DHCPD , dhcp relays on a large network

Alexandru Coseru alexandru.coseru at totaltelecom.ro
Fri Nov 10 14:41:35 UTC 2006


Quote:  "> This doesn't sound right, the server will NOT fail to start 
because a remote subnet is not defined - all that would happen is you would 
get  runtime errors in response to requests for unknown subnets."

It is the other way around.

Srv1 has:

[root at bv ~]# ifconfig
eth0     Link encap:Ethernet  HWaddr 00:0C:46:B3:4F:2F
          inet addr:10.1.1.1  Bcast:10.1.1.255  Mask:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:35 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 b)  TX bytes:5384 (5.2 KiB)
          Interrupt:17 Base address:0x2c00
[root at bv ~]#


cat /etc/dhcpd.conf:

ddns-update-style ad-hoc;
authoritative;
subnet 192.168.1.0 netmask 255.255.255.0{
        option routers                  192.168.1.1;
        option subnet-mask              255.255.255.0;

        option nis-domain               "domain.org";
        option domain-name              "domain.org";
        option domain-name-servers      10.1.1.1;

        default-lease-time 21600;
        max-lease-time 43200;
        host phonehuawei {
                hardware ethernet 88:88:88:88:88:88;
                fixed-address 192.168.1.2;
        }
        host universalphone{
                hardware ethernet 00:13:49:53:C6:3B;
                fixed-address 192.168.1.3;
        }
       host accountingserver{
                hardware ethernet 00:43:29:12:12:FA;
                fixed-address 192.168.1.4;
        }

}


The Layer3 switch has DHCPD relay agent on it... and  2 IP addresses: 
10.1.1.2 on management vlan  (Gbit port) and  192.168.1.1  on clients vlan

The client voip phone  should broadcast a request for an ip address  , the 
L3 SW A is forwarding that request to srv1  (I can see it using tcpdump).
If i'm not declaring the subnet =>runtime errors  , request for unknow 
subnets.


Using the dhcpd config described earlier , i have:
 "No subnet declaration for 192.168.1.0".


Regards
    Alex


----- Original Message ----- 
From: "Simon Hobson" <dhcp1 at thehobsons.co.uk>
To: <dhcp-users at isc.org>
Sent: Friday, November 10, 2006 4:19 PM
Subject: Re: DHCPD , dhcp relays on a large network


> Alexandru Coseru wrote:
>
>>The ideea is that   each switch has a subnet  attached to it..
>>
>>On the backbone   , I have the dhcpd server..
>>
>>The dhcpd server is not directly attached to the subnets , it has to go
>>through a layer3 route to reach them..
>>
>>
>>                  10.1.1.0/24 
>> 192.168.1.0/24
>>srv1     ----------------     L3 SW  A     ------------------------- 
>>subnet1
>
> So far it's looking like a normal routed network.
>
>>The L3 SW A  has  DHCPD relay options enabled  ,  so the DHCPD requests
>>arrives   on  srv1.
>>But srv1  has no knowledge  of  class 192.168.1.0/24  , and the server
>>cannot be started  (Error:   No subnet declaration for 192.168.1.0/24).
>>The server has only one ethernet NIC , with ip 10.1.1.1  on it..
>
> This doesn't sound right, the server will NOT fail to start because a
> remote subnet is not defined - all that would happen is you would get
> runtime errors in response to requests for unknown subnets.
>
>>Can you elaborate more on shared networks ?
>
> It's a common source of confusion, but I don't think you have one. A
> shared subnet is where you have two different IP subnets on the same
> network segment (or if you are being pedantic on the same broadcast
> domain). In Linux for example, you can add additional IP addresses to
> an ethernet port, eg :
>
> eth0 - a.b.c.d/27
> eth0:0 - 192.168.0.1/24
> eth0:1 - 172.16.0.0/12
>
> In this topology, a device attached to this ethernet network could
> have an address in any one or more of the three subnets. This is
> indicated to dhcpd via the shared network construct as described in
> the man pages.
>
>
> Coming back to your problem, can you post the results of 'ifconfig'
> and the contents of your dhcpd.con file ?
>
> Simon
>
>
>
>
> -- 
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.1.409 / Virus Database: 268.14.1/527 - Release Date: 11/9/2006
>
> 



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