When a vmknic is already used as "Obtain settings automatically from a VMkernel network adapter", name servers in /etc/resolv.conf is got from DHCP server. Name servers got from DHCP will not persist to config store. If the vmknic's IP keeps as DHCP, every thing works fine. But if user changes the vmknic from DHCP to static IP, the name servers got from DHCP will be missing in /etc/resolv.conf on next reboot.
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Workaround: When a vmknic is already used as "Obtain settings automatically from a VMkernel network adapter", if user wants to change its IP from DHCP to static or remove the vmknic, user needs to go to VC UI, select "Enter settings manually" on "DNS configuration" for default netstack on VC UI.