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What is DHCP

Dynamic Host setup Protocol (DHCP) is a communication protocol that enables network administrators to centrally manage and automatically assign IP network addresses. In IP networks, each device connected to Internet needs to be assigned a unique IP address. DHCP enables network administrators to monitor and assign IP addresses from a central node. When a computer moves to another location in the network, it automatically receives a new IP address. DHCP uses the concept of a lease, or the validity period of a computer's IP address. Rental time is variable, depending on how long it takes users to connect to Internet somewhere, which is useful for the education industry and other users in an environment that changes frequently. With a short lease, DHCP can dynamically reconfigure the network in an environment where there are more computers than available IP addresses. What is dhcp? DHCP supports the assignment of static addresses to computers, such as DHCP, a Web server that requires a permanent IP address, and BOOTP, another network IP management protocol. At present, the two configuration management protocols are widely used, of which DHCP is more advanced. Some operating systems, such as Windows NT/2000, come with DHCP servers. A DHCP or BOOTP client is a program installed on a computer so that it can be configured. DHCP uniformly uses two IANA-assigned ports as the BOOTP: the server uses 67/udp and the client uses 68/udp. The operation of DHCP is divided into four basic processes: requesting IP lease, providing IP lease, selecting IP lease and confirming IP lease. After the customer has obtained an IP address, they can send an ARP request to avoid IP conflicts caused by overlapping DHCP server address pools.

2. The message of DHCP

There are eight kinds of messages in DHCP, which are DHCP Discover, DHCP Offer, DHCP Request, DHCP ACK, DHCP NAK, DHCP Release, DHCP Decline and DHCP Inform. The functions of each message type are described in the following table.

What is DHCP Relay

In real life, in a slightly more complex network, the server is often concentrated in the server area, and the broadcast packet of DHCP is blocked by layer 3 devices, which will cause HDCP to fail to obtain the address. At this point, the DHCP relay can be configured on the layer 3 device interface closest to the client for secondary addressing to forward the DHCP request broadcast packet.

The DHCP relay agent works as follows:

IV. The role of Snooping

(1) the main function of dhcp-snooping is to isolate illegal dhcp server by configuring untrusted ports.

(2) cooperate with switch DAI to prevent the spread of ARP virus.

(3) establish and maintain a dhcp-snooping binding table, which is generated by the ip and mac addresses in the dhcp ack package, and can be specified manually. This table is the basis for subsequent DAI (dynamic arp inspect) and IPSource Guard. These two similar technologies restrict users from connecting to the network by using this table to determine whether an ip or mac address is legal.

(4) illegal DHCP servers are isolated by establishing trusted ports and untrusted ports. The trusted port forwards DHCP packets normally. After the DHCP offer and DHCPACK of the server response received by the untrusted port, packet loss is handled and no forwarding is carried out.

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