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DHCP
Fichas del conjunto (11)
- 1What is the purpose of DHCP?Dynamic Host Configuration Protocol is used to answer DHCP Requests from clients. It allows to give out IP-Addresses, Gateway, Netmask, DNS, TFTP, NTP Information to a client and configure it so it can access the network correctly.
- 2On which well known ports is DHCP working and which transport protocol is it using?UDP port 67 is the destination port of a server, and UDP port 68 is used by the client.
- 3What messages are exchanged between a client and the DHCP server?DHCP operations fall into four basic phases: server discovery, IP lease offer, IP lease request, and IP lease acknowledgement. These stages are often abbreviated as DORA for Discovery, Offer, Request, and Acknowledgement.
DHCP_DISCOVER
DHCP_OFFER
DHCP_REQUEST
DHCP_ACKNOWLEDGE
DHCP_RELEASE
DHCP_DECLINE - 4Provide at least 3 parameters that can be obtained from a DHCP server.DNS, Netmask, Gateway, TFTP, NTP,
- 5Why does DHCP need to be stateful?Because the DHCP server and the client must both maintain state information to keep addresses from conflicting, to handle leases, and to renew addresses over time.
- 6What happens if two DHCP servers are placed in the same network offering different IP ranges to clients?The client is in control and may accept any of the addresses offered by the DHCP servers.
The client blindly transmits a DHCP DISCOVER. I will receive in return some OFFERs. If there are many servers it may receive many offers. It can then decide which address it would like to use. Some phones for instance will prefer the offer that has special vendor specific attributes in it.
Quite commonly it will be the first that arrives
The client then sends a REQUEST for the address it wants. The appropriate server will ACKNOWLEDGE and the others will silently note that their address wasn't requested and mark it free for someone else. - 7What is a DHCP relay agent and how does it work?In order to allow DHCP clients on subnets not directly served by DHCP servers to communicate with DHCP servers, DHCP relay agents can be installed on these subnets.
The DHCP client broadcasts on the local link; the relay agent receives the broadcast and transmits it to one or more DHCP servers using unicast. The relay agent stores its own IP address in field GIADDR field of the DHCP packet. The DHCP server uses the GIADDR-value to determine the subnet on which the relay agent received the broadcast, and allocates an IP address on that subnet. When the DHCP server replies to the client, it sends the reply to the GIADDR-address, again using unicast. The relay agent then retransmits the response on the local network. - 8What are the advantages and disadvantages of having a DHCP relay?Advantage: DHCP doesn't have to be on the same VLAN or Subnet, thus, reducing deployment complexity
Disadvantage: DHCP Broadcast Traffic is routed into other networks to reach the DHCP. - 9What happens to a DHCP client when it has no IP and there is no response from the DHCP server?Most IPv4 hosts use link-local addressing only as a last resort when a DHCP server is unavailable. In IPv4, the method is called link-local address auto configuration. However, Microsoft refers to this as Automatic Private IP Addressing (APIPA).
APIPA allows the client device to randomly choose one of the 65,534 addresses available in the Class B network address of 169.254.0.0/16. After choosing an address from this range, the computer sends an ARP request to see whether another device on the network is using that address, and if it is not, the client device uses the address.
Even though the device uses this made-up address, it continues to send out DHCP Discover broadcasts to locate a DHCP server on the network as soon as the DHCP server becomes available. - 10What are the commands on CLI to see the IP address (Win/Linux)?Win: ipconfig /all
Linux: ip a or ifconfig (old) - 11hat are the commands on CLI to renew the lease (Win/Linux)?Win: ipconfig /release
Linux: dhclient -r [Interface] && dhclient [Interface]
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- Informatik Fichas sobre DHCP, creado por Simon Perez el 05/12/2019.
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- Simon Perez
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