Re: ifconfig appears to also add entry to routing table

[Ray Olszewski <ray@comarre.com>]

Jim --

First, the basics: What Linux distro, what kernel version? Does it use ifup 
and ifdown, or do its init scripts call ifconfig and route directly?

Second, when you say " ifconfig verifys that this if is good to go" ... are 
you sufficiently expert to be sure of this? If not, please repost with the 
actual output if "ifconfig -a" as it appears just before you run the 
unsuccessful "route" command.

Third, in my experience, I've found "route" to be a bit picky in ways that 
the man page doesn't warn about. In your case, I'd suggest you try this 
form of the command

         route add -host 192.168.1.2 dev eth0

and see if it does any better for you.

(Of course, I can't comment on the "various stabs" that you do not 
describe... it may be that you already tried this, without success.)

Finally, you write:

         Destination  Gateway   Genmask     Flags    Metric   REf  Use   Iface
         192.168.1.0     *    255.255.255.0   U        0       0     0   eth0

         Though this seems sloppy- note that Flags column lacks an "N" for 
Network.

If you look at the list of flags in the man page for "route", you will 
notice that N is not a choice. There is an H for host and a G for gateway, 
but routes are assumed to be networks by default, so not specially labeled. 
So no, it is not "sloppy".

As to the "undocumented" addition of this route ... I'm so used to this 
stuff happening in the background that I don't recall offhand what command 
does what (ifup and ifdown, the commands used on most modern, full-size 
Linux distros, does handle both, and their man pages do say so). So you 
might be more specific about the details of what you did (for example, was 
the routing table empty *before* you brought the interface up?).

As to the source of your router problem, I wouldn't hazard a guess. Your 
guess seems implausible, though ... routers do not usually need to be told 
that they are their own gateways; indeed, I find it hard to imagine a 
circumstance under which this would even work ... but you offer so little 
description of your setup, any comments here are wild guesses. IF you want 
help at that level, please tell us

         what you want this system to route between (2 Ethernets?
                 an Ethernet LAN and a dialup connection
                 to the Internet? something else?) what interfaces
                 does it have (probably eth* and/or ppp*)?
         whether the hosts on the LAN(s) have "real" (routable) IP
                 addresses or you want the router to use IP Masquerading
                 to NAT them for connections to the Internet
         what "ifconfig -a" shows when the router does not route
         what "netstat -nr" shows when the router does not route
         what "cat /proc/sys/net/ipv4/ip_forward" shows when the
                 router does not route
         if this is a connection to the Internet, how the interface to the
                 ISP is supposed to get its IP address (static? DHCP?
                 PPPoE?) and what the ISP says you are supposed to use
                 as a gateway address.
         if this is a connection to the Internet, can you ping the gateway
                 address above? If not, HOW does ping fail (yes, there are
                 many different errors it can report, and they are diagnostic).
         what the routing failures are? For EXAMPLE, can the router itself
                 make Internet connections, but not hosts on the LAN it is
                 supposed to route?

There may be more we need to know, but that will make a good start.

At 01:20 PM 9/1/02 -0600, Jim Earl wrote:
>Hello all,
>
>I have been configuring a linux box to function as a router, ran into some
>problems, and think that I traced the problem to a faulty routing table.  So
>I brought down all my interfaces and routing table ( except lo ) and
>attempted to add them manually.
>
>I was able to add an interface manually, assigning my eth0 device the IP
>addr 192.168.1.2.  ifconfig verifys that this if is good to go.
>
>However, I found that I was unable to give a simple "route" command, for
>example:
>
>route add 192.168.1.2
>
>gives the response:
>
>SIOCADDRT: No such device
>
>the man page for route implies that you can specify the device in the route
>command also:
>
>route add 192.168.1.2 dev eth0
>
>Still no good.  Various stabs at syntax based on the route man page also
>yield nothing
>
>
>I have also found that after issuing the ifconfig command, the routing table
>is automatically updated with an entry for a route to that network:
>
>Destination  Gateway   Genmask     Flags    Metric   REf  Use   Iface
>192.168.1.0     *    255.255.255.0   U        0       0     0   eth0
>
>Though this seems sloppy- note that Flags column lacks an "N" for Network.
>
>This is undocumented behavior, as everywhere I have looked it has been
>implied that one has to bring up an interface and also independently
>establish the route to it.  I would like to be able to add a route to the
>host itself, indicating that it is its own gateway ( I believe this is the
>root of my router problem- no pun intended .  Can someone verify this?)
>
>Of course, Thanks in advance,
>
>Jim




--
-------------------------------------------"Never tell me the odds!"--------
Ray Olszewski					-- Han Solo
Palo Alto, California, USA			  ray@comarre.com
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