dhcpd.conf

The dhcpd.conf file is the configuration file for dhcpd, the ISC DHCP server. This file contains all the configuration settings for the DHCP server, including the IP address ranges (scopes) it will manage, options it will provide to clients, and other operational parameters.

Structure and Syntax

The dhcpd.conf file uses a straightforward, declarative syntax. Here’s an overview of the key components and directives:

1. Global Parameters: These settings apply to the entire DHCP server.

2. Subnet Declarations: These define IP address ranges and options specific to a subnet.

3. Host Declarations: These provide fixed address assignments for specific hosts.

Example Configuration

Here is a basic example of a dhcpd.conf file:

# Global Parameters
default-lease-time 600;
max-lease-time 7200;
option domain-name "example.org";
option domain-name-servers ns1.example.org, ns2.example.org;

# Subnet Declaration
subnet 192.168.1.0 netmask 255.255.255.0 {
    range 192.168.1.10 192.168.1.100;
    option routers 192.168.1.1;
    option broadcast-address 192.168.1.255;
    option subnet-mask 255.255.255.0;
}

# Another Subnet Declaration
subnet 192.168.2.0 netmask 255.255.255.0 {
    range 192.168.2.10 192.168.2.100;
    option routers 192.168.2.1;
    option broadcast-address 192.168.2.255;
    option subnet-mask 255.255.255.0;
}

# Host Declaration
host specialclient {
    hardware ethernet 00:11:22:33:44:55;
    fixed-address 192.168.1.50;
}

Key Components Explained

Global Parameters

• default-lease-time: The default lease time in seconds for IP addresses if the client does not request a specific time.

• max-lease-time: The maximum lease time in seconds for IP addresses.

• option domain-name: The domain name to be assigned to clients.

• option domain-name-servers: The DNS servers to be assigned to clients.

Subnet Declarations

• subnet: Defines a subnet and its netmask.

• range: Specifies the range of IP addresses that the DHCP server can assign to clients within the subnet.

• option routers: The default gateway for clients on this subnet.

• option broadcast-address: The broadcast address for this subnet.

• option subnet-mask: The subnet mask for clients on this subnet.

Host Declarations

• host: Defines a specific client by its MAC address.

• hardware ethernet: The MAC address of the client.

• fixed-address: A fixed IP address to be assigned to this client.

Advanced Configuration Options

Dynamic DNS Updates

The DHCP server can be configured to update DNS records dynamically. This requires additional configuration in both dhcpd.conf and named.conf.

ddns-update-style interim;
ignore client-updates;

zone example.org. {
    primary 192.168.1.1;
    key rndc-key;
}

zone 1.168.192.in-addr.arpa. {
    primary 192.168.1.1;
    key rndc-key;
}

Failover Configuration

For high availability, two DHCP servers can be configured to share load and provide redundancy.

failover peer "dhcp-failover" {
    primary;
    address 192.168.1.1;
    port 519;
    peer address 192.168.1.2;
    peer port 520;
    max-response-delay 30;
    max-unacked-updates 10;
    load balance max seconds 3;
    mclt 3600;
    split 128;
    hba 00:00:00:01:00:00:00:00:00:00:00:00:00:00:00:00;
}

Practical Use Cases

Adding a New Subnet

1. Edit dhcpd.conf:

   sudo nano /etc/dhcp/dhcpd.conf

   Add a new subnet declaration:

   subnet 192.168.3.0 netmask 255.255.255.0 {
       range 192.168.3.10 192.168.3.100;
       option routers 192.168.3.1;
       option broadcast-address 192.168.3.255;
       option subnet-mask 255.255.255.0;
   }

2. Restart the DHCP Server:

   sudo systemctl restart isc-dhcp-server

Assigning a Fixed IP to a Client

1. Edit dhcpd.conf:

   sudo nano /etc/dhcp/dhcpd.conf

   Add a host declaration:

   host printer {
       hardware ethernet 11:22:33:44:55:66;
       fixed-address 192.168.1.60;
   }

2. Restart the DHCP Server:

   sudo systemctl restart isc-dhcp-server

Security Considerations

• Restrict Access: Limit the range of IP addresses to only what is necessary.

• Logging: Enable logging to monitor DHCP server activities.

• Backup: Regularly back up the dhcpd.conf file to prevent data loss.

Conclusion

The dhcpd.conf file is the heart of the ISC DHCP server configuration, allowing administrators to define how IP addresses are allocated, what options are provided to clients, and how the server interacts with other network services. By understanding its structure and options, you can effectively manage and customize your DHCP server to fit your network needs.