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c093f4a658
When configuring a system with multiple network uplinks and default routes, it is often convenient to reference a routing table multiple times - but reject its routing decision if certain constraints are not met by it. Consider this setup: $ ip route add table secuplink default via 10.42.23.1 $ ip rule add pref 100 table main suppress_prefixlength 0 $ ip rule add pref 150 fwmark 0xA table secuplink With this setup, packets marked 0xA will be processed by the additional routing table "secuplink", but only if no suitable route in the main routing table can be found. By suppressing entries with a prefixlength of 0 (or less), the default route (/0) of the table "main" is hidden to packets processed by rule 100; packets traveling to destinations via more specific routes are processed as usual. It is also possible to suppress a routing entry if a device belonging to a specific interface group is to be used: $ ip rule add pref 150 table main suppress_group 1 Signed-off-by: Stefan Tomanek <stefan.tomanek@wertarbyte.de>
271 lines
6.5 KiB
Groff
271 lines
6.5 KiB
Groff
.TH IP\-RULE 8 "20 Dec 2011" "iproute2" "Linux"
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.SH "NAME"
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ip-rule \- routing policy database management
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.SH "SYNOPSIS"
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.sp
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.ad l
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.in +8
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.ti -8
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.B ip
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.RI "[ " OPTIONS " ]"
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.B rule
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.RI " { " COMMAND " | "
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.BR help " }"
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.sp
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.ti -8
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.B ip rule
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.RB " [ " list " | " add " | " del " | " flush " ]"
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.I SELECTOR ACTION
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.ti -8
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.IR SELECTOR " := [ "
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.B from
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.IR PREFIX " ] [ "
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.B to
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.IR PREFIX " ] [ "
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.B tos
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.IR TOS " ] [ "
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.B fwmark
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.IR FWMARK[/MASK] " ] [ "
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.B iif
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.IR STRING " ] [ "
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.B oif
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.IR STRING " ] [ "
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.B pref
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.IR NUMBER " ]"
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.ti -8
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.IR ACTION " := [ "
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.B table
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.IR TABLE_ID " ] [ "
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.B nat
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.IR ADDRESS " ] [ "
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.BR prohibit " | " reject " | " unreachable " ] [ " realms
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.RI "[" SRCREALM "/]" DSTREALM " ]"
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.I SUPPRESSOR
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.ti -8
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.IR SUPPRESSOR " := [ "
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.B suppress_prefixlength
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.IR NUMBER " ] [ "
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.B suppress_ifgroup
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.IR GROUP " ]"
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.ti -8
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.IR TABLE_ID " := [ "
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.BR local " | " main " | " default " |"
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.IR NUMBER " ]"
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.SH DESCRIPTION
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.I ip rule
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manipulates rules
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in the routing policy database control the route selection algorithm.
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.P
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Classic routing algorithms used in the Internet make routing decisions
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based only on the destination address of packets (and in theory,
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but not in practice, on the TOS field).
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.P
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In some circumstances we want to route packets differently depending not only
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on destination addresses, but also on other packet fields: source address,
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IP protocol, transport protocol ports or even packet payload.
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This task is called 'policy routing'.
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.P
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To solve this task, the conventional destination based routing table, ordered
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according to the longest match rule, is replaced with a 'routing policy
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database' (or RPDB), which selects routes by executing some set of rules.
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.P
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Each policy routing rule consists of a
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.B selector
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and an
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.B action predicate.
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The RPDB is scanned in the order of increasing priority. The selector
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of each rule is applied to {source address, destination address, incoming
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interface, tos, fwmark} and, if the selector matches the packet,
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the action is performed. The action predicate may return with success.
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In this case, it will either give a route or failure indication
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and the RPDB lookup is terminated. Otherwise, the RPDB program
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continues on the next rule.
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.P
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Semantically, natural action is to select the nexthop and the output device.
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.P
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At startup time the kernel configures the default RPDB consisting of three
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rules:
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.TP
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1.
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Priority: 0, Selector: match anything, Action: lookup routing
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table
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.B local
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(ID 255).
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The
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.B local
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table is a special routing table containing
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high priority control routes for local and broadcast addresses.
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.sp
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Rule 0 is special. It cannot be deleted or overridden.
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.TP
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2.
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Priority: 32766, Selector: match anything, Action: lookup routing
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table
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.B main
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(ID 254).
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The
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.B main
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table is the normal routing table containing all non-policy
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routes. This rule may be deleted and/or overridden with other
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ones by the administrator.
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.TP
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3.
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Priority: 32767, Selector: match anything, Action: lookup routing
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table
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.B default
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(ID 253).
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The
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.B default
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table is empty. It is reserved for some post-processing if no previous
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default rules selected the packet.
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This rule may also be deleted.
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.P
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Each RPDB entry has additional
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attributes. F.e. each rule has a pointer to some routing
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table. NAT and masquerading rules have an attribute to select new IP
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address to translate/masquerade. Besides that, rules have some
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optional attributes, which routes have, namely
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.BR "realms" .
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These values do not override those contained in the routing tables. They
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are only used if the route did not select any attributes.
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.sp
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The RPDB may contain rules of the following types:
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.in +8
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.B unicast
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- the rule prescribes to return the route found
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in the routing table referenced by the rule.
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.B blackhole
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- the rule prescribes to silently drop the packet.
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.B unreachable
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- the rule prescribes to generate a 'Network is unreachable' error.
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.B prohibit
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- the rule prescribes to generate 'Communication is administratively
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prohibited' error.
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.B nat
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- the rule prescribes to translate the source address
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of the IP packet into some other value.
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.in -8
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.SS ip rule add - insert a new rule
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.SS ip rule delete - delete a rule
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.TP
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.BI type " TYPE " (default)
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the type of this rule. The list of valid types was given in the previous
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subsection.
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.TP
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.BI from " PREFIX"
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select the source prefix to match.
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.TP
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.BI to " PREFIX"
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select the destination prefix to match.
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.TP
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.BI iif " NAME"
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select the incoming device to match. If the interface is loopback,
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the rule only matches packets originating from this host. This means
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that you may create separate routing tables for forwarded and local
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packets and, hence, completely segregate them.
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.TP
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.BI oif " NAME"
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select the outgoing device to match. The outgoing interface is only
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available for packets originating from local sockets that are bound to
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a device.
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.TP
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.BI tos " TOS"
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.TP
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.BI dsfield " TOS"
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select the TOS value to match.
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.TP
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.BI fwmark " MARK"
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select the
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.B fwmark
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value to match.
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.TP
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.BI priority " PREFERENCE"
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the priority of this rule. Each rule should have an explicitly
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set
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.I unique
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priority value.
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The options preference and order are synonyms with priority.
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.TP
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.BI table " TABLEID"
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the routing table identifier to lookup if the rule selector matches.
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It is also possible to use lookup instead of table.
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.TP
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.BI suppress_prefixlength " NUMBER"
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reject routing decisions that have a prefix length of NUMBER or less.
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.TP
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.BI suppress_ifgroup " GROUP"
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reject routing decisions that use a device belonging to the interface
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group GROUP.
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.TP
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.BI realms " FROM/TO"
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Realms to select if the rule matched and the routing table lookup
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succeeded. Realm
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.I TO
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is only used if the route did not select any realm.
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.TP
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.BI nat " ADDRESS"
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The base of the IP address block to translate (for source addresses).
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The
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.I ADDRESS
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may be either the start of the block of NAT addresses (selected by NAT
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routes) or a local host address (or even zero).
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In the last case the router does not translate the packets, but
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masquerades them to this address.
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Using map-to instead of nat means the same thing.
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.B Warning:
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Changes to the RPDB made with these commands do not become active
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immediately. It is assumed that after a script finishes a batch of
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updates, it flushes the routing cache with
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.BR "ip route flush cache" .
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.SS ip rule flush - also dumps all the deleted rules.
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This command has no arguments.
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.SS ip rule show - list rules
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This command has no arguments.
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The options list or lst are synonyms with show.
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.SH SEE ALSO
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.br
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.BR ip (8)
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.SH AUTHOR
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Original Manpage by Michail Litvak <mci@owl.openwall.com>
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