Vol 2 Part 2.5: DHCP Flashcards
Dynamic Host Configuration Protocol (DHCP) assigns host network settings based on configuration at the what?
DHCP Server
What are the 3 main functions of DHCP?
- Reduces workload of manual IP configuration
- Assigns DNS IP addresses
- Recovers no longer used IP addresses
What are the two primary critical IP addresses when using DHCP, and why are they critical?
- 0.0.0- The IP of a host that does not yet have an IP
255. 255.255.255- The local broadcast IP used to locally contact the DHCP server
What are the four critical messages sent between the client and server when using DHCP?
DORA
- Discover- sent by DHCP client to find a willing DHCP server
- Offer- sent by the DHCP server, offering a lease of an IP and other parameters
- Request- sent by the DHCP client, asking the server to lease the IPv4 address listed in the offer
- Acknowledge- sent by the DHCP server, assigns the address and lists the mask, default router, and DNS IP addresses
Routers do not forward broadcasts. Subnets sending discover messages to the 255.255.255.255 broadcast address while a DHCP server is located elsewhere will get no response means that another method must be used for a device on a subnet to hit a DHCP server on a different subnet. What is this method called?
DHCP relay. The router that acts as a middle man.
What command is used to set up the DHCP relay?
ip helper-address SERVER IP
What 4 items does a host need in order to work with DHCP correctly?
- The DNS Server IP for name resolution
- The default gateway IP
- The device’s own IP
- The devices own subnet mask
What interface should the ip dhcp helper-address command go on a router for the subnet that requires network settings from a DHCP server?
The inbound interface from the local network
The MacOS uses what command in order to access the IP configuration commands?
Ifconfig
What IP address does a host use as the source address when sending out a discover message to a DHCP server?
0.0.0.0
DHCP Snooping on a switch acts like a what?
A firewall
What does DHCP snooping do?
It analyzes incoming messages on a specific subset of ports in a VLAN. It filters the DHCP server messages (offer and acknowledge), preventing another device from offering and acknowledging IP network settings from a spurious or rogue DHCP server.
With DHCP snooping, all ports are in what state by default?
untrusted
What are the 3 DHCP snooping rules?
- Discards DHCP messages received on an untrusted port if they are server based DHCP messages
- DHCP messages received on an untrusted port as normally sent by a DHCP client (discover and request) may be filtered if they might be part of an attack
- DHCP messages received on a trusted port will be forwarded. Trusted ports do NOT filer any DHCP messages.
Explain the DHCP Release and DHCP decline messages
DHCP Release- client message telling the server they no longer need the IP address
DHCP Decline- client message turning down an IP offer