route(1M)

NAME

route − manually manipulate the routing tables

SYNOPSIS

/usr/sbin/route [-f] [-n] [-p pmtu] add [net|host] destination [netmask mask] gateway [count]

/usr/sbin/route [-f] [-n] delete [net|host] destination [netmask mask] gateway [count]

/usr/sbin/route -f [-n]

DESCRIPTION

The route command manipulates the network routing tables manually. You must have appropriate privileges.

Subcommands

The following subcommands are supported.

add Add the specified host or network route to the network routing table. If the route already exists, a message is printed and nothing changes.

delete Delete the specified host or network route from the network routing table.

Options and Arguments

route recognizes the following options and arguments.

-f Delete all route table entries that specify a remote host for a gateway. If this is used with one of the subcommands, the entries are deleted before the subcommand is processed.

-n Print any host and network addresses in Internet dot notation, except for the default network address, which is printed as default.

-p pmtu Specifies a path maximum transmission unit (MTU) value for a static host route. The minimum value allowed is 68 bytes; the maximum is the MTU of the outgoing interface for this route. This option only applies to adding a host route. In all other cases, this option is ignored and has no effect on a system.

You can also disable the Path MTU Discovery for a host route by specifying pmtu as zero.

net

host The type of destination address. If this argument is omitted, routes to a particular host are distinguished from those to a network by interpreting the Internet address associated with destination. If the destination has a local address part of INADDR_ANY(0), the route is assumed to be to a network; otherwise, it is treated as a route to a host.

destination The destination host system where the packets will be routed. destination can be one of the following:

• A host name (the official name or an alias, see gethostbyname(3N)).

• A network name (the official name or an alias, see getnetbyname(3N)).

• An Internet address in dot notation (see inet(3N)).

• The keyword default, which signifies the wildcard gateway route (see routing(7)).

netmask

mask The mask that will be bit-wise ANDed with destination to yield a net address where the packets will be routed. mask can be specified as a single hexadecimal number with a leading 0x, with a dot-notation Internet address, or with a pseudo-network name listed in the network table (see networks(4)). The length of the mask, which is the number of contiguous 1’s starting from the leftmost bit position of the 32-bit field, can be shorter than the default network mask for the destination address. (see routing (7)). If the netmask option is not given, mask for the route will be derived from the netmasks associated with the local interfaces. (see ifconfig (1)). mask will be defaulted to the longest netmask of those local interfaces that have the same network address. If there is not any local interface that has the same network address, then mask will be defaulted to the default network mask of destination.

gateway The gateway through which the destination is reached. gateway can be one of the following:

• A host name (the official name or an alias, see gethostbyname(3N)).

• An Internet address in dot notation (see inet(3N)).

count An integer that indicates whether the gateway is a remote host or the local host. If the route leads to a destination through a remote gateway, count should be a number greater than 0. If the route leads to destination and the gateway is the local host, count should be 0. The default for count is zero. The result is not defined if count is negative.

Operation

All symbolic names specified for a destination or gateway are looked up first as a host name using gethostbyname(); if the host name is not found, the destination is searched for as a network name using getnetbyname(). destination and gateway can be in dot notation (see inet(3N)).

If the -n option is not specified, any host and network addresses are displayed symbolically according to the name returned by gethostbyaddr() and getnetbyaddr(), respectively, except for the default network address (printed as default) and addresses that have unknown names. Addresses with unknown names are printed in Internet dot notation (see inet(3N)).

If the -n option is specified, any host and network addresses are printed in Internet dot notation except for the default network address which is printed as default.

If the -f option is specified, route deletes all route table entries that specify a remote host for a gateway. If it is used with one of the subcommands described above, the entries are deleted before the subcommand is processed.

Path MTU Discovery is a technique for discovering the maximum size of an IP datagram that can be sent on an internet path without causing datagram fragmentation in the intermediate routers. In essence, a source host that utilizes this technique initially sends out datagrams up to the the size of the outgoing interface. The Don’t Fragment (DF) bit in the IP datagram header is set. As an intermediate router that supports Path MTU Discovery receives a datagram that is too large to be forwarded in one piece to the next-hop router and the DF bit is set, the router will discard the datagram and send an ICMP Destination Unreachable message with a code meaning "fragmentation needed and DF set". The ICMP message will also contain the MTU of the next-hop router. When the source host receives the ICMP message, it reduces the path MTU of the route to the MTU in the ICMP message. With this technique, the host route in the source host for this path will contain the proper MTU.

By default, Path MTU Discovery is enabled for TCP sockets and disabled for UDP sockets.

If the -p pmtu option is specified for a host route, the pmtu value is considered permanent for the host route. Even if the Path MTU Discovery process discovers a smaller pmtu for this route at a later time, the pmtu field in the host route will not be updated. A warning message will be logged with the new pmtu value.

The -p pmtu option is useful only if you knows the network environment well enough to enter an appropriate pmtu for a host route. IP will fragment a datagram to the pmtu specified for the route on the local host before sending the datagram out to the remote. It will avoid fragmentation by routers along the path, if the pmtu specified in the route command is correct.

ping can be used to find the pmtu information for the route to a remote host. The pmtu information in the routing table can be displayed with the netstat -r command (see netstat(1)).

Output

add destination: gateway gateway

The specified route is being added to the tables.

delete destination: gateway gateway

The specified route is being deleted from the tables.

Flags

The values of the count and destination type fields in the route command determine the presence of the G and H flags in the netstat -r display and thus the route type, as shown in the following table.

Count	Destination Type	Flags	Route Type
=0	network	U	Route to a network directly from the local host
>0	network	UG	Route to a network through a remote host gateway
=0	host	UH	Route to a remote host directly from the local host
>0	host	UGH	Route to a remote host through a remote host gateway
=0	default	U	Wildcard route directly from the local host
>0	default	UG	Wildcard route through a remote host gateway

DIAGNOSTICS

The following error diagnostics can be displayed.

add a route that already exists

The specified entry is already in the routing table.

add too many routes

The routing table is full.

delete a route that does not exist

The specified route was not in the routing table.

WARNINGS

Reciprocal route commands must be executed on the local host, the destination host, and all intermediate hosts if routing is to succeed in the cases of virtual circuit connections or bidirectional datagram transfers.

The HP-UX implementation of route does not presently support a change subcommand.

AUTHOR

route was developed by the University of California, Berkeley.

FILES

/etc/networks
/etc/hosts

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