SYNOPSIS

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

LAYERED_OP := { ip IPHDR_FIELD | ip6 IP6HDR_FIELD | udp UDPHDR_FIELD | 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

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

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:

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.

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 unexpected 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 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 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.

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 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