Generic packet editor action in tc(8)                                  Linux                                 Generic packet editor action in tc(8)

NAME

       pedit - generic packet editor action

SYNOPSIS

       tc ... action pedit [ex] munge { RAW_OP | LAYERED_OP | EXTENDED_LAYERED_OP } [ CONTROL ]

       RAW_OP := offset OFFSET { u8 | u16 | u32 } [ AT_SPEC ] CMD_SPEC

       AT_SPEC := at AT offmask MASK shift SHIFT

       LAYERED_OP := { ip IPHDR_FIELD | ip BEYOND_IPHDR_FIELD } CMD_SPEC

       EXTENDED_LAYERED_OP  :=  { eth ETHHDR_FIELD | ip IPHDR_FIELD | ip EX_IPHDR_FIELD | ip6 IP6HDR_FIELD | tcp TCPHDR_FIELD | udp UDPHDR_FIELD }
               CMD_SPEC

       ETHHDR_FIELD := { src | dst | type }

       IPHDR_FIELD := { src | dst | tos | dsfield | ihl | protocol | precedence | nofrag | firstfrag | ce | df }

       BEYOND_IPHDR_FIELD := { dport | sport | icmp_type | icmp_code }

       EX_IPHDR_FIELD := { ttl }

       IP6HDR_FIELD := { src | dst | flow_lbl | payload_len | nexthdr | hoplimit }

       TCPHDR_FIELD := { sport | dport | flags }

       UDPHDR_FIELD := { sport | dport }

       CMD_SPEC := { clear | invert | set VAL | add VAL | preserve } [ retain RVAL ]

       CONTROL := { reclassify | pipe | drop | shot | continue | pass | goto chain CHAIN_INDEX }

DESCRIPTION

       The pedit action can be used to change arbitrary packet data. The location of data to change can either be specified by  giving  an  offset
       and  size  as  in  RAW_OP,  or  for header values by naming the header and field to edit the size is then chosen automatically based on the
       header field size. Currently this is supported only for IPv4 headers.

OPTIONS

       ex     Use extended pedit.  EXTENDED_LAYERED_OP and the add CMD_SPEC are allowed only in this mode.

       offset OFFSET { u32 | u16 | u8 }
              Specify the offset at which to change data.  OFFSET is a signed integer, it's base is automatically chosen (e.g. hex if prefixed  by
              0x  or  octal  if  prefixed  by 0).  The second argument specifies the length of data to change, that is four bytes (u32), two bytes
              (u16) or a single byte (u8).

       at AT offmask MASK shift SHIFT
              This is an optional part of RAW_OP which allows to have a variable OFFSET depending on packet data at offset  AT,  which  is  binary
              ANDed with MASK and right-shifted by SHIFT before adding it to OFFSET.

       eth ETHHDR_FIELD
              Change an ETH header field. The supported keywords for ETHHDR_FIELD are:

              src
              dst    Source or destination MAC address in the standard format: XX:XX:XX:XX:XX:XX

              type   Ether-type in numeric value

       ip IPHDR_FIELD
              Change an IPv4 header field. The supported keywords for IPHDR_FIELD are:

              src
              dst    Source or destination IP address, a four-byte value.

              tos
              dsfield
              precedence
                     Type Of Service field, an eight-bit value.

              ihl    Change the IP Header Length field, a four-bit value.

              protocol
                     Next-layer Protocol field, an eight-bit value.

              nofrag
              firstfrag
              ce
              df
              mf     Change  IP  header flags. Note that the value to pass to the set command is not just a bit value, but the full byte including
                     the flags field.  Though only the relevant bits of that value are respected, the rest ignored.

       ip BEYOND_IPHDR_FIELD
              Supported only for non-extended layered op. It is passed to the kernel as offsets relative to the beginning of  the  IP  header  and
              assumes the IP header is of minimum size (20 bytes). The supported keywords for BEYOND_IPHDR_FIELD are:

              dport
              sport  Destination  or  source port numbers, a 16-bit value. Indeed, IPv4 headers don't contain this information. Instead, this will
                     set an offset which suits at least TCP and UDP if the IP header is of minimum size (20 bytes). If not,  this  will  do  unex-
                     pected things.

              icmp_type
              icmp_code
                     Again,  this allows to change data past the actual IP header itself. It assumes an ICMP header is present immediately follow-
                     ing the (minimal sized) IP header.  If it is not or the latter is bigger than the minimum of 20 bytes,  this  will  do  unex-
                     pected things. These fields are eight-bit values.

       ip EX_IPHDR_FIELD
              Supported only when ex is used. The supported keywords for EX_IPHDR_FIELD are:

              ttl

       ip6 IP6HDR_FIELD
              The supported keywords for IP6HDR_FIELD are:

              src
              dst
              flow_lbl
              payload_len
              nexthdr
              hoplimit

       tcp TCPHDR_FIELD
              The supported keywords for TCPHDR_FIELD are:

              sport
              dport  Source or destination TCP port number, a 16-bit value.

              flags

       udp UDPHDR_FIELD
              The supported keywords for UDPHDR_FIELD are:

              sport
              dport  Source or destination TCP port number, a 16-bit value.

       clear  Clear the addressed data (i.e., set it to zero).

       invert Swap every bit in the addressed data.

       set VAL
              Set  the  addressed  data to a specific value. The size of VAL is defined by either one of the u32, u16 or u8 keywords in RAW_OP, or
              the size of the addressed header field in LAYERED_OP.

       add VAL
              Add the addressed data by a specific value. The size of VAL is defined by the size of the addressed header  field  in  EXTENDED_LAY-
              ERED_OP.  This operation is supported only for extended layered op.

       preserve
              Keep the addressed data as is.

       retain RVAL
              This optional extra part of CMD_SPEC allows to exclude bits from being changed. Supported only for 32 bits fields or smaller.

       CONTROL
              The following keywords allow to control how the tree of qdisc, classes, filters and actions is further traversed after this action.

              reclassify
                     Restart with the first filter in the current list.

              pipe   Continue with the next action attached to the same filter.

              drop
              shot   Drop the packet.

              continue
                     Continue classification with the next filter in line.

              pass   Finish classification process and return to calling qdisc for further packet processing. This is the default.

EXAMPLES

       Being  able to edit packet data, one could do all kinds of things, such as e.g.  implementing port redirection. Certainly not the most use-
       ful application, but as an example it should do:

       First, qdiscs need to be set up to attach filters to. For the receive path, a simple ingress qdisc will do, for transmit  path  a  classful
       qdisc (HTB in this case) is necessary:

              tc qdisc replace dev eth0 root handle 1: htb
              tc qdisc add dev eth0 ingress handle ffff:

       Finally,  a  filter  with  pedit  action can be added for each direction. In this case, u32 is used matching on the port number to redirect
       from, while pedit then does the actual rewriting:

              tc filter add dev eth0 parent 1: u32 
                   match ip dport 23 0xffff 
                   action pedit pedit munge ip dport set 22
              tc filter add dev eth0 parent ffff: u32 
                   match ip sport 22 0xffff 
                   action pedit pedit munge ip sport set 23
              tc filter add dev eth0 parent ffff: u32 
                   match ip sport 22 0xffff 
                   action pedit ex munge ip dst set 192.168.1.199
              tc filter add dev eth0 parent ffff: u32 
                   match ip sport 22 0xffff 
                   action pedit ex munge ip6 dst set fe80::dacb:8aff:fec7:320e
              tc filter add dev eth0 parent ffff: u32 
                   match ip sport 22 0xffff 
                   action pedit ex munge eth dst set 11:22:33:44:55:66
              tc filter add dev eth0 parent ffff: u32 
                   match ip dport 23 0xffff 
                   action pedit ex munge tcp dport set 22

SEE ALSO

       tc(8), tc-htb(8), tc-u32(8)

iproute2                                                            12 Jan 2015                              Generic packet editor action in tc(8)