ip-rule(8)
NAME
ip-rule - routing policy database management
SYNOPSIS
ip [ OPTIONS ] rule { COMMAND | help }
ip rule [ list | add | del | flush | save ] SELECTOR ACTION
ip rule restore
SELECTOR := [ from PREFIX ] [ to PREFIX ] [ tos TOS ] [ fwmark
FWMARK[/MASK] ] [ iif STRING ] [ oif STRING ] [ pref NUMBER ]
ACTION := [ table TABLE_ID ] [ nat ADDRESS ] [ realms
[SRCREALM/]DSTREALM ] 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
'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. 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.
Rule 0 is special. It cannot be deleted or overridden.
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.
priority PREFERENCE
the priority of this rule. 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.
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.
This command has no arguments.
ip rule show - list rules
This command has no arguments. The options list or lst are
synonyms 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
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.
SEE ALSO
ip(8)
AUTHOR
Original Manpage by Michail Litvak <[email protected]>
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
