Some clients use the User Datagram Protocol (UDP) for DNS communications. However, UDP
specifies a maximum packet size of 512 bytes. When payload lengths exceed 512 bytes, the
client must use the Transmission Control Protocol (TCP). When a client sends the Citrix®
NetScaler® appliance a DNS query, the appliance forwards the query to one of the name
servers. If the response is too large for a UDP packet, the name server sets the truncation
bit in its response to the NetScaler. The truncation bit indicates that the response is too
large for UDP and that the client must send the query over a TCP connection. The NetScaler
relays the response to the client with the truncation bit intact and waits for the client to
initiate a TCP connection with the IP address of the DNS load balancing virtual server, on
port 53. The client sends the request over a TCP connection. The NetScaler appliance then
forwards the request to the name server and relays the response to the client.
To configure the NetScaler to use the TCP protocol for DNS, you must configure a load
balancing virtual server and services, both of type DNS_TCP. You can configure monitors of
type DNS_TCP to check the state of the services. For instructions on creating DNS_TCP
virtual servers, services, and monitors, see "Load Balancing."
For updating the records proactively, the NetScaler uses a TCP connection to the server to
retrieve the records.
Important: To configure the NetScaler to use UDP for DNS and use TCP only when the
payload length of UDP exceeds 512 bytes, you need to configure DNS and DNS_TCP
services. The IP address of the DNS_TCP service must be same as that of the DNS service.
Some clients use the User Datagram Protocol (UDP) for DNS communications. However, UDP
specifies a maximum packet size of 512 bytes. When payload lengths exceed 512 bytes, the
client must use the Transmission Control Protocol (TCP). When a client sends the Citrix®
NetScaler® appliance a DNS query, the appliance forwards the query to one of the name
servers. If the response is too large for a UDP packet, the name server sets the truncation
bit in its response to the NetScaler. The truncation bit indicates that the response is too
large for UDP and that the client must send the query over a TCP connection. The NetScaler
relays the response to the client with the truncation bit intact and waits for the client to
initiate a TCP connection with the IP address of the DNS load balancing virtual server, on
port 53. The client sends the request over a TCP connection. The NetScaler appliance then
forwards the request to the name server and relays the response to the client.
To configure the NetScaler to use the TCP protocol for DNS, you must configure a load
balancing virtual server and services, both of type DNS_TCP. You can configure monitors of
type DNS_TCP to check the state of the services. For instructions on creating DNS_TCP
virtual servers, services, and monitors, see "Load Balancing."
For updating the records proactively, the NetScaler uses a TCP connection to the server to
retrieve the records.
Important: To configure the NetScaler to use UDP for DNS and use TCP only when the
payload length of UDP exceeds 512 bytes, you need to configure DNS and DNS_TCP
services. The IP address of the DNS_TCP service must be same as that of the DNS service.
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