Routing is the mechanism that allows a system to find the network path to another system. A route is a defined pair of addresses which represent the “destination” and a “gateway”. The route indicates that when trying to get to the specified destination, send the packets through the specified gateway. There are three types of destinations: individual hosts, subnets, and “default”. The “default route” is used if no other routes apply. There are also three types of gateways: individual hosts, interfaces, also called links, and Ethernet hardware (MAC) addresses. Known routes are stored in a routing table.
This section provides an overview of routing basics. It then demonstrates how to configure a FreeBSD system as a router and offers some troubleshooting tips.
To view the routing table of a FreeBSD system, use netstat(1):
netstat -rRouting tables Internet: Destination Gateway Flags Refs Use Netif Expire default outside-gw UGS 37 418 em0 localhost localhost UH 0 181 lo0 test0 0:e0:b5:36:cf:4f UHLW 5 63288 re0 77 10.20.30.255 link#1 UHLW 1 2421 example.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.example.com link#1 UC 0 0 224 link#1 UC 0 0
The entries in this example are as follows:
The first route in this table specifies the
defaultroute. When the local system needs to make a connection to a remote host, it checks the routing table to determine if a known path exists. If the remote host matches an entry in the table, the system checks to see if it can connect using the interface specified in that entry.
If the destination does not match an entry, or if all known paths fail, the system uses the entry for the default route. For hosts on a local area network, the
Gatewayfield in the default route is set to the system which has a direct connection to the Internet. When reading this entry, verify that the
Flagscolumn indicates that the gateway is usable (
The default route for a machine which itself is functioning as the gateway to the outside world will be the gateway machine at the Internet Service Provider (ISP).
The second route is the
localhostroute. The interface specified in the
lo0, also known as the loopback device. This indicates that all traffic for this destination should be internal, rather than sending it out over the network.
- MAC address
The addresses beginning with
0:e0:are MAC addresses. FreeBSD will automatically identify any hosts,
test0in the example, on the local Ethernet and add a route for that host over the Ethernet interface,
re0. This type of route has a timeout, seen in the
Expirecolumn, which is used if the host does not respond in a specific amount of time. When this happens, the route to this host will be automatically deleted. These hosts are identified using the Routing Information Protocol (RIP), which calculates routes to local hosts based upon a shortest path determination.
FreeBSD will automatically add subnet routes for the local subnet. In this example,
10.20.30.255is the broadcast address for the subnet
example.comis the domain name associated with that subnet. The designation
link#1refers to the first Ethernet card in the machine.
Local network hosts and local subnets have their routes automatically configured by a daemon called routed(8). If it is not running, only routes which are statically defined by the administrator will exist.
host1line refers to the host by its Ethernet address. Since it is the sending host, FreeBSD knows to use the loopback interface (
lo0) rather than the Ethernet interface.
host2lines represent aliases which were created using ifconfig(8). The
=>symbol after the
lo0interface says that an alias has been set in addition to the loopback address. Such routes only show up on the host that supports the alias and all other hosts on the local network will have a
link#1line for such routes.
The final line (destination subnet
224) deals with multicasting.
Various attributes of each route can be seen in the
Flags column. Table30.1, “Commonly Seen Routing Table Flags”
summarizes some of these flags and their meanings:
|U||The route is active (up).|
|H||The route destination is a single host.|
|G||Send anything for this destination on to this gateway, which will figure out from there where to send it.|
|S||This route was statically configured.|
|C||Clones a new route based upon this route for machines to connect to. This type of route is normally used for local networks.|
|W||The route was auto-configured based upon a local area network (clone) route.|
|L||Route involves references to Ethernet (link) hardware.|
On a FreeBSD system, the default route can defined in
/etc/rc.conf by specifying the
IP address of the default gateway:
It is also possible to manually add the route using
route add default 10.20.30.1
Note that manually added routes will not survive a reboot. For more information on manual manipulation of network routing tables, refer to route(8).
A FreeBSD system can be configured as the default gateway, or router, for a network if it is a dual-homed system. A dual-homed system is a host which resides on at least two different networks. Typically, each network is connected to a separate network interface, though IP aliasing can be used to bind multiple addresses, each on a different subnet, to one physical interface.
In order for the system to forward packets between
interfaces, FreeBSD must be configured as a router. Internet
standards and good engineering practice prevent the FreeBSD
Project from enabling this feature by default, but it can be
configured to start at boot by adding this line to
gateway_enable="YES" # Set to YES if this host will be a gateway
To enable routing now, set the sysctl(8) variable
1. To stop routing, reset this variable to
The routing table of a router needs additional routes so it knows how to reach other networks. Routes can be either added manually using static routes or routes can be automatically learned using a routing protocol. Static routes are appropriate for small networks and this section describes how to add a static routing entry for a small network.
For large networks, static routes quickly become unscalable. FreeBSD comes with the standard BSD routing daemon routed(8), which provides the routing protocols RIP, versions 1 and 2, and IRDP. Support for the BGP and OSPF routing protocols can be installed using the net/zebra package or port.
Consider the following network:
In this scenario,
RouterA is a
FreeBSD machine that is acting as a router to the rest of the
Internet. It has a default route set to
10.0.0.1 which allows it to
connect with the outside world.
RouterB is already configured to use
192.168.1.1 as its
Before adding any static routes, the routing table on
RouterA looks like this:
netstat -nrRouting tables Internet: Destination Gateway Flags Refs Use Netif Expire default 10.0.0.1 UGS 0 49378 xl0 127.0.0.1 127.0.0.1 UH 0 6 lo0 10.0.0.0/24 link#1 UC 0 0 xl0 192.168.1.0/24 link#2 UC 0 0 xl1
With the current routing table,
RouterA does not have a route to the
network. The following command adds the
2 network to
routing table using
192.168.1.2 as the next
route add -net 192.168.2.0/24 192.168.1.2
RouterA can reach any host
However, the routing information will not persist if the FreeBSD
system reboots. If a static route needs to be persistent, add
# Add Internal Net 2 as a persistent static route static_routes="internalnet2" route_internalnet2="-net 192.168.2.0/24 192.168.1.2"
variable is a list of strings separated by a space, where each
string references a route name. The variable
contains the static route for that route name.
Using more than one string in
static_routes creates multiple static
routes. The following shows an example of adding static
routes for the
static_routes="net1 net2" route_net1="-net 192.168.0.0/24 192.168.0.1" route_net2="-net 192.168.1.0/24 192.168.1.1"
When an address space is assigned to a network, the service provider configures their routing tables so that all traffic for the network will be sent to the link for the site. But how do external sites know to send their packets to the network's ISP?
There is a system that keeps track of all assigned address spaces and defines their point of connection to the Internet backbone, or the main trunk lines that carry Internet traffic across the country and around the world. Each backbone machine has a copy of a master set of tables, which direct traffic for a particular network to a specific backbone carrier, and from there down the chain of service providers until it reaches a particular network.
It is the task of the service provider to advertise to the backbone sites that they are the point of connection, and thus the path inward, for a site. This is known as route propagation.
Sometimes, there is a problem with route propagation and
some sites are unable to connect. Perhaps the most useful
command for trying to figure out where routing is breaking
traceroute. It is useful when
traceroute, include the
address of the remote host to connect to. The output will
show the gateway hosts along the path of the attempt,
eventually either reaching the target host, or terminating
because of a lack of connection. For more information, refer
FreeBSD natively supports both multicast applications and multicast routing. Multicast applications do not require any special configuration in order to run on FreeBSD. Support for multicast routing requires that the following option be compiled into a custom kernel:
The multicast routing daemon,
mrouted can be installed using the
net/mrouted package or port. This daemon
implements the DVMRP multicast routing
protocol and is configured by editing
/usr/local/etc/mrouted.conf in order to
set up the tunnels and DVMRP. The
installation of mrouted also
installs map-mbone and
mrinfo, as well as their associated
man pages. Refer to these for configuration examples.
DVMRP has largely been replaced by the PIM protocol in many multicast installations. Refer to pim(4) for more information.