1. Technology
Linux / Unix Command: route
Command Library

NAME

route - show / manipulate the IP routing table

SYNOPSIS

route [-CFvnee]
route
[-v] [-A family] add [-net|-host] target [netmask Nm] [gw Gw] [metric N] [mss M] [window W] [irtt I] [reject] [mod] [dyn] [reinstate] [[dev] If]
route
[-v] [-A family] del [-net|-host] target [gw Gw] [netmask Nm] [metric N] [[dev] If]
route
[-V] [--version] [-h] [--help]

DESCRIPTION

Route manipulates the kernel's IP routing tables. Its primary use is to set up static routes to specific hosts or networks via an interface after it has been configured with the ifconfig(8) program.

When the add or del options are used, route modifies the routing tables. Without these options, route displays the current contents of the routing tables.

OPTIONS

-A family
use the specified address family (eg `inet'; use `route --help' for a full list).
-F
operate on the kernel's FIB (Forwarding Information Base) routing table. This is the default.
-C
operate on the kernel's routing cache.
-v
select verbose operation.
-n
show numerical addresses instead of trying to determine symbolic host names. This is useful if you are trying to determine why the route to your nameserver has vanished.
-e
use netstat(8)-format for displaying the routing table. -ee will generate a very long line with all parameters from the routing table.
del
delete a route.
add
add a new route.
target
the destination network or host. You can provide IP addresses in dotted decimal or host/network names.
-net
the target is a network.
-host
the target is a host.
netmask NM
when adding a network route, the netmask to be used.
gw GW
route packets via a gateway. NOTE: The specified gateway must be reachable first. This usually means that you have to set up a static route to the gateway beforehand. If you specify the address of one of your local interfaces, it will be used to decide about the interface to which the packets should be routed to. This is a BSDism compatibility hack.
metric M
set the metric field in the routing table (used by routing daemons) to M.
mss M
set the TCP Maximum Segment Size (MSS) for connections over this route to M bytes. The default is the device MTU minus headers, or a lower MTU when path mtu discovery occured. This setting can be used to force smaller TCP packets on the other end when path mtu discovery does not work (usually because of misconfigured firewalls that block ICMP Fragmentation Needed)
window W
set the TCP window size for connections over this route to W bytes. This is typically only used on AX.25 networks and with drivers unable to handle back to back frames.
irtt I
set the initial round trip time (irtt) for TCP connections over this route to I milliseconds (1-12000). This is typically only used on AX.25 networks. If omitted the RFC 1122 default of 300ms is used.
reject
install a blocking route, which will force a route lookup to fail. This is for example used to mask out networks before using the default route. This is NOT for firewalling.
mod, dyn, reinstate
install a dynamic or modified route. These flags are for diagnostic purposes, and are generally only set by routing daemons.
dev If
force the route to be associated with the specified device, as the kernel will otherwise try to determine the device on its own (by checking already existing routes and device specifications, and where the route is added to). In most normal networks you won't need this.

If dev If is the last option on the command line, the word dev may be omitted, as it's the default. Otherwise the order of the route modifiers (metric - netmask - gw - dev) doesn't matter.

EXAMPLES

route add -net 127.0.0.0
adds the normal loopback entry, using netmask 255.0.0.0 (class A net, determined from the destination address) and associated with the "lo" device (assuming this device was prviously set up correctly with ifconfig(8)).
route add -net 192.56.76.0 netmask 255.255.255.0 dev eth0
adds a route to the network 192.56.76.x via "eth0". The Class C netmask modifier is not really necessary here because 192.* is a Class C IP address. The word "dev" can be omitted here.
route add default gw mango-gw
adds a default route (which will be used if no other route matches). All packets using this route will be gatewayed through "mango-gw". The device which will actually be used for that route depends on how we can reach "mango-gw" - the static route to "mango-gw" will have to be set up before.
route add ipx4 sl0
Adds the route to the "ipx4" host via the SLIP interface (assuming that "ipx4" is the SLIP host).
route add -net 192.57.66.0 netmask 255.255.255.0 gw ipx4
This command adds the net "192.57.66.x" to be gatewayed through the former route to the SLIP interface.
route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0
This is an obscure one documented so people know how to do it. This sets all of the class D (multicast) IP routes to go via "eth0". This is the correct normal configuration line with a multicasting kernel.
route add -net 10.0.0.0 netmask 255.0.0.0 reject
This installs a rejecting route for the private network "10.x.x.x."

OUTPUT

The output of the kernel routing table is organized in the following columns
Destination
The destination network or destination host.
Gateway
The gateway address or '*' if none set.
Genmask
The netmask for the destination net; '255.255.255.255' for a host destination and '0.0.0.0' for the default route.
Flags
Possible flags include
U (route is up)
H (target is a host)
G (use gateway)
R (reinstate route for dynamic routing)
D (dynamically installed by daemon or redirect)
M (modified from routing daemon or redirect)
A (installed by addrconf)
C (cache entry)
! (reject route)
Metric
The 'distance' to the target (usually counted in hops). It is not used by recent kernels, but may be needed by routing daemons.
Ref
Number of references to this route. (Not used in the Linux kernel.)
Use
Count of lookups for the route. Depending on the use of -F and -C this will be either route cache misses (-F) or hits (-C).
Iface
Interface to which packets for this route will be sent.
MSS
Default maximum segement size for TCP connections over this route.
Window
Default window size for TCP connections over this route.
irtt
Initial RTT (Round Trip Time). The kernel uses this to guess about the best TCP protocol parameters without waiting on (possibly slow) answers.
HH (cached only)
The number of ARP entries and cached routes that refer to the hardware header cache for the cached route. This will be -1 if a hardware address is not needed for the interface of the cached route (e.g. lo).
Arp (cached only)
Whether or not the hardware address for the cached route is up to date.

SEE ALSO

ifconfig(8), netstat(8), arp(8), rarp(8)


Important: Use the man command (% man) to see how a command is used on your particular computer.

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