the further adventures of setting up Ethernet between two Linux m achines

[Mike Bilow]

First of all, if you lose root access, just boot from a floppy.  You can
do anything you want, including hand-editing /etc/passwd or /etc/shadow to
clear a password entirely.

Second, what you are seeing here is deafness: you are misunderstanding
what you are looking at.  The error "no route to host" is a mesage about
an IP routing issue.  This is not directly connected with ARP.  Rather,
ARP is simply the mechanism which is used to temporarily map IP addresses
of machines on your local wire with their Ethernet hardware addresses.

If you are seeing ARP table entries as "incomplete," this means that
either (1) you have send an ARP query that another machine has replied to
but you have not received the reply, or (2) no other machine has replied
to you because either no such machine exists or your ARP query was not
heard by the correct machine.  The most likely reason for ARP failures is
that your interface is misconfigured, on the wrong IRQ, using the wrong
driver, or some such problem.  For example, there are three different
drivers in Linux which are used to support DEC Tulip chipset Ethernet
cards ("tulip," "old_tulip," and "de4x5"), and it is not necessarily easy
to tell which of these will work with your particular hardware.

The first step is to use "ifconfig <ifname>" to see whether there are
actaully frames going through the interface.  If there are frames going
out but not coming in, then you have an IRQ or driver problem.  If there
are no frames going out or coming in, you probably have a routing problem.

Next, try to ping 127.0.0.1, the localhost address.  If you cannot ping
the localhost, then you have a piece of your network stack missing.

Why would that hardware address be at all useful, if you had it?

-- Mike


On Thu, 6 Apr 2000, Gordon, Seth wrote:

> telnet from petunia into horse-nettle.  This led to the error message
> "telnet: Unable to connect to remote host: No route to host".  I looked at
> the routing tables with `arp -v -n`, and got a list of 13 IP addresses
> (including horse-nettle's), all of them with "(incomplete)" in the HWaddress
> column.  Then the really strange things began happening:
> 
> # arp --delete 192.168.1.2
> SIOCDARP(pub): No such file or directory
> 
> # route del 192.168.1.0
> SOICDELRT: No such process
> 
> What now, O network wizards?
> 
> Of course, fool that I was, I didn't take down the hardware address of
> horse-nettle's Ethernet card *before* I shut it down and unplugged the
> monitor from it....