PatrickMcHardykaber@trash.net2008-2014Patrick McHardyPablo NeiraNeira Ayusopablo@netfilter.org2013-2016Pablo Neira Ayusonft8nft
Administration tool for packet filtering and classification
nftdirectoryfilenamecmdnftDescription
nft is used to set up, maintain and inspect packet
filtering and classification rules in the Linux kernel.
Options
For a full summary of options, run nft --help.
Show help message and all options.
Show version.
Show data numerically. When used once (the default behaviour), skip
lookup of addresses to symbolic names. Use twice to also show Internet
services (port numbers) numerically. Use three times to also show
protocols and UIDs/GIDs numerically.
Omit stateful information of rules and stateful objects.
Translate IP addresses to names. Usually requires network traffic for DNS lookup.
Show rule handles in output.
Add the directory directory to the list of directories to be searched for included files.
Read input from filename.
Read input from an interactive readline CLI.
Input file formatLexical conventions
Input is parsed line-wise. When the last character of a line, just before
the newline character, is a non-quoted backslash (\),
the next line is treated as a continuation. Multiple commands on the
same line can be separated using a semicolon (;).
A hash sign (#) begins a comment. All following characters
on the same line are ignored.
Identifiers begin with an alphabetic character (a-z,A-Z),
followed zero or more alphanumeric characters (a-z,A-Z,0-9)
and the characters slash (/), backslash (\),
underscore (_) and dot (.). Identifiers
using different characters or clashing with a keyword need to be enclosed in
double quotes (").
Include filesinclude "filename"
Other files can be included by using the include statement.
The directories to be searched for include files can be specified using
the option.
Symbolic variablesdefinevariable = 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.
Using symbolic variables
define int_if1 = eth0
define int_if2 = eth1
define int_ifs = { $int_if1, $int_if2 }
filter input iif $int_ifs accept
Address families
Address families determine the type of packets which are processed. For each address
family the kernel contains so called hooks at specific stages of the packet processing
paths, which invoke nftables if rules for these hooks exist.
IPv4 address family.
IPv6 address family.
Internet (IPv4/IPv6) address family.
ARP address family, handling packets vi
Bridge address family, handling packets which traverse a bridge device.
Netdev address family, handling packets from ingress.
All nftables objects exist in address family specific namespaces, therefore
all identifiers include an address family. If an identifier is specified without
an address family, the ip family is used by default.
IPv4/IPv6/Inet address families
The IPv4/IPv6/Inet address families handle IPv4, IPv6 or both types of packets. They
contain five hooks at different packet processing stages in the network stack.
IPv4/IPv6/Inet address family hooksHookDescriptionprerouting
All packets entering the system are processed by the prerouting hook. It is invoked
before the routing process and is used for early filtering or changing packet
attributes that affect routing.
input
Packets delivered to the local system are processed by the input hook.
forward
Packets forwarded to a different host are processed by the forward hook.
output
Packets sent by local processes are processed by the output hook.
postrouting
All packets leaving the system are processed by the postrouting hook.
ARP address family
The ARP address family handles ARP packets received and sent by the system. It is commonly used
to mangle ARP packets for clustering.
ARP address family hooksHookDescriptioninput
Packets delivered to the local system are processed by the input hook.
output
Packets send by the local system are processed by the output hook.
Bridge address family
The bridge address family handles ethernet packets traversing bridge devices.
Netdev address family
The Netdev address family handles packets from ingress.
Netdev address family hooksHookDescriptioningress
All packets entering the system are processed by this hook. It is invoked
before layer 3 protocol handlers and it can be used for early filtering and
policing.
Tablesadddeletelistflushtablefamilytable
Tables are containers for chains, sets and stateful objects. They are identified by their address family
and their name. The address family must be one of
ipip6inetarpbridgenetdev.
The inet address family is a dummy family which is used to create
hybrid IPv4/IPv6 tables.
When no address family is specified, ip is used by default.
Add a new table for the given family with the given name.
Delete the specified table.
List all chains and rules of the specified table.
Flush all chains and rules of the specified table.
Chainsaddchainfamilytablechainhookprioritypolicydeviceaddcreatedeletelistflushchainfamilytablechainrenamechainfamilytablechainnewname
Chains are containers for rules. They exist in two kinds,
base chains and regular chains. A base chain is an entry point for
packets from the networking stack, a regular chain may be used
as jump target and is used for better rule organization.
Add a new chain in the specified table. When a hook and priority
value are specified, the chain is created as a base chain and hooked
up to the networking stack.
Simlar to the add command, but returns an error if the
chain already exists.
Delete the specified chain. The chain must not contain any rules or be
used as jump target.
Rename the specified chain.
List all rules of the specified chain.
Flush all rules of the specified chain.
Rulesaddinsertrulefamilytablechainposition positionstatementdeleterulefamilytablechainhandle handle
Rules are constructed from two kinds of components according to a set
of grammatical rules: expressions and statements.
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.
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 the specified rule.
Setsadd setfamilytableset
{
typeflagstimeoutgc-intervalelementssizepolicy
}
deletelistflush setfamilytableset
Sets are elements containers of an user-defined data type, they are uniquely identified by an user-defined name and attached to tables.
Add a new set in the specified table.
Delete the specified set.
Display the elements in the specified set.
Remove all elements from the specified set.
Set specificationsKeywordDescriptionTypetypedata type of set elementsstring: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, markflagsset flagsstring: constant, interval, timeouttimeouttime an element stays in the setstring, decimal followed by unit. Units are: d, h, m, sgc-intervalgarbage collection interval, only available when timeout or flag timeout are activestring, decimal followed by unit. Units are: d, h, m, selementselements contained by the setset data typesizemaximun number of elements in the setunsigned integer (64 bit)policyset policystring: performance [default], memory
Mapsadd mapfamilytablemap
{
typeflagselementssizepolicy
}
deletelist mapfamilytablemap
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 a new map in the specified table.
Delete the specified map.
Display the elements in the specified map.
Map specificationsKeywordDescriptionTypetypedata type of map elementsstring ':' string: ipv4_addr, ipv6_addr, ether_addr, inet_proto, inet_service, mark, counter, quota. Counter and quota can't be used as keysflagsmap flagsstring: constant, intervalelementselements contained by the mapmap data typesizemaximun number of elements in the mapunsigned integer (64 bit)policymap policystring: performance [default], memory
Stateful objectsadddeletelistreset typefamilytableobject
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 a new stateful object in the specified table.
Delete the specified object.
Display stateful information the object holds.
List-and-reset stateful object.
Countercounterpackets bytes
Counter specificationsKeywordDescriptionTypepacketsinitial count of packetsunsigned integer (64 bit)bytesinitial count of bytesunsigned integer (64 bit)
Quotaquotaoveruntilused
Quota specificationsKeywordDescriptionTypequotaquota limit, used as the quota nameTwo arguments, unsigned interger (64 bit) and string: bytes, kbytes, mbytes. "over" and "until" go before these argumentsusedinitial value of used quotaTwo 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 commanddescribeexpression
The describe command shows information about the type of an expression and
its data type.
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
NameKeywordSizeBase typeIntegerintegervariable-
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
The bitmask type (bitmask) is used for bitmasks.
String type
NameKeywordSizeBase typeStringstringvariable-
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.
String specification
# Interface name
filter input iifname eth0
# Weird interface name
filter input iifname "(eth0)"
Link layer address type
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 (:).
Link layer address specification
# Ethernet destination MAC address
filter input ether daddr 20:c9:d0:43:12:d9
IPv4 address type
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.
IPv4 address specification
# dotted decimal notation
filter output ip daddr 127.0.0.1
# host name
filter output ip daddr localhost
IPv6 address type
The IPv6 address type is used for IPv6 addresses. FIXME
IPv6 address specification
# abbreviated loopback address
filter output ip6 daddr ::1
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 expressionsmetalengthnfprotol4protoprotocolprioritymetamarkiifiifnameiiftypeoifoifnameoiftypeskuidskgidnftracertclassidibriportobriportpkttypecpuiifgroupoifgroupcgrouprandom
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.
Meta expression typesKeywordDescriptionTypelengthLength of the packet in bytesinteger (32 bit)protocolEthertype protocol valueether_typepriorityTC packet prioritytc_handlemarkPacket markmarkiifInput interface indexiface_indexiifnameInput interface namestringiiftypeInput interface typeiface_typeoifOutput interface indexiface_indexoifnameOutput interface namestringoiftypeOutput interface hardware typeiface_typeskuidUID associated with originating socketuidskgidGID associated with originating socketgidrtclassidRouting realmrealmibriportInput bridge interface namestringobriportOutput bridge interface namestringpkttypepacket typepkt_typecpucpu number processing the packetinteger (32 bits)iifgroupincoming device groupdevgroupoifgroupoutgoing device groupdevgroupcgroupcontrol group idinteger (32 bits)randompseudo-random numberinteger (32 bits)
Meta expression specific typesTypeDescriptioniface_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).
Using meta expressions
# qualified meta expression
filter output meta oif eth0
# unqualified meta expression
filter output oif eth0
fib expressionsfibsaddrdaddrmarkiifoifoifoifnametype
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.
Using fib expressions
# drop packets without a reverse path
filter prerouting fib saddr . iif oif eq 0 drop
# drop packets to address not configured on ininterface
filter input fib daddr . iif type not { 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 { backhole : drop, prohibit : jump prohibited, unreachable : drop }
Routing expressionsrtclassidnexthop
A routing expression refers to routing data associated with a packet.
Routing expression specific typesTypeDescriptionrealm
Routing Realm (32 bit number). Can be specified numerically
or as symbolic name defined in /etc/iproute2/rt_realms.
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 expressionetherethernet header field
Ethernet header expression typesKeywordDescriptionTypedaddrDestination MAC addressether_addrsaddrSource MAC addressether_addrtypeEtherTypeether_type
VLAN header expressionvlanVLAN header field
VLAN header expressionKeywordDescriptionTypeidVLAN ID (VID)integer (12 bit)cfiCanonical Format Indicatorinteger (1 bit)pcpPriority code pointinteger (3 bit)typeEtherTypeether_type
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.
hbhnexthdrhdrlengthfragnexthdrfrag-offmore-fragmentsidrtnexthdrhdrlengthtypeseg-leftdstnexthdrhdrlengthmhnexthdrhdrlengthchecksumtypetcp optioneolnoopmaxsegwindowsack-permittedsacksack0sack1sack2sack3timestamptcp_option_field
IPv6 extension headersKeywordDescriptionhbhHop by HoprtRouting HeaderfragFragmentation headerdstdst optionsmhMobility Header
finding TCP options
filter input tcp option sack-permitted kind 1 counter
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.
ctstatedirectionstatusmarkexpirationhelperlabell3protoprotocolbytespacketsavgpktzonectoriginalreplyl3protoprotocolsaddrdaddrproto-srcproto-dstbytespacketsavgpktzone
Conntrack expressionsKeywordDescriptionTypestateState of the connectionct_statedirectionDirection of the packet relative to the connectionct_dirstatusStatus of the connectionct_statusmarkConnection markmarkexpirationConnection expiration timetimehelperHelper associated with the connectionstringlabelConnection tracking label bit or symbolic name defined in connlabel.conf in the nftables include pathct_labell3protoLayer 3 protocol of the connectionnf_protosaddrSource address of the connection for the given directionipv4_addr/ipv6_addrdaddrDestination address of the connection for the given directionipv4_addr/ipv6_addrprotocolLayer 4 protocol of the connection for the given directioninet_protoproto-srcLayer 4 protocol source for the given directioninteger (16 bit)proto-dstLayer 4 protocol destination for the given directioninteger (16 bit)packetspacket count seen in the given direction or sum of original and replyinteger (64 bit)bytesbytecount seen, see description for packets keywordinteger (64 bit)avgpktaverage bytes per packet, see description for packets keywordinteger (64 bit)zoneconntrack zoneinteger (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.
acceptdropqueuecontinuereturnjumpgotochain
Terminate ruleset evaluation and accept the packet.
Terminate ruleset evaluation and drop the packet.
Terminate ruleset evaluation and queue the packet to userspace.
Continue ruleset evaluation with the next rule. FIXME
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.
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.
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.
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.
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
Set IPv4 DSCP header field
ip forward ip dscp set 42
Log statementlogprefix
quoted_stringlevel
syslog-levelflags
log-flagsloggroup
nflog_groupprefix
quoted_stringqueue-threshold
valuesnaplen
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.
log statement optionsKeywordDescriptionTypeprefixLog message prefixquoted stringsyslog-levelSyslog level of loggingstring: emerg, alert, crit, err, warn [default], notice, info, debuggroupNFLOG group to send messages tounsigned integer (16 bit)snaplenLength of packet payload to include in netlink messageunsigned integer (32 bit)queue-thresholdNumber of packets to queue inside the kernel before sending them to userspaceunsigned integer (32 bit)
log-flagsFlagDescriptiontcp sequenceLog TCP sequence numbers.tcp optionsLog options from the TCP packet header.ip optionsLog options from the IP/IPv6 packet header.skuidLog the userid of the process which generated the packet.etherDecode MAC addresses and protocol.allEnable all log flags listed above.
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 statementrejectwithicmpicmp6icmpxtypeicmp_typeicmp6_typeicmpx_typerejectwithtcpreset
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.
reject statement type (ip)ValueDescriptionTypeicmp_typeICMP type response to be sent to the hostnet-unreachable, host-unreachable, prot-unreachable, port-unreachable [default], net-prohibited, host-prohibited, admin-prohibited
reject statement type (ip6)ValueDescriptionTypeicmp6_typeICMPv6 type response to be sent to the hostno-route, admin-prohibited, addr-unreachable, port-unreachable [default], policy-fail, reject-route
reject statement type (inet)ValueDescriptionTypeicmpx_typeICMPvXtype 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.
counterpackets
numberbytes
numberConntrack statement
The conntrack statement can be used to set the conntrack mark and conntrack labels.
ctmarklabelsetvalue
The ct statement sets meta data associated with a connection.
Meta statement typesKeywordDescriptionValuemarkConnection tracking markmarklabelConnection tracking labellabel
save packet nfmark in conntrack
ct set mark meta mark
Meta statement
A meta statement sets the value of a meta expression.
The existing meta fields are: priority, mark, pkttype, nftrace.
metamarkprioritypkttypenftracesetvalue
A meta statement sets meta data associated with a packet.
Meta statement typesKeywordDescriptionValuepriorityTC packet prioritytc_handlemarkPacket markmarkpkttypepacket typepkt_typenftraceruleset packet tracing on/off. Use monitor trace command to watch traces0, 1
Limit statementlimitrateoverpacket_number/secondminutehourdayburst packet_number packetslimitrateoverbyte_numberbyteskbytesmbytes/secondminutehourdayweekburst 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.
limit statement valuesValueDescriptionTypepacket_numberNumber of packetsunsigned integer (32 bit)byte_numberNumber of bytesunsigned integer (32 bit)
NAT statementssnatto
address:portpersistent, random, fully-randomsnatto
address - address:port - portpersistent, random, fully-randomdnatto
address:portpersistent, random, fully-randomdnatto
address:port - portpersistent, random, fully-random
The nat statements are only valid from nat chain types.
The snat statement is only valid in the postrouting and input hooks, it specifies that the source address of the packet should be modified. The dnat statement is only valid in the prerouting and output chains, it specifies 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.
NAT statement valuesExpressionDescriptionTypeaddressSpecifies 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 }portSpecifies that the source/destination address of the packet should be modified.port number (16 bits)
NAT statement flagsFlagDescriptionpersistentGives a client the same source-/destination-address for each connection.randomIf used then port mapping will be randomized using a random seeded MD5 hash mix using source and destination address and destination port.fully-randomIf used then port mapping is generated based on a 32-bit pseudo-random algorithm.
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.
queuenum
queue_numberbypassqueuenum
queue_number_from - queue_number_tobypass,fanout
queue statement valuesValueDescriptionTypequeue_numberSets queue number, default is 0.unsigned integer (16 bit)queue_number_fromSets initial queue in the range, if fanout is used.unsigned integer (16 bit)queue_number_toSets closing queue in the range, if fanout is used.unsigned integer (16 bit)
queue statement flagsFlagDescriptionbypassLet packets go through if userspace application cannot back off. Before using this flag, read libnetfilter_queue documentation for performance tuning recomendations.fanoutDistribute packets between several queues.