phpman > man > tc-u32(8)

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 ] [ clas‐‐
               sid CLASSID ] [ divisor uint_value ] [ order u32_value ] [ ht HANDLE ] [  sample  SE‐
               LECTOR  [ 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 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  be‐
       comes  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 fil‐
       ter 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 indi‐
       cate the required format and/or maximum allowed numeric value: Prefixes u32, u16 and u8 indi‐
       cate 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. Oth‐
       erwise, the value's base is automatically detected, i.e. values prefixed with 0x are  consid‐
       ered  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  for‐
       matting  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  ex‐
              tracted  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 in‐
              verses 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 ex‐
              ponent 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 divi‐‐
              sor 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  de‐
              fault, 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 ex‐
              tract (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  be‐
              fore 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 Desti‐
                     nation 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 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 up‐
                     per 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 ear‐
       lier).  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 di‐‐
       visor 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 sup‐
       ports 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 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)