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Linux route command

News Routing Recommended Links Static Routing Typical Operations Making routing entries permanent
ip command Linux ifconfig netstat Static Routing Default Route Suse static routes table
Policy Routing Suse network configuration Redhat Network Configuration Traceroute tcpdump RIP Daemon
martian source nmap Horror Stories Unix History Humor Etc

Introduction

Like in other Unixes, route is an entry in routing table specifying the target where Linux kernel sends IP packet based on it the destination address. Linux inherited set of classic Unix utilities such as route, netstat, ifconfig from GNU project and like other utilities from this project they have idiosyncrasies and extensions in comparison with POSIX utilities of the same name.  For example, one difference between route command in Solaris and route in Linux is that you need keyword gw before specifying the gateway. Other then that two version are pretty compatible.

For basic info see Route command. Here we will discuss mainly Linux-specific issues.

Traditional routing vs. policy routing

Most Linux distributions support two types of routing:

Typical Operations

To check the content of the routing table in Linux (see netstat):

netstat -r # with DNS names
netstat -rn # with IP addresses

Adding and Removing a Network in Linux

route add -net 10.10.10.0/24 gw 192.168.0.1
route del -net 10.10.10.0/24 gw 192.168.0.1

Adding and Removing a specific host is Linux-flavor specific:

route add -host 10.10.10.45 gw 192.168.0.1
route del -host 10.10.10.45 gw 192.168.0.1

You can also specify netmask and interface

route del -net 10.1.0.0 netmask 255.255.0.0 gw 10.2.0.1 eth0 
route del -host 10.10.0.5 netmask 255.255.0.0 gw 10.2.0.1 eth0 

Adding a Default GW in Linux

route add default gw 192.168.0.1
route del default gw 192.168.0.1

You can also specify it in /etc/sysconfig/network

NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=box17
GATEWAY=10.194.176.1

Flashing routing table

ip route flush

Note:

Making routing entries permanent

In Solaris and other Unixes to make routing entry permanent you need to use -p option of the route command. That's it. Linux guys decided to reinvent the bicycle and that shows.

In Suse to make routing entries permanent you need to put them into a special table of static routes that will be read on boot. It is stored in /etc/sysconfig/network/route file. The file has the format of output of the route -rn command, so it's a pretty elegant approach. But only if you know about it ;-). See Suse static routes table for more details.

In Red Hat, Fedora, Centos, and Oracle Linux this issue is an over-engineered mess. There is no system wide table to store static route information. It is stored on interface basis. For each interface you need to define and maintain /etc/sysconfig/network-scripts/route-eth<number> (or other network interface) file. Such interface config file exists for each valid network interface card.  For example, static routes for the eth0 interface would be stored in the /etc/sysconfig/network-scripts/route-eth0 file.

There are two formats acceptable in this file

We will start with the new format as it causes less allergy. It is available since Red Hat 8, I think. In this case the route-interface file has two types of directives: one for default router and the other for network/netmask directives. Here is an example from Centos deployment guide:

The following is a sample route-eth0 file using the IP command arguments format. The default gateway is 192.168.0.1, interface eth0. The two static routes are for the 10.10.10.0/24 and 172.16.1.0/24 networks:
default 192.168.0.1 dev eth0
10.10.10.0/24 via 192.168.0.1 dev eth0
172.16.1.0/24 via 192.168.0.1 dev eth0
There is also older, pretty stupid format that for compatibility is still accepted. You should never use it, but it might be useful to be aware about its existence:

You can also use the network/netmask directives format for route-interface files. The following is a template for the network/netmask format, with instructions following afterwards:

ADDRESS0=X.X.X.X
NETMASK0=X.X.X.X
GATEWAY0=X.X.X.X
Where:

The following is a sample route-eth0 file using the network/netmask directives format. The default gateway is 192.168.0.1, interface eth0. The two static routes are for the 10.10.10.0/24 and 172.16.1.0/24 networks. However, as mentioned before, this example is not necessary as the 10.10.10.0/24 and 172.16.1.0/24 networks would use the default gateway anyway:

ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=192.168.0.1
ADDRESS1=172.16.1.0
NETMASK1=255.255.255.0
GATEWAY1=192.168.0.1

Subsequent static routes must be numbered sequentially, and must not skip any values. For example, ADDRESS0, ADDRESS1, ADDRESS2, and so on. --[That make deletion a labor intensive operation --NNB;-)]

Below is an example of setting static routes to a different subnet, on a machine in the 192.168.0.0/24 subnet. The example machine has an eth0 interface in the 192.168.0.0/24 subnet, and an eth1 interface (10.10.10.1) in the 10.10.10.0/24 subnet:

ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=10.10.10.1

What's really funny is that this horrible way of specifying static routes was essentially a change from Suse-style that was used in Red Hat 7 (after all Suse is a Red Hat derivative). Yes, Red Hat 7 used to have a "normal" way to define static routes using /etc/sysconfig/static-routes table (Static Routes in Red Hat 8.0):

As of Red Hat 8.0, Red Hat has changed the way in which non-default static routes are initialized and added to the routing table on startup. Since this process is not documented, I've made a few notes here.

Traditionally, static routes were added in /etc/sysconfig/static-routes, in the form:

iface type dest-addr netmask netmask gw gateway-addr 
	... 

such as this example, taken from a real system:

eth0 net 192.168.170.0 netmask 255.255.255.0 gw 192.168.168.1

This would cause the startup scripts to execute a command like this
route add -type dest-addr netmask netmask gw gateway-addr ... iface

Notice the ellipsis at the end of the line there - this means that other options for the route add command can be specified in static-routes, which is particularly useful for specifying metrics - something that is quite common in moderately complex intranets. Other options, such as maximum segment size, initial window size and initial round-trip time, may also be useful.

In Red Hat 8.0, attempts to add interface-specific routes in static-routes will fail. Instead, static routes must be specified as multiple variables in multiple files in /etc/sysconfig/networking/devices. For example, a static route for the eth0 device must be specified in a file called eth0.route, like this:

ADDRESS0=192.168.170.0
NETMASK0=255.255.255.0
GATEWAY0=192.168.168.1

No other variables are supported, although additional routes can be specified as ADDRESS1, NETMASK1, etc. Clearly, this means that metrics and other parameters cannot be set at this point.

Way to go, Red Hat. More complexity, with less functionality. . . Sigh . . .

Dr Nikolai Bezroukov


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Old News

[Feb 19, 2011] Static Routes in Redhat 9 - New isn't always better

Looks like the degeneration of Red Hat and loss of architectural thinking started around 2003... Now with systemd it is in full swing ;-)
Overview

With the introduction of Redhat version 8 and continued into version 9, the /etc/sysconfig/static-routes file no longer seems to function correctly.

Linux static routes changed in 8.0 to a new format. Now you are to create a file in /etc/sysconfig/network-scripts for each Ethernet interface you wish to create static routes on.

Example:

touch /etc/sysconfig/network-scripts/route-eth0

The syntax for this file is different from the traditional route format used in /etc/sysconfig/static-routes . Redhat has yet to document the change on their web site as of June 2003.

Syntax based on a usenet post go to /etc/sysconfig/network-scripts, make a file called route-devicename (ex: route-eth0) and populate it with your static routes for that device so if you wanted to make a static route to the 192.168.0.0/24 network through 152.3.182.5 type:

192.168.0.0/24 via 152.3.182.5

Persistent static routes for ANY linux distribution

You may use this method to add static routes and it will work under any Linux distribution. However, it is considered by some a 'hack' or the 'ugly way'.

Edit your /etc/rc.local file and add your static routes using the route statement.

Example:

route add -net 10.10.98.0 netmask 255.255.255.0 gw 10.164.234.132 dev eth1
route add -net 10.164.234.96 netmask 255.255.255.252 gw 10.164.234.132 dev eth1
route add -net 10.164.234.112 netmask 255.255.255.240 gw 10.164.234.132 dev eth1

Force the old static-routes file to work under Redhat 9

Clear out the new /etc/sysconfig/network-scripts/ifup-routes script so that you can populate it with the original shell script from Redhat 7.x.

cat /dev/null > /etc/sysconfig/network-scripts/ifup-routes
vi /etc/sysconfig/network-scripts/ifup-routes

type in the following (or copy and paste) not including the tilde lines:

#!/bin/sh

# adds static routes which go through device $1

if [ "$1" = "" ]; then
echo "usage: $0 "
exit 1
fi

if [ ! -f /etc/sysconfig/static-routes ]; then
exit 0
fi

# note the trailing space in the grep gets rid of aliases
grep "^$1 " /etc/sysconfig/static-routes | while read device args; do
/sbin/route add -$args $device
done
grep "^any " /etc/sysconfig/static-routes | while read ignore type net netmask mask bogus dev ; do
if [ "$dev" = "$1" ]; then
/sbin/route add -$type $net $netmask $mask $dev
fi
done 

Remember to use /etc/sysconfig/network for your default gateway

If you only intend to add one route, your default gateway, then you need not worry about the static routes file or using the route command. Simply add your default gateway in /etc/sysconfig/network.

Example

NETWORKING=yes
HOSTNAME="hostname.linux.org"
GATEWAY="10.164.234.1"
GATEWAYDEV="eth0"
FORWARD_IPV4="yes" 

[Feb 19, 2011] Red Hat Deployment Guide/Configuring Static Routes

Routing will be configured on routing devices, therefore it should not be necessary to configure static routes on Red Hat Enterprise Linux servers or clients. However, if static routes are required they can be configured for each interface. This can be useful if you have multiple interfaces in different subnets. Use the route command to display the IP routing table.

Static route configuration is stored in a /etc/sysconfig/network-scripts/route-interface file. For example, static routes for the eth0 interface would be stored in the /etc/sysconfig/network-scripts/route-eth0 file. The route-interface file has two formats: IP command arguments and network/netmask directives.

IP Command Arguments Format
Define a default gateway on the first line. This is only required if the default gateway is not set via DHCP:
default X.X.X.X dev interface
X.X.X.X is the IP address of the default gateway. The interface is the interface that is connected to, or can reach, the default gateway.

Define a static route. Each line is parsed as an individual route:

X.X.X.X/X via X.X.X.X dev interface
X.X.X.X/X is the network number and netmask for the static route. X.X.X.X and interface are the IP address and interface for the default gateway respectively. The X.X.X.X address does not have to be the default gateway IP address. In most cases, X.X.X.X will be an IP address in a different subnet, and interface will be the interface that is connected to, or can reach, that subnet. Add as many static routes as required.

The following is a sample route-eth0 file using the IP command arguments format. The default gateway is 192.168.0.1, interface eth0. The two static routes are for the 10.10.10.0/24 and 172.16.1.0/24 networks:

default 192.168.0.1 dev eth0
10.10.10.0/24 via 192.168.0.1 dev eth0
172.16.1.0/24 via 192.168.0.1 dev eth0
Static routes should only be configured for other subnets. The above example is not necessary, since packets going to the 10.10.10.0/24 and 172.16.1.0/24 networks will use the default gateway anyway. Below is an example of setting static routes to a different subnet, on a machine in a 192.168.0.0/24 subnet. The example machine has an eth0 interface in the 192.168.0.0/24 subnet, and an eth1 interface (10.10.10.1) in the 10.10.10.0/24 subnet:
10.10.10.0/24 via 10.10.10.1 dev eth1

Duplicate Default Gateways

If the default gateway is already assigned from DHCP, the IP command arguments format can cause one of two errors during start-up, or when bringing up an interface from the down state using the ifup command: "RTNETLINK answers: File exists" or 'Error: either "to" is a duplicate, or "X.X.X.X" is a garbage.', where X.X.X.X is the gateway, or a different IP address. These errors can also occur if you have another route to another network using the default gateway. Both of these errors are safe to ignore.
Network/Netmask Directives Format
You can also use the network/netmask directives format for route-interface files. The following is a template for the network/netmask format, with instructions following afterwards:
ADDRESS0=X.X.X.X
NETMASK0=X.X.X.X
GATEWAY0=X.X.X.X
The following is a sample route-eth0 file using the network/netmask directives format. The default gateway is 192.168.0.1, interface eth0. The two static routes are for the 10.10.10.0/24 and 172.16.1.0/24 networks. However, as mentioned before, this example is not necessary as the 10.10.10.0/24 and 172.16.1.0/24 networks would use the default gateway anyway:
ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=192.168.0.1
ADDRESS1=172.16.1.0
NETMASK1=255.255.255.0
GATEWAY1=192.168.0.1
Subsequent static routes must be numbered sequentially, and must not skip any values. For example, ADDRESS0, ADDRESS1, ADDRESS2, and so on.

Below is an example of setting static routes to a different subnet, on a machine in the 192.168.0.0/24 subnet. The example machine has an eth0 interface in the 192.168.0.0/24 subnet, and an eth1 interface (10.10.10.1) in the 10.10.10.0/24 subnet:

ADDRESS0=10.10.10.0
NETMASK0=255.255.255.0
GATEWAY0=10.10.10.1
DHCP should assign these settings automatically, therefore it should not be necessary to configure static routes on Red Hat Enterprise Linux servers or clients.

[Oct 20, 2008] Add Persistant Route - openSUSE Forums

Q:

Hello,

I want SuSE to automatically remember some routes I always have to feed it when it restarts. How do I do that?

example:

route add -net 134.135.2.0 gw 136.131.1.22 netmask 255.255.255.0 dev eth0
In MS DOS I can just "route add -p" for persistant. man route didn't help much (I loathe man pages - the way they are written is overly geeky and 87% of the time I haven't got a clue what they're on about. And they are so DULL to read).

Any help greatly appreciated!
Thanks.

A:

Yast -> Network Devices -> Network card -> Edit -> Routing

You can also edit /etc/sysconfig/network/routes if you prefer the CLI.

HOWTO add a static route

Gentoo Linux Wiki

What is a route

A route is a rule used by your kernel to determine how to get someplace on a network. This HOWTO covers IP routes (routes on an IP network) but there are other types of routable networks. Routes are stored in the Linux kernel are accessible for viewing and editing to users.

Viewing your routing table

The easiest way to view your routes is to use the command:

/sbin/route -n

The table looks something like this:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1
127.0.0.0       127.0.0.1       255.0.0.0       UG    0      0        0 lo
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth1

The Destination is the network address for the routing entry, combined with the genmask (netmask) you can see what network is being routed to the Gateway listed. Please read the man page for route for more information on the meanings of these fields.

Adding a route

To add a route you must first know the network address of the network you wish to add, and the gateway to that network. In our case we are going to use the network 10.0.0.0 with a netmask of 255.255.255.0. We have a firewall on our network with an IP address of 192.168.1.50 that is the gateway to this 10.0.0.0 network.

To add the route manually we use the command:

route add -net 10.0.0.0 netmask 255.255.255.0 gw 192.168.1.50

Our route table now looks like this:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
10.0.0.0        192.168.1.50    255.255.255.0   UG    0      0        0 eth1
192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1
127.0.0.0       127.0.0.1       255.0.0.0       UG    0      0        0 lo
0.0.0.0         192.168.1.1     0.0.0.0         UG    0      0        0 eth1

As you can see the network 10.0.0.0 with a Genmask of 255.255.255.0 shows up with a gateway of 192.168.1.50. But how do we make sure this happens on boot?

Re Static routes for eth0 at startup

Please note that this recommendation is obsolite for RHEL 5 and later.

On Mon, 2004-07-12 at 14:24, Ben Adams wrote:
> I have a server that uses the system default gateway for the vast
> majority of its traffic, but needs a different gateway for a handful of
> specific hosts. I want to establish specific routes for these hosts,
> which I would normally do from the command line like so:
>
> route add -host A.B.C.D gw X.X.X.X
> route add -host E.F.G.H gw X.X.X.X
> . . . etc . . .
>
> I could just tack these commands onto the end of rc.local (or some other
> such hack), but I know there's got to be something more elegant.
>
> What's the proper place to list these routes so that they will be
> applied to eth0 at boot?

/etc/sysconfig/network-scripts/routes-eth0

ADDRESS0=192.168.0.0
NETMASK0=255.255.255.0
GATEWAY0=10.10.10.1
ADDRESS1=...
NETMASK1=...
GATEWAY1=...

It's not well documented, but you will see this if you read
/etc/sysconfig/network-scripts/ifup-routes

--
Chris Kloiber, RHCT
Global Support Services
Red Hat, Inc.

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