route
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Guide to IP Layer Network Administration with Linux
Version 0.4.5
Author: Martin A. Brown
D.1. route
In the same way that ifconfig is the venerable utility for IP address management, route is a tremendously useful command for manipulating and displaying IP routing tables.
Here we'll look at several tasks you can perform with route. You can display routes, add routes (most importantly, the default route), remove routes, andexamine the routing cache. I will switch between traditional and CIDR notation for network addressing in this (and subsequent) sections, so the reader unaware of these notations is encouraged to refer liberally to the links provided in Section I.1.3, “General IP Networking Resources”.
When using route and ip route on the same machine, it is important to understand that not all routing table entries can be shown with route. The key distinction is that route only displays information in the main routing table. NAT routes, and routes in tables other than the main routing table must be managed and viewed separately with the ip route tool.
D.1.1. Displaying the routing table with route
By far the simplest and most common task one performs with route is viewing the routing table. On a single-homed desktop like tristan, the routing table will be very simple, probably comprised of only a few routes. Compare this to a complex routing table on a host with multiple interfaces and static routes to internal networks, such as masq-gw. It is by using the route command that you can determine where a packet goes when it leaves your machine.
Example D.1. Viewing a simple routing table with route
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In the simplest routing tables, as in tristan's case, you'll see three separate routes. The route which is customarily present on all machines (and which I'll not remark on after this) is the route to the loopback interface. The loopback interface is an IP interface completely local to the host itself. Most commonly, loopback is configured as a single IP address in a class A-sized network. This entire network has been set aside for use on loopback devices. The address used is usually 127.0.0.1/8, and the device name under all default installations of linux I have seen is lo. It is not at all unheard of for people to host services on loopback which are intended only for consumption on that machine, e.g., SMTP on tcp/25.
The remaining two lines define how tristan should reach any other IP address anywhere on the Internet. These two routing table entries divide the world into two different categories: a locally reachable network (192.168.99.0/24) and everything else. If an address falls within the 192.168.99.0/24 range, tristanknows it can reach the IP range directly on the wire, so any packets bound for this range will be pushed out onto the local media.
If the packet falls in any other range tristan will consult its routing table and find no single route that matches. In this case, the default route functions as a terminal choice. If no other route matches, the packet will be forwarded to this destination address, which is usually a router to another set of networks and routers (which eventually lead to the Internet).
Viewing a complex routing table is no more difficult than viewing a simple routing table, although it can be a bit more diffiult to read, interpret, and sometimes even find the route you wish to examine.
Example D.2. Viewing a complex routing table with route
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The above routing table shows a more complex set of static routes than one finds on a single-homed host. By comparing the network mask of the routes above, we can see that the network mask is listed from the most specific to the least specific. Refer to Section 4.5, “Route Selection” for more discussion.
A quick glance down this routing table also provides us with a good deal of knowledge about the topology of the network. Immediately we can identify four separate Ethernet interfaces, 3 locally connected class C sized networks, and one tiny subnet (192.168.100.0/30). We can also determine that there are two networks reachable via static routes behind internal routers.
Now that we have taken a quick glance at the output from the route command, let's examine a bit more systematically what it's reporting to us.
D.1.2. Reading route's output
For this discussion refer to the network map in the appendix, and also to Example D.2, “Viewing a complex routing table with route”. route is a venerable command, one which can manipulate routing tables for protocols other than IP. If you wish to know what other protocols are supported, try route --help at your leisure. Fortunately, route defaults to inet (IPv4) routes if no other address family is specified.
By combining the values in columns one and three you can determine the destination network or host address. The first line in masq-gw's routing table shows 192.168.100.0/255.255.255.252, which is more conveniently written in CIDR notation as 192.168.100.0/30. This is the smallest possible network according toRFC 1878. The only two useable addresses are 192.168.100.1 (service-router) and 192.168.100.2 (masq-gw).
The second column holds the IP address of the gateway to the destination if the destination is not a locally connected network. If there is a value other than 0.0.0.0 in this field, the kernel will address the outbound packet for this device (a router of some kind) rather than directly for the destination. The column after the netmask column (Flags) should always contain a G for destination not locally connected to the linux machine.
The fields Metric, Ref and Use are not generally used in simple or even moderately complex routing tables, however, we will discuss the Use column further in Section D.1.3, “Using route to display the routing cache”.
The final field in the route output contains the name of the interface through which the destination is reachable. This can be any interface known to the kernel which has an IP address. In Example D.2, “Viewing a complex routing table with route” we can learn immediately that 192.168.98.0/24 is reachable through interface eth2.
After this brief examination of the commonest of output from route, let's look at some of the other things we can learn from route and also how we can change the routing table.
D.1.3. Using route to display the routing cache
The routing cache is used by the kernel as a lookup table analogous to a quick reference card. It's faster for the kernel to refer to the cache (internally implemented as a hash table) for a recently used route than to lookup the destination address again. Routes existing in the route cache are periodically expired. If you need to clean out the routing cache entirely, you'll want to become familiar with ip route flush cache.
At first, it might surprise you to learn that there are no entries for locally connected networks in a routing cache. After a bit of reflection, you come to realize that there is on need to cache an IP route for a locally connected network because the machine is connected to the same Ethernet. So, any given destination has an entry in either the arp table or in the routing cache. For a clearer picture of the differences between each of the cached routse, I'd suggest adding a -e switch.
Example D.3. Viewing the routing cache with route
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FIXME! I don't really know why there are three entries in the routing cache for each destination. Here, for example, we see three entries in the routing cache for 194.52.197.133 (a Swedish destination).
The MSS column tells us what the path MTU discovery has determined for a maximum segment size for the route to this destination. By discovering the proper segment size for a route and caching this information, we can make most efficient use of bandwidth to the destination, without incurring the overhead of packet fragmentation enroute. See Section 4.10.1, “MTU, MSS, and ICMP” for a more complete discussion of MSS and MTU.
FIXME! There has to be more we can say about the routing cache here.
D.1.4. Creating a static route with route add
Static routes are explicit routes to non-local destinations through routers or gateways which are not the default gateway. The case of the routing table ontristan is a classic example of the need for a static route. There are two routers in the same network, masq-gw and isdn-router. If tristan has packets for the 192.168.98.0/24 network, they should be routed to 192.168.99.1 (isdn-router). Refer also to Section 1.3.3, “Adding and removing a static route” for this example.
As with ifconfig, route has a syntax unlike most standard unix command line utilities, mixing options and arguments with less regularity. Note the mandatory -net or -host options when adding or removing any route other than the default route.
In order to add a static route to the routing table, you'll need to gather several pieces of information about the remote network.
In our example network, masq-gw can only reach 10.38.0.0/16 through service-router. Let's add a static route to the masquerading firewall to ensure that 10.38.0.0/16 is reachable. Our intended routing table will look like the routing table in Example D.2, “Viewing a complex routing table with route”. Let's also view the output if we mistype the IP address of the default gateway and specify an address which is not a locally reachable address.
Example D.4. Adding a static route to a network route add
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It should be clear now that the gateway address must be reachable on a locally connected network for a static route to be useable (or even make sense). In the first line, where we mistyped, the route could not be added to the routing table because the gateway address was not a reachable address.
Now, instead of sending packets with a destination of 10.38.0.0/16 to the default gateway, wan-gw, masq-gw will send this traffic to service-router at IP address 192.168.100.1.
The above is a simple example of routing a network to a separate gateway, a gateway other than the default gateway. This is a common need on networks central to an operation, and less common in branch offices and remote networks.
Occasionally, however, you'll have a single machine with an IP address in a different range on the same Ethernet as some other machines. Or you might have a single machine which is reachable via a router. Let's look at these two scenarios to see how we can create static routes to solve this routing need.
Occasionally, you may have a desire to restrict communication from one network to another by not including routes to the network. In our sample network,tristan may be a workstation of an employee who doesn't need to reach any machines in the branch office. Perhaps this employee needs to periodically access some data or service supplied on 192.168.98.101. We'll need to add a static route to allow this machine to access this single host IP in the branch office network [47].
Here's a summary of the required data for our static route. The destination is 192.168.98.101/32 and the gateway is 192.168.99.1.
Example D.5. Adding a static route to a host with route add
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Now, we have successfully altered the routing table to include a host route for the single machine we want our employee to be able to reach.
Even rarer, you may encounter a situation where a single Ethernet network is used to host multiple IP networks.