Bug 85625

Created attachment 90472 [details]
patch to fix dynamic DNS documentation in dhcpd.conf.5

dhcpd.conf misled the user into using the wrong TSIG algorithm for dynamic
updates in conjunction with dhcpd.

I have fixed the Man Page in Rawhide.  Tammy could you take a look at the
documentation problem.

Thanks.

Good point. I'll work on adding it for the next version of the manual.

In regards to your RHL docs bugs:

>In Chapter 17 of the Redhat documentation for /etc/sysconfig doesn't have much
>to say about the /etc/sysconfig/network-scripts/ifcfg-* configuration for
>DHCP_HOSTNAME --   This would be a prime place to put that information! 
>(http://www.redhat.com/docs/manuals/linux/RHL-8.0-Manual/custom-guide/s1-dhcp-configuring-client.html)

I will add an explanation of this option here and also make sure the option is
added to the Reference Guide. However, our DHCP server does not honor this
option. It is only there for DHCP servers that require a hostname from the client.

>If one wants DNS to be dynamically updated through dhcpd, tinkering with
>dhclient.conf all day will not do the trick -- making a change to the sysconfig
>scripts will. 
>
>Add this to /etc/sysconfig/network-scripts/ifcfg-eth0 : 
>DHCP_HOSTNAME="${HOSTNAME%%.*}"

On a RHL DHCP server, DNS information is passed to the client if it exists for
the client in the DHCP configuration file. If a RHL DHCP client receives DNS
information from the server, it will use it. I don't understand how you think
the DHCP_HOSTNAME option will allow the client to grab the DNS information since
it does without the option. Maybe your server requires a hostname to be passed
to it to get the DNS information?

Hi Tammy. 
Thanks for all the great work you have done with documenting Linux over the years. 

> On a RHL DHCP server, DNS information is passed to the client if it exists for
> the client in the DHCP configuration file.

Yes,  provided there are DNS entries for the host in question. :) 

> I don't understand how you think the DHCP_HOSTNAME option will allow the 
> client  to grab the DNS information since it does without the option. 
> Maybe your server requires a hostname to be passed to it to get the DNS 
> information?

(The server in question is RHL 8.0)

The DHCP_HOSTNAME option is for when the client wishes to assert a hostname.
(not when it is willing to have its hostname handed-out from the DHCP server).

If my RHL workstation (that acts as a dhcp client) has a name associated with it
(Just like a Windows 2000 workstation, for instance), I can assert to the DHCP
server what is contained in the DHCP_HOSTNAME variable. 

I did a lot of packet sniffs and analysis with ethereal and tcpdump.  I noticed
that the Windows 2000 box would always assert to the DHCP server "Hello! My name
is pbarber-lt-30; please update your DNS accordingly" and (when I got the
dynamic updates working correctly with the dhcp server), the assertions were
dynamically updated into the DNS. 

However, with the RedHat linux workstations, the packet sniffs showed a very
different conversation: the hostname's assertion was never a part of the
request.  The RHL dhcp client scuffed its feet in the dirt, and allowed the DHCP
server to tell it what its name was.  That's not very assertive, is it?

I looked into the dhclient.conf man pages and noticed that there are places to
notify the dhcp server of the client's hostname.  Making changes to
dhclient.conf exactly as the "Dynamic updates" section of the dhclient.conf man
page didn't quite do the trick --  The assertions for hostname were never sent
to the dhcp server in the same way the Windows 2000 client did. 

HOWEVER, 
The init script that starts and stops the network appends to the dhclient.conf
file whenever the init script is run.   

The section of code that does this is in the ifup script (/sbin/ifup)

Here it is: 

if ! grep "send *host-name *\"${DHCP_HOSTNAME}\"" /etc/dhclient-${DEVICE}.conf >
/dev/null 2>&1 ; then
    echo "send host-name \"${DHCP_HOSTNAME}\";  # temporary RHL ifup addition"
>> /etc/dhclient-${DEVICE}.conf
fi


The dhclient.conf man page says that in order to assert a hostname, one has to
use this syntax: 

man dhclient.conf---
DYNAMIC DNS
       The client now has some very limited support for doing DNS updates when
       a lease is acquired.   This is prototypical, and  probably  doesnât  do
       what  you want.   It also only works if you happen to have control over
       your DNS server, which isnât very likely.  (Now THAT'S a ringing
endorsement!  --pb)

         send fqdn.fqdn "grosse.fugue.com.";
         send fqdn.encoded on;
         send fqdn.serverâupdate off;

That seems to be getting ignored, I didn't see the appropriate thing in the
packet sniffs for hostname assertion. Also, the $DHCP_HOSTNAME addition with the
ifup script seems to override the "send" entries in dhclient.conf.  (the
dhclient.conf man page doesn't say anything about 'send host-name
"hammy.pietbarber.com"', at least until the SAMPLE section of the man page.

I hope I 'splained what I needed to say adequately. :)

Steve, can you look at this and tell me what you think?

This is now fixed in all current dhcp-3.0.1+ releases.