The Universal/Ugly 32bit filter allows to match arbitrary bitfields
in the packet. Due to breaking everything down to values, masks and
offsets, It is equally powerful and hard to use. Luckily many
abstracting directives are present which allow defining rules on a
higher level and therefore free the user from having to fiddle with
bits and masks in many cases.
There are two general modes of invocation: The first mode creates a
new filter to delegate packets to different destinations. Apart from
the obvious ones, namely classifying the packet by specifying a
CLASSID or calling an action, one may link one filter to another one
(or even a list of them), effectively organizing filters into a tree-
Typically filter delegation is done by means of a hash table, which
leads to the second mode of invocation: it merely serves to set up
these hash tables. Filters can select a hash table and provide a key
selector from which a hash is to be computed and used as key to
lookup the table's bucket which contains filters for further
processing. This is useful if a high number of filters is in use, as
the overhead of performing the hash operation and table lookup
becomes negligible in that case. Using hashtables with u32 basically
involves the following pattern:
(1) Creating a new hash table, specifying it's size using the divisor
parameter and ideally a handle by which the table can be
identified. If the latter is not given, the kernel chooses one on
it's own, which has to be guessed later.
(2) Creating filters which link to the created table in (1) using the
link parameter and defining the packet data which the kernel will
use to calculate the hashkey.
(3) Adding filters to buckets in the hash table from (1). In order
to avoid having to know how exactly the kernel creates the hash
key, there is the sample parameter, which gives sample data to
hash and thereby define the table bucket the filter should be
In fact, even if not explicitly requested u32 creates a hash table
for every priority a filter is being added with. The table's size is
1 though, so it is in fact merely a linked list.
Options and selectors require values to be specified in a specific
format, which is often non-intuitive. Therefore the terminals in
SYNOPSIS have been given descriptive names to indicate the required
format and/or maximum allowed numeric value: Prefixes u32, u16 and u8
indicate four, two and single byte unsigned values. E.g. u16
indicates a two byte-sized value in range between 0 and 65535
(0xFFFF) inclusive. A prefix of int indicates a four byte signed
value. A middle part of _hex_ indicates that the value is parsed in
hexadecimal format. Otherwise, the value's base is automatically
detected, i.e. values prefixed with 0x are considered hexadecimal, a
leading 0 indicates octal format and decimal format otherwise. There
are some values with special formatting as well: ip_address and
netmask are in dotted-quad formatting as usual for IPv4 addresses. An
ip6_address is specified in common, colon-separated hexadecimal
format. Finally, prefixlen is an unsigned, decimal integer value in
range from 0 to the address width in bits (32 for IPv4 and 128 for
Sometimes values need to be dividable by a certain number. In that
case a name of the form N*val was chosen, indicating that val must be
dividable by N. Or the other way around: the resulting value must be
a multiple of N.
U32 recognizes the following options:
The handle is used to reference a filter and therefore must be
unique. It consists of a hash table identifier htid and
optional hash (which identifies the hash table's bucket) and
nodeid. All these values are parsed as unsigned, hexadecimal
numbers with length 12bits ( htid and nodeid) or 8bits (
hash). Alternatively one may specify a single, 32bit long hex
number which contains the three fields bits in concatenated
form. Other than the fields themselves, it has to be prefixed
Set an offset which defines where matches of subsequent
filters are applied to. Therefore this option is useful only
when combined with link or a combination of ht and sample.
The offset may be given explicitly by using the plus keyword,
or extracted from the packet data with at. It is possible to
mangle the latter using mask and/or shift keywords. By
default, this offset is recorded but not implicitly applied.
It is used only to substitute the nexthdr+ statement. Using
the keyword eat though inverses this behaviour: the offset is
applied always, and nexthdr+ will fall back to zero.
Spefify what packet data to use to calculate a hash key for
bucket lookup. The kernel adjusts the value according to the
hash table's size. For this to work, the option link must be
Classify matching packets into the given CLASSID, which
consists of either 16bit major and minor numbers or a single
32bit value combining both.
Specify a modulo value. Used when creating hash tables to
define their size or for declaring a sample to calculate hash
table keys from. Must be a power of two with exponent not
A value to order filters by, ascending. Conflicts with handle
which serves the same purpose.
Used together with ht to specify which bucket to add this
filter to. This allows one to avoid having to know how exactly
the kernel calculates hashes. The additional divisor defaults
to 256, so must be given for hash tables of different size.
Delegate matching packets to filters in a hash table. HANDLE
is used to only specify the hash table, so only htid may be
given, hash and nodeid have to be omitted. By default, bucket
number 0 will be used and can be overridden by the hashkey
Filter on the incoming interface of the packet. Obviously
works only for forwarded traffic.
help Print a brief help text about possible options.
Basically the only real selector is u32 . All others merely provide
a higher level syntax and are internally translated into u32 .u32 VAL_MASK_32u16 VAL_MASK_16u8 VAL_MASK_8
Match packet data to a given value. The selector name defines
the sample length to extract (32bits for u32, 16bits for u16
and 8bits for u8). Before comparing, the sample is binary
AND'ed with the given mask. This way uninteresting bits can be
cleared before comparison. The position of the sample is
defined by the offset specified in AT.
ip IPip6 IP6
Assume packet starts with an IPv4 ( ip) or IPv6 ( ip6) header.
IP/IP6 then allows to match various header fields:
src ADDRdst ADDR
Compare Source or Destination Address fields against
the value of ADDR. The reserved words default, any and
all effectively match any address. Otherwise an IP
address of the particular protocol is expected,
optionally suffixed by a prefix length to match whole
subnets. In case of IPv4 a netmask may also be given.
IPv4 only. Match the packet header's DSCP/ECN field.
Synonyms to this are tos and precedence.
IPv4 only. Match the Internet Header Length field. Note
that the value's unit is 32bits, so to match a packet
with 24byte header length u8_value has to be 6.
Match the Protocol (IPv4) or Next Header (IPv6) field
value, e.g. 6 for TCP.
icmp_type VAL_MASK_8icmp_code VAL_MASK_8
Assume a next-header protocol of icmp or ipv6-icmp and
match Type or Code field values. This is dangerous, as
the code assumes minimal header size for IPv4 and lack
of extension headers for IPv6.
sport VAL_MASK_16dport VAL_MASK_16
Match layer four source or destination ports. This is
dangerous as well, as it assumes a suitable layer four
protocol is present (which has Source and Destination
Port fields right at the start of the header and 16bit
in size). Also minimal header size for IPv4 and lack
of IPv6 extension headers is assumed.
nofragfirstfragdfmf IPv4 only, check certain flags and fragment offset
values. Match if the packet is not a fragment (nofrag),
the first fragment (firstfrag), if Don't Fragment (df)
or More Fragments (mf) bits are set.
IPv6 only. Match the header's Traffic Class field,
which has the same purpose and semantics of IPv4's ToS
field since RFC 3168: upper six bits are DSCP, the
lower two ECN.
IPv6 only. Match the Flow Label field's value. Note
that Flow Label itself is only 20bytes long, which are
the least significant ones here. The remaining upper
12bytes match Version and Traffic Class fields.
tcp TCPUDPudp TCPUDP
Match fields of next header of protocol TCP or UDP. The
possible values for TCPDUP are:
Match on Source Port field value.
Match on Destination Port field value.
Match fields of next header of protocol ICMP. The possible
values for ICMP are:
Match on ICMP Type field.
Match on ICMP Code field.
Match on netfilter fwmark value.
Match on ethernet header fields. Possible values for ETHER
src ether_address ATdst ether_address AT
Match on source or destination ethernet address. This
is dangerous: It assumes an ethernet header is present
at the start of the packet. This will probably lead to
unexpected things if used with layer three interfaces
like e.g. tun or ppp.
tc filter add dev eth0 parent 999:0 prio 99 protocol ip u32 \
match ip src 192.168.8.0/24 classid 1:1
This attaches a filter to the qdisc identified by 999:0. It's prior‐
ity is 99, which affects in which order multiple filters attached to
the same parent are consulted (the lower the earlier). The filter
handles packets of protocol type ip, and matches if the IP header's
source address is within the 192.168.8.0/24 subnet. Matching packets
are classified into class 1.1. The effect of this command might be
surprising at first glance:
filter parent 1: protocol ip pref 99 u32
filter parent 1: protocol ip pref 99 u32 \
fh 800: ht divisor 1
filter parent 1: protocol ip pref 99 u32 \
fh 800::800 order 2048 key ht 800 bkt 0 flowid 1:1 \
match c0a80800/ffffff00 at 12
So parent 1: is assigned a new u32 filter, which contains a hash ta‐
ble of size 1 (as the divisor indicates). The table ID is 800. The
third line then shows the actual filter which was added above: it
sits in table 800 and bucket 0, classifies packets into class ID 1:1
and matches the upper three bytes of the four byte value at offset 12
to be 0xc0a808, which is 192, 168 and 8.
Now for something more complicated, namely creating a custom hash ta‐
tc filter add dev eth0 prio 99 handle 1: u32 divisor 256
This creates a table of size 256 with handle 1: in priority 99. The
effect is as follows:
filter parent 1: protocol all pref 99 u32
filter parent 1: protocol all pref 99 u32 fh 1: ht divisor 256
filter parent 1: protocol all pref 99 u32 fh 800: ht divisor 1
So along with the requested hash table (handle 1:), the kernel has
created his own table of size 1 to hold other filters of the same
The next step is to create a filter which links to the created hash
tc filter add dev eth0 parent 1: prio 1 u32 \
link 1: hashkey mask 0x0000ff00 at 12 \
match ip src 192.168.0.0/16
The filter is given a lower priority than the hash table itself so
u32 consults it before manually traversing the hash table. The
options link and hashkey determine which table and bucket to redirect
to. In this case the hash key should be constructed out of the second
byte at offset 12, which corresponds to an IP packet's third byte of
the source address field. Along with the match statement, this effec‐
tively maps all class C networks below 192.168.0.0/16 to different
buckets of the hash table.
Filters for certain subnets can be created like so:
tc filter add dev eth0 parent 1: prio 99 u32 \
ht 1: sample u32 0x00000800 0x0000ff00 at 12 \
match ip src 192.168.8.0/24 classid 1:1
The bucket is defined using the sample option: In this case, the sec‐
ond byte at offset 12 must be 0x08, exactly. In this case, the
resulting bucket ID is obviously 8, but as soon as sample selects an
amount of data which could exceed the divisor, one would have to know
the kernel-internal algorithm to deduce the destination bucket. This
filter's match statement is redundant in this case, as the entropy
for the hash key does not exceed the table size and therefore no col‐
lisions can occur. Otherwise it's necessary to prevent matching
Matching upper layer fields is problematic since IPv4 header length
is variable and IPv6 supports extension headers which affect upper
layer header offset. To overcome this, there is the possibility to
specify nexthdr+ when giving an offset, and to make things easier
there are the tcp and udp matches which use nexthdr+ implicitly. This
offset has to be calculated in beforehand though, and the only way to
achieve that is by doing it in a separate filter which then links to
the filter which wants to use it. Here is an example of doing so:
tc filter add dev eth0 parent 1:0 protocol ip handle 1: \
u32 divisor 1
tc filter add dev eth0 parent 1:0 protocol ip \
u32 ht 1: \
match tcp src 22 FFFF \
tc filter add dev eth0 parent 1:0 protocol ip \
u32 ht 800: \
match ip protocol 6 FF \
match ip firstfrag \
offset at 0 mask 0f00 shift 6 \
This is what is being done: In the first call, a single element sized
hash table is created so there is a place to hold the linked to fil‐
ter and a known handle (1:) to reference to it. The second call then
adds the actual filter, which pushes packets with TCP source port 22
into class 1:2. Using ht, it is moved into the hash table created by
the first call. The third call then does the actual magic: It matches
IPv4 packets with next layer protocol 6 (TCP), only if it's the first
fragment (usually TCP sets DF bit, but if it doesn't and the packet
is fragmented, only the first one contains the TCP header), and then
sets the offset based on the IP header's IHL field (right-shifting by
6 eliminates the offset of the field and at the same time converts
the value into byte unit). Finally, using link, the hash table from
first call is referenced which holds the filter from second call.
This page is part of the iproute2 (utilities for controlling TCP/IP
networking and traffic) project. Information about the project can
be found at
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iproute2 25 Sep 20U1n5iversal 32bit classifier in tc(8)