An enterprise network administrator has been notified by multiple users that they have been redirected to malicious websites despite entering the correct URLs. Upon investigation, it was discovered that the local DNS server's cache may have been poisoned. Which of the following actions should the network administrator prioritize to mitigate this issue?
Change the local DNS server's cache settings to refuse responses from unauthorized or external sources.
Implement DNSSEC for all DNS queries.
Immediately switch to a different DNS provider.
Flush the DNS cache on all affected machines.