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# Configuring the unicast list of NTP servers

If SNTP unicast mode is enabled via the CYGPKG_NET_SNTP_UNICAST option, the SNTP client can be configured with a list of NTP servers to contact for time updates.

By default, this list is configured with NTP server information received from DHCP. The number of NTP servers that are extracted from DHCP can be configured with the CYGOPT_NET_SNTP_UNICAST_MAXDHCP option. This option can also be used to disable DHCP usage entirely.

The list of NTP servers can be manually configured with the following API function. Note that manual configuration will override any servers that were automatically configured by DHCP. But later reconfigurations by DHCP will override manual configurations. Hence it is not recommended to manually configure servers when CYGOPT_NET_SNTP_UNICAST is enabled.

    #include <cyg/sntp/sntp.h>

    void cyg_sntp_set_servers(struct sockaddr *server_list, cyg_uint32 num_servers);

This function takes an array of sockaddr structures specifying the IP address and UDP port of each NTP server to query. Currently, both IPv4 and IPv6 sockaddr structures are supported. The num_servers argument specifies how many sockaddr's are contained in the array. The server_list array must be maintained by the caller. Once the array is registered with this function, it must not be modified by the caller until it is replaced or unregistered by another call to this function.

Calling this function with a server_list of NULL and a num_servers value of 0 unregisters any previously configured server_list array.

Finally, note that if this function is called with a non-empty server list, it will implicitly start the SNTP client if it has not already been started (i.e. it will call cyg_sntp_start()).

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