sent to the 6 internal firewall’s external interface.
Route Flags The Flags column indicates how the routes in a system were generated or used. You can find a full listing of all route flags in route(4) and related manual pages, but some of the common ones are listed in Table 8-2. You don’t need to understand what each of these flags means at this point. Just be familiar with the flags for each route that normally appears on your system, and if something different appears, start digging for more information. Table 8-2: Common route flags Flag Description U The route is usable G This route is a gateway S This route is static (e.g., not added dynamically by a routing protocol) L This route is a protocol-to-link-address translation (i.e., the MAC address used to reach an IP address) H This route is for a particular host C This route is used when you dynamically create new routes (e.g., a gateway) c This route is used for protocol-specific new routes (e.g., how to reach the gateway) W This route was cloned from another route Adding Routes Adding routes is very simple with the “route add” command. All you need to know is the network block you want to route, the netmask for that block, and the IP address you want them routed to. # route add 172.16.1.0 -netmask 255.255.255.0 192.168.1.254 add net 172.16.1.0: gateway 192.168.1.254 # If you go back and look at your routing table, you’ll see that route. Packets will start to flow back to your internal network. Congratulations! To have this happen automatically at boot, just add the route command to /etc/rc.local. Deleting Routes Take a good close look at the route we added in the last example. Our internal network is 172.16.0.0/24, not 172.16.1.0/24. Oops! To delete a route table entry, you just need the network block and the netmask for that block. # route delete 172.16.1.0 -netmask 255.255.255.0 delete net 172.16.1.0 # Route(8) has many more useful functions; check the man page for full details. Now that you understand the bare bones of the theory of networking, in the next chapter we’ll see how this works out in practice. Page 176
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