ROUTESection: Linux Programmer's Manual (8)
Index | Return to Main Contents
NAMEroute - show / manipulate the IP routing table
- [-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]
- [-v] [-A family] del [-net|-host] target [gw Gw] [netmask Nm] [metric N] [[dev] If]
- [-V] [--version] [-h] [--help]
DESCRIPTIONRoute 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
- operate on the kernel's FIB (Forwarding Information Base) routing table. 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.
- the target is a network.
- 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 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 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.
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 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 126.96.36.199 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 188.8.131.52 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 184.108.40.206 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."
OUTPUTThe 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 default route.
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)
- 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.
SEE ALSOifconfig(8), netstat(8), arp(8), rarp(8)
HISTORYRoute for Linux was originally written by Fred N. van Kempen, <firstname.lastname@example.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.
AUTHORCurrently maintained by Phil Blundell <Philip.Blundell@pobox.com> and Bernd Eckenfels <email@example.com>.
Return to Main Contents