destroy" (delete-without-ENOENT) command that exists in newer versions of nft is not currently supported because it would be unexpectedly heavyweight to emulate on systems that don't have it, so it is better (for now) to force callers to implement it by hand.

ListRules returns Rule objects without the Rule field filled in, because it uses the JSON API to list the rules, but there is no easy way to convert the JSON rule representation back into plaintext form. This means that it is only useful when either (a) you know the order of the rules in the chain, but want to know their handles, or (b) you can recognize the rules you are looking for by their comments, rather than the rule bodies.

Design Notes

The library works by invoking the nft binary. "Write" operations are implemented with the ordinary plain-text API, while "read" operations are implemented with the JSON API, for parseability.

The fact that the API uses functions and objects (e.g. tx.Add(&knftables.Chain{...})) rather than just specifying everything as textual input to nft (e.g. tx.Exec("add chain ...")) is mostly just because it's much easier to have a fake implementation for unit tests this way.

License

danwinship/knftables

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