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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXAMPLES | SEE ALSO | COLOPHON |
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Generic packe... action in tc(8) Linux Generic packe... action in tc(8)
pedit - generic packet editor action
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 }
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.
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.
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
tc(8), tc-htb(8), tc-u32(8)
This page is part of the iproute2 (utilities for controlling
TCP/IP networking and traffic) project. Information about the
project can be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
[email protected], [email protected]. This page was
obtained from the project's upstream Git repository
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-11.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
iproute2 12 Jan 201G5eneric packe... action in tc(8)
Pages that refer to this page: tc-actions(8), tc-csum(8), tc-skbedit(8), tc-skbmod(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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