Problem with DHCP, /etc/hosts and GNOME

Tom 'Needs A Hat' Mitchell mitch48 at sbcglobal.net
Fri Apr 9 05:09:36 UTC 2004


On Thu, Apr 08, 2004 at 09:24:58PM +0100, Julien Olivier wrote:
> > 
> 
> It's there but it doesn't work, at least for me. Whichever hostname I
> put in it, I always get the modem provided hostname. Can anyone confirm
> that it works for him ?

Goodness what a long thread.

What most are missing is that the DNS name thing is a way to associate
names to IP addresses not to the machine.

Another way to think about it is that in IP lan-d the network ports
have names not the machines itself.

A router can have six or sixty ports each with names that can be
looked up via DNS.

With DHCP you ask for a valid address on the network you plugged into.
The IP-address likely has a name just so they can keep track of it.
Long and silly looking names....

Windows mostly ignores DNS and switches to a WINS naming trick.  Which
works within broadcast domains and with a WINS server.  The WINS
addresses are often separate and distinct from the IPaddress name
space.  WINS has a smallish local view of the world.  If you fully use
DNS in windows land all these issues exist there too.

DHCP + DynamicDNS is slightly different.  You use DHCP to get an address
and pass to the DHCP/DNS server that you want your current leased address
to be mapped to such and such a name.

For some DHCP+router+RFC-1918 (private address spaces) tangle things
further.  The router gets an IP address via DHCP.  The name of that
port is likely a name given to this number by the ISP.  It might look
like: (adsl-68-22-33-177.dsl.bcvloh.ameritech.net [68.22.33.177]) Now
the Linksys (to pick a brand) connects to and authenticates to
ameritech.net and requests an IPaddress.  It gets 68.22.33.177 back
that is in a massive table and has a name
"adsl-68-22-33-177.dsl.bcvloh.ameritech.net" so forward and reverse
lookups happen.

Next the Linksys gets a DHCP requests for one of it's RFC-1918 network
numbers.  It gets 192.168.1.100... Now things are tangled because
DNS service will not have an name for this number and the  192.168.1.xxx
network is not routed.  Gateway has a public name and address.  Other things
are hidden behind it.

Of interest with /etc/hosts or a local DNS server you can name your
RFC-1918 network numbers.  You can even setup a dynamic DNS service
for them that goes no place because of the nature of RFC-1918 network
numbers.

There can be interesting effects with multiple IPaddresses.  For example
by selecting the DNS name of a multi-adapter NFS server you can control
to a limited extent which path the bits take into the server.  Routers
have a thing called host routes that can appear to make all the ports
have the same name.

When connecting to a public net think about things in stages.  First
localhost.localdomain and establish a host name as an alias for
localhost.localdomain.  With that done sendmail can hook up an
identity, X can latch to an identity, etc.  Next ADD an interface and
IP address DHCP, dialup ppp.... what ever.   

Then there is sendmail... and MX records and.... mail agents...

There are some experts out there... have at it some more.

-- 
   T o m M i t c h e l l 
   /dev/null the ultimate in secure storage.





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