nftables

From ArchWiki
Jump to: navigation, search

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

It 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, libnl contains the low-level functions for communicating with the kernel, and the nftables front-end is what the user interacts with via nft.

You can also visit the official nftables wiki page for more information.

Installation

Install the userspace utilities package nftables or the git version nftables-gitAUR.

Tango-view-fullscreen.pngThis article or section needs expansion.Tango-view-fullscreen.png

Reason: Can an example package be named which works? (also see release info and keep in mind Arch package uses legacy so far.) (Discuss in Talk:Nftables#)

Some iptables#front-ends support nftables.

Usage

nftables makes a distinction between temporary rules made in the commandline and permanent ones loaded from or saved to a file. The default file is /etc/nftables.conf which already contains a simple ipv4/ipv6 firewall table named "inet filter".

To use it start/enable the nftables.service.

You can check the ruleset with

# nft list ruleset
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'
Otherwise, you could end up with the dreaded Error: Could not process rule: No such file or directory error.

Configuration

nftables' user-space utility nft performs most of the rule-set evaluation before handing rule-sets to the kernel. Rules are stored in chains, which in turn are stored in tables. The following sections indicate how to create and modify these constructs.

All changes below are temporary. To make changes permanent, save your ruleset to /etc/nftables.conf which is loaded by nftables.service:

# nft list ruleset > /etc/nftables.conf
Note: nft list does not output variable definitions, if you have any in /etc/nftables.conf they will be lost. Any variables used in rules will be replaced by that variable value.

To read input from a file use the -f flag:

# nft -f filename

Note that any rules already loaded are not automatically flushed.

See nft(8) for a complete list of all commands.

Tables

Tables hold #Chains. Unlike tables in iptables, there are no built-in tables in nftables. The number of tables and their names is up to the user. However, each table only has one address family and only applies to packets of this family. Tables can have one of five families specified:

nftables family iptables utility
ip iptables
ip6 ip6tables
inet iptables and ip6tables
arp arptables
bridge ebtables

ip (i.e. IPv4) is the default family and will be used if family is not specified.

To create one rule that applies to both IPv4 and IPv6, use inet. inet allows for the unification of the ip and ip6 families to make defining rules for both easier.

Note: inet does not work for nat-type chains, only for filter-type chains. (source)

See the section ADDRESS FAMILIES in nft(8) for a complete description of address families.

In all of the following, family is optional, and if not specified is set to ip.

Create table

The following adds a new table:

# nft add table family table

List tables

To list all tables:

# nft list tables

List chains and rules in a table

To list all chains and rules of a specified table do:

# nft list table family table

For example, to list all the rules of the filter table of the inet family:

# nft list table inet filter

Delete table

To delete a table do:

# nft delete table family table

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

Flush table

To flush all rules from a table do:

# nft flush table family table

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.

Chains have two types. A base chain is an entry point for packets from the networking stack, where a hook value is specified. A regular chain may be used as a jump target for better organization.

In all of the following family is optional, and if not specified is set to ip.

Create chain

Regular chain

The following adds a regular chain named chain to the table named table:

# nft add chain family table chain

For example, to add a regular chain named tcpchain to the filter table of the inet address family do:

# nft add chain inet filter tcpchain
Base chain

To add a base chain specify hook and priority values:

# nft add chain family table chain { type type hook hook priority priority \; }

type can be filter, route, or nat.

For IPv4/IPv6/Inet address families hook can be prerouting, input, forward, output, or postrouting. See nft(8) for a list of hooks for other families.

priority takes an integer value. Chains with lower numbers are processed first and can be negative. [1]

For example, to add a base chain that filters input packets:

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

Replace add with create in any of the above to add a new chain but return an error if the chain already exists.

List rules

The following lists all rules of a chain:

# nft list chain family table chain

For example, the following lists the rules of the chain named output in the inet table named filter:

# nft list chain inet filter output

Edit a chain

To edit a chain, simply call it by its name and define the rules you want to change.

# nft chain <family> <table> <chain> { [ type <type> hook <hook> device <device> priority <priority> \; policy <policy> \; ] }

If for example, you just want to change the input chain policy of the default table from "accept" to "drop"

# nft chain inet filter input { policy drop \; }

Delete a chain

To delete a chain do:

# nft delete chain family table chain

The chain must not contain any rules or be a jump target.

Flush rules from a chain

To flush rules from a chain do:

# nft flush chain family table chain

Rules

Rules are either constructed from expressions or statements and are contained within chains.

Add rule

Tip: The iptables-translate utility translates iptables rules to nftables format.

To add a rule to a chain do:

# nft add rule family table chain handle handle statement

The rule is appended at handle, which is optional. If not specified, the rule is appended to the end of the chain.

To prepend the rule to the position do:

# nft insert rule family table chain handle handle statement

If handle is not specified, the rule is prepended to the chain.

Expressions

Typically a statement includes some expression to be matched and then a verdict statement. Verdict statements include accept, drop, queue, continue, return, jump chain, and goto chain. Other statements than verdict statements are possible. See nft(8) for more information.

There are various expressions 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. 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 (for a more complete list, see nft(8)):

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>
Note: nft does not use /etc/services to match port numbers with names, instead it uses an internal list. To show the port mappings from the command line, use e.g. nft describe tcp dport.

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 fltrTable {
     chain input {
          type filter hook input priority 0;
          ip saddr 127.0.0.1 accept # handle 10
     }
}
# nft delete rule fltrTable 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.

Atomic reloading

Flush the current ruleset:

# echo "flush ruleset" > /tmp/nftables 

Dump the current ruleset:

# nft list ruleset >> /tmp/nftables

Now you can edit /tmp/nftables and apply your changes with:

# nft -f /tmp/nftables

Examples

Workstation

/etc/nftables.conf
flush ruleset

table inet filter {
        chain input {
                type filter hook input priority 0;

                # accept any localhost traffic
                iif lo accept

                # accept traffic originated from us
                ct state established,related accept

		# accept ICMP & IGMP
		ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, parameter-problem, mld-listener-query, mld-listener-report, mld-listener-reduction, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert, mld2-listener-report } accept
		ip protocol icmp icmp type { destination-unreachable, router-solicitation, router-advertisement, time-exceeded, parameter-problem } accept
		ip protocol igmp accept

                # activate the following line to accept common local services
                #tcp dport { 22, 80, 443 } ct state new accept

                # count and drop any other traffic
                counter drop
        }
}

Simple IPv4/IPv6 firewall

firewall.rules
# A simple firewall

flush ruleset

table inet filter {
	chain input {
		type filter hook input priority 0; policy drop;

		# established/related connections
		ct state established,related accept

		# invalid connections
		ct state invalid drop
		
		# loopback interface
		iif lo accept

		# ICMP & IGMP
		ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, parameter-problem, mld-listener-query, mld-listener-report, mld-listener-reduction, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert, mld2-listener-report } accept
		ip protocol icmp icmp type { destination-unreachable, router-solicitation, router-advertisement, time-exceeded, parameter-problem } accept
		ip protocol igmp accept

		# SSH (port 22)
		tcp dport ssh accept

		# HTTP (ports 80 & 443)
		tcp dport { http, https } accept
	}

	chain forward {
		type filter hook forward priority 0; policy drop;
	}

	chain output {
		type filter hook output priority 0; policy accept;
	}

}

Limit rate IPv4/IPv6 firewall

firewall.2.rules
table inet filter {
	chain input {
		type filter hook input priority 0; policy drop;

		ct state invalid drop

		iif lo accept

		# no ping floods:
		ip protocol icmp icmp type echo-request limit rate over 10/second burst 4 packets  drop
		ip6 nexthdr icmpv6 icmpv6 type echo-request limit rate over 10/second burst 4 packets drop

		ct state established,related accept

		# ICMP & IGMP
		ip6 nexthdr icmpv6 icmpv6 type { destination-unreachable, packet-too-big, time-exceeded, parameter-problem, mld-listener-query, mld-listener-report, mld-listener-reduction, nd-router-solicit, nd-router-advert, nd-neighbor-solicit, nd-neighbor-advert, ind-neighbor-solicit, ind-neighbor-advert, mld2-listener-report } accept
		ip protocol icmp icmp type { destination-unreachable, router-solicitation, router-advertisement, time-exceeded, parameter-problem } accept
		ip protocol igmp accept

		# avoid brute force on ssh:
		tcp dport ssh ct state new limit rate 15/minute accept

	}

	chain forward {
		type filter hook forward priority 0; policy drop;
	}

	chain output {
		type filter hook output priority 0; policy accept;
	}

}

Jump

When using jumps in config file, it is necessary to define the target chain first. Otherwise one could end up with Error: Could not process rule: No such file or directory.

jump.rules
table inet filter {
    chain web {
        tcp dport http accept
        tcp dport 8080 accept
    }
    chain input {
        type filter hook input priority 0;
        ip saddr 10.0.2.0/24 jump web
        drop
    }
}

Different rules for different interfaces

If your box has more than one network interface, and you would like to use different rules for different interfaces, you may want to use a "dispatching" filter chain, and then interface-specific filter chains. For example, let us assume your box acts as a home router, you want to run a web server accessible over the LAN (interface nsp3s0), but not from the public internet (interface enp2s0), you may want to consider a structure like this:

table inet filter {
  chain input { # this chain serves as a dispatcher
    type filter hook input priority 0;

    iif lo accept # always accept loopback
    iifname enp2s0 jump input_enp2s0
    iifname enp3s0 jump input_enp3s0

    reject with icmp type port-unreachable # refuse traffic from all other interfaces
  }
  chain input_enp2s0 { # rules applicable to public interface interface
    ct state {established,related} accept
    ct state invalid drop
    udp dport bootpc accept
    tcp dport bootpc accept
    reject with icmp type port-unreachable # all other traffic
  }
  chain input_enp3s0 {
    ct state {established,related} accept
    ct state invalid drop
    udp dport bootpc accept
    tcp dport bootpc accept
    tcp port http accept
    tcp port https accept
    reject with icmp type port-unreachable # all other traffic
  }
  chain ouput { # we let everything out
    type filter hook output priority 0;
    accept
  }
 }

Alternatively you could choose only one iifname statement, such as for the single upstream interface, and put the default rules for all other interfaces in one place, instead of dispatching for each interface.

Masquerading

nftables has a special keyword masquerade "where the source address is automagically set to the address of the output interface" (source). This is particularly useful for situations in which the IP address of the interface is unpredictable or unstable, such as the upstream interface of routers connecting to many ISPs. Without it, the Network Address Translation rules would have to be updated every time the IP address of the interface changed.

To use it:

  • make sure masquerading is enabled in the kernel (true if you use the default kernel), otherwise during kernel configuration, set
CONFIG_NFT_MASQ=m
  • the masquerade keyword can only be used in chains of type nat, which in turn cannot be contained in a table with family inet. Use a table with family ip and/or ip6 instead.
  • masquerading is a kind of source NAT, so only works in the output path.

Example for a machine with two interfaces: LAN connected to nsp3s0, and public internet connected to enp2s0:

table ip nat {
  chain prerouting {
    type nat hook prerouting priority 0;
  }
  chain postrouting {
    type nat hook postrouting priority 0;
    oifname "enp0s2" masquerade
  }
}

Tips and tricks

Simple stateful firewall

See Simple stateful firewall for more information.

Single machine

Flush the current ruleset:

# nft flush ruleset

Add a table:

# nft add table inet filter

Add the input, forward, and output base chains. The policy for input and forward will be to drop. The policy for output will be to accept.

# nft add chain inet filter input { type filter hook input priority 0 \; policy drop \; }
# nft add chain inet filter forward { type filter hook forward priority 0 \; policy drop \; }
# nft add chain inet filter output { type filter hook output priority 0 \; policy accept \; }

Add two regular chains that will be associated with tcp and udp:

# nft add chain inet filter TCP
# nft add chain inet filter UDP

Related and established traffic will be accepted:

# nft add rule inet filter input ct state related,established accept

All loopback interface traffic will be accepted:

# nft add rule inet filter input iif lo accept

Drop any invalid traffic:

# nft add rule inet filter input ct state invalid drop

New echo requests (pings) will be accepted:

# nft add rule inet filter input ip protocol icmp icmp type echo-request ct state new accept

New udp traffic will jump to the UDP chain:

# nft add rule inet filter input ip protocol udp ct state new jump UDP

New tcp traffic will jump to the TCP chain:

# nft add rule inet filter input ip protocol tcp tcp flags \& \(fin\|syn\|rst\|ack\) == syn ct state new jump TCP

Reject all traffic that was not processed by other rules:

# nft add rule inet filter input ip protocol udp reject
# nft add rule inet filter input ip protocol tcp reject with tcp reset
# nft add rule inet filter input counter reject with icmp type prot-unreachable

At this point you should decide what ports you want to open to incoming connections, which are handled by the TCP and UDP chains. For example to open connections for a web server add:

# nft add rule inet filter TCP tcp dport 80 accept

To accept HTTPS connections for a webserver on port 443:

# nft add rule inet filter TCP tcp dport 443 accept

To accept SSH traffic on port 22:

# nft add rule inet filter TCP tcp dport 22 accept

To accept incoming DNS requests:

# nft add rule inet filter TCP tcp dport 53 accept
# nft add rule inet filter UDP udp dport 53 accept

Be sure to make your changes permanent when satisifed.

Prevent brute-force attacks

Sshguard is program that can detect brute-force attacks and modify firewalls based on IP addresses it temporarily blacklists. See Sshguard#nftables on how to set up nftables to be used with it.

See also