Generic packet editor action in tc
Section: Linux (8)
Updated: 12 Jan 2015
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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
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.
- 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_LAYERED_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 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
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)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- EXAMPLES
-
- SEE ALSO
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