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- Table of Contents
-
- H3C S9500 Operation Manual-Release2132[V2.03]-02 IP Services Volume
- 00-1Cover
- 01-ARP Configuration
- 02-DHCP Configuration
- 03-DNS Configuration
- 04-IP Addressing Configuration
- 05-IP Performance Configuration
- 06-UDP Helper Configuration
- 07-IPv6 Basics Configuration
- 08-Dual Stack Configuration
- 09-Tunneling Configuration
- 10-Adjacency Table Configuration
- Related Documents
-
| Title | Size | Download |
|---|---|---|
| 02-DHCP Configuration | 358.08 KB |
1.2.2 Dynamic IP Address Allocation Procedure
1.2.3 IP Address Lease Extension
Chapter 2 DHCP Server Configuration
2.1 Introduction to DHCP Server
2.1.3 IP Address Allocation Sequence
2.2 DHCP Server Configuration Task List
2.4 Enabling the DHCP Server on an Interface
2.5 Configuring an Address Pool for the DHCP Server
2.5.2 Creating a DHCP Address Pool
2.5.3 Configuring an Address Allocation Mechanism
2.5.4 Configuring a Domain Name for the Client
2.5.5 Configuring DNS Servers for the Client
2.5.6 Configuring WINS Servers and NetBIOS Node Type for the Client
2.5.7 Configuring the BIMS server Information for the Client
2.5.8 Configuring Gateways for the Client
2.5.9 Configuring the TFTP Server Address and Bootfile Name for the Client
2.5.10 Configuring Self-Defined DHCP Options
2.6 Configuring the DHCP Server Security Functions
2.6.1 Configuration Prerequisites
2.6.2 Enabling Unauthorized DHCP Server Detection
2.6.3 Configuring IP Address Conflict Detection
2.7 Enabling the DHCP Sever to Handle Option 82
2.8 Displaying and Maintaining the DHCP Server
2.9 DHCP Server Configuration Examples
2.9.1 Static IP Address Assignment Configuration Example
2.9.2 Dynamic IP Address Assignment Configuration Example
2.10 Troubleshooting DHCP Server Configuration
Chapter 3 DHCP Relay Agent Configuration
3.1 Introduction to DHCP Relay Agent
3.2 Configuring DHCP Relay Agent
3.2.1 DHCP Relay Agent Configuration Task List
3.2.3 Enabling the DHCP Relay Agent on Interfaces
3.2.4 Correlating a DHCP Server Group with Relay Agent Interfaces
3.2.5 Configuring the DHCP Relay Agent to Send a DHCP-Release Request
3.2.6 Configuring the DHCP Relay Agent Security Functions
3.2.7 Configuring the DHCP Relay Agent to Support Option 82
3.3 Displaying and Maintaining DHCP Relay Agent Configuration
3.4 DHCP Relay Agent Configuration Example
3.5 Troubleshooting DHCP Relay Agent Configuration
Chapter 1 DHCP Overview
When configuring DHCP, go to these sections for information you are interested in:
1.1 Introduction to DHCP
The fast expansion and growing complexity of networks result in scarce IP addresses assignable to hosts. Meanwhile, many people need to take their laptops across networks, and the IP addresses need be changed accordingly. Therefore, the related configurations on hosts become more complex. Dynamic host configuration protocol (DHCP) was introduced to provide a framework for passing configuration information to hosts on a TCP/IP network.
DHCP is built on a client-server model, in which the client sends a configuration request and then the server returns a reply to send configuration parameters such as an IP address to the client.
A typical DHCP application, as shown in Figure 1-1, includes a DHCP server and multiple clients (PCs and laptops).
Figure 1-1 A typical DHCP application
1.2 DHCP Address Allocation
1.2.1 Allocation Mechanisms
Different DHCP clients need to occupy their IP addresses for different time durations. For example, some servers need fixed IP addresses, some hosts need to use dynamic IP addresses for a long time, and some clients only need temporary IP addresses. DHCP supports three mechanisms for IP address allocation based on various needs.
l Manual allocation: The network administrator assigns an IP address to a client like a WWW server, and DHCP conveys the assigned address to the client.
l Automatic allocation: DHCP assigns a permanent IP address to a client.
l Dynamic allocation: DHCP assigns an IP address to a client for a limited period of time, which is called a lease. Most clients obtain their addresses in this way.
1.2.2 Dynamic IP Address Allocation Procedure
Figure 1-2 Dynamic IP address allocation process
As shown in the figure above, a DHCP client obtains an IP address from a DHCP server via four steps:
1) The client broadcasts a DHCP-DISCOVER message to locate a DHCP server.
2) A DHCP server offers configuration parameters such as an IP address to the client in a DHCP-OFFER message.
3) If several DHCP servers send offers to the client, the client accepts the first received offer, and broadcasts it in a DHCP-REQUEST message to formally request the IP address.
4) All DHCP servers receive the DHCP-REQUEST message, but only the server to which the client sent a formal request for the offered IP address returns a DHCP-ACK message to the client, confirming that the IP address has been allocated to the client, or returns a DHCP-NAK unicast message, denying the IP address allocation.
& Note:
l After the client receives the DHCP-ACK message, it will probe whether the IP address assigned by the server is in use by broadcasting gratuitous ARP. If the client receives no response within specified time, the client can use this IP address. Otherwise, the client sends a DHCP-DECLINE message to the server to request an IP address again.
l If there are multiple DHCP servers, IP addresses offered by other DHCP servers are assignable to other clients.
1.2.3 IP Address Lease Extension
The IP address dynamically allocated by a DHCP server to a client has a lease. After the lease duration elapses, the IP address will be reclaimed by the DHCP server. If the client wants to use the IP address again, it has to extend the lease duration.
After the half lease duration elapses, the DHCP client will send the DHCP server a DHCP-REQUEST unicast message to extend the lease duration. Upon availability of the IP address, the DHCP server returns a DHCP-ACK unicast confirming that the client’s lease duration has been extended, or a DHCP-NAK unicast denying the request.
If the client receives the DHCP-NAK message, it will broadcast another DHCP-REQUEST message for lease extension after 7/8 lease duration elapses. The DHCP server will handle the request as above mentioned.
1.3 DHCP Message Format
Figure 1-3 gives the DHCP message format, which is based on the BOOTP message format and involves eight types. These types of messages have the same format except that some fields have different values. The numbers in parentheses indicate the size of each field in octets.
Figure 1-3 DHCP message format
l op: Message type defined in option field. 1 = REQUEST, 2 = REPLY
l htype,hlen: Hardware address type and length of a DHCP client.
l hops: Number of relay agents a request message traveled.
l xid: Transaction ID, a random number chosen by the client to identify an IP address allocation.
l secs: Filled in by the client, the number of seconds elapsed since the client began address acquisition or renewal process. Currently this field is reserved and set to 0.
l flags: The leftmost bit is defined as the BROADCAST (B) flag. If this flag is set to 0, the DHCP server sent a reply back by unicast; if this flag is set to 1, the DHCP server sent a reply back by broadcast. The remaining bits of the flags field are reserved for future use.
l ciaddr: Client IP address.
l yiaddr: 'your' (client) IP address, assigned by the server.
l siaddr: Server IP address, from which the clients obtained configuration parameters.
l giaddr: The first relay agent IP address a request message traveled.
l chaddr: Client hardware address.
l sname: The server host name, from which the client obtained configuration parameters.
l file: Bootfile name and routing information, defined by the server to the client.
l options: Optional parameters field that is variable in length, which includes the message type, lease, DNS IP address, WINS IP address and so forth.
1.4 Protocols and Standards
l RFC 2131: Dynamic Host Configuration Protocol
l RFC 2132: DHCP Options and BOOTP Vendor Extensions
l RFC 1542: Clarifications and Extensions for the Bootstrap Protocol
l RFC 3046: DHCP Relay Agent Information Option
Chapter 2 DHCP Server Configuration
When configuring the DHCP server, go to these sections for information you are interested in:
l DHCP Server Configuration Task List
l Enabling the DHCP Server on an Interface
l Configuring an Address Pool for the DHCP Server
l Configuring the DHCP Server Security Functions
l Enabling the DHCP Sever to Handle Option 82
l Displaying and Maintaining the DHCP Server
l DHCP Server Configuration Example
l Troubleshooting DHCP Server Configuration
& Note:
The DHCP server configuration is supported only on VLAN interfaces.
2.1 Introduction to DHCP Server
2.1.1 Application Environment
The DHCP server is well suited to the network where:
l It is hard to implement manual configuration and centralized management.
l The hosts are more than the assignable IP addresses and it is impossible to assign a fixed IP address to each host. For example, an ISP limits the number of hosts to access the Internet at a time, so lots of hosts need to acquire IP addresses dynamically.
l A few hosts need fixed IP addresses.
2.1.2 DHCP Address Pool
I. Address pool structure
In response to a client’s request, the DHCP server selects an idle IP address from an address pool and sends it together with other parameters such as lease and DNS server address to the client.
The address pool database is organized as a tree. The root of the tree is the address pool for natural networks, branches are address pools for subnets, and leaves are addresses statically bound to clients. For the same level address pools, a previously configured pool has a higher selection priority than a new one.
At the very beginning, subnetworks inherit network parameters and clients inherit subnetwork parameters. Therefore, common parameters, for example the domain name, should be configured at the highest (network or subnetwork) level of the tree.
After establishment of the inheritance relationship, the new configuration at the higher level of the tree will be:
l Inherited if the lower level has no such configuration, or
l Overridden if the lower level has such configuration.
& Note:
The IP address lease does not have any inheritance.
II. Principles for selecting an address pool
The DHCP server observes the following principles to select an address pool to assign IP addresses to clients:
1) If there is an address pool where IP addresses are statically bound to the MAC addresses or IDs of clients, the DHCP server will select this address pool and assign statically bound IP addresses to clients. For the configuration of this address pool, refer to section 2.5.3 I. “Configuring manual address allocation”.
2) Otherwise, the DHCP server will select the smallest address pool that contains the IP address of the interface receiving DHCP requests, regardless of the mask. If no IP address is available in the smallest address pool, the DHCP server will fail to assign addresses to clients because it will not assign those in the father address pool to clients. For the configuration of the smallest address pool, refer to section 2.5.3 II. “Configuring dynamic address allocation”.
For example, two address pools are configured on the DHCP server. The ranges of IP addresses that can be dynamically assigned are 1.1.1.0/24 and 1.1.1.0/25 respectively. If the IP address of the interface receiving DHCP requests is 1.1.1.1/25, the DHCP server will select IP addresses for clients from the 1.1.1.0/25 address pool. If no IP address is available in the 1.1.1.0/25 address pool, the DHCP server will fail to assign addresses to clients. If the IP address of the interface receiving DHCP requests is 1.1.1.130/25, the DHCP server will select IP addresses for clients from the 1.1.1.0/24 address pool.
& Note:
Keep the IP addresses for dynamic allocation within the subnet where the interface of the DHCP server or DHCP relay agent resides to avoid wrong IP address allocation.
2.1.3 IP Address Allocation Sequence
A DHCP server assigns an IP address to a client according to the following sequence:
1) The IP address manually bound to the client’s MAC address or ID
2) The IP address that was ever assigned to the client
3) The IP address designated by the Option 50 field in a DHCP-DISCOVER message
4) The first assignable IP address found in a proper DHCP address pool
5) The IP address that was a conflict or passed its lease duration
If no IP address is assignable, the server will not respond.
2.2 DHCP Server Configuration Task List
To configure the DHCP server feature, perform the tasks described in the following sections:
|
Remarks |
|
|
Required |
|
|
Optional |
|
|
Optional |
|
|
Optional |
|
|
Optional |
2.3 Enabling DHCP
Enable DHCP before performing other configurations.
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enable DHCP |
dhcp enable |
Required Disabled by default |
2.4 Enabling the DHCP Server on an Interface
With the DHCP server enabled on an interface, upon receiving a client’s request, the DHCP server will assign an IP address from its address pool to the DHCP client.
Follow these steps to enable the DHCP server on an interface:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter VLAN interface view |
interface interface-type interface-number |
— |
|
Enable the DHCP server on an interface |
dhcp select server global-pool [ subaddress ] |
Optional Enabled by default. |
& Note:
The subaddress keyword is valid only when the server and client are on the same subnet. If a DHCP relay agent exists in between, regardless of subaddress, the DHCP server will select an IP address from the address pool of the subnet which contains the primary IP address of the DHCP relay agent’s interface (connected to the client).
When the DHCP server and client are on the same subnet, the server will:
l With subaddress specified, assign an IP address from the address pool of the subnet which the secondary IP address of the server’s interface connected to the client belongs to, or assign from the first secondary IP address if several secondary IP addresses exist. If no secondary IP address is configured for the interface, the server is unable to assign an IP address to the client.
l Without subaddress specified, assign an IP address from the address pool of the subnet which the primary IP address of the server’s interface (connected to the client) belongs to.
2.5 Configuring an Address Pool for the DHCP Server
2.5.1 Configuration Task List
To configure an address pool, perform the tasks described in the following sections:
|
Task |
Remarks |
|
|
Required |
||
|
Required to configure either of the two |
||
|
Optional |
||
|
Configuring WINS Servers and NetBIOS Node Type for the Client |
||
|
Configuring the TFTP Server Address and Bootfile Name for the Client |
||
2.5.2 Creating a DHCP Address Pool
To create a DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Create a DHCP address pool and enter its view |
dhcp server ip-pool pool-name |
Required No DHCP address pool is created by default. |
2.5.3 Configuring an Address Allocation Mechanism
Caution:
You can configure either the static binding or dynamic address allocation for an address pool as needed.
It is required to specify an address range for the dynamic address allocation. A static binding is a special address pool containing only one IP address.
I. Configuring manual address allocation
Some DHCP clients such as a WWW server need fixed IP addresses. You can create a static binding of a client’s MAC or ID to IP address in the DHCP address pool.
When the client with the MAC address or ID requests an IP address, the DHCP server will find the IP address from the binding for the client.
A DHCP address pool now supports only one static binding, which can be a MAC-to-IP or ID-to-IP binding.
To configure the static binding in a DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
|
Enter system view |
system-view |
— |
|
|
Enter DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
|
Bind IP addresses statically |
static-bind ip-address ip-address [ mask-length | mask mask ] |
Required No IP addresses are statically bound by default |
|
|
Bind MAC addresses or IDs statically |
Specify the MAC address |
static-bind mac-address mac-address |
Required to configure either of the two Neither is bound statically by default |
|
Specify the ID |
static-bind client-identifier client-identifier |
||
& Note:
l Use the static-bind ip-address command together with static-bind mac-address or static-bind client-identifier command to accomplish a static binding configuration.
l If you use the static-bind ip-address, static-bind mac-address, or static-bind client-identifier command repeatedly in the DHCP address pool, the new configuration will overwrite the previous one.
l The IP address of the static binding cannot be an interface address of the DHCP server. Otherwise, an IP address conflict may occur and the bound client cannot obtain an IP address correctly.
l The ID of the static binding must be identical to the ID displayed by using the display dhcp client verbose command on the client. Otherwise, the client cannot obtain an IP address.
II. Configuring dynamic address allocation
You need to specify one and only one address range using a mask for the dynamic address allocation.
To avoid address conflicts, the DHCP server excludes IP addresses used by the GW, FTP server and so forth from dynamic allocation.
You can specify the lease duration for a DHCP address pool different from others, and a DHCP address pool can only have the same lease duration. A lease does not enjoy the inheritance attribute.
To configure the dynamic address allocation, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
Specify an IP address range |
network network-address [ mask-length | mask mask ] |
Required Not specified by default, meaning no assignable address |
|
Specify the address lease duration |
expired { day day [ hour hour [ minute minute ] ] | unlimited } |
Optional One day by default |
|
Return to system view |
quit |
— |
|
Exclude IP addresses from automatic allocation |
dhcp server forbidden-ip low-ip-address [ high-ip-address ] |
Optional Except IP addresses of the DHCP server interfaces, all addresses in the DHCP address pool are assignable by default. |
& Note:
l In DHCP address pool view, using the network command repeatedly overwrites the previous configuration.
l Using the dhcp server forbidden-ip command repeatedly can specify multiple IP address ranges not assignable.
2.5.4 Configuring a Domain Name for the Client
You can specify a domain name in each DHCP address pool on the DHCP server. To configure the domain name in the DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter the DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
Specify the domain name for the client |
domain-name domain-name |
Required Not specified by default |
2.5.5 Configuring DNS Servers for the Client
When a DHCP client wants to access a host on the Internet via the host name, it contacts a domain name system (DNS) server holding host name-to-IP address mappings to get the host IP address. You can specify up to eight DNS servers in the DHCP address pool.
To configure DNS servers in the DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
Specify DNS servers for the client |
dns-list ip-address&<1-8> |
Required Not specified by default |
2.5.6 Configuring WINS Servers and NetBIOS Node Type for the Client
A Microsoft Windows DHCP client using NetBIOS protocol contacts a Windows Internet Naming Service (WINS) server for name resolution. Therefore, the DHCP server should assign a WINS server address when assigning an IP address to the client.
You can specify up to eight WINS servers in a DHCP address pool.
You need to specify in a DHCP address pool a NetBIOS node type for the client to approach name resolution. There are four NetBIOS node types:
l b (broadcast)-node: The b-node client sends the destination name in a broadcast message. The destination returns its IP address to the client after receiving the message.
l p (peer-to-peer)-node: The p-node client sends the destination name in a unicast message to the WINS server, and the WINS server returns the destination IP address.
l m (mixed)-node: A combination of broadcast first and peer-to-peer second. The m-node client broadcasts the destination name, if no response, then unicasts the destination name to the WINS server to get the destination IP address.
l h (hybrid)-node: A combination of peer-to-peer first and broadcast second. The h-node client unicasts the destination name to the WINS server, if no response, then broadcasts it to get the destination IP address.
To configure WINS servers and NetBIOS node type in the DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
Specify WINS server IP addresses for the client |
nbns-list ip-address&<1-8> |
Required (optional for b-node) No address is specified by default |
|
Specify the NetBIOS node type |
netbios-type { b-node | h-node | m-node | p-node } |
Required Not specified by default |
& Note:
If b-node is specified for the client, you need to specify no WINS server address.
2.5.7 Configuring the BIMS server Information for the Client
A DHCP client performs regular software update and backup using configuration files obtained from a branch intelligent management system (BIMS) server. Therefore, the DHCP server needs to offer DHCP clients the BIMS server IP address, port number, shared key from the DHCP address pool.
To configure the BIMS server IP address, port number, and shared key in the DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
Specify the BIMS server IP address, port number, and shared key |
bims-server ip ip-address [ port port-number ] sharekey key |
Required Not specified by default |
2.5.8 Configuring Gateways for the Client
DHCP clients wanting to access hosts outside the local subnet request gateways to forward data. You can specify gateways in each address pool for clients. Up to eight gateways can be specified in a DHCP address pool.
To configure the gateways in the DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
Specify gateways |
gateway-list ip-address&<1-8> |
Required No gateway is specified by default. |
& Note:
For a Virtual Router Redundancy Protocol (VRRP) network, you are recommended to specify the virtual IP address of the VRRP group as the gateway of the address pool, or reserve the virtual IP address. Otherwise, the virtual IP address of the VRRP group may be assigned to a client (Linux client, for example), which does not perform address conflict detection after obtaining the IP address through DHCP, thus causing network disruption.
2.5.9 Configuring the TFTP Server Address and Bootfile Name for the Client
This task is to specify the IP address and name of a TFTP server and the bootfile name in the DHCP address pool. The DHCP clients use these parameters to contact the TFTP server, requesting the configuration file used for system initialization, which is called autoconfiguration. The request process of the client is described below:
1) When a router starts up without loading any configuration file, the system sets an active interface (such as the interface of the default VLAN or a Layer 3 Ethernet interface) as the DHCP client to request from the DHCP server parameters such as an IP address and name of a TFTP server, and the bootfile name.
2) After getting related parameters, the DHCP client will send a TFTP request to obtain the configuration file from the specified TFTP server for system initialization. If the client cannot get such parameters, it will perform system initialization without loading any configuration file.
Note that if the client cannot get related parameters, it will use the empty configuration file for system initialization.
When option 55 in the requesting client message contains parameters of option 66, option 67, or option 150, the DHCP server will return the IP address and name of the specified TFTP server, and bootfile name to the client.
To configure the IP address and name of the TFTP server and the bootfile name in the DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
Specify the TFTP server |
tftp-server ip-address ip-address |
Optional Not specified by default |
|
Specify the name of the TFTP server |
tftp-server domain-name domain-name |
Optional Not specified by default |
|
Specify the bootfile name |
bootfile-name bootfile-name |
Optional Not specified by default |
2.5.10 Configuring Self-Defined DHCP Options
By configuring self-defined DHCP options, you can
l Define new DHCP options. New configuration options will come out with DHCP development. To support these new options, you can add them into the attribute list of the DHCP server.
l Expand existing DHCP options. When the current DHCP options cannot meet the customer’s requirements (for example, you cannot use the dns-list command to configure more than eight DNS server addresses), you can expand these options.
To configure a self-defined DHCP option in the DHCP address pool, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter DHCP address pool view |
dhcp server ip-pool pool-name |
— |
|
Configure a self-defined DHCP option |
option code { ascii ascii-string | hex hex-string&<1-16> | ip-address ip-address&<1-8> } |
Required No DHCP option is configured by default |
Table 2-1 Description of common options
|
Option |
Corresponding Name in RFC |
Corresponding command |
Command parameter |
|
3 |
Router Option |
gateway-list |
ip-address |
|
6 |
Domain Name Server Option |
dns-list |
ip-address |
|
15 |
Domain Name |
domain-name |
ascii |
|
44 |
NetBIOS over TCP/IP Name Server Option |
nbns-list |
ip-address |
|
46 |
NetBIOS over TCP/IP Node Type Option |
netbios-type |
hex |
|
58 |
Renewal (T1) Time Value |
expired |
hex |
|
59 |
Rebinding (T2) Time Value |
expired |
hex |
|
66 |
TFTP server name |
tftp-server |
ascii |
|
67 |
Bootfile name |
bootfile-name |
ascii |
Configure self-defined DHCP options with caution because the configuration of these options may affect DHCP operation.
2.6 Configuring the DHCP Server Security Functions
This configuration is necessary to secure DHCP services on the DHCP server.
2.6.1 Configuration Prerequisites
Before this configuration, complete the following tasks on the DHCP server:
l Enable DHCP
l Configure the DHCP address pool
2.6.2 Enabling Unauthorized DHCP Server Detection
There are unauthorized DHCP servers on networks, which reply DHCP clients with wrong IP addresses.
With this feature enabled, when receiving a DHCP message with the siaddr field not being 0 from a client, the DHCP server will record the value of the siaddr field in the message and the receiving interface. The administrator can use this information to check out any DHCP unauthorized servers.
To enable unauthorized DHCP server detection, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enable unauthorized DHCP server detection |
dhcp server detect |
Required Disabled by default |
& Note:
With the unauthorized DHCP server detection enabled, the device puts a record once for each DHCP server. The administrator needs to find unauthorized DHCP servers from the log information.
2.6.3 Configuring IP Address Conflict Detection
To avoid IP address conflicts, the DHCP server checks whether the address to be assigned is in use via sending ping packets.
The DHCP server pings the IP address to be assigned using ICMP. If the server gets a response within the specified period, the server will ping another IP address; otherwise, the server will ping the IP addresses once again until the specified number of ping packets are sent. If still no response, the server will assign the IP address to the requesting client (The DHCP client probes the IP address by sending gratuitous ARP packets).
To configure IP address conflict detection, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Specify the number of ping packets |
dhcp server ping packets number |
Optional One ping packet by default. The value “0” indicates that no ping operation is performed. |
|
Configure a timeout waiting for ping responses |
dhcp server ping timeout milliseconds |
Optional 500 ms by default. The value “0” indicates that no ping operation is performed. |
2.7 Enabling the DHCP Sever to Handle Option 82
If the server is configured to ignore Option 82, it assigns an IP address to the client without adding Option 82 in the response message.
I. Configuration prerequisites
Before performing this configuration, complete the following tasks on the DHCP server:
l Enable DHCP
l Configure the DHCP address pool
II. Configuration procedure
Follow these steps to enable the DHCP server to handle Option 82:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enable the DHCP server to handle Option 82 |
dhcp server relay information enable |
Optional Enabled by default. |
& Note:
To support Option 82, it is required to perform related configurations on both the DHCP server and relay agent. For related configuration on the DHCP relay agent, refer to 3.2.7 “Configuring the DHCP Relay Agent to Support Option 82”.
2.8 Displaying and Maintaining the DHCP Server
|
To do… |
Use the command… |
Remarks |
|
Display information about IP address conflicts |
display dhcp server conflict { all | ip ip-address } |
Available in any view |
|
Display information about lease expiration |
display dhcp server expired { ip ip-address | pool [ pool-name ] | all } |
|
|
Display information about assignable IP addresses |
display dhcp server free-ip |
|
|
Display IP addresses excluded from dynamic allocation in the DHCP address pool |
display dhcp server forbidden-ip |
|
|
Display information about bindings |
display dhcp server ip-in-use { ip ip-address | pool [ pool-name ] | all } |
|
|
Display information about DHCP server statistics |
display dhcp server statistics |
|
|
Display information about the address pool tree organization |
display dhcp server tree { pool [ pool-name ] | all } |
|
|
Clear information about IP address conflicts |
reset dhcp server conflict { all | ip ip-address } |
Available in user view |
|
Clear information about dynamic bindings |
reset dhcp server ip-in-use { ip ip-address | pool [ pool-name ] | all } |
|
|
Clear information about DHCP server statistics |
reset dhcp server statistics |
& Note:
Using the save command does not save DHCP server lease information. Therefore, when the system boots up or the reset dhcp server ip-in-use command is executed, no lease information will be available in the configuration file. In this case, the server will deny the request for lease extension from a client and the client needs to request an IP address again.
2.9 DHCP Server Configuration Examples
2.9.1 Static IP Address Assignment Configuration Example
I. Network requirements
Switch B (DHCP client) obtains a static IP address, DNS server address, and gateway address from Switch A (DHCP server).
II. Network diagram
Figure 2-1 Network diagram for static IP address assignment
III. Configuration procedure
1) Configure the IP address of VLAN-interface 2 on Switch A.
<SwitchA> system-view
[SwitchA] interface vlan-interface 2
[SwitchA-Vlan-interface2] ip address 10.1.1.1 25
[SwitchA-Vlan-interface2] quit
2) Configure the DHCP server
# Enable DHCP.
[SwitchA] dhcp enable
# Create DHCP address pool 0, configure a static IP-MAC binding, DNS server and gateway in it.
[SwitchA] dhcp server ip-pool 0
[SwitchA-dhcp-pool-0] static-bind ip-address 10.1.1.5
[SwitchA-dhcp-pool-0] static-bind mac-address 000f-e200-0002
[SwitchA-dhcp-pool-0] dns-list 10.1.1.2
[SwitchA-dhcp-pool-0] gateway-list 10.1.1.126
[SwitchA-dhcp-pool-0] quit
2.9.2 Dynamic IP Address Assignment Configuration Example
I. Network requirements
l The DHCP server (Switch A) assigns IP address to clients in subnet 10.1.1.0/24, which is subnetted into 10.1.1.0/25 and 10.1.1.128/25.
l The IP addresses of VLAN interfaces 1 and 2 on Switch A are 10.1.1.1/25 and 10.1.1.129/25 respectively.
l In the address pool 10.1.1.0/25, the address lease duration is ten days and twelve hours, domain name aabbcc.com, DNS server address 10.1.1.2/25, gateway 10.1.1.126/25, and WINS server 10.1.1.4/25.
l In the address pool 10.1.1.128/25, the address lease duration is five days, domain name aabbcc.com, DNS server address 10.1.1.2/25, and gateway address 10.1.1.254/25, and there is no WINS server address.
l The domain name and DNS server address on the subnets 10.1.1.0/25 and 10.1.1.128/25 are the same. Therefore, a domain name and DNS server address can be configured only for the subnet 10.1.1.0/24, and the subnet 10.1.1.0/25 and 10.1.1.128/25 can inherit the configuration of the subnet 10.1.1.0/24.
& Note:
In this example, the number of requesting clients connected to VLAN-interface 1 should not exceed 122, and that of clients connected to VLAN-interface 2 should not exceed 124.
II. Network diagram
III. Configuration procedure
1) Specify VLAN interfaces and IP addresses for VLAN interfaces (omitted).
2) Configure the DHCP server
# Enable DHCP
<Sysname> system-view
[Sysname] dhcp enable
# Exclude IP addresses (addresses of the DNS server, WINS server and gateways).
[Sysname] dhcp server forbidden-ip 10.1.1.2
[Sysname] dhcp server forbidden-ip 10.1.1.4
[Sysname] dhcp server forbidden-ip 10.1.1.126
[Sysname] dhcp server forbidden-ip 10.1.1.254
# Configure DHCP address pool 0 (address range, client domain name, and DNS server address).
[Sysname] dhcp server ip-pool 0
[Sysname-dhcp-pool-0] network 10.1.1.0 mask 255.255.255.0
[Sysname-dhcp-pool-0] domain-name aabbcc.com
[Sysname-dhcp-pool-0] dns-list 10.1.1.2
[Sysname-dhcp-pool-0] quit
# Configure DHCP address pool 1 (address range, gateway, lease duration, and WINS server).
[Sysname] dhcp server ip-pool 1
[Sysname-dhcp-pool-1] network 10.1.1.0 mask 255.255.255.128
[Sysname-dhcp-pool-1] gateway-list 10.1.1.126
[Sysname-dhcp-pool-1] expired day 10 hour 12
[Sysname-dhcp-pool-1] nbns-list 10.1.1.4
[Sysname-dhcp-pool-1] quit
# Configure DHCP address pool 2 (address range, gateway, and lease duration).
[Sysname] dhcp server ip-pool 2
[Sysname-dhcp-pool-2] network 10.1.1.128 mask 255.255.255.128
[Sysname-dhcp-pool-2] expired day 5
[Sysname-dhcp-pool-2] gateway-list 10.1.1.254
2.10 Troubleshooting DHCP Server Configuration
I. Symptom
A client’s IP address obtained from the DHCP server conflicts with another IP address.
II. Analysis
A host on the subnet may have the same IP address.
III. Solution
1) Disconnect the client’s network cable and ping the client’s IP address on another host with a long timeout time to check whether there is a host using the same IP address.
2) If a ping response is received, the IP address has been manually configured on the host. Execute the dhcp server forbidden-ip command on the DHCP server to exclude the IP address from dynamic allocation.
3) Connect the client’s network cable. Release the IP address and obtain another one on the client. Take WINDOW XP as an example, run cmd to enter into DOS window. Type ipconfig/release to relinquish the IP address and then ipconfig/renew to obtain another IP address.
Chapter 3 DHCP Relay Agent Configuration
When configuring the DHCP relay agent, go to these sections for information you are interested in:
l Introduction to DHCP Relay Agent
l Configuring DHCP Relay Agent
l Displaying and Maintaining DHCP Relay Agent Configuration
l DHCP Relay Agent Configuration Example
l Troubleshooting DHCP Relay Agent Configuration
The DHCP relay agent configuration is supported only on VLAN interfaces.
3.1 Introduction to DHCP Relay Agent
3.1.1 Application Environment
Since DHCP clients request IP addresses via broadcast messages, the DHCP server and clients must be on the same subnet. Therefore, a DHCP server must be available on each subnet. It is not practical.
DHCP relay agent solves the problem. Via a relay agent, DHCP clients communicate with a DHCP server on another subnet to obtain configuration parameters. Thus, DHCP clients on different subnets can contact the same DHCP server for ease of centralized management and cost reduction.
3.1.2 Fundamentals
Figure 3-1 shows a typical application of the DHCP relay agent.
Figure 3-1 DHCP relay agent application
No matter whether a relay agent exists or not, the DHCP server and client interact with each other in a similar way (see section 1.2.2 “Dynamic IP Address Allocation Procedure”). The following describes the forwarding process on the DHCP relay agent.
Figure 3-2 DHCP relay agent work process
As shown in the figure above, the DHCP relay agent works as follows:
1) After receiving a DHCP-DISCOVER or DHCP-REQUEST broadcast message from a DHCP client, the DHCP relay agent fills the giaddr field of the message with its IP address and forwards the message to the designated DHCP server in unicast mode.
2) Based on the giaddr field, the DHCP server returns an IP address and other configuration parameters to the relay agent, which conveys them to the client.
3.2 Configuring DHCP Relay Agent
3.2.1 DHCP Relay Agent Configuration Task List
Complete the following tasks to configure the DHCP relay agent:
|
Task |
Remarks |
|
Required |
|
|
Required |
|
|
Required |
|
|
Configuring the DHCP Relay Agent to Send a DHCP-Release Request |
Optional |
|
Optional |
|
|
Optional |
3.2.2 Enabling DHCP
Enable DHCP before performing other DHCP-related configurations.
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enable DHCP |
dhcp enable |
Required Disabled by default |
3.2.3 Enabling the DHCP Relay Agent on Interfaces
With this task completed, upon receiving a DHCP request from an enabled interface, the relay agent will forward the request to a DHCP server for address allocation.
To enable the DHCP relay agent on interfaces, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter VLAN interface view |
Interface interface-type interface-number |
— |
|
Enable the DHCP relay agent on the current interface |
dhcp select relay |
Required With DHCP enabled, interfaces work in the DHCP server mode. |
& Note:
l If the DHCP client obtains an IP address via the DHCP relay agent, the address pool of the subnet which the IP address of the DHCP relay agent belongs to must be configured on the DHCP server. Otherwise, the DHCP client cannot obtain a correct IP address.
l When the DHCP relay agent receives a request from a DHCP client, it adds its primary IP address into the request before forwarding the packet to the DHCP server. Then, the DHCP server assigns an IP address on the same network segment as the added IP address to the client.
l When receiving a request of a DHCP client from a DHCP relay agent, a DHCP server assigns an IP address on the same network segment as the primary IP address (instead of a secondary IP address) of the DHCP relay agent to the DHCP client.
3.2.4 Correlating a DHCP Server Group with Relay Agent Interfaces
To improve reliability, you can specify several DHCP servers as a group on the DHCP relay agent and correlate a relay agent interface with the server group. When the interface receives requesting messages from clients, the relay agent will forward them to all the DHCP servers of the group.
To correlate a DHCP server group with relay agent interfaces, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Specify a DHCP server group number and servers in the group |
dhcp relay server-group group-id ip ip-address |
Required Not specified by default |
|
Enter VLAN interface view |
interface interface-type interface-number |
— |
|
Correlate the DHCP server group with the current interface |
dhcp relay server-select group-id |
Required By default, no interface is correlated with any DHCP server group. |
& Note:
l You can specify at most twenty DHCP server groups on S9500 series routing switches.
l Up to eight DHCP server addresses can be configured for each DHCP server group. The IP addresses of DHCP servers that belong to the DHCP server group and those of relay agent’s interfaces cannot be on the same subnet. Otherwise, the client cannot obtain an IP address.
l A DHCP server group can correlate with one or multiple DHCP relay agent interfaces, while a relay agent interface can only correlate with one DHCP server group. Using the dhcp relay server-select command repeatedly overwrites the previous configuration. However, if the specified DHCP server group does not exist, the interface still uses the previous correlation.
l The group-id in the dhcp relay server-select command was specified by the dhcp relay server-group command.
3.2.5 Configuring the DHCP Relay Agent to Send a DHCP-Release Request
Sometimes, you need to release a client’s IP address manually on the DHCP relay agent. With this task completed, the DHCP relay agent can actively send a DHCP-RELEASE request that contains the client’s IP address to be released. Upon receiving the DHCP-RELEASE request, the DHCP server then releases the IP address for the client.
With this feature enabled in system view, the DHCP-RELEASE request will be sent to those DHCP servers correlated with the DHCP relay agent interfaces.
To configure the DHCP relay agent in system view to send a DHCP-RELEASE request, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Configure the DHCP relay agent in system view to send a DHCP-RELEASE request |
dhcp relay release ip client-ip |
Required |
3.2.6 Configuring the DHCP Relay Agent Security Functions
I. Create static bindings and enable invalid IP address check
The DHCP relay agent dynamically records clients’ IP-to-MAC bindings to generate a dynamic binding after clients got IP addresses. It also supports static binding, which means you can manually configure IP-to-MAC bindings on the DHCP relay agent, so that users can access external network using fixed IP addresses.
For avoidance of invalid IP address configuration, you can configure the DHCP relay agent to check whether a requesting client’s IP and MAC addresses match a binding on it (both dynamic and static bindings). If not, the client cannot access outside networks via the DHCP relay agent.
To create a static binding and enable invalid IP address check, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Create a static binding |
dhcp relay security static ip-address mac-address |
Optional No static binding is created by default |
|
Enter VLAN interface view |
interface vlan-interface interface-number |
— |
|
Enable invalid IP address check |
dhcp relay address-check { enable | disable } |
Required Disabled by default |
& Note:
l The dhcp relay address-check enable command is independent of other commands of the DHCP relay agent. That is, the invalid address check takes effect when this command is executed, regardless of whether other commands are used.
l You are recommended to configure IP address check on the interface enabled with the DHCP relay agent; otherwise, legal DHCP clients may not be capable of accessing networks.
II. Configure dynamic binding update interval
Via the DHCP relay agent, a DHCP client sends a DHCP-RELEASE unicast message to the DHCP server to relinquish its IP address. In this case the DHCP relay agent simply conveys the message to the DHCP server, thus it does not remove the IP address from its bindings. To solve this, the DHCP relay agent can update dynamic bindings at a specified interval.
The DHCP relay agent uses the IP address of a client and the MAC address of the its interface to regularly send a DHCP-REQUEST message to the DHCP server.
l If the server returns a DHCP-ACK message or does not return any message within a specified interval, which means the IP address is assignable now, the DHCP relay agent will age out the entry of the IP address.
l If the server returns a DHCP-NAK message, which means the IP address is still in use, the relay agent will not age it out.
To configure dynamic binding update interval, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Configure binding update interval |
dhcp relay security tracker { interval | auto } |
Optional auto by default (auto interval is calculated by the relay agent according to the number of bindings) |
& Note:
A large number of binding entries may result in a slow refreshing speed, so you are recommended to use the default refreshing interval.
III. Enable unauthorized DHCP servers detection
There are invalid DHCP servers on networks, which reply DHCP clients with wrong IP addresses. These invalid DHCP servers are unauthorized DHCP servers.
With this feature enabled, upon receiving a DHCP message with the siaddr field (IP address of the server assigning IP addresses to clients) not being 0 from a client, the DHCP relay agent will record the value of the siaddr field and the information on the interface receiving the DHCP message. The administrator can use this information to check out any DHCP unauthorized servers.
To enable unauthorized DHCP server detection, use the following commands:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enable unauthorized DHCP server detection |
dhcp relay server-detect |
Required Disabled by default |
& Note:
With the unauthorized DHCP server detection enabled, the device puts a record once for each DHCP server. The administrator needs to find unauthorized DHCP servers from the log information. After the recorded information of a DHCP server is cleared, a new record will be put for the DHCP server.
3.2.7 Configuring the DHCP Relay Agent to Support Option 82
I. Introduction to Option 82
Option 82 is the relay agent option in the Options field of the DHCP message. It records the location information of the DHCP client. When a DHCP relay agent receives a client’s request, it adds Option 82 to the request message and then forwards the message to the DHCP server, so that the administrator can locate the DHCP client to further implement security control and accounting.
Option 82 involves at most 255 sub-options. At least one sub-option must be defined. Now S9500 series routing switches support two sub-options: sub-option 1 (Circuit ID) and sub-option 2 (Remote ID).
Option 82 has no unified definition. Its padding formats vary with vendors. Currently the device supports two padding formats: normal and verbose.
1) Normal padding format
The padding contents for sub-options in the normal padding format are as follows:
l sub-option 1: Padded with the VLAN ID and interface number of the interface that received the client’s request. The following figure gives its format. The content in the brackets is the fixed value of the field.
Figure 3-3 Sub-option 1 in normal padding format
l sub-option 2: Padded with the MAC address of the interface that received the client’s request. The following figure gives its format. The content in the brackets is the fixed value of the field.
Figure 3-4 Sub-option 2 in normal padding format
2) Verbose padding format
The padding contents for sub-options in the verbose padding format are as follows:
l sub-option 1: Padded with the user-specified access node identifier, and the type, number, PVC identifier (used when the interface type is ATM), and VLAN ID of the interface that received the client’s request. Its format is shown in the following figure.
Figure 3-5 Sub-option 1 in verbose padding format
& Note:
In the above figure, except that the VLAN ID field has a fixed length of 2 bytes, all the other fields of sub-option 1 are length variable.
sub-option 2: Padded with the MAC address of the DHCP relay agent interface or the MAC address of the DHCP snooping device that received the client’s request. It has the same format as normal padding format, as shown in Figure 3-4.
II. Handling strategies for Option 82 on the relay agent
If the DHCP relay agent supports Option 82, it will handle a client’s request according to the contents defined in Option 82, if any. The handling strategies are described in the table below.
If a reply returned by the DHCP server contains Option 82, the DHCP relay agent will remove Option 82 before forwarding the reply to the client.
|
If a client’s request message has… |
Handling strategy |
Padding format |
The DHCP relay agent will… |
|
Option 82 |
Drop |
— |
Drop the message. |
|
Keep |
— |
Forward the message without changing Option 82. |
|
|
Replace |
Normal |
Forward the message after replacing the original Option 82 with the Option 82 padded in normal format. |
|
|
Verbose |
Forward the message after replacing the original Option 82 with the Option 82 padded in verbose format. |
||
|
no Option 82 |
— |
Normal |
Forward the message after adding the Option 82 padded in normal format. |
|
— |
Verbose |
Forward the message after adding the Option 82 padded in verbose format. |
III. Prerequisites
You need to complete the following tasks before configuring the DHCP relay agent to support Option 82.
l Enabling DHCP
l Enabling the DHCP relay agent on the specified interface
l Correlating a DHCP server group with relay agent interfaces
IV. Configuring the DHCP relay agent to support Option 82
Use the following commands for this configuration:
|
To do… |
Use the command… |
Remarks |
|
Enter system view |
system-view |
— |
|
Enter VLAN interface view |
interface vlan-interface interface-number |
— |
|
Enable the relay agent to support Option 82 |
dhcp relay information enable |
Required Disabled by default |
|
Configure the handling strategy for requesting messages containing Option 82 |
dhcp relay information strategy { drop | keep | replace } |
Optional replace by default |
|
Configure the padding format for Option 82 |
dhcp relay information format { normal | verbose [ node-identifier { mac | sysname | user-defined node-identifier } ] } |
Optional normal by default |
& Note:
l To support Option 82, it is required to perform related configuration on both the DHCP server and relay agent. Refer to 2.7 “Enabling the DHCP Sever to Handle Option 82” for DHCP server configuration information.
l If the handling strategy of the DHCP relay agent is configured as replace, you need to configure a padding format for Option 82. If the handling strategy is keep or drop, you need not configure any padding format.
l If sub-option 1 (node identifier) of Option 82 is padded with the device name (sysname) of a node, the device name must contain no spaces. Otherwise, the DHCP relay agent will drop the message.
3.3 Displaying and Maintaining DHCP Relay Agent Configuration
|
To do… |
Use the command… |
Remarks |
|
Display information about DHCP server groups correlated to a specified or all interfaces |
display dhcp relay { interface interface-type interface-number | all } |
Available in any view |
|
Display information about bindings of DHCP relay agents |
display dhcp relay security [ dynamic | static | ip-address ] |
Available in any view |
|
Display statistics information about bindings of DHCP relay agents |
display dhcp relay security statistics |
Available in any view |
|
Display information about the refreshing interval for entries of dynamic IP-to-MAC bindings |
display dhcp relay security tracker |
Available in any view |
|
Display information about the configuration of a specified or all DHCP server groups |
display dhcp relay server-group { group-id | all } |
Available in any view |
|
Display packet statistics on relay agent |
display dhcp relay statistics [ server-group { group-id | all } ] |
Available in user view |
|
Clear packet statistics from relay agent |
reset dhcp relay statistics [ server-group group-id ] |
Available in user view |
3.4 DHCP Relay Agent Configuration Example
I. Prerequisites
l VLAN-interface 1 on the DHCP relay agent (Switch A) connects to the network where DHCP clients reside. The IP address of VLAN-interface 1 is 10.10.1.1/24 and IP address of VLAN-interface 2 is 10.1.1.2/24.
l The IP address of the DHCP server 10.1.1.1/24.
l Switch A forwards messages between DHCP clients and the DHCP server, so that the DHCP client can obtain an IP address of the network segment 10.10.1.0/24 and related configuration information from the DHCP server.
II. Network requirements
Enable Switch A to forward DHCP messages so that the DHCP clients can obtain IP addresses and corresponding configuration information from the DHCP server.
III. Network diagram
Figure 3-6 Network diagram for DHCP relay agent
IV. Configuration procedure
# Enable DHCP.
<Sysname> system-view
[Sysname] dhcp enable
# Enable the DHCP relay agent on VLAN-interface 1.
[Sysname] interface vlan-interface 1
[Sysname-Vlan-interface1] dhcp select relay
[Sysname-Vlan-interface1] quit
# Configure DHCP server group 1 with the DHCP server 10.1.1.1, and correlate the DHCP server group 1 with VLAN-interface 1.
[Sysname] dhcp relay server-group 1 ip 10.1.1.1
[Sysname] interface vlan-interface 1
[Sysname-Vlan-interface1] dhcp relay server-select 1
& Note:
Performing the configuration on the DHCP server is also required to guarantee the client-server communication via the relay agent. Refer to 2.9 “DHCP Server Configuration Examples” for DHCP server configuration information.
3.5 Troubleshooting DHCP Relay Agent Configuration
I. Symptom
DHCP clients cannot obtain any configuration parameters via the DHCP relay agent.
II. Analysis
Some problems may occur with the DHCP relay agent or server configuration. Enable debugging and execute the display command on the DHCP relay agent to view the debugging information and interface state information for locating the problem.
III. Solution
Check that:
l The DHCP is enabled on the DHCP server and relay agent.
l The address pool on the same subnet where DHCP clients reside is available on the DHCP server.
l The routes between the DHCP server and DHCP relay agent are reachable.
l The relay agent interface connected to DHCP clients is correlated with correct DHCP server group and IP addresses for the group members are correct.
