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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 list.

       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

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

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