route (8)


       route - show / manipulate the IP routing table


       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]


       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.


       -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) rout-
              ing 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.

              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  dae-
              mons) 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 diag-
              nostic 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.


       route add -net
              adds the normal loopback entry, using netmask (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 netmask 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 ipx4 sl0
              Adds the route to the "ipx4" host via the SLIP interface (assum-
              ing that "ipx4" is the SLIP host).

       route add -net netmask 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 netmask 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 netmask reject
              This   installs  a  rejecting  route  for  the  private  network


       The output of the kernel routing table is organized  in  the  following

              The destination network or destination host.

              The gateway address or '*' if none set.

              The  netmask  for  the  destination net; '' for a
              host destination and '' 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 dae-

       Ref    Number of references to this route. (Not used in the Linux  ker-

       Use    Count  of lookups for the route.  Depending on the use of -F and
       irtt   Initial  RTT  (Round  Trip  Time). The kernel uses this to guess
              about the best TCP protocol parameters without waiting on  (pos-
              sibly 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)


       Route  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  Ecken-


       Currently maintained by Phil Blundell <>.

net-tools                       2 January 2000                        route(8)