nftables：nft man文档阅读笔记

发表信息: by

利用空闲时间学习了nftables的基础知识，其中官方的man page中包含了大量信息，在阅读过程中整理了一份带中文注释的笔记，以辅助加深记忆。

工具名称

nft –  包过滤规则管理工具

基本用法

nft [ -n | --numeric ] [ -s | --stateless ] [ [-I | --includepath] directory ] [ [-f | --file] filename | [-i | --interactive] | cmd ]

nft [ -h | --help ] [ -v | --version ]

工具描述

nftables 作为新一代的防火墙策略框架，旨在替代之前的各种防火墙工具诸如iptables/ebtables等，而且提供了类似tc的带宽限速能力。而nft则提供了nftables的命令行入口，是用户空间的管理工具。

选项说明

执行nft --help查看完整帮助信息

-h, --help

查看帮助信息.

-v, --version

查看版本号.

-n, --numeric

以数值方式展示数据，可重复使用，一个-n表示不解析域名，第二次不解析端口号，第三次不解析协议和uid/gid。

-s, --stateless

省略规则和有状态对象的状态信息

-N

将ip地址解析成域名，依赖dns解析。

-a, --handle

输出内容中展示规则handle信息

-I, --includepath directory

添加include文件搜索目录

-f, --file filename

从文件获取输入

-i, --interactive

从交互式cli获取输入

文件格式

语法规定

单行过长可用\换行连接； 多个命令写到同一行可用分号; 分隔； 注释使用井号#打头； 标识符用大小写字母打头，后面跟数字字母下划线正斜杠反斜杠以及点号; 用双引号引起来表示纯字符串。

文件引用

include "filename"

可由外部文件通过include导入到当前文件，用-I/--includepath指定导入文件所在目录，如果include后面接的是目录而非文件，则整个目录的文件将以字母顺序依次导入。

符号变量

define variable = expr

$variable

Symbolic variables can be defined using the define statement. Variable references are expressions and can be used initialize other variables. The scope of a definition is the current block and all blocks contained within. 变量使用define定义，变量引用属于表达式，可以用于初始化其他变量，变量的生效范围在当前block以及被包含的所有block内。

示例 1. 使用符号变量

define int_if1 = eth0
define int_if2 = eth1
define int_ifs = { $int_if1, $int_if2 }

filter input iif $int_ifs accept

地址族

根据处理的包的种类不同可以将其分为不同的地址族。不同的地址族在内核中包含有特定阶段的处理路径和hook点，当对应hook的规则存在时则会被nftables处理。具体类型如下：

ip

IPv4 地址族

ip6

IPv6 地址族

inet

Internet (IPv4/IPv6) 地址族

arp

ARP 地址族

bridge

Bridge 地址族

netdev

Netdev 地址族

所有nftables对象存在于特定的地址族namespace中，换言之所有identifier都含有一个特定的地址族，如果未指定则默认使用ip地址族

IPv4/IPv6/Inet address families

IPv4/IPv6/Inet 地址族用于处理 IPv4和IPv6包，其在network stack中在不同的包处理阶段一共包含了5个hook.

Table 1. IPv4/IPv6/Inet 地址类hook列表

Hook名称 描述
prerouting 所有进入到系统的包都会被prerouting hook进行处理. 它在routing流程之前就被发起，用于靠前阶段的包过滤或者更改影响routing的包属性.
input 发往本地系统的包将被input hook处理.
forward 被转发到其他主机的包会经由forward hook处理.
output 由本地进程发送出去的包将被output hook处理.
postrouting 所有离开系统的包都将被postrouting hook处理.

ARP address family

ARP地址族用于处理经由系统接收和发送的ARP包。一般在集群环境中对ARP包进行mangle处理以支持clustering。

Table 2. ARP address family hooks

Hook 描述
input 分发到本机的包会经过input hook.
output 由本机发出的包会经过output hook.

Bridge address family

bridge地址族处理通过桥接设备的ethernet包。

Netdev address family

Netdev地址族处理从ingress过来的包。

Table 3. Netdev address family hooks

Hook Description
ingress 所有进入系统的包都将被ingress hook处理。它在进入layer 3之前的阶段就开始处理。

Tables

{add | delete | list | flush} table [family] {table}

table是chain/set/stateful object的容器，table由其地址族和名字做标识。地址族必须属于ip, ip6, arp, bridge, netdev中的一种，inet地址族是一个虚拟地址族，同来创建同时包含IPv4和IPv6的table，如果没有指定地址族则默认使用ip地址族。

add

添加指定地址族，指定名称的table

delete

删除指定的table

list

列出指定table中的所有chain和rule

flush

清除指定table中的所有chain和rule

Chains

{add} chain [family] {table} {chain} {hook} {priority} {policy} {device}

{add | create | delete | list | flush} chain [family] {table} {chain}

{rename} chain [family] {table} {chain} {newname}

chain是rule的容器，他们存在于两种类型，基础链（base chain）和常规链（regular chain）。base chain是网络栈中数据包的入口点，regular chain则可用于jump的目标并对规则进行更好地组织。

add

在指定table中添加新的链，当hook和权重值被指定时，添加的chain为base chain，将在网络栈中hook相关联。

create

与`add`命令类似，不同之处在于当创建的chain存在时会返回错误。

delete

删除指定的chain，被删除的chain不能有规则且不能是跳转目标chain。

rename

重命名chain

list

列出指定chain中的所有rule

flush

清除指定chain中所有rule

Rules

[add | insert] rule [family] {table} {chain} [position position] {statement...}

{delete} rule [family] {table} {chain} {handle handle}

Rules are constructed from two kinds of components according to a set of grammatical rules: expressions and statements.

add

Add a new rule described by the list of statements. The rule is appended to the given chain unless a position is specified, in which case the rule is appended to the rule given by the position.

insert

Similar to the ***add*** command, but the rule is prepended to the beginning of the chain or before the rule at the given position.

delete

Delete the specified rule.

Sets

{add} set family] {table} {set}{ {type} [flags] [timeout] [gc-interval] [elements] [size] [policy]}

{delete | list | flush} set [family] {table} {set}

{add | delete} element [family] {table} {set}{ {elements}}

Sets are elements containers of an user-defined data type, they are uniquely identified by an user-defined name and attached to tables. sets 是用户定义的数据类型的容器，具有用户定义的唯一标识，被应用到table上。

add

在指定的table中添加一个新的set

delete

删除指定set

list

查看set内的元素

flush

清空整个set

add element

往set中添加元素，多个使用逗号分隔

delete element

从set中删除元素，多个使用逗号分隔

Table 4. Set 参数

关键字 描述 类型
type 元素的数据类型 string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark
flags set flags string: constant, interval, timeout
timeout 元素在set中的存活时间 string, 带单位的小数. 单位: d, h, m, s
gc-interval 垃圾回收间隔, 仅当timeout或flag timeout设置时生效 string, decimal followed by unit. Units are: d, h, m, s
elements set中包含的元素 set data type
size set可存放的最大元素个数 unsigned integer (64 bit)
policy set policy string: performance [default], memory

Maps

{add} map [family] {table} {map}{ {type} [flags] [elements] [size] [policy]}

{delete | list | flush} map [family] {table} {map}

{add | delete} element [family] {table} {map}{ {elements}}

Maps store data based on some specific key used as input, they are uniquely identified by an user-defined name and attached to tables.

add

Add a new map in the specified table.

delete

Delete the specified map.

list

Display the elements in the specified map.

flush

Remove all elements from the specified map.

add element

Comma-separated list of elements to add into the specified map.

delete element

Comma-separated list of element keys to delete from the specified map.

Table 5. Map specifications

Keyword Description Type
type data type of map elements string ':' string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark, counter, quota. Counter and quota can't be used as keys
flags map flags string: constant, interval
elements elements contained by the map map data type
size maximun number of elements in the map unsigned integer (64 bit)
policy map policy string: performance [default], memory

Stateful objects

{add | delete | list | reset} type [family] {table} {object}

Stateful objects are attached to tables and are identified by an unique name. They group stateful information from rules, to reference them in rules the keywords "type name" are used e.g. "counter name".

add

Add a new stateful object in the specified table.

delete

Delete the specified object.

list

Display stateful information the object holds.

reset

List-and-reset stateful object.

Ct

ct {helper} {type} {type} {protocol} {protocol} [l3proto] [family]

Ct helper is used to define connection tracking helpers that can then be used in combination with the "ct helper set" statement. type and protocol are mandatory, l3proto is derived from the table family by default, i.e. in the inet table the kernel will try to load both the ipv4 and ipv6 helper backends, if they are supported by the kernel.

Table 6. conntrack helper specifications

Keyword Description Type
type name of helper type quoted string (e.g. "ftp")
protocol layer 4 protocol of the helper string (e.g. tcp)
l3proto layer 3 protocol of the helper address family (e.g. ip)

示例 2. defining and assigning ftp helper

Unlike iptables, helper assignment needs to be performed after the conntrack lookup has completed, for example with the default 0 hook priority.

table inet myhelpers {
  ct helper ftp-standard {
     type "ftp" protocol tcp
  }
  chain prerouting {
      type filter hook prerouting priority 0;
      tcp dport 21 ct helper set "ftp-standard"
  }
}

Counter

counter [packets bytes]

Table 7. Counter specifications

Keyword Description Type
packets initial count of packets unsigned integer (64 bit)
bytes initial count of bytes unsigned integer (64 bit)

Quota

quota [over until] [used]

Table 8. Quota specifications

Keyword Description Type
quota quota limit, used as the quota name Two arguments, unsigned interger (64 bit) and string: bytes, kbytes, mbytes. "over" and "until" go before these arguments
used initial value of used quota Two arguments, unsigned interger (64 bit) and string: bytes, kbytes, mbytes

Expressions

Expressions represent values, either constants like network addresses, port numbers etc. or data gathered from the packet during ruleset evaluation. Expressions can be combined using binary, logical, relational and other types of expressions to form complex or relational (match) expressions. They are also used as arguments to certain types of operations, like NAT, packet marking etc.

Each expression has a data type, which determines the size, parsing and representation of symbolic values and type compatibility with other expressions.

describe command

describe {expression}

The describe command shows information about the type of an expression and its data type.

示例 3. The describe command

$ nft describe tcp flags
payload expression, datatype tcp_flag (TCP flag) (basetype bitmask, integer), 8 bits

pre-defined symbolic constants:
fin                               0x01
syn                               0x02
rst                               0x04
psh                               0x08
ack                               0x10
urg                               0x20
ecn                               0x40
cwr                               0x80

Data types

Data types determine the size, parsing and representation of symbolic values and type compatibility of expressions. A number of global data types exist, in addition some expression types define further data types specific to the expression type. Most data types have a fixed size, some however may have a dynamic size, f.i. the string type.

Types may be derived from lower order types, f.i. the IPv4 address type is derived from the integer type, meaning an IPv4 address can also be specified as an integer value.

In certain contexts (set and map definitions) it is necessary to explicitly specify a data type. Each type has a name which is used for this.

Integer type

Table 9.

Name Keyword Size Base type
Integer integer variable -

The integer type is used for numeric values. It may be specified as decimal, hexadecimal or octal number. The integer type doesn't have a fixed size, its size is determined by the expression for which it is used.

Bitmask type

Table 10.

Name Keyword Size Base type
Bitmask bitmask variable integer

The bitmask type (bitmask) is used for bitmasks.

String type

Table 11.

Name Keyword Size Base type
String string variable -

The string type is used to for character strings. A string begins with an alphabetic character (a-zA-Z) followed by zero or more alphanumeric characters or the characters /, -, _ and .. In addition anything enclosed in double quotes (") is recognized as a string.

示例 4. String specification


# Interface name
filter input iifname eth0

# Weird interface name
filter input iifname "(eth0)"

Table 12.

Name Keyword Size Base type
Link layer address lladdr variable integer

The link layer address type is used for link layer addresses. Link layer addresses are specified as a variable amount of groups of two hexadecimal digits separated using colons (:).

示例 5. Link layer address specification


# Ethernet destination MAC address
filter input ether daddr 20:c9:d0:43:12:d9

IPv4 address type

Table 13.

Name Keyword Size Base type
IPv4 address ipv4_addr 32 bit integer

The IPv4 address type is used for IPv4 addresses. Addresses are specified in either dotted decimal, dotted hexadecimal, dotted octal, decimal, hexadecimal, octal notation or as a host name. A host name will be resolved using the standard system resolver.

示例 6. IPv4 address specification

# dotted decimal notation
filter output ip daddr 127.0.0.1

# host name
filter output ip daddr localhost

IPv6 address type

Table 14.

Name Keyword Size Base type
IPv6 address ipv6_addr 128 bit integer

The IPv6 address type is used for IPv6 addresses. FIXME

示例 7. IPv6 address specification

# abbreviated loopback address
filter output ip6 daddr ::1

Boolean type

Table 15.

Name Keyword Size Base type
Boolean boolean 1 bit integer

The boolean type is a syntactical helper type in user space. It's use is in the right-hand side of a (typically implicit) relational expression to change the expression on the left-hand side into a boolean check (usually for existence).

The following keywords will automatically resolve into a boolean type with given value:

Table 16.

Keyword Value
exists 1
missing 0

示例 8. Boolean specification

The following expressions support a boolean comparison:

Table 17.

Expression Behaviour
fib Check route existence.
exthdr Check IPv6 extension header existence.
tcp option Check TCP option header existence. 
# match if route exists
filter input fib daddr . iif oif exists

# match only non-fragmented packets in IPv6 traffic
filter input exthdr frag missing

# match if TCP timestamp option is present
filter input tcp option timestamp exists

ICMP Type type

Table 18.

Name Keyword Size Base type
ICMP Type icmp_type 8 bit integer

The ICMP Type type is used to conveniently specify the ICMP header's type field.

The following keywords may be used when specifying the ICMP type:

Table 19.

Keyword Value
echo-reply 0
destination-unreachable 3
source-quench 4
redirect 5
echo-request 8
router-advertisement 9
router-solicitation 10
time-exceeded 11
parameter-problem 12
timestamp-request 13
timestamp-reply 14
info-request 15
info-reply 16
address-mask-request 17
address-mask-reply 18

示例 9. ICMP Type specification


# match ping packets
filter output icmp type { echo-request, echo-reply }

ICMPv6 Type type

Table 20.

Name Keyword Size Base type
ICMPv6 Type icmpv6_type 8 bit integer

The ICMPv6 Type type is used to conveniently specify the ICMPv6 header's type field.

The following keywords may be used when specifying the ICMPv6 type:

Table 21.

Keyword Value
destination-unreachable 1
packet-too-big 2
time-exceeded 3
parameter-problem 4
echo-request 128
echo-reply 129
mld-listener-query 130
mld-listener-report 131
mld-listener-done 132
mld-listener-reduction 132
nd-router-solicit 133
nd-router-advert 134
nd-neighbor-solicit 135
nd-neighbor-advert 136
nd-redirect 137
router-renumbering 138
ind-neighbor-solicit 141
ind-neighbor-advert 142
mld2-listener-report 143

示例 10. ICMPv6 Type specification


# match ICMPv6 ping packets
filter output icmpv6 type { echo-request, echo-reply }

Primary expressions

The lowest order expression is a primary expression, representing either a constant or a single datum from a packet's payload, meta data or a stateful module.

Meta expressions

meta {length | nfproto | l4proto | protocol | priority}

[meta] {mark | iif | iifname | iiftype | oif | oifname | oiftype} [meta] {skuid | skgid | nftrace | rtclassid | ibriport | obriport | pkttype | cpu | iifgroup | oifgroup | cgroup | random}

A meta expression refers to meta data associated with a packet.

There are two types of meta expressions: unqualified and qualified meta expressions. Qualified meta expressions require the meta keyword before the meta key, unqualified meta expressions can be specified by using the meta key directly or as qualified meta expressions.

Table 22. Meta expression types

Keyword Description Type
length Length of the packet in bytes integer (32 bit)
protocol Ethertype protocol value ether_type
priority TC packet priority tc_handle
mark Packet mark mark
iif Input interface index iface_index
iifname Input interface name string
iiftype Input interface type iface_type
oif Output interface index iface_index
oifname Output interface name string
oiftype Output interface hardware type iface_type
skuid UID associated with originating socket uid
skgid GID associated with originating socket gid
rtclassid Routing realm realm
ibriport Input bridge interface name string
obriport Output bridge interface name string
pkttype packet type pkt_type
cpu cpu number processing the packet integer (32 bits)
iifgroup incoming device group devgroup
oifgroup outgoing device group devgroup
cgroup control group id integer (32 bits)
random pseudo-random number integer (32 bits)

Table 23. Meta expression specific types

Type Description
iface_index Interface index (32 bit number). Can be specified numerically or as name of an existing interface.
ifname Interface name (16 byte string). Does not have to exist.
iface_type Interface type (16 bit number).
uid User ID (32 bit number). Can be specified numerically or as user name.
gid Group ID (32 bit number). Can be specified numerically or as group name.
realm Routing Realm (32 bit number). Can be specified numerically or as symbolic name defined in /etc/iproute2/rt_realms.
devgroup_type Device group (32 bit number). Can be specified numerically or as symbolic name defined in /etc/iproute2/group.
pkt_type Packet type: Unicast (addressed to local host), Broadcast (to all), Multicast (to group).

示例 11. Using meta expressions


# qualified meta expression
filter output meta oif eth0

# unqualified meta expression
filter output oif eth0

fib expressions

fib {saddr daddr [mark iif oif]} {oif oifname type}

A fib expression queries the fib (forwarding information base) to obtain information such as the output interface index a particular address would use. The input is a tuple of elements that is used as input to the fib lookup functions.

Table 24. fib expression specific types

Keyword Description Type
oif Output interface index integer (32 bit)
oifname Output interface name string
type Address type fib_addrtype

示例 12. Using fib expressions


# drop packets without a reverse path
filter prerouting fib saddr . iif oif missing drop

# drop packets to address not configured on ininterface
filter prerouting fib daddr . iif type != { local, broadcast, multicast } drop

# perform lookup in a specific 'blackhole' table (0xdead, needs ip appropriate ip rule)
filter prerouting meta mark set 0xdead fib daddr . mark type vmap { blackhole : drop, prohibit : jump prohibited, unreachable : drop }

Routing expressions

rt {classid nexthop}

A routing expression refers to routing data associated with a packet.

Table 25. Routing expression types

Keyword Description Type
classid Routing realm realm
nexthop Routing nexthop ipv4_addr/ipv6_addr

Table 26. Routing expression specific types

Type Description
realm Routing Realm (32 bit number). Can be specified numerically or as symbolic name defined in /etc/iproute2/rt_realms.

示例 13. Using routing expressions


# IP family independent rt expression
filter output rt classid 10

# IP family dependent rt expressions
ip filter output rt nexthop 192.168.0.1
ip6 filter output rt nexthop fd00::1
inet filter meta nfproto ipv4 output rt nexthop 192.168.0.1
inet filter meta nfproto ipv6 output rt nexthop fd00::1

Payload expressions

Payload expressions refer to data from the packet's payload.

Ethernet header expression

ether [ethernet header field]

Table 27. Ethernet header expression types

Keyword Description Type
daddr Destination MAC address ether_addr
saddr Source MAC address ether_addr
type EtherType ether_type

VLAN header expression

vlan [VLAN header field]

Table 28. VLAN header expression

Keyword Description Type
id VLAN ID (VID) integer (12 bit)
cfi Canonical Format Indicator integer (1 bit)
pcp Priority code point integer (3 bit)
type EtherType ether_type

ARP header expression

arp [ARP header field]

Table 29. ARP header expression

Keyword Description Type
htype ARP hardware type integer (16 bit)
ptype EtherType ether_type
hlen Hardware address len integer (8 bit)
plen Protocol address len integer (8 bit)
operation Operation arp_op

IPv4 header expression

ip [IPv4 header field]

Table 30. IPv4 header expression

Keyword Description Type
version IP header version (4) integer (4 bit)
hdrlength IP header length including options integer (4 bit) FIXME scaling
dscp Differentiated Services Code Point dscp
ecn Explicit Congestion Notification ecn
length Total packet length integer (16 bit)
id IP ID integer (16 bit)
frag-off Fragment offset integer (16 bit)
ttl Time to live integer (8 bit)
protocol Upper layer protocol inet_proto
checksum IP header checksum integer (16 bit)
saddr Source address ipv4_addr
daddr Destination address ipv4_addr

ICMP header expression

icmp [ICMP header field]

Table 31. ICMP header expression

Keyword Description Type
type ICMP type field icmp_type
code ICMP code field integer (8 bit)
checksum ICMP checksum field integer (16 bit)
id ID of echo request/response integer (16 bit)
sequence sequence number of echo request/response integer (16 bit)
gateway gateway of redirects integer (32 bit)
mtu MTU of path MTU discovery integer (16 bit)

IPv6 header expression

ip6 [IPv6 header field]

Table 32. IPv6 header expression

Keyword Description Type
version IP header version (6) integer (4 bit)
dscp Differentiated Services Code Point dscp
ecn Explicit Congestion Notification ecn
flowlabel Flow label integer (20 bit)
length Payload length integer (16 bit)
nexthdr Nexthdr protocol inet_proto
hoplimit Hop limit integer (8 bit)
saddr Source address ipv6_addr
daddr Destination address ipv6_addr

ICMPv6 header expression

icmpv6 [ICMPv6 header field]

Table 33. ICMPv6 header expression

Keyword Description Type
type ICMPv6 type field icmpv6_type
code ICMPv6 code field integer (8 bit)
checksum ICMPv6 checksum field integer (16 bit)
parameter-problem pointer to problem integer (32 bit)
packet-too-big oversized MTU integer (32 bit)
id ID of echo request/response integer (16 bit)
sequence sequence number of echo request/response integer (16 bit)
max-delay maximum response delay of MLD queries integer (16 bit)

TCP header expression

tcp [TCP header field]

Table 34. TCP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
sequence Sequence number integer (32 bit)
ackseq Acknowledgement number integer (32 bit)
doff Data offset integer (4 bit) FIXME scaling
reserved Reserved area integer (4 bit)
flags TCP flags tcp_flag
window Window integer (16 bit)
checksum Checksum integer (16 bit)
urgptr Urgent pointer integer (16 bit)

UDP header expression

udp [UDP header field]

Table 35. UDP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
length Total packet length integer (16 bit)
checksum Checksum integer (16 bit)

UDP-Lite header expression

udplite [UDP-Lite header field]

Table 36. UDP-Lite header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
checksum Checksum integer (16 bit)

SCTP header expression

sctp [SCTP header field]

Table 37. SCTP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service
vtag Verfication Tag integer (32 bit)
checksum Checksum integer (32 bit)

DCCP header expression

dccp [DCCP header field]

Table 38. DCCP header expression

Keyword Description Type
sport Source port inet_service
dport Destination port inet_service

Authentication header expression

ah [AH header field]

Table 39. AH header expression

Keyword Description Type
nexthdr Next header protocol inet_proto
hdrlength AH Header length integer (8 bit)
reserved Reserved area integer (16 bit)
spi Security Parameter Index integer (32 bit)
sequence Sequence number integer (32 bit)

Encrypted security payload header expression

esp [ESP header field]

Table 40. ESP header expression

Keyword Description Type
spi Security Parameter Index integer (32 bit)
sequence Sequence number integer (32 bit)

IPcomp header expression

comp [IPComp header field]

Table 41. IPComp header expression

Keyword Description Type
nexthdr Next header protocol inet_proto
flags Flags bitmask
cpi Compression Parameter Index integer (16 bit)

Extension header expressions

Extension header expressions refer to data from variable-sized protocol headers, such as IPv6 extension headers and TCPs options.

nftables currently supports matching (finding) a given ipv6 extension header or TCP option.

hbh {nexthdr | hdrlength}

frag {nexthdr | frag-off | more-fragments | id}

rt {nexthdr | hdrlength | type | seg-left}

dst {nexthdr | hdrlength}

mh {nexthdr | hdrlength | checksum | type}

tcp option {eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 | timestamp} [_tcp_option_field_]

The following syntaxes are valid only in a relational expression with boolean type on right-hand side for checking header existence only:

exthdr {hbh | frag | rt | dst | mh}

tcp option {eol | noop | maxseg | window | sack-permitted | sack | sack0 | sack1 | sack2 | sack3 | timestamp}

Table 42. IPv6 extension headers

Keyword Description
hbh Hop by Hop
rt Routing Header
frag Fragmentation header
dst dst options
mh Mobility Header

Table 43. TCP Options

Keyword Description TCP option fields
eol End of option list kind
noop 1 Byte TCP No-op options kind
maxseg TCP Maximum Segment Size kind, length, size
window TCP Window Scaling kind, length, count
sack-permitted TCP SACK permitted kind, length
sack TCP Selective Acknowledgement (alias of block 0) kind, length, left, right
sack0 TCP Selective Acknowledgement (block 0) kind, length, left, right
sack1 TCP Selective Acknowledgement (block 1) kind, length, left, right
sack2 TCP Selective Acknowledgement (block 2) kind, length, left, right
sack3 TCP Selective Acknowledgement (block 3) kind, length, left, right
timestamp TCP Timestamps kind, length, tsval, tsecr

示例 14. finding TCP options

filter input tcp option sack-permitted kind 1 counter

示例 15. matching IPv6 exthdr

ip6 filter input frag more-fragments 1 counter

Conntrack expressions

Conntrack expressions refer to meta data of the connection tracking entry associated with a packet.

There are three types of conntrack expressions. Some conntrack expressions require the flow direction before the conntrack key, others must be used directly because they are direction agnostic. The packets, bytes and avgpkt keywords can be used with or without a direction. If the direction is omitted, the sum of the original and the reply direction is returned. The same is true for the zone, if a direction is given, the zone is only matched if the zone id is tied to the given direction.

ct {state | direction | status | mark | expiration | helper | label | l3proto | protocol | bytes | packets | avgpkt | zone}

ct {original | reply} {l3proto | protocol | saddr | daddr | proto-src | proto-dst | bytes | packets | avgpkt | zone}

Table 44. Conntrack expressions

Keyword Description Type
state State of the connection ct_state
direction Direction of the packet relative to the connection ct_dir
status Status of the connection ct_status
mark Connection mark mark
expiration Connection expiration time time
helper Helper associated with the connection string
label Connection tracking label bit or symbolic name defined in connlabel.conf in the nftables include path ct_label
l3proto Layer 3 protocol of the connection nf_proto
saddr Source address of the connection for the given direction ipv4_addr/ipv6_addr
daddr Destination address of the connection for the given direction ipv4_addr/ipv6_addr
protocol Layer 4 protocol of the connection for the given direction inet_proto
proto-src Layer 4 protocol source for the given direction integer (16 bit)
proto-dst Layer 4 protocol destination for the given direction integer (16 bit)
packets packet count seen in the given direction or sum of original and reply integer (64 bit)
bytes bytecount seen, see description for packets keyword integer (64 bit)
avgpkt average bytes per packet, see description for packets keyword integer (64 bit)
zone conntrack zone integer (16 bit)

Statements

Statements represent actions to be performed. They can alter control flow (return, jump to a different chain, accept or drop the packet) or can perform actions, such as logging, rejecting a packet, etc.

Statements exist in two kinds. Terminal statements unconditionally terminate evaluation of the current rule, non-terminal statements either only conditionally or never terminate evaluation of the current rule, in other words, they are passive from the ruleset evaluation perspective. There can be an arbitrary amount of non-terminal statements in a rule, but only a single terminal statement as the final statement.

Verdict statement

The verdict statement alters control flow in the ruleset and issues policy decisions for packets.

{accept | drop | queue | continue | return}

{jump | goto} {chain}

accept

Terminate ruleset evaluation and accept the packet.

drop

Terminate ruleset evaluation and drop the packet.

queue

Terminate ruleset evaluation and queue the packet to userspace.

continue

Continue ruleset evaluation with the next rule. FIXME

return

Return from the current chain and continue evaluation at the next rule in the last chain. If issued in a base chain, it is equivalent to ***accept***.

jump chain

Continue evaluation at the first rule in chain. The current position in the ruleset is pushed to a call stack and evaluation will continue there when the new chain is entirely evaluated of a ***return*** verdict is issued.

goto chain

Similar to ***jump***, but the current position is not pushed to the call stack, meaning that after the new chain evaluation will continue at the last chain instead of the one containing the goto statement.

示例 16. Verdict statements

# process packets from eth0 and the internal network in from_lan
# chain, drop all packets from eth0 with different source addresses.

filter input iif eth0 ip saddr 192.168.0.0/24 jump from_lan
filter input iif eth0 drop

Payload statement

The payload statement alters packet content. It can be used for example to set ip DSCP (differv) header field or ipv6 flow labels.

示例 17. route some packets instead of bridging

# redirect tcp:http from 192.160.0.0/16 to local machine for routing instead of bridging
# assumes 00:11:22:33:44:55 is local MAC address.
bridge input meta iif eth0 ip saddr 192.168.0.0/16 tcp dport 80 meta pkttype set unicast ether daddr set 00:11:22:33:44:55

示例 18. Set IPv4 DSCP header field

ip forward ip dscp set 42

Log statement

log [prefix _quoted_string_] [level _syslog-level_] [flags log-flags]

log [group _nflog_group_] [prefix _quoted_string_] [queue-threshold value] [snaplen size]

The log statement enables logging of matching packets. When this statement is used from a rule, the Linux kernel will print some information on all matching packets, such as header fields, via the kernel log (where it can be read with dmesg(1) or read in the syslog). If the group number is specified, the Linux kernel will pass the packet to nfnetlink_log which will multicast the packet through a netlink socket to the specified multicast group. One or more userspace processes may subscribe to the group to receive the packets, see libnetfilter_queue documentation for details. This is a non-terminating statement, so the rule evaluation continues after the packet is logged.

Table 45. log statement options

Keyword Description Type
prefix Log message prefix quoted string
syslog-level Syslog level of logging string: emerg, alert, crit, err, warn [default], notice, info, debug
group NFLOG group to send messages to unsigned integer (16 bit)
snaplen Length of packet payload to include in netlink message unsigned integer (32 bit)
queue-threshold Number of packets to queue inside the kernel before sending them to userspace unsigned integer (32 bit)

Table 46. log-flags

Flag Description
tcp sequence Log TCP sequence numbers.
tcp options Log options from the TCP packet header.
ip options Log options from the IP/IPv6 packet header.
skuid Log the userid of the process which generated the packet.
ether Decode MAC addresses and protocol.
all Enable all log flags listed above.

示例 19. Using log statement

# log the UID which generated the packet and ip options
ip filter output log flags skuid flags ip options

# log the tcp sequence numbers and tcp options from the TCP packet
ip filter output log flags tcp sequence,options

# enable all supported log flags
ip6 filter output log flags all

Reject statement

reject [with] {icmp | icmp6 | icmpx} [type] {icmp_type | icmp6_type | icmpx_type}

reject [with] {tcp} {reset}

A reject statement is used to send back an error packet in response to the matched packet otherwise it is equivalent to drop so it is a terminating statement, ending rule traversal. This statement is only valid in the input, forward and output chains, and user-defined chains which are only called from those chains.

Table 47. reject statement type (ip)

Value Description Type
icmp_type ICMP type response to be sent to the host net-unreachable, host-unreachable, prot-unreachable, port-unreachable [default], net-prohibited, host-prohibited, admin-prohibited

Table 48. reject statement type (ip6)

Value Description Type
icmp6_type ICMPv6 type response to be sent to the host no-route, admin-prohibited, addr-unreachable, port-unreachable [default], policy-fail, reject-route

Table 49. reject statement type (inet)

Value Description Type
icmpx_type ICMPvXtype abstraction response to be sent to the host, this is a set of types that overlap in IPv4 and IPv6 to be used from the inet family. port-unreachable [default], admin-prohibited, no-route, host-unreachable

Counter statement

A counter statement sets the hit count of packets along with the number of bytes.

counter {packets _number_ } {bytes _number_ }

Conntrack statement

The conntrack statement can be used to set the conntrack mark and conntrack labels.

ct {mark | eventmask | label | zone} [set] value

The ct statement sets meta data associated with a connection. The zone id has to be assigned before a conntrack lookup takes place, i.e. this has to be done in prerouting and possibly output (if locally generated packets need to be placed in a distinct zone), with a hook priority of -300.

Table 50. Conntrack statement types

Keyword Description Value
eventmask conntrack event bits bitmask, integer (32 bit)
helper name of ct helper object to assign to the connection quoted string
mark Connection tracking mark mark
label Connection tracking label label
zone conntrack zone integer (16 bit)

示例 20. save packet nfmark in conntrack

ct mark set meta mark

示例 21. set zone mapped via interface

table inet raw {
  chain prerouting {
      type filter hook prerouting priority -300;
      ct zone set iif map { "eth1" : 1, "veth1" : 2 }
  }
  chain output {
      type filter hook output priority -300;
      ct zone set oif map { "eth1" : 1, "veth1" : 2 }
  }
}

示例 22. restrict events reported by ctnetlink

ct eventmask set new or related or destroy

Meta statement

A meta statement sets the value of a meta expression. The existing meta fields are: priority, mark, pkttype, nftrace.

meta {mark | priority | pkttype | nftrace} [set] value

A meta statement sets meta data associated with a packet.

Table 51. Meta statement types

Keyword Description Value
priority TC packet priority tc_handle
mark Packet mark mark
pkttype packet type pkt_type
nftrace ruleset packet tracing on/off. Use monitor trace command to watch traces 0, 1

Limit statement

limit [rate] [over]_packet_number_ [/] {second | minute | hour | day} [burst _packet_number_ packets]

limit [rate] [over]_byte_number_ {bytes | kbytes | mbytes} [/] {second | minute | hour | day | week} [burst _byte_number_ bytes]

A limit statement matches at a limited rate using a token bucket filter. A rule using this statement will match until this limit is reached. It can be used in combination with the log statement to give limited logging. The over keyword, that is optional, makes it match over the specified rate.

Table 52. limit statement values

Value Description Type
packet_number Number of packets unsigned integer (32 bit)
byte_number Number of bytes unsigned integer (32 bit)

NAT statements

snat [to _address_ [:port]] [persistent, random, fully-random]

snat [to _address_ - _address_ [:_port_ - _port_]] [persistent, random, fully-random]

dnat [to _address_ [:_port_]] [persistent, random, fully-random]

dnat [to _address_ [:_port_ - _port_]] [persistent, random, fully-random]

masquerade [to [:_port_]] [persistent, random, fully-random]

masquerade [to [:_port_ - _port_]] [persistent, random, fully-random]

redirect [to [:_port_]] [persistent, random, fully-random]

redirect [to [:_port_ - _port_]] [persistent, random, fully-random]

The nat statements are only valid from nat chain types.

The snat and masquerade statements specify that the source address of the packet should be modified. While snat is only valid in the postrouting and input chains, masquerade makes sense only in postrouting. The dnat and redirect statements are only valid in the prerouting and output chains, they specify that the destination address of the packet should be modified. You can use non-base chains which are called from base chains of nat chain type too. All future packets in this connection will also be mangled, and rules should cease being examined.

The masquerade statement is a special form of snat which always uses the outgoing interface's IP address to translate to. It is particularly useful on gateways with dynamic (public) IP addresses.

The redirect statement is a special form of dnat which always translates the destination address to the local host's one. It comes in handy if one only wants to alter the destination port of incoming traffic on different interfaces.

Note that all nat statements require both prerouting and postrouting base chains to be present since otherwise packets on the return path won't be seen by netfilter and therefore no reverse translation will take place.

Table 53. NAT statement values

Expression Description Type
address Specifies that the source/destination address of the packet should be modified. You may specify a mapping to relate a list of tuples composed of arbitrary expression key with address value. ipv4_addr, ipv6_addr, eg. abcd::1234, or you can use a mapping, eg. meta mark map { 10 : 192.168.1.2, 20 : 192.168.1.3 }
port Specifies that the source/destination address of the packet should be modified. port number (16 bits)

Table 54. NAT statement flags

Flag Description
persistent Gives a client the same source-/destination-address for each connection.
random If used then port mapping will be randomized using a random seeded MD5 hash mix using source and destination address and destination port.
fully-random If used then port mapping is generated based on a 32-bit pseudo-random algorithm.

示例 23. Using NAT statements

# create a suitable table/chain setup for all further examples
add table nat
add chain nat prerouting { type nat hook prerouting priority 0; }
add chain nat postrouting { type nat hook postrouting priority 100; }

# translate source addresses of all packets leaving via eth0 to address 1.2.3.4
add rule nat postrouting oif eth0 snat to 1.2.3.4

# redirect all traffic entering via eth0 to destination address 192.168.1.120
add rule nat prerouting iif eth0 dnat to 192.168.1.120

# translate source addresses of all packets leaving via eth0 to whatever
# locally generated packets would use as source to reach the same destination
add rule nat postrouting oif eth0 masquerade

# redirect incoming TCP traffic for port 22 to port 2222
add rule nat prerouting tcp dport 22 redirect to :2222

Queue statement

This statement passes the packet to userspace using the nfnetlink_queue handler. The packet is put into the queue identified by its 16-bit queue number. Userspace can inspect and modify the packet if desired. Userspace must then drop or reinject the packet into the kernel. See libnetfilter_queue documentation for details.

queue [num _queue_number_] [bypass]

queue [num _queue_number_from_ - _queue_number_to_] [bypass,fanout]

Table 55. queue statement values

Value Description Type
queue_number Sets queue number, default is 0. unsigned integer (16 bit)
queue_number_from Sets initial queue in the range, if fanout is used. unsigned integer (16 bit)
queue_number_to Sets closing queue in the range, if fanout is used. unsigned integer (16 bit)

Table 56. queue statement flags

Flag Description
bypass Let packets go through if userspace application cannot back off. Before using this flag, read libnetfilter_queue documentation for performance tuning recomendations.
fanout Distribute packets between several queues.

Additional commands

These are some additional commands included in nft.

export

Export your current ruleset in XML or JSON format to stdout.

Examples:

[...]
% nft export json
[...]

monitor

The monitor command allows you to listen to Netlink events produced by the nf_tables subsystem, related to creation and deletion of objects. When they ocurr, nft will print to stdout the monitored events in either XML, JSON or native nft format.

To filter events related to a concrete object, use one of the keywords 'tables', 'chains', 'sets', 'rules', 'elements'.

To filter events related to a concrete action, use keyword 'new' or 'destroy'.

Hit ^C to finish the monitor operation.

示例 24. Listen to all events, report in native nft format

% nft monitor

示例 25. Listen to added tables, report in XML format

% nft monitor new tables xml

示例 26. Listen to deleted rules, report in JSON format

% nft monitor destroy rules json

示例 27. Listen to both new and destroyed chains, in native nft format

% nft monitor chains

Error reporting

When an error is detected, nft shows the line(s) containing the error, the position of the erroneous parts in the input stream and marks up the erroneous parts using carrets (^). If the error results from the combination of two expressions or statements, the part imposing the constraints which are violated is marked using tildes (~).

For errors returned by the kernel, nft can't detect which parts of the input caused the error and the entire command is marked.

示例 28. Error caused by single incorrect expression

<cmdline>:1:19-22: Error: Interface does not exist
filter output oif eth0
                  ^^^^

示例 29. Error caused by invalid combination of two expressions

<cmdline>:1:28-36: Error: Right hand side of relational expression (==) must be constant
filter output tcp dport == tcp dport
                        ~~ ^^^^^^^^^

示例 30. Error returned by the kernel

<cmdline>:0:0-23: Error: Could not process rule: Operation not permitted
filter output oif wlan0
^^^^^^^^^^^^^^^^^^^^^^^

退出状态码

On success, nft exits with a status of 0. Unspecified errors cause it to exit with a status of 1, memory allocation errors with a status of 2, unable to open Netlink socket with 3.

See Also

iptables(8), ip6tables(8), arptables(8), ebtables(8), ip(8), tc(8)

There is an official wiki at: wiki.nftables.org

Authors

nftables was written by Patrick McHardy and Pablo Neira Ayuso, among many other contributors from the Netfilter community.

nftables is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License version 2 as published by the Free Software Foundation.

This documentation is licenced under the terms of the Creative Commons Attribution-ShareAlike 4.0 license, CC BY-SA 4.0.