Generic packet editor action in tc(8)LinuGeneric packet editor action in tc(8)NAME
pedit - generic packet editor action
SYNOPSIS
tc ... action pedit [ex] munge { RAW_OP | LAYERED_OP | EXTENDED_LAY‐
ERED_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 | prece‐
dence | 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 loca‐
tion 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 vari‐
able 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 ETH‐
HDR_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 sup‐
ported 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 imme‐
diately 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 fil‐
ter.
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 ALSOtc(8), tc-htb(8), tc-u32(8)iproute2 12 Jan Generic packet editor action in tc(8)
