Generic packet editor action in tc(8) Linux
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 | traffic_class | flow_lbl | payload_len |
nexthdr | hoplimit }
TCPHDR_FIELD := { sport | dport | flags }
UDPHDR_FIELD := { sport | dport }
CMD_SPEC := { clear | invert | set VAL | add VAL | decrement |
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.
OPTIONS
ex Use extended pedit. EXTENDED_LAYERED_OP and the add/decrement
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 one 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 one 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
traffic_class
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.
decrement
Decrease the addressed data by one. This operation is supported
only for ip ttl and ip6 hoplimit.
preserve
Keep the addressed data as is.
retain RVAL
This optional extra part of CMD_SPEC allows one to exclude bits
from being changed. Supported only for 32 bits fields or
smaller.
CONTROL
The following keywords allow one 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
To rewrite just part of a field, use the retain directive. E.g. to
overwrite the DSCP part of a dsfield with $DSCP, without touching ECN:
tc filter add dev eth0 ingress flower ... \
action pedit ex munge ip dsfield set $((DSCP << 2)) retain 0xfc
And vice versa, to set ECN to e.g. 1 without impacting DSCP:
tc filter add dev eth0 ingress flower ... \
action pedit ex munge ip dsfield set 1 retain 0x3
SEE ALSO
tc(8), tc-htb(8), tc-u32(8)
iproute2 12 Jan 2015
Generic packet editor action in tc(8)
pedit - generic packet editor action