route8
ROUTE(8) Linux Programmer's Manual ROUTE(8)
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.
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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 gate
way 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 connec
tions 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 con
nections 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.
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dev If force the route to be associated with the specified
device, as the kernel will otherwise try to deter
mine the device on its own (by checking already
existing routes and device specifications, and
where the route is added to). In most normal net
works 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 desti
nation address) and associated with the "lo" device
(assuming this device was prviously set up cor
rectly 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 neces
sary 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 gate
wayed through the former route to the SLIP inter
face.
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 (multi
cast) IP routes to go via "eth0". This is the cor
rect normal configuration line with a multicasting
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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.
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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.
FILES
/proc/net/ipv6_route
/proc/net/route
/proc/net/rt_cache
SEE ALSO
ifconfig(8), netstat(8), arp(8), rarp(8)
HISTORY
Route for Linux was originally written by Fred N. van
Kempen, <waltje@uwalt.nl.mugnet.org> and then modified by
Johannes Stille and Linus Torvalds for pl15. Alan Cox
added the mss and window options for Linux 1.1.22. irtt
support and merged with netstat from Bernd Eckenfels.
AUTHOR
Currently maintained by Phil Blundell <Philip.Blun
dell@pobox.com>.
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