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

NAME         top

       pedit - generic packet editor action

SYNOPSIS         top

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

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

       AT_SPEC := at AT offmask MASK shift SHIFT

               | tcp TCPHDR_FIELD | icmp ICMPHDR_FIELD } CMD_SPEC

       IPHDR_FIELD := { src | dst | tos | dsfield | ihl | protocol |
               precedence | nofrag | firstfrag | ce | df | mf | dport |
               sport | icmp_type | icmp_code }

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

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

DESCRIPTION         top

       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

OPTIONS         top

       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.

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

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

                     Type Of Service field, an eight-bit value.

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

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

              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

              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 unexpected things.

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

       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

              Keep the addressed data as is.

       retain RVAL
              This optional extra part of CMD_SPEC allows to exclude bits
              from being changed.

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

                     Restart with the first filter in the current list.

              pipe   Continue with the next action attached to the same

              shot   Drop the packet.

                     Continue classification with the next filter in line.

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

EXAMPLES         top

       Being able to edit packet data, one could do all kinds of things,
       such as e.g.  implementing port redirection. Certainly not the most
       useful 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

SEE ALSO         top

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

COLOPHON         top

       This page is part of the iproute2 (utilities for controlling TCP/IP
       networking and traffic) project.  Information about the project can
       be found at 
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iproute2                         12 JanG2e0n1e5ric packet editor action in tc(8)