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 'routing policy database' (or RPDB), which selects routes
by executing some set of rules.
Each policy routing rule consists of a selector and an action
predicate. 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, destination 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
output 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 table containing high priority control
routes for local and broadcast addresses.
2. Priority: 32766, Selector: match anything, Action: lookup
routing 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
routing 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 values 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 routing tables for
forwarded and local packets and, hence, completely
segregate them.
oif NAME
select the outgoing device to match. The outgoing
interface 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.
uidrange NUMBER-NUMBER
select the uid value to match.
ipproto PROTOCOL
select the ip protocol value to match.
sport NUMBER | NUMBER-NUMBER
select the source port value to match. supports
port range.
dport NUMBER | NUMBER-NUMBER
select the destination port value to match.
supports port range.
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 priority.
table TABLEID
the routing table identifier to lookup if the rule
selector matches. It is also possible to use
lookup instead of table.
protocol PROTO
the routing protocol who installed the rule in
question. As an example when zebra installs a rule
it would get RTPROT_ZEBRA as the installing
protocol.
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 table 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 routing cache with ip route
flush cache.
ip rule flush - also dumps all the deleted rules.
protocol PROTO
Select the originating protocol.
ip rule show - list rules
This command has no arguments. The options list or lst
are synonyms with show.
ip rule save
protocol PROTO
Select the originating protocol.
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 information exactly as it was at the time of the
save. Any rules already in the table are left unchanged,
and duplicates are not ignored.
