Universal 32bit classifier in tc(8)  Linux Universal 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 | 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 ]

DESCRIPTION
       The Universal/Ugly 32bit filter allows one 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 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 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 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 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.
       Otherwise, 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 formatting 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
       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).
	      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
	      Specify 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
	      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 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 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.

       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.

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 one 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 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 of a fragmented packet (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
		     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 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.

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
       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 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 effectively 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 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 u16 0 1fff at 6 \
		      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
       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/

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