DHCP in Linux

Lease Process

Installation: Use the command sudo apt install isc-dhcp-server to install the daemon (dhcpd).
Logging: Server messages are sent to the system log at /var/log/syslog.
Configuration File: Primary settings are managed in /etc/dhcp/dhcpd.conf.
Restarting: Apply changes using sudo systemctl restart isc-dhcp-server.service.

The configuration file consists of statements divided into two categories:

Parameters: Define how or if to do something (e.g., lease duration, allowing unknown clients).
Declarations: Describe the network layout using keywords like subnet, host, group, and pool.
- Key Global Parameters
  - default-lease-time: The duration a client gets if they do not request a specific time.
  - max-lease-time: The absolute limit for a lease; if a client requests more, they are capped at this value.
  - authoritative: Should be uncommented if this is the primary DHCP server for the local network.
- Subnet & Host Declarations
  - subnet [addr] netmask [mask]: Describes the network segment for leasing.
  - range [first] [last]: Defines the pool of available addresses allocated sequentially.
  - option routers: Defines the default gateway for clients.
  - option domain-name-servers: Specifies DNS server addresses.
  - host [Name]: Assigns a predefined IP (fixed-address) to a specific device based on its hardware ethernet (MAC address).
- Example: dhcpd.conf
  Terminal window
```
default-lease-time 600;
max-lease-time 7200;
authoritative;

subnet **192.168.1.0** netmask **255.255.255.0**
{
    range **192.168.1.150 192.168.1.200**;
    option routers **192.168.1.254**;
    option domain-name-servers **192.168.1.1**, **192.168.1.2**;
    option domain-name **"mydomain.example"**;
}

host Client1
{
    hardware ethernet 00:02:3f:3d:73:b3;
    fixed-address 192.168.1.100;
}
```

Interfaces: Edit /etc/default/isc-dhcp-server to specify which network card (e.g., enp0s3) the server should listen on.
Pools: Used to treat groups of addresses differently within the same subnet.
Groups: Apply parameters to a collection of declarations not related by subnet.
Allow/Deny Unknown Clients: The unknown-clients flag determines if devices without a specific host declaration can receive an IP.

Ubuntu: Set dhcp4: true (or yes) within a Netplan YAML file in /etc/netplan/*.yaml.
CentOS: Edit the interface script (e.g., /etc/sysconfig/network-scripts/ifcfg-enp0s3) and set BOOTPROTO=dhcp.

Active Leases: Use dhcp-lease-list to see currently assigned IPs, hostnames, and expiration times.
Lease History: The file /var/lib/dhcp/dhcpd.leases stores the full history and current states (e.g., active, free) of all IP assignments.