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My network is operated by Cisco Catalyst with several VLANs and one DHCP server. On the Catalyst, for each vlan is set ip address-helper with the IP of DHCP server. isc-dhcp-server on ubuntu 14.04 is used as DHCP server.

I have several network subnets set as can be seen below and all of them work without problem except one and I cant figure out why.

The network which does not work properly is 10.12.35.0/24 The messages I got from syslog are like: Jan 6 06:57:54 dhcp-server-1 dhcpd: DHCPDISCOVER from 00:0c:29:33:3c:bb via 10.12.35.1: unknown network segment

And as I already mentioned - all other subnets work well

Thanks! I really appreciate your help!

config file:

option ntp-servers 0.pool.ntp.org, 1.pool.ntp.org, 2.pool.ntp.org;
authoritative;
allow client-updates;
allow unknown-clients;
ddns-updates on;
ddns-domainname "domain.do";

key rndc-key {
secret notgonnatellyou;
algorithm hmac-md5;
}

ddns-update-style interim;

option domain-name "domain.do";
option domain-name-servers 10.12.240.29, 10.12.32.41;

default-lease-time 600;
max-lease-time 7200;

log-facility local7;


subnet 10.12.4.0 netmask 255.255.252.0 {
option domain-name-servers 8.8.8.8;
option routers 10.12.4.1;
option subnet-mask 255.255.252.0;
option broadcast-address 10.12.7.255;
    pool {
        range 10.12.4.100 10.12.5.250;
        }
    }
subnet 10.12.35.0 netmask 255.255.255.0 {
    ddns-updates off;
    option routers 10.12.35.1;
    option subnet-mask 255.255.255.0;
    option broadcast-address 10.12.35.255;
    pool {
        range 10.12.35.100 10.12.35.254;
        }
    }
subnet 10.12.96.0 netmask 255.255.255.0 {
    option routers 10.12.96.1;
    option subnet-mask 255.255.255.0;
    option broadcast-address 10.12.96.255;
    pool {
        range 10.12.96.100 10.12.96.200;
        }
    }
subnet 10.12.32.0 netmask 255.255.255.0 {
    option routers 10.12.32.1;
    option subnet-mask 255.255.255.0;
    option broadcast-address 10.12.32.255;
    pool {
        range 10.12.32.200 10.12.32.220;
        }
    }
subnet 10.12.0.0 netmask 255.255.252.0 {
    option routers 10.12.0.1;
    option subnet-mask 255.255.252.0;
    option broadcast-address 10.12.3.255;
    pool {
        range 10.12.0.10 10.12.2.255;
        }
host some-host {
    hardware ethernet 00:0c:29:52:1c:a7;
    fixed-address 10.12.3.0;
    }
host someother-host {
    hardware ethernet 00:50:56:9f:04:26;
    fixed-address 10.12.3.1;
    }
host evenother-host {
    hardware ethernet 10:dd:b1:e7:b2:e0;
    fixed-address 10.12.0.200;
    }
}

zone domain.do. {
primary 10.12.240.29;
key rndc-key;
}
zone 12.10.in-addr.arpa. {
primary 10.12.240.29;
key rndc-key;
}
  • As the subnet declaration for net 10.12.35.0 is there and looks correct, there can only be problem on the relay agent side I guess. Check the device with ip 10.12.35.1 and the network connected to it. You can also run a tcpdump to see the actual request content. – Diamond Jan 6 '16 at 10:53
  • 10.12.35.1 is the address of the catalyst switch.. (as is the 10.12.32.1, 96.1 and so on) but I will try the trick with wireshark a compare it to each other – user3337015 Jan 6 '16 at 12:30
2

The configuration was OK all along. The problem was with "ghost process". I dont know how that happened, but I ran two dhcpd processes and the first one was really old... from the time when the subnet was not defined in conf

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