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NAME | SYNOPSIS | DESCRIPTION | VALUES | OPTIONS | SELECTORS | EXAMPLES | SEE ALSO | COLOPHON |
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Universal 32b...ssifier in tc(8) Linux Universal 32b...ssifier in tc(8)
u32 - universal 32bit traffic control filter
tc filter ... [ handle HANDLE ] u32 OPTION_LIST [ offset OFFSET ]
[ hashkey HASHKEY ] [ classid CLASSID ] [ divisor
uint_value ] [ order u32_value ] [ ht HANDLE ] [ sample
SELECTOR [ divisor uint_value ] ] [ link HANDLE ] [ indev
ifname ] [ skip_hw | skip_sw ] [ help ]
HANDLE := { u12_hex_htid:[u8_hex_hash:[u12_hex_nodeid] |
0xu32_hex_value }
OPTION_LIST := [ OPTION_LIST ] OPTION
HASHKEY := [ mask u32_hex_value ] [ at 4*int_value ]
CLASSID := { root | none | [u16_major]:u16_minor | u32_hex_value }
OFFSET := [ plus int_value ] [ at 2*int_value ] [ mask
u16_hex_value ] [ shift int_value ] [ eat ]
OPTION := { match SELECTOR | action ACTION }
SELECTOR := { u32 VAL_MASK_32 | u16 VAL_MASK_16 | u8 VAL_MASK_8 |
ip IP | ip6 IP6 | { tcp | udp } TCPUDP | icmp ICMP | mark
VAL_MASK_32 | ether ETHER }
IP := { { src | dst } { default | any | all | ip_address [ / {
prefixlen | netmask } ] } AT | { dsfield | ihl | protocol
| precedence | icmp_type | icmp_code } VAL_MASK_8 | {
sport | dport } VAL_MASK_16 | nofrag | firstfrag | df | mf
}
IP6 := { { src | dst } { default | any | all | ip6_address
[/prefixlen ] } AT | priority VAL_MASK_8 | { protocol |
icmp_type | icmp_code } VAL_MASK_8 | flowlabel VAL_MASK_32
| { sport | dport } VAL_MASK_16 }
TCPUDP := { src | dst } VAL_MASK_16
ICMP := { type VAL_MASK_8 | code VAL_MASK_8 }
ETHER := { src | dst } ether_address AT
VAL_MASK_32 := u32_value u32_hex_mask [ AT ]
VAL_MASK_16 := u16_value u16_hex_mask [ AT ]
VAL_MASK_8 := u8_value u8_hex_mask [ AT ]
AT := [ at [ nexthdr+ ] int_value ]
The Universal/Ugly 32bit filter allows one 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-like hierarchy.
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 added to.
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 IPv6).
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:
handle HANDLE
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 by 0x.
offset OFFSET
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.
hashkey HASHKEY
Specify 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 given.
classid CLASSID
Classify matching packets into the given CLASSID, which
consists of either 16bit major and minor numbers or a
single 32bit value combining both.
divisor u32_value
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 exceeding eight.
order u32_value
A value to order filters by, ascending. Conflicts with
handle which serves the same purpose.
sample SELECTOR
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.
link HANDLE
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 option.
indev ifname
Filter on the incoming interface of the packet. Obviously
works only for forwarded traffic.
skip_sw
Do not process filter by software. If hardware has no
offload support for this filter, or TC offload is not
enabled for the interface, operation will fail.
skip_hw
Do not process filter by hardware.
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_32
u16 VAL_MASK_16
u8 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 IP
ip6 IP6
Assume packet starts with an IPv4 ( ip) or IPv6 ( ip6)
header. IP/IP6 then allows one to match various header
fields:
src ADDR
dst 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.
dsfield VAL_MASK_8
IPv4 only. Match the packet header's DSCP/ECN field.
Synonyms to this are tos and precedence.
ihl VAL_MASK_8
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.
protocol VAL_MASK_8
Match the Protocol (IPv4) or Next Header (IPv6)
field value, e.g. 6 for TCP.
icmp_type VAL_MASK_8
icmp_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_16
dport 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.
nofrag
firstfrag
df
mf IPv4 only, check certain flags and fragment offset
values. Match if the packet is not a fragment
(nofrag), the first fragment of a fragmented packet
(firstfrag), if Don't Fragment (df) or More
Fragments (mf) bits are set.
priority VAL_MASK_8
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.
flowlabel VAL_MASK_32
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 TCPUDP
udp TCPUDP
Match fields of next header of protocol TCP or UDP. The
possible values for TCPDUP are:
src VAL_MASK_16
Match on Source Port field value.
dst VALMASK_16
Match on Destination Port field value.
icmp ICMP
Match fields of next header of protocol ICMP. The possible
values for ICMP are:
type VAL_MASK_8
Match on ICMP Type field.
code VAL_MASK_8
Match on ICMP Code field.
mark VAL_MASK_32
Match on netfilter fwmark value.
ether ETHER
Match on ethernet header fields. Possible values for ETHER
are:
src ether_address AT
dst 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
priority 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
table 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
table:
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
priority.
The next step is to create a filter which links to the created
hash table:
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 effectively 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
second 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 collisions can occur. Otherwise it's necessary to
prevent matching unwanted packets.
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 \
classid 1:2
tc filter add dev eth0 parent 1:0 protocol ip \
u32 ht 800: \
match ip protocol 6 FF \
match u16 0 1fff at 6 \
offset at 0 mask 0f00 shift 6 \
link 1:
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 filter 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.
tc(8),
cls_u32.txt at http://linux-tc-notes.sourceforge.net/
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-08.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a
mail to man-pages@man7.org
iproute2 25 Sep 2015Universal 32b...ssifier in tc(8)
Pages that refer to this page: tc(8), tc-gact(8), tc-ife(8), tc-mirred(8), tc-pedit(8), tc-skbmod(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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