On Fri, Jun 13, 2008 at 7:21 PM, Michael ODonnell <[EMAIL PROTECTED]> wrote: > I believe the route command is used to identify/control > which interface to use to reach a particular host/network > [and which one(s) to handoff to in the default case(s)] for > connections originating on the machine in question. I believe > that's distinct from forwarding, which determines whether/how > inbound traffic destined for other hosts/networks is handled, > and is managed by manipulating /proc/sys/net/ipv4/ip_forward > and friends via the sysctl command.
Clarification: The kernel routing table determines the next-hop of all packets the system transmits[1], regardless of whether they are originating locally or being forwarded. The net.ipv4.ip_forward kernel option simply turns forwarding on or off. When forwarding is off, if the kernel receives a packet for another host, the packet is dropped[2]. When forwarding is on, the kernel will use the routing table to decide where such a packet should go for the next hop. That next-hop could be a node reachable via a network interface on the system (such as a host on a locally connected Ethernet, or the other end of a PPP link). In that case, the routing table will simply tell the kernel to transmit the packet to that node via the appropriate interface. Alternatively, the next-hop could be a gateway -- i.e., not the final destination, but an intermediate node. In that case, the kernel also uses the routing table to decide how to reach the gateway. In simpler terms, a routing table entry -- a route -- simply tells the kernel, "to reach this destination, use this interface" or "to reach this destination, use this gateway". A default route is simply a route which matches any destination. There is absolutely no difference between a default route and any other route. There is nothing special about a default route. A default route is simply the least-specific possible route. Since more specific routes are always preferred, any other route will be preferred over a default route. Did I mention that a default route works like any other route? (Sorry to belabor this, but this is a common source of confusion.) Footnotes --------- [1] Technically speaking, things like iptables (NAT, mangle, etc.), policy routing, and multiple routing tables can also influence routing. Presumably, if you're using them, you know about that. [2] Or maybe an ICMP message gets sent; I forget. Either way, the packet is not forwarded. -- Ben _______________________________________________ gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/
