eG Monitoring
 

Measures reported by NSdhcpTest

DHCP provides an automated way to distribute and update IP addresses and other configuration information on a network. When you deploy Dynamic Host Configuration Protocol (DHCP) servers on your network, you can automatically provide client computers and other TCP/IP based network devices with valid IP addresses. You can also provide the additional configuration parameters these clients and devices need, called DHCP options, that allow them to connect to other network resources, such as DNS servers, WINS servers, and routers.

DHCP is a client-server technology that allows DHCP servers to assign, or lease, IP addresses to computers and other devices that are enabled as DHCP clients. With DHCP, you can do the following:

  • Lease IP addresses for a specific amount of time to DHCP clients, and then automatically renew the IP addresses when the client requests a renewal.

  • Update DHCP client parameters automatically by changing a server or scope option at the DHCP server rather than performing this action individually on all DHCP clients.

  • Reserve IP addresses for specific computers or other devices so that they always have the same IP address and also receive the most up-to-date DHCP options.

  • Exclude IP addresses or address ranges from distribution by the DHCP server so that these IP addresses and ranges can be used to statically configure servers, routers, and other devices that require static IP addresses.

  • Provide DHCP services to many subnets, if all routers between the DHCP server and the subnet for which you want to provide service are configured to forward DHCP messages.

  • Configure the DHCP server to perform DNS name registration services for DHCP clients.

  • Provide multicast address assignment for IP-based DHCP clients.

A DHCP client initiates a conversation with a DHCP server when it is seeking a new lease, renewing a lease, rebinding, or restarting. The DHCP conversation consists of a series of DHCP messages passed between the DHCP client and DHCP servers. In an environment where the Citrix NetScaler VPX/MDX is deployed, the DHCP client-server communication happens via the NetScaler appliance. Whenever communication between a DHCP client and a DHCP server suffers a setback, say for example, a DHCP client cannot renew the acquired lease as the DHCP server that provided the original lease is offline, then, the NetScaler appliance is required to initiate a communication between the DHCP client and another DHCP server. If the DHCP client has to wait for a longer period to acquire a lease, then the DHCP client jmay move out of the network by releasing its lease. If too many DHCP clients face difficulty in acquiring a lease or renewing the lease, then severe bottlenecks will be detected in the communication between the DHCP client and DHCP server. To proactively detect such communication failure, administrators can use the NSdhcpTest test!

This test scans the syslog file for the communication between a DHCP client and DHCP server via the NetScaler and reports the number of times the DHCP server has sent an invalid setting in lieu of a DHCP request; the number of times a lease was acquired and released by a DHCP client and the number of times a Policy Based Routing policy is still dependent on a leased IP even after the release of the lease. Using this test, administrators can analyze the communication between the DHCP server and DHCP client, identify if there are any performance bottlenecks between the DHCP client and server and rectify the same.

For this test to run and report metrics, the NetScaler device should be configured to create a Syslog file in a remote Syslog server, where the details of all interactions with the NetScaler appliance will be logged. To know how to configure the Syslog server where this Syslog file should be created, Click here.

Outputs of the test : One set of results for the NetScaler appliance being monitored

The measures made by this test are as follows:

Measurement Description Measurement Unit Interpretation
DHCP_send_invalid Indicates the number of times the DHCP client acquired a lease. Number A DHCP-enabled client obtains a lease for an IP address from a DHCP server. Before the lease expires, the DHCP server must renew the lease for the client or the client must obtain a new lease. Leases are retained in the DHCP server database approximately one day after expiration. This grace period protects a client's lease in case the client and server are in different time zones, their internal clocks are not synchronized, or the client is off the network when the lease expires.
DHCP_acquires_lease Indicates the number of times the DHCP client released a lease. Number The DHCP client acquires a lease for an IP address from the DHCP server and configures its TCP/IP properties by using the DHCP option information in the reply received from the DHCP server, and completes its initialization of TCP/IP. The IP address obtained as lease from the DHCP server remains allocated to the client until the client manually releases the address, or until the lease time expires after which the DHCP server cancels the lease. Mostly, the DHCP client releases a lease when moving to a different network. A high value for this measure therefore may indicate that the DHCP clients are moving away from the network or the DHCP server is currently offline.
DHCP_release_lease Indicates the number of times the DHCP client released a lease while a Policy Based Routing (PPBR) is still dependent on the leased IP. Number PBR is a concept that closely relates to Access Control List (ACL) on a NetScaler appliance. PBR can be leveraged to take routing decision (next hop router) based on certain criteria such as Source IP, Source Port, Destination IP, Destination Port, Protocol, Interface, VLAN and Source MAC. Using PBR, a NetScaler appliance can either ALLOW or DENY access to network packets.
PBR_Dependent Indicates the number of times the DHCP server sent an invalid setting. Number The DHCP client acquires a lease from the DHCP server and configures its TCP/IP properties by using the DHCP option information in the reply received from the DHCP server, and completes its initialization of TCP/IP. In rare cases, a DHCP server might return a negative acknowledgment to the client. This can happen if a client requests an invalid or duplicate address. If a client receives a negative acknowledgment (DHCPNack), the client must begin the entire lease process again.

This measure is a good indicator of a conflict in communication between the DHCP client and DHCP server.