wiki-grav/pages/02.linux/dhcp-server-and-routing/default.en.md

---
title: "DHCP Server and Routing"
visible: true
---

[toc]

## Installation

```sh
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

```sh
ifup {INTERFACE}
```

Restart DHCP Server

```sh
systemctl restart isc-dhcp-server.service
```

### Enable routing

```sh
echo "net.ipv4.ip_forward=1" >> /etc/sysctl.d/80-forwarding.conf
sysctl -p /etc/sysctl.d/80-forwarding.conf
```

```sh
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

```sh
apt install iptables-persistent
```

### Enable DHCP-managed fixed IP address

```
host (hostname) {
  hardware ethernet (mac);
  fixed-address (ip address);
}
```

### Dynamic DNS

_Needs a supported DNS like BIND or PowerDNS_  
[Configure your DNS server to accept updates](https://wiki.realstickman.net/e/en/linux/services/powerdns)  
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

```sh
dhclient -v
```

Release IP

```sh
# dhclient -v -r
```