ifconfig appears to also add entry to routing table

DISCUSSION Jim Earl 5 participants 7 messages Sep 1, 2002 – Oct 5, 2002

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Jim Earl

author

2002-09-01 19:20 UTC #0 lore raw

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

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pa3gcu

2002-09-01 19:41 UTC #1 lore raw

Quoted 6 lines · Jim Earl

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.

A complete routing table would have been of more help here;

Quoted 8 lines

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

There is nothing wrong there as what would a route to "yourself" be of any use to you.

Quoted 4 lines

gives the response:
SIOCADDRT: No such device

Yup thats normal with that command.

Quoted 8 lines

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

As above, you cant and dont need a route to you "own interface".

Quoted 7 lines

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.

No not sloppy at all, note your netmask 255.255.255.0 So ifconfig adds a route accordingly.
If you dislike the above use the netmask option with "ifconfig"

Quoted 6 lines

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?)

Not at all, a route is assigned according to the netmask used with ifconfig.

I think it would be better if you stated your router problem and supply details of the routing table and output of ifconfig.
route -ne
ifconfig -a
+ of course a description of just what you want to do.

The problem which most folks have is that they need to define a default route via an interface.
Simply upping an interface on a machine does not automaticly mean you have a working routing table.

Quoted 5 lines

Of course, Thanks in advance,
Jim

Signature · 9 lines

Regards Richard
[email protected]
http://people.zeelandnet.nl/pa3gcu/

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Ray Olszewski

2002-09-01 20:04 UTC #2 lore raw

In reply to Jim Earl

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.

Quoted 42 lines · Jim Earl

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?)

Quoted 3 lines

Of course, Thanks in advance,
Jim

--
-------------------------------------------"Never tell me the odds!"--------
Ray Olszewski -- Han Solo
Palo Alto, California, USA [email protected]
-------------------------------------------------------------------------------

Abhijit Vijay

2002-10-04 16:26 UTC #3 lore raw

Make Linux bootdisk

In reply to Jim Earl

Hi All,

Could anyone please tell me how to make a linux bootdisk? Is it true that a linux bootdisk made for one computer cannot be used to boot another linux machine?

Regards,
Thanks in Advance,
Abhijit.

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Ray Olszewski

2002-10-04 18:45 UTC #4 lore raw

Re: Make Linux bootdisk

Quoted 6 lines · Abhijit Vijay

Hi All,
Could anyone please tell me how to make a linux bootdisk? Is it true that a linux bootdisk made for one computer cannot be used to boot another linux machine?

At the level of generality you ask, there is no simple answer, other than suggesting you read the Bootdisk HowTo (available at ibiblio.org, for example).

There are many types of bootdisks. Some (rescue disks like tomsrtbt, for example, or floppy-based distros like LEAF) are quite general and can boot on most any machine. Others, designed to boot a specific configuration on a particular machine, will work on a different machine only with luck (the two machines need not be identifal, but they need to be quite similar).

So ... read the HowTo for general advice. Repost here with more specific questions.

Chuck Gelm

2002-10-04 20:06 UTC #5 lore raw

Re: Make Linux bootdisk

In reply to Abhijit Vijay

Howdy, Abhijit Vijay:

Can you please tell use which distribution of linux that you have?

Chuck

Quoted 12 lines · Abhijit Vijay

Hi All,
Could anyone please tell me how to make a linux bootdisk? Is it true that a linux bootdisk made for one computer cannot be used to boot another linux machine?
Regards,
Thanks in Advance,
Abhijit.

Quoted 9 lines

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pa3gcu

2002-10-05 08:08 UTC #6 lore raw

Re: Make Linux bootdisk

In reply to Abhijit Vijay

Quoted 6 lines · Abhijit Vijay

Hi All,
Could anyone please tell me how to make a linux bootdisk? Is it true that a linux bootdisk made for one computer cannot be used to boot another linux machine?

Ray explained in great detail, however to expand on what he said about booting other computers, the created bootdisk will only boot another machine to start with if it has the same root partition, for example /dev/hda3 would be the root partition, if another machine has /dev/hda2 as it root partition then it would boot the other machine as is, you would need to use 'rdev' to change the boot params of the disk.

'man rdev' and 'rdev -h' will help you more.

Then as Ray said the machines would need to be somewhat the same.

Quoted 5 lines

Regards,
Thanks in Advance,
Abhijit.

Signature · 9 lines

Regards Richard
[email protected]
http://people.zeelandnet.nl/pa3gcu/

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