tc-u32(8) — Linux manual page


Universal 32bit classifier in tc(8) LinuxUniversal 32bit classifier in tc(8)

NAME         top

       u32 - universal 32bit traffic control filter

SYNOPSIS         top

       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 }


       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 ]

DESCRIPTION         top

       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-
       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.

VALUES         top

       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.

OPTIONS         top

       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
              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

       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

       indev ifname
              Filter on the incoming interface of the packet. Obviously
              works only for forwarded traffic.

              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.

              Do not process filter by hardware.

       help   Print a brief help text about possible options.

SELECTORS         top

       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 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.

              mf     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.

              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

              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.

EXAMPLES         top

              tc filter add dev eth0 parent 999:0 prio 99 protocol ip u32 \
                      match ip src 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 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

       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 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 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
       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 ip firstfrag \
                      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 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.

SEE ALSO         top

       cls_u32.txt at 

COLOPHON         top

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iproute2                         25 Sep 20U1n5iversal 32bit classifier in tc(8)

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