META EXPRESSIONS
~~~~~~~~~~~~~~~~
[verse]
*meta* {length | nfproto | l4proto | protocol | priority}
[meta] {mark | iif | iifname | iiftype | oif | oifname | oiftype |
skuid | skgid | nftrace | rtclassid | ibrname | obrname | pkttype | cpu
| iifgroup | oifgroup | cgroup | random | secpath}

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 l4proto is useful to match a
particular transport protocol that is part of either an IPv4 or IPv6 packet. It
will also skip any IPv6 extension headers present in an IPv6 packet.

.Meta expression types
[options="header"]
|==================
|Keyword | Description | Type
|length|
Length of the packet in bytes|
integer (32 bit)
|nfproto|
real hook protocol family, useful only in inet table|
integer (32 bit)
|l4proto|
layer 4 protocol, skips ipv6 extension headers|
integer (8 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 |
ifname
|iiftype|
Input interface type|
iface_type
|oif|
Output interface index|
iface_index
|oifname|
Output interface name|
ifname
|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
|ibrname|
Input bridge interface name|
ifname
|obrname|
Output bridge interface name|
ifname
|pkttype|
packet type|
pkt_type
|cpu|
cpu number processing the packet|
integer (32 bit)
|iifgroup|
incoming device group|
devgroup
|oifgroup|
outgoing device group|
devgroup
|cgroup|
control group id |
integer (32 bit)
|random|
pseudo-random number|
integer (32 bit)
|secpath|
boolean|
boolean (1 bit)
|====================

.Meta expression specific types
[options="header"]
|==================
|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).
|=============================

.Using meta expressions
-----------------------
# qualified meta expression
filter output meta oif eth0

# unqualified meta expression
filter output oif eth0

# packed was subject to ipsec processing
raw prerouting meta secpath exists accept
-----------------------

SOCKET EXPRESSION
~~~~~~~~~~~~~~~~~
[verse]
*socket* \{transparent\}

Socket expression can be used to search for an existing open TCP/UDP socket and
its attributes that can be associated with a packet. It looks for an established
or non-zero bound listening socket (possibly with a non-local address).

.Available socket attributes
[options="header"]
|==================
|Name |Description| Type
|transparent|
Value of the IP_TRANSPARENT socket option in the found socket. It can be 0 or 1.|
boolean (1 bit)
|mark| Value of the socket mark (SOL_SOCKET, SO_MARK). | mark
|==================

.Using socket expression
------------------------
# Mark packets that correspond to a transparent socket
table inet x {
    chain y {
	type filter hook prerouting priority -150; policy accept;
        socket transparent 1 mark set 0x00000001 accept
    }
}

# Trace packets that corresponds to a socket with a mark value of 15
table inet x {
    chain y {
        type filter hook prerouting priority -150; policy accept;
        socket mark 0x0000000f nftrace set 1
    }
}

# Set packet mark to socket mark
table inet x {
    chain y {
        type filter hook prerouting priority -150; policy accept;
        tcp dport 8080 mark set socket mark
    }
}
----------------------

OSF EXPRESSION
~~~~~~~~~~~~~~
[verse]
osf {name}

The osf expression does passive operating system fingerprinting. This
expression compares some data (Window Size, MSS, options and their order, DF,
and others) from packets with the SYN bit set.

.Available osf attributes
[options="header"]
|==================
|Name |Description| Type
|name|
Name of the OS signature to match. All signatures can be found at pf.os file.|
Use "unknown" for OS signatures that the expression could not detect.
|==================

.Using osf expression
---------------------
# Accept packets that match the "Linux" OS signature.
table inet x {
    chain y {
	type filter hook input priority 0; policy accept;
        osf "Linux"
    }
}
-----------------------

FIB EXPRESSIONS
~~~~~~~~~~~~~~~
[verse]
*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.

.fib expression specific types
[options="header"]
|==================
|Keyword| Description| Type
|oif|
Output interface index|
integer (32 bit)
|oifname|
Output interface name|
string
|type|
Address type |
fib_addrtype
|=======================

.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
~~~~~~~~~~~~~~~~~~~
[verse]
*rt* {classid | nexthop}

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

.Routing expression types
[options="header"]
|=======================
|Keyword| Description| Type
|classid|
Routing realm|
realm
|nexthop|
Routing nexthop|
ipv4_addr/ipv6_addr
|mtu|
TCP maximum segment size of route |
integer (16 bit)
|=================================

.Routing expression specific types
[options="header"]
|=======================
|Type| Description
|realm|
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 output rt ip nexthop 192.168.0.1
inet filter output rt ip6 nexthop fd00::1
--------------------------