16.1.1. An example

To illustrate different aspects of routing, we will use the following example which is the output of the command netstat -r:

Destination      Gateway            Flags     Refs     Use     Netif Expire

default          outside-gw         UGSc       37      418      ppp0
localhost        localhost          UH          0      181       lo0
test0            0:e0:b5:36:cf:4f   UHLW        5    63288       ed0     77
10.20.30.255     link#1             UHLW        1     2421 
foobar.com       link#1             UC          0        0 
host1            0:e0:a8:37:8:1e    UHLW        3     4601       lo0
host2            0:e0:a8:37:8:1e    UHLW        0        5       lo0 =>
host2.foobar.com link#1             UC          0        0
224              link#1	            UC          0        0

The first two lines specify the default route (which we will cover in the next section) and the localhost route.

The interface (Netif column) that it specifies to use for localhost is lo0, also known as the loopback device. This says to keep all traffic for this destination internal, rather than sending it out over the LAN, since it will only end up back where it started anyway.

The next thing that stands out are the ``0:e0:...'' addresses. These are ethernet hardware addresses. FreeBSD will automatically identify any hosts (test0 in the example) on the local ethernet and add a route for that host, directly to it over the ethernet interface, ed0. There is also a timeout (Expire column) associated with this type of route, which is used if we fail to hear from the host in a specific amount of time. In this case the route will be automatically deleted. These hosts are identified using a mechanism known as RIP (Routing Information Protocol), which figures out routes to local hosts based upon a shortest path determination.

FreeBSD will also add subnet routes for the local subnet (10.20.30.255 is the broadcast address for the subnet 10.20.30, and foobar.com is the domain name associated with that subnet). The designation link#1 refers to the first ethernet card in the machine. You will notice no additional interface is specified for those.

Both of these groups (local network hosts and local subnets) have their routes automatically configured by a daemon called routed. If this is not run, then only routes which are statically defined (ie. entered explicitly) will exist.

The host1 line refers to our host, which it knows by ethernet address. Since we are the sending host, FreeBSD knows to use the loopback interface (lo0) rather than sending it out over the ethernet interface.

The two host2 lines are an example of what happens when we use an ifconfig alias (see the section of ethernet for reasons why we would do this). The => symbol after the lo0 interface says that not only are we using the loopback (since this is address also refers to the local host), but specifically it is an alias. Such routes only show up on the host that supports the alias; all other hosts on the local network will simply have a link#1 line for such.

The final line (destination subnet 224) deals with MultiCasting, which will be covered in a another section.

The other column that we should talk about are the Flags. Each route has different attributes that are described in the column. Below is a short table of some of these flags and their meanings:

U

Up: The route is active.

H

Host: The route destination is a single host.

G

Gateway: Send anything for this destination on to this remote system, which will figure out from there where to send it.

S

Static: This route was configured manually, not automatically generated by the system.

C

Clone: Generates a new route based upon this route for machines we connect to. This type of route is normally used for local networks.

W

WasCloned Indicated a route that was auto-configured based upon a local area network (Clone) route.

L

Link: Route involves references to ethernet hardware.