Universal 32bit classifier in t(c2(58)SU<en/piav>2e0r1s5a)l 32bit classifier in tc(8)

     NAME
          u32 - universal 32bit traffic control filter

     SYNOPSIS
          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 | ip_address [  / {
                  prefixlen | netmask } ] } AT | { dsfield | ihl |
                  protocol | icmp_type | icmp_code } VAL_MASK_8 | {
                  sport | dport } VAL_MASK_16 |  nofrag | firstfrag |
                  df |

          IP6 := { {  src | dst } { default | 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 [

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          VAL_MASK_16 := u16_value u16_hex_mask [

          VAL_MASK_8 := u8_value u8_hex_mask [

          AT := [  at [ nexthdr+ ] int_value ]

     DESCRIPTION
          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 desti-
          nations. 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

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          list.

     VALUES
          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 sin-
          gle 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 hexadec-
          imal format. Otherwise, the value's base is automatically
          detected, i.e. values prefixed with 0x are considered hex-
          adecimal, a leading 0 indicates octal format and decimal
          format otherwise. There are some values with special format-
          ting as well: ip_address and netmask are in dotted-quad for-
          matting 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.

     OPTIONS
          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 use-
               ful 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 off-
               set is recorded but not implicitly applied. It is used

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               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 accord-
               ing 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 cal-
               culate 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 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 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. Obvi-
               ously 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.

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     SELECTORS
          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 des-
                    tination ports. This is dangerous as well, as it

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                    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
                    (firstfrag), if Don't Fragment (df) or More Frag-
                    ments (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 pos-
               sible 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:

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               src ether_address AT
                    dst ether_address AT Match on source or destina-
                    tion 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.

     EXAMPLES
               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 sur-
          prising 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 ker-
          nel has created his own table of size 1 to hold other fil-
          ters of the same priority.

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          The next step is to create a filter which links to the cre-
          ated 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 effec-
          tively 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 algo-
          rithm 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 calcu-
          lated 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 \

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                       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 ele-
          ment 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.

     SEE ALSO
          tc(8),
          cls_u32.txt at http://linux-tc-notes.sourceforge.net/

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