# nftables

Related articles

- Firewalls
- iptables

nftables is a netfilter project that aims to replace the existing ip-, ip6-, arp-, and ebtables framework. It provides a new packet filtering framework, a new user-space utility (nft), and a compatibility layer for ip- and ip6tables. It uses the existing hooks, connection tracking system, user-space queueing component, and logging subsystem of netfilter.

The first release is available in Linux 3.13, which is in the core repository (linux), and nftables (the user-space components) is available in the community repository (nftables), and on the AUR in package nftables-git.

## Overview

nftables consists of three main components: a kernel implementation, the libnl netlink communication and the nftables user-space front-end. The kernel provides a netlink configuration interface, as well as run-time rule-set evaluation using a small classification language interpreter. libnl contains the low-level functions for communicating with the kernel; the nftables front-end is what the user interacts with.

## nft

nftables' user-space utility `nft` now performs most of the rule-set evaluation before handing rule-sets to the kernel. Because of this, nftables provides no default tables or chains; although, a user can emulate an iptables-like setup.

It works in a fashion similar to ifconfig or iproute2. The commands are a long, structured sequence rather than using argument switches like in iptables. For example:

    nft add rule ip6 filter input ip saddr ::1 accept

`add` is the command. `rule` is a subcommand of `add`. `ip6` is an argument of `rule`, telling it to use the ip6 family. `filter` and `input` are arguments of `rule` specifying the table and chain to use, respectively. The rest that follows is a rule definition, which includes matches (`ip`), their parameters (`saddr`), parameter arguments (`::1`), and jumps (`accept`).

The following is an incomplete list of the commands available in nft:

    list
      tables [family]
      table [family] <name>
      chain [family] <table> <name>

    add
      table [family] <name>
      chain [family] <table> <name> [chain definitions]
      rule [family] <table> <chain> <rule definition>

    table [family] <name> (shortcut for `add table`)

    insert
      rule [family] <table> <chain> <rule definition>

    delete
      table [family] <name>
      chain [family] <table> <name>
      rule [family] <table> <handle>

    flush
      table [family] <name>
      chain [family] <table> <name>

`family` is optional, but it will default to `ip`.

## Tables

The purpose of tables is to hold chains. Unlike tables in iptables, there are no built-in tables in nftables. Tables can have one of four families specified, which unifies the various iptables utilities into one:

- ip (iptables)
- ip6 (ip6tables)
- arp (arptables)
- bridge (ebtables)

`ip` is the default family. A fifth family is scheduled for Linux 3.15 that allows for the unification of the ip and ip6 families to make defining rules for both easier.

### Listing

You can list the current tables in a family with the `nft list` command.

    # nft list tables
    # nft list tables ip6

You can list a full table definition by specifying a table name:

    # nft list table foo
    # nft list table ip6 foo

### Creation

Tables can be added via two commands — one just being a shortcut for the other. Here is an example of how to add an ip table called foo and an ip6 table called foo:

    # nft add table foo
    # nft table ip6 foo

You can have two tables with the same name as long as they are in different families.

### Deletion

Tables can only be deleted if there are no chains in them.

    # nft delete table foo
    # nft delete table ip6 foo

## Chains

The purpose of chains is to hold rules. Unlike chains in iptables, there are no built-in chains in nftables. This means that if no chain uses any types or hooks in the netfilter framework, packets that would flow through those chains will not be touched by nftables, unlike iptables.

### Listing

The `nft list table foo` command will list all the chains in the foo table. You can also list rules from an individual chain.

    # nft list chain foo bar
    # nft list chain ip6 foo bar

These commands will list the `bar` chains in the ip and ip6 `foo` tables.

### Creation

Chains can be added when a table is created in a file definition or one at time via the `nft add chain` command.

    # nft add chain foo bar
    # nft add chain ip6 foo bar

These commands will add a chain called `bar` to the ip and ip6 `foo` tables.

#### Properties

Because nftables has no built-in chains, it allows chains to access certain features of the netfilter framework.

    # nft add chain filter input { type filter hook input priority 0; }

This command tells nftables to add a chain called `input` to the `filter` table and defines its type, hook, and priority. These properties essentially replace the built-in tables and chains in iptables.

##### Types

There are three types a chain can have and they correspond to the tables used in iptables:

- filter
- nat
- route (mangle)

##### Hooks

There are five hooks a chain can use and they correspond to the chains used in iptables:

- input
- output
- forward
- prerouting
- postrouting

##### Priorities

Note: Priorities do not currently appear to have any effect on which chain sees packets first.
Note: Since the priority seems to be an unsigned integer, negative priorities will be converted into very high priorities.

Priorities tell nftables which chains packets should pass through first. They are integers, and the higher the integer, the higher the priority.

### Deletion

Chains can only be deleted if there are no rules in them.

    # nft delete chain foo bar
    # nft delete chain ip6 foo bar

These commands delete the `bar` chains from the ip and ip6 `foo` tables.

## Rules

The purpose of rules is to identify packets (match) and carry out tasks (jump). Like in iptables, there are various matches and jumps available, though not all of them are feature-complete in nftables.

### Listing

You can list the current rules in a table with the `nft list` command, using the same method as listing a table. You can also list rules from an individual chain.

    # nft list chain foo bar
    # nft list chain ip6 foo bar

These commands will list the rules in the `bar` chains in the ip and ip6 `foo` tables.

### Creation

Rules can be added when a table is created in a file definition or one at time via the `nft add rule` command.

    # nft add rule foo bar ip saddr 127.0.0.1 accept
    # nft add rule ip6 foo bar ip saddr ::1 accept

These commands will add a rule to the `bar` chains in the ip and ip6 `foo` tables that matches an `ip` packet when its `saddr` (source address) is 127.0.0.1 (IPv4) or ::1 (IPv6) and accepts those packets.

#### Matches

There are various matches available in nftables and, for the most part, coincide with their iptables counterparts. The most noticeable difference is that there are no generic or implicit matches anymore. A generic match was one that was always available, such as `--protocol` or `--source`. Implicit matches were protocol-specific, such as `--sport` when a packet was determined to be TCP.

The following is an incomplete list of the matches available:

- meta (meta properties, e.g. interfaces)
- icmp (ICMP protocol)
- icmpv6 (ICMPv6 protocol)
- ip (IP protocol)
- ip6 (IPv6 protocol)
- tcp (TCP protocol)
- udp (UDP protocol)
- sctp (SCTP protocol)
- ct (connection tracking)

The following is an incomplete list of match arguments:

    meta:
      oif <output interface INDEX>
      iif <input interface INDEX>
      oifname <output interface NAME>
      iifname <input interface NAME>

      (oif and iif accept string arguments and are converted to interface indexes)
      (oifname and iifname are more dynamic, but slower because of string matching)

    icmp:
      type <icmp type>

    icmpv6:
      type <icmpv6 type>

    ip:
      protocol <protocol>
      daddr <destination address>
      saddr <source address>

    ip6:
      daddr <destination address>
      saddr <source address>

    tcp:
      dport <destination port>
      sport <source port>

    udp:
      dport <destination port>
      sport <source port>

    sctp:
      dport <destination port>
      sport <source port>

    ct:
      state <new | established | related | invalid>

#### Jumps

Jumps work the same as they do in iptables, except multiple jumps can now be used in one rule.

    # nft add rule filter input tcp dport 22 log accept

The following is an incomplete list of jumps:

- accept (accept a packet)
- reject (reject a packet)
- drop (drop a packet)
- snat (perform source NAT on a packet)
- dnat (perform destination NAT on a packet)
- log (log a packet)
- counter (keep a counter on a packet; counters are optional in nftables)
- return (stop traversing the chain)

### Insertion

Rules can be prepended to chains with the `nft insert rule` command.

    # nft insert rule filter input ct state established,related accept

### Deletion

Individual rules can only be deleted by their handles. The `nft --handle list` command must be used to determine rule handles. Note the `--handle` switch, which tells `nft` to list handles in its output.

The following determines the handle for a rule and then deletes it. The `--number` argument is useful for viewing some numeric output, like unresolved IP addresses.

    # nft --handle --numeric list chain filter input

    table ip filter {
         chain input {
              type filter hook input priority 0;
              ip saddr 127.0.0.1 accept # handle 10
         }
    }

    # nft delete rule filter input handle 10

All the chains in a table can be flushed with the `nft flush table` command. Individual chains can be flushed using either the `nft flush chain` or `nft delete rule` commands.

    # nft flush table foo
    # nft flush chain foo bar
    # nft delete rule ip6 foo bar

The first command flushes all of the chains in the ip `foo` table. The second flushes the `bar` chain in the ip `foo` table. The third deletes all of the rules in `bar` chain in the ip6 `foo` table.

## File Definitions

Warning: The `nft -f` command, despite what the netfilter wiki says, is NOT atomic. This means you will have a small window between deleting the old tables and when the new ruleset is loaded where all packets will be accepted.
Note: You must delete all conflicting tables before using the `nft -f` command.

File definitions can be used by the `nft -f` command, which acts like the `iptables-restore` command.

    /etc/nftables/filter.rules

    table ip filter {
         chain input {
              type filter hook input priority 0;
              ct state established,related accept
              ip saddr 127.0.0.1 accept
              tcp dport 22 log accept
              reject
         }
    }

## Getting Started

To get an iptables-like chain set up, you will first need to use the provided IPv4 filter file:

    # nft -f /etc/nftables/ipv4-filter

To list the resulting chain:

    # nft list table filter

Drop output to a destination:

    # nft add rule ip filter output ip daddr 1.2.3.4 drop

Drop packets destined for local port 80:

    # nft add rule ip filter input tcp dport 80 drop

Delete all rules in a chain:

    # nft delete rule filter output

## Samples

### Simple IP/IPv6 Firewall

    firewall.rules

    # A simple firewall

    table firewall {
      chain incoming {
        type filter hook input priority 0;

        # established/related connections
        ct state {established, related} accept

        # invalid connections
        ct state invalid drop

        # loopback interface
        iifname lo accept

        # icmp
        ip protocol icmp accept

        # open tcp ports: sshd (22), httpd (80)
        tcp dport {ssh, http} accept

        # everything else
        reject
      }
    }

    table ip6 firewall {
      chain incoming {
        type filter hook input priority 0;

        # established/related connections
        ct state {established, related} accept

        # invalid connections
        ct state invalid drop

        # loopback interface
        iifname lo accept

        # icmp
        ip6 nexthdr icmpv6 accept

        # open tcp ports: sshd (22), httpd (80)
        tcp dport {ssh, http} accept

        # everything else
        reject
      }
    }

### Limit rate and tcp flags IP/IPv6 Firewall

    firewall.2.rules

    table firewall {
        chain incoming {
            type filter hook input priority 0;

            # bad tcp -> avoid network scanning:
            tcp flags & (fin|syn) == (fin|syn)			drop
            tcp flags & (syn|rst) == (syn|rst)			drop
            tcp flags & (fin|syn|rst|psh|ack|urg) < (fin)		drop # == 0 would be better, not supported yet.
            tcp flags & (fin|syn|rst|psh|ack|urg) == (fin|psh|urg)	drop

            # no ping floods:
            ip protocol icmp limit rate 10/second accept
            ip protocol icmp drop

            ct state {established, related} accept
            ct state invalid drop

            iifname lo accept

    	# avoid brute force on ssh:
            tcp dport {ssh} limit rate 15/minute accept

            reject
        }
    }

    table ip6 firewall {
        chain incoming {
            type filter hook input priority 0;

            # bad tcp:
            tcp flags & (fin|syn) == (fin|syn)			drop
            tcp flags & (syn|rst) == (syn|rst)			drop
            tcp flags & (fin|syn|rst|psh|ack|urg) < (fin)		drop # == 0 would be better, not supported yet.
            tcp flags & (fin|syn|rst|psh|ack|urg) == (fin|psh|urg)	drop

            # no ping floods:
            ip6 nexthdr icmpv6 limit rate 10/second accept
            ip6 nexthdr icmpv6 drop

            ct state {established, related} accept
            ct state invalid drop

            # loopback interface
            iifname lo accept

    	# avoid brute force on ssh:
            tcp dport {ssh} limit rate 15/minute accept

            reject
        }
    }

### Priority-based Atomic Fix

If priorities ever actually take effect, this may be a workaround for `nft -f`'s lack of true atomicness (being able to replace all the current rules with new ones in one go):

    atomic.rules

    table atomic {
      chain incoming {
        type filter hook input priority 0;
        ct state new reject
      }
    }

    table ip6 atomic {
      chain incoming {
        type filter hook input priority 0;
        ct state new reject
      }
    }

Set the priority of other chains that hook input to higher than 0. This should block new connections while no other input chains are loaded.

### Rules Script with Atomic Fix

Because using `nft -f` to reload rulesets is time consuming, it's far easier to script it. This will include an atomic fix not based on priorities. It uses the two rules files from above.

    firewall.sh

    #!/bin/sh

    # Load atomic rules first
    nft -f atomic.rules

    # New incoming traffic should now be stopped

    # Get rid of both the ip and ip6 firewall tables

    nft flush table firewall 2>/dev/null
    nft delete chain firewall incoming 2>/dev/null
    nft delete table firewall 2>/dev/null

    nft flush table ip6 firewall 2>/dev/null
    nft delete chain ip6 firewall incoming 2>/dev/null
    nft delete table ip6 firewall 2>/dev/null

    # Reload the firewall rules
    nft -f firewall.rules

    # Get rid of both the ip and ip6 atomic tables

    nft flush table atomic 2>/dev/null
    nft delete chain atomic incoming 2>/dev/null
    nft delete table atomic 2>/dev/null

    # New incoming IP traffic should be working

    nft flush table ip6 atomic 2>/dev/null
    nft delete chain ip6 atomic incoming 2>/dev/null
    nft delete table ip6 atomic 2>/dev/null

    # New incoming IPv6 traffic should be working

This should take anywhere from 100ms to 400ms, which is clearly unacceptable, but the only apparent solution.

## Loading rules at boot

To automatically load rules on system boot, nftables-systemd-git can be used. Further install instructions can be found on the corresponding github page.

Note: While the name suggests that it works only for systemd, it can be adapted for other init systems as well.
Note: You may have to create `/etc/modules-load.d/nftables.conf` with all of the nftables related modules you require as entries for the systemd service to work correctly. You can get a list of modules using this command:

    lsmod | grep nf

Otheriwse, you could end up with the dreaded `Error: Could not process rule: No such file or directory` error.

## Logging to Syslog

For logging to Syslog, the `xt_LOG` module needs to be loaded.

## See also

- netfilter nftables wiki
- First release of nftables
- nftables quick howto
- The return of nftables

Category:

- Firewalls