i have a dhcp server runs on linux server. this server works as a proxy server for a private network
the /etc/dhcpd.conf has the configuration:

option domain-name "my-network.net";
option domain-name-servers 10.0.0.1;
option routers 10.0.0.1;
ddns-update-style interim;
ddns-updates on;
ignore client-updates;
default-lease-time 259200; # 3 days
max-lease-time 604800; # 1 week
authoritative ;
subnet 10.0.0.0 netmask 255.255.255.0 {
range dynamic-bootp 10.10.10.2 10.10.10.100;
}
host tech {
hardware ethernet <mac addr of tech>;
fixed-address 10.0.0.235;
}

when i try to start dhcp server it fails , and gives me the error:
.
.
No subnet declaration for eth-<mac address of the external card> (0.0.0.0).
** Ignoring requests on eth-<mac address of the external card>. If this is not what
you want, please write a subnet declaration
in your dhcpd.conf file for the network segment
to which interface eth-<mac address of the external card> is attached. **

Not configured to listen on any interfaces!

I'm guessing here, but it looks like your system has an additional interface that has not been configured. The output of "ifconfig" should verify this. If so, you can either add the subnet declaration as pointed out in the error message or simply specify a list of interfaces for dhcpd to bind to on startup through the dhcpd command line. i.e. dhcpd eth0

Either way you would like to set wich interface the DCHP server listens to as state above. This is a security issue as the DHCP server only should serve your local network.
The setting for NIC's to bind to usually is found in the /etc/sysconfig/ directory.

can i do that by an option in /etc/dhcpd.conf? what is that option?

You set this in the file /etc/sysconfig/dhcpd file, add the line