Theroute Command#

In computer networking, a router is a device responsible for forwarding network traffic. When a data packet arrives at a router, it must determine the best way to route it to its destination. The route command is used to view and modify the kernel routing table. The command syntax varies on different systems, so it's practical to check the command's help to determine the specific usage.

Syntax#

route [-CFvnee]
route [-v] [-A family] add [-net|-host] target [netmask Nm] [gw Gw] 
      [metric N] i [mss M] [window W] [irtt m] [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]

Parameters#

• (none): View all current route definitions.
• -A family: Use the specified address family. You can check the supported address families by using route --help, commonly supported address families include inet (DARPA Internet), inet6 (IPv6), ax25 (AMPR AX.25), netrom (AMPR NET/ROM), ipx (Novell IPX), ddp (Appletalk DDP), x25 (CCITT X.25).
• -F: Operate on the kernel's FIB forwarding information base routing table, which is the default.
• -C: Operate on the kernel's route cache.
• -v: Output verbose operation details.
• -n: Show numerical addresses instead of trying to determine symbolic host names; this will be useful to detect why routing to a name server is failing.
• -e: Display the route table in netstat format; -ee will generate a very long line with all parameters from the route table.
• del: Delete a route.
• add: Add a route.
• target: Target network or host, can be provided as an IP address in dotted decimal form or a host/network name.
• -net: The target being added is a network.
• -host: The target being added is a host.
• netmask NM: When adding a network route, the network mask to be used.
• gw GW: Route packets through a gateway; the gateway must be accessible first, usually meaning a static route to the gateway must be set beforehand. If you specify an address of one of the local interfaces, it will be used to determine which interface to route the packet on.
• 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 routed through this route to M bytes. The default is the device MTU minus the headers, or a lower MTU in case of Path MTU Discovery. When Path MTU Discovery doesn't work (typically due to misconfigured firewalls blocking the required ICMP fragments), this setting can be used to force the use of smaller TCP packets on the other end.
• window W: Set the TCP window size for connections routed through this route to W bytes; only used on AX.25 networks when the driver can't handle back-to-back frames.
• irtt m: Set the Initial Round Trip Time (irtt) for TCP connections routed through this route to m milliseconds (1-12000); only used on AX.25 networks. If omitted, the default value as per RFC 1122 of 300ms is used.
• reject: Install a blocking route, which forces a route lookup failure. This can be used to block a network before using a default route. It's important to note that this is not intended for firewall purposes.
• mod, dyn, reinstate: Install a dynamic or modified route; these flags are used for diagnostic purposes and are typically set only by routing daemons.
• dev If: Force the route to be associated with the specified device; otherwise, the kernel will try to determine the device on its own (by checking existing routes and device specifications, and the position of adding the route). This functionality is rarely needed in most typical networks. If dev If is the last option on the command line, the word dev can be omitted as it is the default. Otherwise, the order of route modifiers metric - netmask - gw - dev is irrelevant.
• -h, --help: Output help information.

Examples#

Display the routing table for all IPs bound to the server.

route -n

Add a default gateway, which assigns a gateway address to forward all packets not belonging to this network.

route add default gw 169.213.0.0

List kernel's route cache information; the kernel retains route cache information for faster packet routing.

route -Cn

Reject routing to a specific host or network.

route add -host 192.168.1.51 reject

Delete the default gateway, which may cause some internet issues, so take note of the default gateway before proceeding with this command.

route del default

When we connect a PPTP, we typically configure adding a route for a network segment to specify data to go through ppp0.

route add -net 192.168.0.0/16 dev ppp0

Configure adding a route for a domain name to specify data to go through ppp0.

route add -host jwgl.sdust.edu.cn dev ppp0

Set all Class D multicast IP routes to go through eth0.

route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0

Daily Question#

References#