IP-RULE(8) Linux IP-RULE(8)
NAME
ip-rule - routing policy database management
SYNOPSIS
ip [ OPTIONS ] rule { COMMAND | help }
ip rule [ list [ SELECTOR ]]
ip rule { add | del } SELECTOR ACTION
ip rule { flush | save | restore }
SELECTOR := [ not ] [ from PREFIX ] [ to PREFIX ] [ tos TOS ] [ fwmark
FWMARK[/MASK] ] [ iif STRING ] [ oif STRING ] [ pref NUMBER ] [
l3mdev ]
ACTION := [ table TABLE_ID ] [ nat ADDRESS ] [ realms
[SRCREALM/]DSTREALM ] [ goto NUMBER ] SUPPRESSOR
SUPPRESSOR := [ suppress_prefixlength NUMBER ] [ suppress_ifgroup GROUP
]
TABLE_ID := [ local | main | default | NUMBER ]
DESCRIPTION
ip rule manipulates rules in the routing policy database control the
route selection algorithm.
Classic routing algorithms used in the Internet make routing decisions
based only on the destination address of packets (and in theory, but
not in practice, on the TOS field).
In some circumstances we want to route packets differently depending
not only on destination addresses, but also on other packet fields:
source address, IP protocol, transport protocol ports or even packet
payload. This task is called 'policy routing'.
To solve this task, the conventional destination based routing table,
ordered according to the longest match rule, is replaced with a 'rout-
ing policy database' (or RPDB), which selects routes by executing some
set of rules.
Each policy routing rule consists of a selector and an action predi-
cate. The RPDB is scanned in order of decreasing priority (note that
lower number means higher priority, see the description of PREFERENCE
below). The selector of each rule is applied to {source address, desti-
nation address, incoming interface, tos, fwmark} and, if the selector
matches the packet, the action is performed. The action predicate may
return with success. In this case, it will either give a route or
failure indication and the RPDB lookup is terminated. Otherwise, the
RPDB program continues with the next rule.
Semantically, the natural action is to select the nexthop and the out-
put device.
At startup time the kernel configures the default RPDB consisting of
three rules:
1. Priority: 0, Selector: match anything, Action: lookup routing
table local (ID 255). The local table is a special routing ta-
ble containing high priority control routes for local and broad-
cast addresses.
2. Priority: 32766, Selector: match anything, Action: lookup rout-
ing table main (ID 254). The main table is the normal routing
table containing all non-policy routes. This rule may be deleted
and/or overridden with other ones by the administrator.
3. Priority: 32767, Selector: match anything, Action: lookup rout-
ing table default (ID 253). The default table is empty. It is
reserved for some post-processing if no previous default rules
selected the packet. This rule may also be deleted.
Each RPDB entry has additional attributes. F.e. each rule has a pointer
to some routing table. NAT and masquerading rules have an attribute to
select new IP address to translate/masquerade. Besides that, rules have
some optional attributes, which routes have, namely realms. These val-
ues do not override those contained in the routing tables. They are
only used if the route did not select any attributes.
The RPDB may contain rules of the following types:
unicast - the rule prescribes to return the route found in the
routing table referenced by the rule.
blackhole - the rule prescribes to silently drop the packet.
unreachable - the rule prescribes to generate a 'Network is
unreachable' error.
prohibit - the rule prescribes to generate 'Communication is
administratively prohibited' error.
nat - the rule prescribes to translate the source address of the
IP packet into some other value.
ip rule add - insert a new rule
ip rule delete - delete a rule
type TYPE (default)
the type of this rule. The list of valid types was given
in the previous subsection.
from PREFIX
select the source prefix to match.
to PREFIX
select the destination prefix to match.
iif NAME
select the incoming device to match. If the interface is
loopback, the rule only matches packets originating from
this host. This means that you may create separate rout-
ing tables for forwarded and local packets and, hence,
completely segregate them.
oif NAME
select the outgoing device to match. The outgoing inter-
face is only available for packets originating from local
sockets that are bound to a device.
tos TOS
dsfield TOS
select the TOS value to match.
fwmark MARK
select the fwmark value to match.
priority PREFERENCE
the priority of this rule. PREFERENCE is an unsigned
integer value, higher number means lower priority, and
rules get processed in order of increasing number. Each
rule should have an explicitly set unique priority value.
The options preference and order are synonyms with prior-
ity.
table TABLEID
the routing table identifier to lookup if the rule selec-
tor matches. It is also possible to use lookup instead
of table.
suppress_prefixlength NUMBER
reject routing decisions that have a prefix length of
NUMBER or less.
suppress_ifgroup GROUP
reject routing decisions that use a device belonging to
the interface group GROUP.
realms FROM/TO
Realms to select if the rule matched and the routing ta-
ble lookup succeeded. Realm TO is only used if the route
did not select any realm.
nat ADDRESS
The base of the IP address block to translate (for source
addresses). The ADDRESS may be either the start of the
block of NAT addresses (selected by NAT routes) or a
local host address (or even zero). In the last case the
router does not translate the packets, but masquerades
them to this address. Using map-to instead of nat means
the same thing.
Warning: Changes to the RPDB made with these commands do
not become active immediately. It is assumed that after a
script finishes a batch of updates, it flushes the rout-
ing cache with ip route flush cache.
ip rule flush - also dumps all the deleted rules.
This command has no arguments.
ip rule show - list rules
This command has no arguments. The options list or lst are syn-
onyms with show.
ip rule save
save rules table information to stdout
This command behaves like ip rule show except that the output is
raw data suitable for passing to ip rule restore.
ip rule restore
restore rules table information from stdin
This command expects to read a data stream as returned from ip
rule save. It will attempt to restore the rules table informa-
tion exactly as it was at the time of the save. Any rules
already in the table are left unchanged, and duplicates are not
ignored.
SEE ALSO
ip(8)
AUTHOR
Original Manpage by Michail Litvak <mci AT owl.com>
iproute2 20 Dec 2011 IP-RULE(8)
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