route executes one of four commands on a route to a destination. Two additional commands operate globally on all routing information. The six commands are:

add

Add a route.

change

Change aspects of a route (such as its gateway).

delete

Delete a specific route.

flush

Remove all gateway entries from the routing table.

get

Lookup and display the route for a destination.

monitor

Continuously report any changes to the routing information base, routing lookup misses, or suspected network partitionings.

The add, delete, and change commands have the following syntax:

route [ -fnvq ] add | delete [ -net | -host ] destination gateway [args]

or

route [ -fnvq ] change | get [ -net | -host ] destination gateway [args]

By default, a destination is looked up under the AF_INET address family or as an IPv4 address. All symbolic names specified for a destination or gateway are looked up first as a host name, using getipnodebyname(3SOCKET). If this lookup fails in the AF_INET case, getnetbyname(3SOCKET) is used to interpret the name as that of a network.

An optional modifier may be included on the command line before a destination, to force how route interprets a destination:

-host

Forces the destination to be interpreted as a host.

-net

Forces the destination to be interpreted as a network.

-inet

Forces the destination to be interpreted under the AF_INET address family or as an IPv4 address.

-inet6

Forces the destination to be interpreted under the AF_INET6 address family or as an IPv6 address.

In the case of the AF_INET address family or an IPv4 address, routes to a particular host may be distinguished from those to a network by interpreting the Internet address specified as the destination. If the destination has a "local address part" of INADDR_ANY, or if the destination is the symbolic name of a network, then the route is assumed to be to a network; otherwise, it is presumed to be a route to a host.

For example:

If the destination is directly reachable by way of an interface requiring no intermediary system to act as a gateway, this can be indicated by including one of two optional modifiers after the destination: The -interface modifier can be included or a metric of 0 can be specified. These modifiers are illustrated in the following alternative examples:

example% route add default hostname -interface
example% route add default hostname 0

hostname is the name or IP address associated with the network interface all packets should be sent over. On a host with a single network interface, hostname is normally the same as the nodename returned by uname -n (see uname(1)).

In the above examples, the route does not refer to a gateway, but rather to one of the machine's interfaces. Destinations matching such a route are sent out on the interface identified by the gateway address. For interfaces using the ARP protocol, this type of route is used to specify all destinations are local. That is, a host should ARP for all addresses by adding a default route using one of the two commands listed above.

With the AF_INET address family or an IPv4 address, the optional -netmask qualifier is intended to manually add subnet routes with netmasks different from that of the implied network interface. The implicit network mask generated in the AF_INET case can be overridden by making sure this option, and an ensuing address parameter (to be interpreted as a network mask), follows the destination parameter.

Alternatively, the length of the netmask may be supplied by appending a slash character and the length immediately after the destination. For example:

example% route add 192.0.2.32/27 somegateway

will create an IPv4 route to the destination 192.0.2.32 with a netmask of 255.255.255.224, and

example% route add -inet6 3ffe::/16 somegateway

will create an IPv6 route to the destination 33fe:: with a netmask of 16 one-bits followed by 112 zero-bits.

Routes have associated flags which influence operation of the protocols when sending to destinations matched by the routes. These flags may be set (or sometimes cleared) by including the following corresponding modifiers on the command line:

The optional modifiers -rtt, -rttvar, -sendpipe, -recvpipe, -mtu, -hopcount, -expire, and -ssthresh provide initial values to quantities maintained in the routing entry by transport level protocols, such as TCP. These may be individually locked either by preceding each modifier to be locked by the -lock meta-modifier, or by specifying that all ensuing metrics may be locked by the -lockrest meta-modifier.

The optional modifiers are defined as follows:

-expire

Lifetime for the entry. This optional modifier is not currently supported.

-hopcount

Maximum hop count. This optional modifier is not currently supported.

-mtu

Maximum MTU in bytes.

-recvpipe

Receive pipe size in bytes.

-rtt

Round trip time in microseconds.

-rttvar

Round trip time variance in microseconds.

-sendpipe

Send pipe size in bytes.

-ssthresh

Send pipe size threshold in bytes.

Some transport layer protocols may support only some of these metrics.

In a change or add command where the destination and gateway are not sufficient to specify the route (for example, , when several interfaces have the same address), the -ifp or -ifa modifiers may be used to determine the interface or interface address.