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 ] [ 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.
       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
              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.

       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
                     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.

       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

       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

       cls_u32.txt at

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