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 ] [  sam‐
               ple 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 [ / { 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 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

       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  ide‐
           ally  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

       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 hexa‐
       decimal format. Otherwise, the value's base is automatically detected,  i.e.  values  pre‐
       fixed  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 spec‐
       ified 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 ta‐
              ble'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 ker‐
              nel  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

       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 addi‐
              tional 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 spec‐
              ify  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

       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. 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

              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 pur‐
                     pose 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 remain‐
                     ing 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

              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 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  classi‐
       fied 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

       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 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 state‐
       ment  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 sep‐
       arate 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 cre‐
       ated 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  sec‐
       ond call.

       cls_u32.txt at http://linux-tc-notes.sourceforge.net/

iproute2                                   25 Sep 2015        Universal 32bit classifier in tc(8)


Designed by SanjuD(@ngineerbabu)