Two elementary questions on LANs

[Ed Greshko]

On 06/18/2011 09:00 PM, Timothy Murphy wrote:
> Ed Greshko wrote:
>
>>> Just to be clear: dhcp wasn't running at the point I am talking about.
>>> NM did delete the nameserver information in /etc/resolv.conf .
>> What precisely do you mean when you say "dhcp wasn't running at the
>> point I am talking about"?
>>
>> Do you mean your DHCP server was off?  Or, do you mean that you had
>> BOOTPROTO=none set in your ifcfg- file?
> I tried both.
> DHCP was not running because the DHCPD server on my router
> did not appear to be functioning, although dhcp was enabled.

Well...  I can tell you this....

If your ifcfg- contains BOOTPROTO=dhcp and no dhcp server is available
the interface won't come up.

>> When you bring it down....NM will rewrite it to read....
>>
>> # No nameservers found; try putting DNS servers into your
>> # ifcfg files in /etc/sysconfig/network-scripts like so:
>> #
>> # DNS1=xxx.xxx.xxx.xxx
>> # DNS2=xxx.xxx.xxx.xxx
>> # DOMAIN=lab.foo.com bar.foo.com
> NM certainly didn't write that in my ifcfg-wlan0
> or I would have followed its advice.
> IIRC, NM simply deleted the nameservers in /etc/resolv.conf 
> that I had given.

That is because that isn't what I wrote....

I wrote that is what is written in /etc/resolv.conf  !

I have never seen a case where /etc/resolv.conf was totally "blanked".  
It either contains the information supplied by DHCP, the information
supplied in the ifcfg- file, or the stanza I noted above.

> I'm pretty sure I did add DNS1,DNS2 to ifcfg-wlan0 at one point
> (I know I added them, but it could have been on another machine),
> and it didn't seem to stop NM deleting the nameservers in /etc/resolv.conf .
>
> My solution which seems to work is to go into NM's Manage Connections,
> and give the IP address and nameservers there.
>
> But I'd have to say that I can't see any point in NM deleting
> the nameservers in /etc/resolv.conf .
> I can't think of any circumstances in which one would be better off
> with no nameservers listed.
> It seems to me that the rational action for NM to take
> would be to leave /etc/resolv.con as it is
> if it cannot find any nameservers where it looks for them.
>
>

Well, if your interface isn't up then it doesn't matter what is in that
file in any event.