nftables

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nftables is the successor to iptables. It replaces the existing iptables, ip6tables, arptables, and ebtables framework. It uses the Linux kernel and a new userspace utility called nft. nftables provides a compatibility layer for the ip(6)tables and framework.

Introduction

As with the iptables framework, nftables is built upon rules which specify actions. These rules are attached to chains. A chain can contain a collection of rules and is registered in the netfilter hooks. Chains are stored inside tables. A table is specific for one of the layer 3 protocols. One of the main differences with iptables is that there are no predefined tables and chains anymore.

CODE Configuration hierarchy
Table
├─Chain1
│ ├─Rule1a
│ └─Rule1b
├─Chain2
│ ├─Rule2a
│ ├─Rule2b
│ └─Rule2c
└─Chain3
  └─Rule3a
Note
Both iptables and nftables use the Netfilter framework. Because iptables does not allow for the manual configuration of hooks - the default tables are used for tapping into the Netfilter framework, while nftables requires the hooks to be defined inside of user-defined chains. Iptables also does not support dual stack (inet) configurations or netdev. Other than these differences, they are functionally identical, acting as interfaces for Netfilter.

Tables

A table is nothing more than a container for your chains. Unlike iptables, nftables has no predefined tables (filter, raw, mangle...). An iptables-like structure can be used, but this is not required. Tables must be defined with an address family and name, which can be anything.

The address families used by nftables are documented under man nft 8:

Address family Description
ip Used for IPv4 related chains.
ip6 Used for IPv6 related chains.
inet Mixed IPv4/IPv6 chains (kernel 3.14 and up).
arp Used for ARP related chains.
bridge Used for bridging related chains.
netdev Used for chains that filter early in the stack (kernel 4.2 and up).
Note
If a family is not specified, ip is used.

Chains

Chains are used to group rules. As with the tables, nftables does not have any predefined chains. Chains are grouped in base and non-base types. Base chains are registered in one of the netfilter hooks, non-base chains are not. base chains must be defined with a hook type, and priority. In contrast, non-base chains are not attached to a hook and they don't see any traffic by default. They can be used as jump targets to arrange a rule-set in a tree of chains.

The chains used by nftables are documented under man nft 8:

Chain Families Hooks Description
filter all all Standard chain, generally used for filtering.
nat ip, ip6, nat prerouting, input, output, postrouting Used to perform Native Address Translation using conntrack. Only the first packet of a connection uses this chain.
route ip, ip6 output Packets that traverse this chain type, if about to be accepted, trigger a route lookup if the IP header has changed.

Each address family has different hook capabilities, defined under the respective {name} ADDRESS FAMILY section of man nft 8:

IPv4/IPv6/INET/Bridge hooks

Hook Description
prerouting Processes all packets entering the system, invoked before the routing process, used for early filtering or changing attributes which would affect routing.
input Processes packets destined for the local system.
forward Processes packets received by the local system, but destined for another one.
output Processes packets sent by the local system (includes NATed packets).
postrouting Processes all packets leaving the system, regardless of the source.
ingress (Since 5.10) Processes all packets entering the system, before prerouting (and all other layer 3 handlers). Only available to the inet address family.

ARP hooks

Hook Description
input Processes ARP packets the local system receives.
output Processes ARP packets leaving the local system.

Netdev hooks

Hook Description
ingress Processes all packets entering the system. Invoked after network taps such as tcpdump, and before layer 3 handlers.
egress Processes all packets leaving the system. Invoked before tcpdump egress.

Rules

Rules specify which action has to be taken for which packets. Rules are attached to chains. Each rule can have an expression to match packets and one or more actions to perform when matching. One main difference to iptables is that it is possible to specify multiple actions per rule. Another is that by default counters are off. A counter must be specified explicitly in each rule for which you want packet- and byte-counters.

Each rule has a unique handle number by which it can be distinguished.

The following matches are available:

  • ip: IP protocol.
  • ip6: IPv6 protocol.
  • tcp: TCP protocol.
  • udp: UDP protocol.
  • udplite: UDP-lite protocol.
  • sctp: SCTP protocol.
  • dccp: DCCP protocol.
  • ah: Authentication headers.
  • esp: Encrypted security payload headers.
  • ipcomp: IPcomp headers.
  • icmp: icmp protocol.
  • icmpv6: icmpv6 protocol.
  • ct: Connection tracking.
  • meta: meta properties such as interfaces.

Matches

Match Arguments Description/Example
ip version Ip Header version
hdrlength IP header length
tos Type of Service
length Total packet length
id IP ID
frag-off Fragmentation offset
ttl Time to live
protocol Upper layer protocol
checksum IP header checksum
saddr Source address
daddr Destination address
ip6 version IP header version
priority
flowlabel Flow label
length Payload length
nexthdr Next header type (Upper layer protocol number)
hoplimit Hop limit
saddr Source Address
daddr Destination Address
tcp sport Source port
dport Destination port
sequence Sequence number
ackseq Acknowledgement number
doff Data offset
flags TCP flags
window Window
checksum Checksum
urgptr Urgent pointer
udp sport Source port
dport destination port
length Total packet length
checksum Checksum
udplite sport Source port
dport destination port
cscov Checksum coverage
checksum Checksum
sctp sport Source port
dport destination port
vtag Verification tag
checksum Checksum
dccp sport Source port
dport destination port
ah nexthdr Next header protocol (Upper layer protocol)
hdrlength AH header length
spi Security Parameter Index
sequence Sequence Number
esp spi Security Parameter Index
sequence Sequence Number
ipcomp nexthdr Next header protocol (Upper layer protocol)
flags Flags
cfi Compression Parameter Index
icmp type icmp packet type
icmpv6 type icmpv6 packet type
ct state State of the connection
direction Direction of the packet relative to the connection
status Status of the connection
mark Connection mark
expiration Connection expiration time
helper Helper associated with the connection
l3proto Layer 3 protocol of the connection
saddr Source address of the connection for the given direction
daddr Destination address of the connection for the given direction
protocol Layer 4 protocol of the connection for the given direction
proto-src Layer 4 protocol source for the given direction
proto-dst Layer 4 protocol destination for the given direction
meta length Length of the packet in bytes: meta length > 1000
protocol ethertype protocol: meta protocol vlan
priority TC packet priority
mark Packet mark
iif Input interface index
iifname Input interface name
iiftype Input interface type
oif Output interface index
oifname Output interface name
oiftype Output interface hardware type
pkttype Packet type: unicast, multicast or broadcast
skuid UID associated with originating socket
skgid GID associated with originating socket
rtclassid Routing realm

Statements

Statements represent the action to be performed when the rule matches. They exist in two kinds: Terminal statements, unconditionally terminate the evaluation of the current rules and non-terminal statements that either conditionally or never terminate the current rules. There can be an arbitrary amount of non-terminal statements, but there must be only a single terminal statement. The terminal statements can be:

  • accept: Accept the packet and stop the ruleset evaluation.
  • drop: Drop the packet and stop the ruleset evaluation.
  • reject: Reject the packet with an icmp message.
  • queue: Queue the packet to userspace and stop the ruleset evaluation.
  • continue:
  • return: Return from the current chain and continue at the next rule of the last chain. In a base chain it is equivalent to accept.
  • jump <chain>: Continue at the first rule of <chain>. It will continue at the next rule after a return statement is issued.
  • goto <chain>: Similar to jump, but after the new chain the evaluation will continue at the last chain instead of the one containing the goto statement.

Sets

nftables allows defining anonymous and named sets (dictionaries and maps). For example, the following nft script defines the fullbogons set, adds elements to it and drops packages from the IPs conforming the set.

FILE rules.nft
#!/sbin/nft
add set filter fullbogons { type ipv4_addr; flags interval; }
add element filter fullbogons {0.0.0.0/8}
add element filter fullbogons {10.0.0.0/8}
add element filter fullbogons {41.62.0.0/16}
add element filter fullbogons {41.67.64.0/20}
add rule filter input iifname eth0 ct state new ip saddr @fullbogons counter drop comment "drop from blacklist"

Installation

Kernel

Example Config: A bare minimum for basic IPv4 firewalling with NAT:

KERNEL Nftables kernel requirements
[*] Networking support  --->
    Networking options  --->
        [*] Network packet filtering framework (Netfilter)  --->
            Core Netfilter Configuration  --->
                <M> Netfilter connection tracking support
                <M> Netfilter nf_tables support
                <M>   Netfilter nf_tables conntrack module
                <M>   Netfilter nf_tables counter module
                <M>   Netfilter nf_tables log module
                <M>   Netfilter nf_tables limit module
                <M>   Netfilter nf_tables masquerade support
                <M>   Netfilter nf_tables nat module
            IP: Netfilter Configuration  --->
                <M> IPv4 nf_tables support
                <M> IPv4 packet rejection
                <M> IP tables support (required for filtering/masq/NAT)
                <M>   Packet filtering
                <M>     REJECT target support
                <M>   iptables NAT support
                <M>     MASQUERADE target support

Additional Common Config: For mixed IPv4 and IPv6 rules combined into one table: CONFIG_NF_TABLES_INET

(If family inet is not enabled, only families ip and ip6 can be used individually)

KERNEL Nftables inet family
[*] Networking support  --->
    Networking options  --->
        [*] Network packet filtering framework (Netfilter)  --->
            Core Netfilter Configuration  --->
                <M> Netfilter nf_tables support
                [*]   Netfilter nf_tables mixed IPv4/IPv6 tables support

Additional Optional Config: Early filtering based on network device requires netdev tables support: CONFIG_NF_TABLES_NETDEV

KERNEL Nftables netdev family
[*] Networking support  --->
    Networking options  --->
        [*] Network packet filtering framework (Netfilter)  --->
            Core Netfilter Configuration  --->
                <M> Netfilter nf_tables support
                [*]   Netfilter nf_tables netdev tables support

Nftables is very modular, and has many more options than mentioned here. Certain software likely requires additional features.

Depending on your intended purposes, you can go as bare minimum as listed here, OR you can enable other things as modules, because the kernel will load them as needed.

Disclaimer: This network section of the kernel changes a lot - these options are subject to change OFTEN; more modules = more compatibility. They only load when requested.

Emerge

Install net-firewall/nftables:

root #emerge --ask net-firewall/nftables

Configuration

OpenRC

The init script supports the following actions:

  • /etc/init.d/nftables save stores the currently loaded ruleset in /var/lib/nftables/rules-save
  • /etc/init.d/nftables reload loads the currently loaded ruleset in /var/lib/nftables/rules-save
  • /etc/init.d/nftables stop is intended to be called on system shutdown, verifies if SAVE_ON_STOP is enabled in /etc/conf.d/nftables and saves the ruleset
  • /etc/init.d/nftables start is intended to be called on system boot and loads the last saved ruleset
  • /etc/init.d/nftables clear flushes the currently loaded ruleset
  • /etc/init.d/nftables list lists the currently loaded ruleset

Don't forget to add nftables service to startup:

root #rc-update add nftables default
Warning
Failing to invoke /etc/init.d/nftables save manually after altering the ruleset could result in an older ruleset being loaded if there is an issue during system shutdown and saving the ruleset fails.

systemd

After first setup:

root #touch /var/lib/nftables/rules-save
root #systemctl enable --now nftables-restore

Usage

All nftable commands are executed with the nft utility from net-firewall/nftables.

Tables

Creating tables

The following command adds a table called base_table for the IPv4 and IPv6 layers:

root #nft add table inet base_table

Likewise, a table for arp can be created with

root #nft add table arp base_table
Note
The name "base_table" used here is completely arbitrary. Any name can be used.

Listing tables

The following command lists all tables:

root #nft list tables
table inet base_table
table arp base_table

The tables can be filtered by type by adding the address family as an argument:

root #nft list tables inet
table inet base_table

The contents of the table base_table can be listed with:

root #nft list table inet base_table
table inet base_table {
        chain input_filter {
                 type filter hook input priority 0;
                 ct state established,related accept
                 iifname "lo" accept
                 ip protocol icmp accept
                 drop
        }
}
Tip
Adding -a as an argument to the nft command displays the rule handle after the rule.
root #nft -a list table inet base_table
table inet base_table {
        chain input_filter {
                 type filter hook input priority 0;
                 ct state established,related accept # handle 2
                 iifname "lo" accept # handle 3
                 ip protocol icmp accept # handle 4
                 drop # handle 5
        }
}

Deleting tables

The following command deletes the table called base_table, which is part of the address family inet:

root #nft delete table inet base_table

Chains

Adding chains

The following command adds a base chain called input_filter to the inet base_table table. It is registered to the input hook with priority 0, and type filter.

root #nft add chain inet base_table input_filter "{type filter hook input priority 0;}"
Note
A non-base chain can be added by not specifying the chain configurations between the curly braces.
Important
Chains cannot be redefined once made, that means the hook type and priority are immutable. The chain must be deleted and redefined to keep the name but change these parameters. Deletion requires all references to the chain are also removed!

Listing chain rules

The rules for a chain can be listed using:

root #nft list chain inet base_table input_filter
table inet base_filter {
	chain input_filter {
	        type filter hook input priority 0;
        }
}

Removing chains

The created chain can be removed with:

root #nft delete chain inet base_table input_filter
Note
Chains can only be deleted if there are no rules in them, and they are not used as a jump target.
Tip
destroy can be used instead of delete to ensure a chain does not exist. Unlike delete, destroy will not return an error if the chain did not already exist.

Rules

Adding rules

The following command adds a rule to the chain called input_filter, on the base_table table, dropping all incoming traffic to port 80:

root #nft add rule inet base_table input_filter tcp dport 80 drop

Listing all rules

The following command can be used to list all rules in the current ruleset (with handles):

root #nft -a list ruleset
table inet base_table {
        chain input_filter { # handle 1
                 type filter hook input priority 0; # handle 2
                 tcp dport http drop # handle 3
        }
        chain output_filter { # handle 4
                 type filter hook input priority 0; # handle 5
                 tcp sport http drop # handle 6
        }
}

Deleting rules

To delete a rule, the rule's handle number is required:

root #nft delete rule inet base_table input_filter handle 3

Management

Atomic rule loading

nft supports atomic rule replacement by using nft -f. Thus it is possible to conveniently manage the rules in a text file. Comments may be added to the file by prefixing them with #, as with shell scripts; they can also be appended to the end of rules as comment "<arbitrary string>", and these will be preserved as-is in nft list output.

Compared to building a ruleset with multiple nft calls in a shell script, this also ensures that failures in such a script do not end with an only partially applied ruleset.

FILE /etc/nftables-localskeleton nftables config file
#! /sbin/nft -f

# this is a skeleton file for an nftables ruleset
# load it with nft -f /etc/nftables-local

# it is supported to define variables here, that can later on be
# expanded in rule definitions
define http_ports = {80, 443} 

flush ruleset

table inet local {

  chain input {
    type filter hook input priority 0; policy drop;
    tcp dport $http_ports counter accept comment "incoming http traffic";
  }
  chain output {
    type filter hook output priority 0; policy drop;
  }

}

Backup

You can also backup your rules:

root #echo "flush ruleset" > backup.nft
root #nft list ruleset >> backup.nft
Note
If you are loading your ruleset with nft -f from a file, do not overwrite this file with the nft list ruleset output. This overwrites comments and variable definitions.

Logging

Start logging

Logging of e.g. dropped packages is possible by adding a line with the keyword log at the end of the rule-set, e.g. log prefix "nft.dropinput";.

Adding a prefix will produce a log entry to /var/log/messages, such as:

FILE /var/log/messagesexample entry in messages file
Jun 07 13:35:19 host kernel: nft.dropinput IN=eno1 OUT= MAC=...

Configure syslog-ng

Logging will be written by default to messages file and will fill up the file with annoying information. Based on using the prefix, the syslog-ng filters will be used to redirect those to its own file nft.log.

FILE /etc/syslog-ng/syslog-ng.confentries for logging nft entries to its own file
#add destination for nft messages
destination netfilter { file("/var/log/nft.log"); };

#add filter for nft (netfilter) messages and messages
filter f_netfilter { message("nft"); };
filter f_messages { not message("nft"); };

#use filter for nft logging
log { source(src); filter(f_netfilter); destination(netfilter); };

#modify original lines to filter out nft messages
log { source(src); filter(f_messages); destination(messages); };
log { source(src); filter(f_messages); destination(console_all); };

Examples

See the Nftables examples article.

Troubleshooting

Before loading new or edited rules check them with nft

user $nft -c -f ruleset

No such file or directory

If this error is printed for every chain of a table definition make sure, that the table's family is available through the kernel. This happens for example if the table uses family inet and the kernel configuration did not enable mixed IPv4 and IPv6 rules (CONFIG_NF_TABLES_INET).

Conflicting intervals

A set definition of IP ranges causes this error if ranges overlap. For example 224.0.0.0/3 and 240.0.0.0/5 overlap completely. Either add auto-merge to the set's options, drop the range that is fully included or change syntax to 224.0.0.0-255.255.255.255.

CODE Sample Set
table netdev filter {
  set blocked_ipv4 {
    169.254.0.0/16,
    224.0.0.0/3,
    240.0.0.0/5
  }
  # fix the last two IPs overlapping
  auto-merge
}

Restart of nftables or reboot cause blocked connections

Default configuration of the save and restore function use numeric mode to store the rule set. The persisted rule set could have changed from the original upload from a manually written file. Such a transformation might break things. Therefore make sure:

  1. that /etc/conf.d/nftables contains the parameter -n for the SAVE_OPTIONS
  2. and loading your rule set as root yields a working configuration
  3. and the save and restore cycle of restarting nftables service causes the issue

If all three conditions are met remove the -n parameter from SAVE_OPTIONS in /etc/conf.d/nftables. Then load your rule set again from the manually written file and restart the service again. This cycles through save and restore and should create a fully working rule set.

This affected at least version 0.9.9, see bug #819456.

Family netdev and ingress hook

Broken packets should be rejected early which requires an ingress hook for family netdev. This sets up a chain that acts for a dedicated network device before packets enter further processing – improved performance. The configuration looks like this:

CODE Family netdev and ingress chain
table netdev filter {
  chain ingress {
    type filter hook ingress device enp4s0 priority -500;

    # Drop all fragments.
    ip frag-off & 0x1fff != 0 counter drop

    # Drop XMAS packets.
    tcp flags & (fin|syn|rst|psh|ack|urg) == fin|syn|rst|psh|ack|urg counter drop

    # Drop NULL packets.
    tcp flags & (fin|syn|rst|psh|ack|urg) == 0x0 counter drop

    # Drop uncommon MSS values.
    tcp flags syn tcp option maxseg size 1-535 counter drop

  }
}

Mind the device name enp4s0. If this changes for example when changing hardware or an upgrade changed device naming this family is broken. In turn none of the rules will be loaded. The error looks like this (filename and line numbers differ depending on the host configuration):

CODE Error at chains instead of non-existing device
/etc/nftables.conf:94:9-15: Error: Could not... 
chain ingress {
      ^^^^^^^
/etc/nftables.conf:94:9-15: Error: Could not...
  chain ingress {
        ^^^^^^^
/etc/nftables.conf:94:9-15: Error: Could not...
  chain ingress {
        ^^^^^^^
/etc/nftables.conf:94:9-15: Error: Could not...
  chain ingress {
        ^^^^^^^
/etc/nftables.conf:94:9-15: Error: Could not...
  chain ingress {
        ^^^^^^^

Check the device name is actually correct and exists, e.g. ip addr list.

See also

  • Iptables — a program used to configure and manage the kernel's netfilter modules. Contains a section about migration to nftables

External Resources

Retrieved from "https://wiki.gentoo.org/index.php?title=Nftables&oldid=1228352"