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Changes in dhcpd.conf do not make a difference in DHCP service behaviour


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# 1  
Changes in dhcpd.conf do not make a difference in DHCP service behaviour

Hi Experts,

Our DHCP server currently answers the DHCP Discover requests from ServerX. In our dhcpd.conf file there are parameters defined for ServerX.
Now we introduced some additional Servers into the network and want them to get service from the same DHCP server.
Similar configuration parameters as currently present for ServerX were added for the other Servers into the dhcpd.conf file.
The dhcpd process was restarted.
However, in the /var/log/messages, we could see the incoming DHCP Discover messages for the newly added servers but no OFFER for them.

Code:
Mar  7 11:18:05 trber dhcpd: DHCPDISCOVER from mac:mac:mac:mac via IP:IP:IP:IP:IP: unknown network segment

The system keeps sending OFFER for the ServerX only.

We rebooted the DHCP server. It is up and the result is still same.
For confirming that the dhcpd service could use the conf file properly, we removed the entries for ServerX in the dhcpd.conf file and restarted the dhcpd process.
Still DHCP server continue to OFFER for ServerX while there are no parameters for it in the conf file.

It makes me feel that dhcpd process is not interested in any change at dhcpd.conf file, even not use it.

How could it happen?

Our dhcpd.conf file stays in /etc directory : /etc/dhcpd.conf
In some server I could see the directory used is /etc/dhcp/ : /etc/dhcp/dhcpd.conf

Do you have any idea who dhcpd service does not use it's conf file?




Moderator's Comments:
Mod Comment Please use CODE tags as required by forum rules!

Last edited by RudiC; 03-07-2019 at 05:05 AM.. Reason: Added CODE tags.
# 2  
Hi ekorgur and welcome to the forum.

Quote:
Originally Posted by ekorgur
Our dhcpd.conf file stays in /etc directory : /etc/dhcpd.conf
In some server I could see the directory used is /etc/dhcp/ : /etc/dhcp/dhcpd.conf

Do you have any idea who dhcpd service does not use it's conf file?
Perhaps that depends on the type of DHCP-server software you use. It seems the best, IMHO, to look at its man page or other accompanying documentation to resolve that. Actually i have - in various environments - seen both /etc/dhcpd.conf and /etc/dhcpd/dhcpd.conf. If the documentation is unclear about this (i doubt that, but who knows?) you can create /etc/dhcpd/dhcpd.conf as a symlink to /etc/dhcpd.conf and have it both ways.

2 additional thoughts that might or might not be relevant for your problem:

For DHCP to work properly routers in between the server and the client have to be able to work as a "bootp-relay-agent" because the DHCPrequest packets by which the client asks for an IP-address are broadcasts without a source-IP (obviously). See RFC 1533 or its successor RFC2132 "DHCP Options and bootp Vendor Extensions" or RFC1534 "Interoperation Between DHCP and BOOTP". If the two servers are in different subnets this might affect the operation, if they are in the same subnet then this point is moot, of course.

DHCP (as well as bootp, of which DHCP is a superset) allows to base client configuration on MAC-addresses so that a certain client always gets the same config. It is also possible to configure the server so that it ignores all requests not coming from a defined set of such addresses. This might also be a reason why your new system are being ignored.

I hope this helps.

bakunin
This User Gave Thanks to bakunin For This Post:
# 3  
They are in same subnet. No issues at there.

I will copy the dhcpd.conf to /etc/dhcp/ directory too and try restarting the dhcpd service. I will check it but not much hope as the working ServerX was fine with the file /etc/dhcpd.conf.
We did the modification on this file /etc/dhcpd.conf but no changes on the behaviour as I mentioned in my first entry.

I didnt get the explanation of why could the server ignore the requests not coming from a defined set of addresses.
For the MAC addresses DISCOVERed in the incoming relayed packet, we have entries in dhcpd.conf and an OFFER should occur.
However, as I tried to explain, anything we do in dhcpd.conf doesnt make any change in DHCP behaviour.

Besides being unable to get OFFER for the new system, another important output is that we removed the entry for the working DHCP client in /etc/dhcpd.conf file but the DHCP server still providing OFFER to that server. I would expect that it starts to give error for the incoming DHCP requests since then.


Thanks a lot for your time by the way @bakunin
# 4  
Quote:
Originally Posted by ekorgur
I will copy the dhcpd.conf to /etc/dhcp/ directory too and try restarting the dhcpd service.
At this point, to get more qualified help, you might consider telling us a bit more about your environment:

OS + version (of the DHCP server)?
DHCP server software used + version?

Again, where the configuration files can be found depends on the implementation, so don't expect more than generic suggestions as long as you do not provide more specific information.

In general, a daemon will re-read its configuration only if made to do so. The common way of doing so is to send signal 1 (i.e. kill -1 <PID-of-dhcpd>) but maybe your implementation does it differently - again: tell us what you use and we may offer better/less generic help.

Quote:
Originally Posted by ekorgur
I will check it but not much hope as the working ServerX was fine with the file /etc/dhcpd.conf.
We did the modification on this file /etc/dhcpd.conf but no changes on the behaviour as I mentioned in my first entry.
If you hoped for something like press the foo-key three times, turn right, spit over your left shoulder and all is good i have to disappoint you. ;-)) I am, at this point, at the same loss you are and by suggesting whatever comes to my mind as remotely possible i hope to help you to get the right idea eventually.

Quote:
Originally Posted by ekorgur
I didnt get the explanation of why could the server ignore the requests not coming from a defined set of addresses.
For the MAC addresses DISCOVERed in the incoming relayed packet, we have entries in dhcpd.conf and an OFFER should occur.
It is possible to enter rules into the configuration so that "MAC-address X" is always given "IP-address Y (plus other specific information in the DHCP option fields: default route, DNS server, ...)". It is also possible to configure the DHCP server to only answer requests from clients explicitly defined in that way - which implies ignoring all the others. So, provided you have such a setup (and maybe you are not aware of that) and if you did not enter explicit configuration for the new servers it might lead to what you saw. If you have configs for the new systems, this point is also moot.

Another possibility crossed my mind: if you have a firewall between you and the client it might block some of the traffic. This is rather unlikely after what you said as you come across pretty sure the server itself doesn't answer but again: i try to come up with ideas, you have to test them.

Quote:
Originally Posted by ekorgur
Thanks a lot for your time by the way
You are welcome. If - once you found the solution - tell us what it was and how you resolved to problem we all have learned something and we have to in fact thank you for making us wiser.

I hope this helps.

bakunin

Last edited by bakunin; 03-08-2019 at 02:36 AM.. Reason: typo
# 5  
I couldnt get the OS version yet. DHCP server is administrated by somebody else. I will update it soon.
DHCP Server 4.1.1-P1

I mentioned before that I was going to copy the /etc/dhcpd.conf to /etc/dhcp/dhcpd.conf
I did that and restarted the service.
Now I started to receive a new kind of error and the service doesnt start. How it was working before with /etc/dhcpd.conf is still something I have no idea.

Code:
Mar  7 16:19:30 trber dhcpd: Internet Systems Consortium DHCP Server 4.1.1-P1
Mar  7 16:19:30 trber dhcpd: Copyright 2004-2010 Internet Systems Consortium.
Mar  7 16:19:30 trber dhcpd: All rights reserved.
Mar  7 16:19:30 trber dhcpd: For info, please visit .......
Mar  7 16:19:30 trber dhcpd: WARNING: Host declarations are global.  They are not limited to the scope you declared them in.
Mar  7 16:19:30 trber dhcpd: Not searching LDAP since ldap-server, ldap-port and ldap-base-dn were not specified in the config file
Mar  7 16:19:30 trber dhcpd: Wrote 0 deleted host decls to leases file.
Mar  7 16:19:30 trber dhcpd: Wrote 0 new dynamic host decls to leases file.
Mar  7 16:19:30 trber dhcpd: Wrote 0 leases to leases file.
Mar  7 16:19:30 trber dhcpd:
Mar  7 16:19:30 trber dhcpd: No subnet declaration for eth0 (10.210.148.7).
Mar  7 16:19:30 trber dhcpd: ** Ignoring requests on eth0.  If this is not what
Mar  7 16:19:30 trber dhcpd:    you want, please write a subnet declaration
Mar  7 16:19:30 trber dhcpd:    in your dhcpd.conf file for the network segment
Mar  7 16:19:30 trber dhcpd:    to which interface eth0 is attached. **
Mar  7 16:19:30 trber dhcpd:
Mar  7 16:19:30 trber dhcpd:
Mar  7 16:19:30 trber dhcpd: Not configured to listen on any interfaces!
Mar  7 16:19:30 trber dhcpd:
Mar  7 16:19:30 trber dhcpd: This version of ISC DHCP is based on the release available
Mar  7 16:19:30 trber dhcpd: on ftp.isc.org.  Features have been added and other changes
Mar  7 16:19:30 trber dhcpd: have been made to the base software release in order to make
Mar  7 16:19:30 trber dhcpd: it work better with this distribution.
Mar  7 16:19:30 trber dhcpd:
Mar  7 16:19:30 trber dhcpd: Please report for this software via the Red Hat Bugzilla site:
Mar  7 16:19:30 trber dhcpd:     ......
Mar  7 16:19:30 trber dhcpd:
Mar  7 16:19:30 trber dhcpd: exiting.

Subnet declaration was defined as below.

Code:
ddns-update-style none;

subnet 172.17.126.192 netmask 255.255.255.224 {
   option broadcast-address 172.17.126.223;
   next-server 172.17.126.195;
   option routers 172.17.126.195;

   host fsb1 {
      option host-name "fsb1";
      option root-path "/gsn/nodes/fsb1";
      hardware ethernet XX:XX:XX:XX:45:06;
      fixed-address 172.17.126.200;
      filename "/BootFiles/fsb_gep3/linux/kernel/pxelinux.0";
   }

Because of the error seen in messages logs, or something else, the dhcpd service doesnt work anymore.
# 6  
Without being able to substantially contribute as my dhcpd days are long gone, I stumble over the obvious discrepancies between eth0 IP (10.210.148.7) and DHCP subnet (172.17.126.192) definition. There must be a router between the two, and bakunin's comment in post #2 becomes noteworthy. Worthwhile digging deeper?

Last edited by RudiC; 03-08-2019 at 07:03 AM..
# 7  
Code:
Mar  7 09:05:04 trber dhcpd: DHCPDISCOVER from XX:XX:XX:XX:45:06 via 172.17.126.195: unknown network segment
Mar  7 09:05:04 trber dhcpd: DHCPDISCOVER from XX:XX:XX:XX:45:06 via 172.17.126.194: unknown network segment

Before I was receiving those DISCOVER messages from the interface.
The DHCP broadcast from the client is relayed to 10.210.148.7 .
Requests were received however the dhcp server was not offering an IP while I added the required config into /etc/dhcpd.conf

As this modification did not help, I copied the modified /etc/dhcpd.conf to /etc/dhcp/dhcpd.conf. But now, the process does not go up and give the "no subnet decleration" failure.
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