Section: Linux Programmer's Manual (8)
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route - show / manipulate the IP routing table
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
options are used,
modifies the routing tables. Without these options,
displays the current contents of the routing tables.
- -A family
use the specified address family (eg `inet'; use `route --help' for a full
operate on the kernel's FIB (Forwarding Information Base) routing
This is the default.
operate on the kernel's routing cache.
select verbose operation.
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.
for displaying the routing table.
will generate a very long line with all parameters from the routing table.
delete a route.
add a new route.
the destination network or host. You can provide IP addresses in dotted
decimal or host/network names.
is a network.
is a host.
- netmask NM
when adding a network route, the netmask to be used.
- gw GW
route packets via a gateway.
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
- 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.
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.
is the last option on the command line, the word
may be omitted, as it's the default. Otherwise the order of the route
modifiers (metric - netmask - gw - dev) doesn't matter.
- route add -net 127.0.0.0 netmask 255.0.0.0 dev lo
adds the normal loopback entry, using netmask 255.0.0.0 and associated with the
"lo" device (assuming this device was previously set up correctly with
- route add -net 22.214.171.124 netmask 255.255.255.0 dev eth0
adds a route to the local network 192.56.76.x via
"eth0". The word "dev" can be omitted here.
- route del default
deletes the current default route, which is labeled "default" or 0.0.0.0
in the destination field of the current routing table.
- 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 126.96.36.199 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 188.8.131.52 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."
The output of the kernel routing table is organized in the following columns
The destination network or destination host.
The gateway address or '*' if none set.
The netmask for the destination net; '255.255.255.255' for a host destination
and '0.0.0.0' for the
Possible flags include
(target is a
route for dynamic routing)
installed by daemon or redirect)
from routing daemon or redirect)
The 'distance' to the target (usually counted in hops). It is not used by
recent kernels, but may be needed by routing daemons.
Number of references to this route. (Not used in the Linux kernel.)
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).
Interface to which packets for this route will be sent.
Default maximum segment size for TCP connections over this route.
Default window size for TCP connections over this route.
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.
ifconfig(8), netstat(8), arp(8), rarp(8)
for Linux was originally written by Fred N. van Kempen,
> 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.
Currently maintained by Phil Blundell <Philip.Blundell@pobox.com
> and Bernd Eckenfels <email@example.com
- SEE ALSO