# DHCP
autoconfig client network settings such as IP address, subnet mask, default gateway, DNS server...


# Implementations
[bind](bind) is usually a sane choice on linux, BSD.


# DHCPv4
1. **DHCP Discover**
	- Client sends broadcast message, looking for any DHCP server
2. **DHCP Offer**
	- Server responds with address offer
	- Server creates entry in ARP table with offered IP
3. **DHCP Request**
	- Client request mentioned address or another
4. **DHCP ACK/NACK**
	- Server accepts/rejects IP request

## Router as DHCP server
A dedicated DHCP server can be configured for IPv4 networks, however the router usually serves this purpose, while medium-sized networks generally should have a seperate DHCP server.

## Renew lease
In order to renew a DHCP IP lease, the host periodically sends **Request** messages (time specified by server) and the server responds with **Acknowledge**


# DHCPv6
1. **DHCPv6 SOLICIT**
	- Host sends multicast to `ff02::1:2`
2. **DHCPv6 ADVERTISE**
	- Server is available
3. **DHCPv6 REQUEST**
	- Host wants an IPv6 address or other parameters
4. **DHCPv6 REPLY**
	- Server responds with config information

## ICMPv6 specific messages
Router Advertisement messages use three flags in order to specify which method hosts will use in order to get an IPv6 address:
- Flag ***A***
	- *Address Autoconfiguration Flag*
	- Used for SLAAC 
- Flag ***O***
	- *Other Configuration Flag*
	- Used for **stateless DHCP** ***(SLAAC + DHCPv6)***
- Flag ***M***
	- *Managed Address Configuration Flag*
	- Used for **stateful DHCP**



# IOS Configuration
## DHCPv4 Server
```sh
R1(config)#ip dhcp excluded-address 192.168.10.1 192.168.10.9
R1(config)#ip dhcp excluded-address 192.168.10.254
R1(config)#ip dhcp pool LAN-POOL-1
R1(dhcp-config)#network 192.168.10.0 255.255.255.0
R1(dhcp-config)#default-router 192.168.0.1
R1(dhcp-config)#dns-server 192.168.11.5
R1(dhcp-config)#domain-name example.com
R1(dhcp-config)#exit
```
## DHCPv4 Relay
```sh
R1(config)#interface g0/0/0
R1(config)#ip helper-address 192.168.11.6
R1(config-if)#exit
```
## Router as DHCPv4 Client
```sh
Client(config)#interface g0/0/0
Client(config-if)#ip address dhcp
Client(config-if)#no shutdown
Client(config-if)#end
Client#show ip interface g0/0/1
```

```
GigabitEthernet0/0/1 is up, line protocol is up
Internet address is 209.165.201.12/27
Broadcast address is 255.255.255.255
Address determined by DHCP
```

## Stateless DHCPv6 Server
```sh
R1(config)#ipv6 unicast-routing
R1(config)#ipv6 dhcp pool IPV6-STATELESS
R1(config-dhcpv6)#dns-server 2001:db8:acad:1::254
R1(config-dhcpv6)#domain-name example.com
R1(config-dhcpv6)#exit
R1(config)#interface g0/0/1
R1(config-if)#ipv6 dhcp server IPV6-STATELESS
R1(config-if)#ipv6 nd other-config-flag
```
## Stateless DHCPv6 - Client
```sh
R3(config)#ipv6 unicast-routing
R3(config)#interface g0/0/1
R3(config-if)#ipv6 enable
R3(config-if)#ipv6 address autoconfig
R3(config-if)#do show ipv6 interface brief
```

```
GigabitEthernet0/0/1
[up/up]
FE80::2FC:BAFF:FE94:29B1
2001:DB8:ACAD:1:2FC:BAFF:FE94:29B1
R3#
```
## Stateful DHCPv6 - Server
```sh
R1(config)#ipv6 unicast-routing
R1(config)#ipv6 dhcp pool IPV6-STATEFUL
R1(config-dhcpv6)#address prefix 2001:db8:acad:1::/64
R1(config-dhcpv6)#dns-server 2001:4860:4860::8888
R1(config-dhcpv6)#domain-name example.com
R1(config-dhcpv6)#exit
R1(config)#interface g0/0/1
R1(config-if)#ipv6 dhcp server IPV6-STATEFUL
R1(config-if)#ipv6 nd managed-config-flag
R1(config-if)#ipv6 dhcp nd prefix default no-autoconfig
```
## Stateful DHCPv6 - Client
```sh
R3(config)#ipv6 unicast-routing
R3(config)#interface g0/0/1
R3(config-if)#ipv6 enable
R3(config-if)#ipv6 address dhcp
R3(config-if)#do show ipv6 interface brief
```

```
GigabitEthernet0/0/1
[up/up]
FE80::2FC:BAFF:FE94:29B1
2001:DB8:ACAD:1:B4CB:25FA:3C9:747C
```
### Relay
```sh
R1(config)#interface g0/0/1
R1(config-if)#ipv6 dhcp relay destination 2001:db8:acad:1::2 g0/0/0
```

## Debugging
`R1#show ip dhcp binding` and `R1#show ipv6 dhcp binding`

```
Client: FE80::5C43:EE7C:2959:DA68
DUID: 0001000124F5CEA2005056B3636D
Username : unassigned
VRF : default
IA NA: IA ID 0x03000C29, T1 43200, T2 69120
Address: 2001:DB8:ACAD:2:9C3C:64DE:AADA:7857
preferred lifetime 86400, valid lifetime 172800
expires at Sep 29 2019 08:26 PM (172710 seconds)
```

```sh
R1#show ipv6 dhcp interface
```
```
GigabitEthernet0/0/1 is in relay mode
Relay destinations:
2001:DB8:ACAD:1::2
2001:DB8:ACAD:1::2 via GigabitEthernet0/0/0
```