Universal 32bit classifier in tc(8)  Linux Universal 32bit classifier 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 }


       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 [ / { pre-
               fixlen | netmask } ] } AT | { dsfield | ihl | protocol | prece-
               dence  | 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 to match arbitrary bitfields in
       the packet. Due to breaking everything down to values, masks  and  off-
       sets,  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  hierar-

       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

       (1) Creating  a  new hash table, specifying it's size using the divisor
           parameter and ideally a handle by which the table  can  be  identi-
           fied.  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 for-
       mat, 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.  Other-
       wise,  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 for-
       matting 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 divid-
       able by N.  Or the other way around: the resulting value must be a mul-
       tiple 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).  Alterna-
              tively 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 com-
              bined 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 lat-
              ter using mask and/or shift keywords. By default, this offset is
              recorded but not implicitly applied. It is used only to  substi-
              tute  the  nexthdr+  statement.  Using  the  keyword  eat though
              inverses this behaviour: the offset is applied always, and  nex-
              thdr+ 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

       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 ker-
              nel 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.

              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.

       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. Syn-
                     onyms 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 val-
                     ues. 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

              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 val-
              ues 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 unex-
                     pected  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 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 subnet. Matching packets are clas-
       sified  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 ta-
       ble  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 cre-
       ated his own table of size 1 to hold other filters of the  same  prior-

       The next step is to create a filter which links to the created hash ta-

              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 off-
       set 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 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 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 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  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 frag-
       ment (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.

       cls_u32.txt at

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