mirror_iproute2/man/man8/tc-pedit.8

.TH "Generic packet editor action in tc" 8 "12 Jan 2015" "iproute2" "Linux"

.SH NAME
pedit - generic packet editor action
.SH SYNOPSIS
.in +8
.ti -8
.BR tc " ... " "action pedit [ex] munge " {
.IR RAW_OP " | " LAYERED_OP " | " EXTENDED_LAYERED_OP " } [ " CONTROL " ]"

.ti -8
.IR RAW_OP " := "
.BI offset " OFFSET"
.RB "{ " u8 " | " u16 " | " u32 " } ["
.IR AT_SPEC " ] " CMD_SPEC

.ti -8
.IR AT_SPEC " := "
.BI at " AT " offmask " MASK " shift " SHIFT"

.ti -8
.IR LAYERED_OP " := { "
.BI ip " IPHDR_FIELD"
|
.BI ip " BEYOND_IPHDR_FIELD"
.RI } " CMD_SPEC"

.ti -8
.IR EXTENDED_LAYERED_OP " := { "
.BI ip " IPHDR_FIELD"
.RI } " CMD_SPEC"

.ti -8
.IR IPHDR_FIELD " := { "
.BR src " | " dst " | " tos " | " dsfield " | " ihl " | " protocol " |"
.BR precedence " | " nofrag " | " firstfrag " | " ce " | " df " }"

.ti -8
.IR BEYOND_IPHDR_FIELD " := { "
.BR dport " | " sport " | " icmp_type " | " icmp_code " }"

.ti -8
.IR CMD_SPEC " := {"
.BR clear " | " invert " | " set
.IR VAL " | "
.BR preserve " } [ " retain
.IR RVAL " ]"

.ti -8
.IR CONTROL " := {"
.BR reclassify " | " pipe " | " drop " | " shot " | " continue " | " pass " }"
.SH DESCRIPTION
The
.B 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
.IR 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.
.SH OPTIONS
.TP
.B ex
Use extended pedit.
.I EXTENDED_LAYERED_OP
is allowed only in this mode.
.TP
.BI offset " OFFSET " "\fR{ \fBu32 \fR| \fBu16 \fR| \fBu8 \fR}"
Specify the offset at which to change data.
.I OFFSET
is a signed integer, it's base is automatically chosen (e.g. hex if prefixed by
.B 0x
or octal if prefixed by
.BR 0 ).
The second argument specifies the length of data to change, that is four bytes
.RB ( u32 ),
two bytes
.RB ( u16 )
or a single byte
.RB ( u8 ).
.TP
.BI at " AT " offmask " MASK " shift " SHIFT"
This is an optional part of
.IR RAW_OP
which allows to have a variable
.I OFFSET
depending on packet data at offset
.IR AT ,
which is binary ANDed with
.I MASK
and right-shifted by
.I SHIFT
before adding it to
.IR OFFSET .
.TP
.BI ip " IPHDR_FIELD"
Change an IPv4 header field. The supported keywords for
.I IPHDR_FIELD
are:
.RS
.TP
.B src
.TQ
.B dst
Source or destination IP address, a four-byte value.
.TP
.B tos
.TQ
.B dsfield
.TQ
.B precedence
Type Of Service field, an eight-bit value.
.TP
.B ihl
Change the IP Header Length field, a four-bit value.
.TP
.B protocol
Next-layer Protocol field, an eight-bit value.
.TP
.B nofrag
.TQ
.B firstfrag
.TQ
.B ce
.TQ
.B df
.TQ
.B mf
Change IP header flags. Note that the value to pass to the
.B 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.
.RE
.TP
.BI 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
.I BEYOND_IPHDR_FIELD
are:
.RS
.TP
.B dport
.TQ
.B 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.
.TP
.B icmp_type
.TQ
.B 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.
.RE
.TP
.B clear
Clear the addressed data (i.e., set it to zero).
.TP
.B invert
Swap every bit in the addressed data.
.TP
.BI set " VAL"
Set the addressed data to a specific value. The size of
.I VAL
is defined by either one of the
.BR u32 ", " u16 " or " u8
keywords in
.IR RAW_OP ,
or the size of the addressed header field in
.IR LAYERED_OP .
.TP
.B preserve
Keep the addressed data as is.
.TP
.BI retain " RVAL"
This optional extra part of
.I CMD_SPEC
allows to exclude bits from being changed.
.TP
.I CONTROL
The following keywords allow to control how the tree of qdisc, classes,
filters and actions is further traversed after this action.
.RS
.TP
.B reclassify
Restart with the first filter in the current list.
.TP
.B pipe
Continue with the next action attached to the same filter.
.TP
.B drop
.TQ
.B shot
Drop the packet.
.TP
.B continue
Continue classification with the next filter in line.
.TP
.B pass
Finish classification process and return to calling qdisc for further packet
processing. This is the default.
.RE
.SH 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
.B ingress
qdisc will do, for transmit path a classful qdisc
.RB ( HTB
in this case) is necessary:

.RS
.EX
tc qdisc replace dev eth0 root handle 1: htb
tc qdisc add dev eth0 ingress handle ffff:
.EE
.RE

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

.RS
.EX
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
.EE
.RE
.SH SEE ALSO
.BR tc (8),
.BR tc-htb (8),
.BR tc-u32 (8)