DHCP Server and Routing

Table of Contents

Installation

apt install isc-dhcp-server

Configuration

Edit /etc/default/isc-dhcp-server

INTERFACESv4="[INTERFACE 1] [INTERFACE 2]"

Edit /etc/dhcp/dhcpd.conf to set a subnet

subnet [NETADDRESS] netmask [SUBNETMASK] {
    range [FIRST DHCP] [LAST DHCP];
    option subnet-mask [SUBNETMASK];
    option routers [GATEWAY];
    option domain-name "[NAME]";
    option domain-name-servers [DNS SERVER];
}

Edit /etc/network/interfaces

auto [INTERFACE]
iface [INTERFACE] inet static
    address [ADDRESS]
    network [NETADDRESS]
    netmask [NETMASK]
    broadcast [BROADCAST]

Enable the interface

ifup [INTERFACE]

Restart DHCP Server

systemctl restart isc-dhcp-server.service

Enable routing

echo "net.ipv4.ip_forward=1" >> /etc/sysctl.d/80-forwarding.conf
sysctl -p /etc/sysctl.d/80-forwarding.conf
iptables -t nat -A POSTROUTING -o [WAN INTERFACE] -j MASQUERADE
iptables -A FORWARD -i [LAN INTERFACE] -j ACCEPT

Make iptables permanent
Select Yes during the installation to save current rules

apt install iptables-persistent

Enable DHCP-managed fixed IP address

host [HOSTNAME] {
  hardware ethernet [MAC ADDRESS];
  fixed-address [IP ADDRESS];
}

Dynamic DNS

Needs a supported DNS like BIND or PowerDNS
Configure your DNS server to accept updates
Add the following snippet to your /etc/dhcp/dhcpd.conf file
How to generate the key is also described in the DNS article

ddns-updates on;
ddns-update-style interim;
update-static-leases on;

ddns-domainname "testpdns";
ddns-rev-domainname "in-addr.arpa.";

key "[KEYNAME]" {
    algorithm hmac-md5;
    secret "[KEY]";
};

zone testpdns {
    primary 127.0.0.1;
    key [KEYNAME];
}

zone 7.168.192.in-addr.arpa. {
    primary 127.0.0.1;
    key [KEYNAME];
}

Client

DHCP Request

dhclient -v

Release IP

dhclient -v -r