Linux kernel 2.2 and 2.4 support multiple routing tables [21]. 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)
destination address; a network or host address (primary key)
tos; Type of Service
output interface
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 [22].
Kernels supporting multiple routing tables refer to routing tables by unique integer slots between 0 and 255 [23]. 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, Example 10.3 and Example 10.4. 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.
Example 4.6. Typical content of /etc/iproute2/rt_tables
# # reserved values # 255 local |
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.
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 [24].
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.
- 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 [25] and is typically handled by the kernel.
- 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 [26].
- 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. [27].
- 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.
- 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.
- blackhole
A packet matching a route with the route type blackhole is discarded. No ICMP is sent and no packet is forwarded.
- 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.
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.
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, 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
[root@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
[root@real-server]# ip route show dev eth1
10.10.20.0/24 proto kernel scope link src 10.10.20.89
[root@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
[root@real-server]# ip address add 192.168.254.254/24 brd + dev eth1
[root@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
[root@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
[root@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, 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 and Example D.22 for example output.
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 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.
[21] The kernel must be compiled with the option CONFIG_IP_MULTIPLE_TABLES=y. This is common in vendor and stock kernels, both 2.2 and 2.4.
[22] If somebody has used scope or oif as additional keys in a routing table, and has an example, I'd love to see it, for possible inclusion in this documentation.
[23] Can anybody describe to me what is in table 0? It looks almost like an aggregation of the routing entries in routing tables 254 and 255.
[24] Once again, I recommend caution when altering the local routing table. Removing local route types from the local routing table can break networking in strange and wonderful ways.
[25] OK, I'm not absolutely sure you can't use the broadcast route in other routing tables, but I believe you can't. Testing forthcoming...
[26] Ibid. I'm not sure that local route types can be used in any routing table other than the local routing table. Testing forthcoming...
[27] Ibid. nat route types might be ineffectual outside the local routing table. Testing forthcoming...