Linux kernel 2.2 and 2.4 support multiple routing tables [22].
Beyond the two commonly used routing tables (the local and main routing tables), the
kernel supports up to 252 additional routing tables.
The multiple routing table system provides a flexible
infrastructure on top of which to implement policy routing. By allowing
multiple traditional routing tables (keyed primarily to destination
address) to be combined with the routing policy database
(RPDB) (keyed primarily to source address), the kernel supports a
well-known and well-understood interface while simultaneously expanding
and extending its routing capabilities. Each routing table still
operates in the traditional and expected fashion. Linux simply allows
you to choose from a number of routing tables, and to traverse routing
tables in a user-definable sequence until a matching route is found.
Any given routing table can contain an arbitrary number of entries,
each of which is keyed on the following characteristics (cf. Table 4.1,
“Keys used for hash table lookups during route selection”)
For practical purposes, this means that (even) a single routing
table can contain multiple routes to the same destination if the ToS
differs on each route or if the route applies to a different interface [23].
Kernels supporting multiple routing tables refer to routing tables
by unique integer slots between 0 and 255 [24].
The two routing tables normally employed are table 255, the local routing table, and table 254, the main routing table. For examples of using
multiple routing tables, see Chapter 9, Advanced IP
Management, in particular, Example 10.1,
“Multiple Outbound Internet links, part I; ip route”,
Example 10.3,
“Multiple Outbound Internet links, part III; ip rule”
and Example 10.4,
“Multiple Internet links, inbound traffic; using iproute2
only ”. Also be sure to read Section 10.3,
“Using the Routing Policy Database and Multiple Routing Tables” and
Section 4.9, “Routing
Policy Database (RPDB)”.
The ip route and ip rule commands have built in support
for the special tables main and local. Any other routing tables can be
referred to by number or an administratively maintained mapping file, /etc/iproute2/rt_tables.
The format of this file is extraordinarily simple. Each line
represents one mapping of an arbitrary string to an integer. Comments
are allowed.
The routing table manipulated by the conventional route command is the main routing table. Additionally, the use of
both ip address
and ifconfig
will cause the kernel to alter the local routing table (and usually the
main routing table). For further documentation on how to manipulate the
other routing tables, see the command description of ip route.
4.8.1. Routing Table Entries (Routes)
Each routing table can contain an arbitrary number of route
entries. Aside from the local routing table, which
is maintained by the kernel, and the main routing table which is
partially maintained by the kernel, all routing tables are controlled
by the administrator or routing software. All routes on a machine can
be changed or removed [25].
Each of the following route types is available for use with the ip route command. Each route type
causes a particular sort of behaviour, which is identified in the
textual description. Compare the route types described below with the rule
types available for use in the RPDB.
- unicast
-
A unicast route is the most common route in routing tables.
This is a typical route to a destination network address, which
describes the path to the destination. Even complex routes, such as
nexthop routes are considered unicast routes. If no route type is
specified on the command line, the route is assumed to be a unicast
route.
Example 4.7. unicast route types
ip route add unicast 192.168.0.0/24 via 192.168.100.5
ip route add default via 193.7.255.1
ip route add unicast default via 206.59.29.193
ip route add 10.40.0.0/16 via 10.72.75.254
|
- broadcast
-
This route type is used for link layer devices (such as
Ethernet cards) which support the notion of a broadcast address. This
route type is used only in the local routing table [26]
and is typically handled by the kernel.
Example 4.8. broadcast route types
ip route add table local broadcast 10.10.20.255 dev eth0 proto kernel scope link src 10.10.20.67
ip route add table local broadcast 192.168.43.31 dev eth4 proto kernel scope link src 192.168.43.14
|
- local
-
The kernel will add entries into the local routing table when
IP addresses are added to an interface. This means that the IPs are
locally hosted IPs [27].
Example 4.9. local route types
ip route add table local local 10.10.20.64 dev eth0 proto kernel scope host src 10.10.20.67
ip route add table local local 192.168.43.12 dev eth4 proto kernel scope host src 192.168.43.14
|
- nat
-
This route entry is added by the kernel in the local routing
table, when the user attempts to configure stateless NAT. See Section 5.3, “Stateless NAT
with iproute2” for a
fuller discussion of network address translation in general. [28].
Example 4.10. nat route types
ip route add nat 193.7.255.184 via 172.16.82.184
ip route add nat 10.40.0.0/16 via 172.40.0.0
|
- unreachable
-
When a request for a routing decision returns a destination
with an unreachable route type, an ICMP unreachable is generated and
returned to the source address.
Example 4.11. unreachable route types
ip route add unreachable 172.16.82.184
ip route add unreachable 192.168.14.0/26
ip route add unreachable 209.10.26.51
|
- prohibit
-
When a request for a routing decision returns a destination
with a prohibit route type, the kernel generates an ICMP prohibited to
return to the source address.
Example 4.12. prohibit route types
ip route add prohibit 10.21.82.157
ip route add prohibit 172.28.113.0/28
ip route add prohibit 209.10.26.51
|
- blackhole
-
A packet matching a route with the route type blackhole is
discarded. No ICMP is sent and no packet is forwarded.
Example 4.13. blackhole route types
ip route add blackhole default
ip route add blackhole 202.143.170.0/24
ip route add blackhole 64.65.64.0/18
|
- throw
-
The throw route type is a convenient route type which causes a
route lookup in a routing table to fail, returning the routing selection process to the
RPDB. This is useful when there are additional routing tables. Note
that there is an implicit throw if no default route exists in a routing
table, so the route created by the first command in the example is
superfluous, although legal.
Example 4.14. throw route types
ip route add throw default
ip route add throw 10.79.0.0/16
ip route add throw 172.16.0.0/12
|
The power of these route types when combined with the routing policy database
can hardly be understated. All of these route types can be used without
the RPDB, although the throw route doesn't make much sense outside of a
multiple routing table installation.
4.8.2. The Local Routing Table
The local routing table is maintained by the kernel. Normally, the
local routing table should not be manipulated, but it is available for
viewing. In Example D.12,
“Viewing the local routing table with ip route show table local”,
you'll see two of the common uses of the local routing table. The first
common use is the specification of broadcast address, necessary only
for link layers which support broadcast addressing. The second common
type of entry in a local routing table is a route to a locally hosted
IP.
The route types found in the local routing table are local, nat and broadcast. These route types are not relevant
in other routing tables, and other route types cannot be used in the
local routing table.
If the the machine has several IP addresses on one Ethernet
interface, there will be a route to each locally hosted IP in the local
routing table. This is a normal side
effect of bringing up an IP address on an interface under linux.
Maintenance of the broadcast and local routes in the local routing
table can only be done by the kernel.
Example 4.15. Kernel maintenance of the local routing table
[r...@real-server]# ip address show dev eth1
6: eth1: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 100
link/ether 00:80:c8:e8:1e:fc brd ff:ff:ff:ff:ff:ff
inet 10.10.20.89/24 brd 10.10.20.255 scope global eth1
[r...@real-server]# ip route show dev eth1
10.10.20.0/24 proto kernel scope link src 10.10.20.89
[r...@real-server]# ip route show dev eth1 table local
broadcast 10.10.20.0 proto kernel scope link src 10.10.20.89
broadcast 10.10.20.255 proto kernel scope link src 10.10.20.89
local 10.10.20.89 proto kernel scope host src 10.10.20.89
[r...@real-server]# ip address add 192.168.254.254/24 brd + dev eth1
[r...@real-server]# ip address show dev eth1
6: eth1: <BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast qlen 100
link/ether 00:80:c8:e8:1e:fc brd ff:ff:ff:ff:ff:ff
inet 10.10.20.89/24 brd 10.10.20.255 scope global eth1
inet 192.168.254.254/24 brd 192.168.254.255 scope global eth1
[r...@real-server]# ip route show dev eth1
10.10.20.0/24 proto kernel scope link src 10.10.20.89
192.168.254.0/24 proto kernel scope link src 192.168.254.254
[r...@real-server]# ip route show dev eth1 table local
broadcast 10.10.20.0 proto kernel scope link src 10.10.20.89
broadcast 192.168.254.0 proto kernel scope link src 192.168.254.254
broadcast 10.10.20.255 proto kernel scope link src 10.10.20.89
local 192.168.254.254 proto kernel scope host src 192.168.254.254
local 10.10.20.89 proto kernel scope host src 10.10.20.89
broadcast 192.168.254.255 proto kernel scope link src 192.168.254.254
|
Note in Example 4.15,
“Kernel maintenance of the local routing
table”, that the kernel adds not only the route for the locally
connected network in the main routing
table, but also the three required special addresses in the local routing table. Any IP addresses which
are locally hosted on the box will have local
entries in the local table. The network
address and broadcast
address are both entered as broadcast
type addresses on the interface to which they have been bound.
Conceptually, there is significance to the distinction between a
network and broadcast address, but practically, they are treated
analogously, by other networking gear as well as the linux kernel.
There is one other type of route which commonly ends up in the local routing table. When using iproute2 NAT, there will be entries in
the local routing table for each network address translation. Refer to Example D.21,
“Creating a NAT route for a single IP with ip route add nat”
and Example D.22,
“Creating a NAT route for an entire network with ip route add nat”
for example output.
4.8.3. The Main Routing Table
The main routing table is the
routing table most people think of when considering a linux routing
table. When no table is specified to an ip
route command, the kernel assumes the main routing table. The route command only manipulates the main routing table.
Similarly to the local table, the main table is populated automatically by the
kernel when new interfaces are brought up with IP addresses. Consult
the main routing table before and after ip address add 192.168.254.254/24 brd + dev
eth1 in Example 4.15,
“Kernel maintenance of the local routing
table” for a concrete example of this kernel behaviour. Also, visit
this
summary of side effects of interface definition and activation with
ifconfig or ip address.
