Linux and Unix route command

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About route
Syntax
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About route

Show or manipulate the IP routing table.

Overview

In computer networking, a router is a device responsible for forwarding network traffic. When datagrams arrive at a router, the router must determine the best way to route them to their destination.

On Linux, BSD, and other Unix-like systems, the route command is used to view and make changes to the kernel routing table. The command syntax is different on different systems; here, when it comes to specific command syntax, we'll be discussing the Linux version.

Running route at the command line without any options will display the routing table entries:

route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         192.168.1.2     0.0.0.0         UG    1024   0        0 eth0
192.168.1.0     *               255.255.255.0   U     0      0        0 eth0

This shows us how the system is currently configured. If a packet comes into the system and has a destination in the range 192.168.2.0 through 192.168.2.255, then it is forwarded to the gateway *, which is 0.0.0.0 — a special address which represents an invalid or non-existant destination. So, in this case, our system will not route these packets.

If the destination is not in this IP address range, it is forwarded to the default gateway (in this case, 192.168.1.2, and that system will determine how to forward the traffic on to the next step towards its destination.

Command Syntax

route [-CFvnee]
route [-v] [-A family] add [-net|-host] target [netmask Nm] [gw Gw] 
      [metric N] i [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]

Technical 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 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-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 BSD-style 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 occurred. 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 -n

Shows routing table for all IPs bound to the server.

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 -net 224.0.0.0 netmask 240.0.0.0 dev eth0

This command sets all of the class D (multicast) IP routes to go via "eth0". This is the correct normal configuration for a multicasting kernel.

Related commands

arp — Manipulate the system ARP cache.
ifconfig — View or modify the configuration of network interfaces.
ip — Display and manipulate information about routing, devices, policy routing and tunnels.
netstat — Print information about network connections, routing tables, interface statistics, masquerade connections, and multicast memberships.