There are several possible causes for DHCP failure on an NGAF (Next-Generation Application Firewall) after a machine restarts and doesn't receive an IP address within an hour. Here are some of the most common causes:

DHCP server is not available: The DHCP server might be down or offline, or there might be network connectivity issues preventing the client from communicating with the server.

IP address conflicts: The DHCP server may have already assigned the IP address to another device, causing a conflict. This can happen if the DHCP server is not configured to check for existing IP address assignments before assigning new addresses.

DHCP server configuration issues: There may be misconfigurations or issues with the DHCP server's settings, such as incorrect IP address ranges, subnet masks, or DNS server settings.

Firewall rules: Firewall rules or policies on the NGAF may be blocking DHCP traffic, preventing the DHCP server from responding to the client's request for an IP address.

To troubleshoot this issue, you can try the following:

Verify network connectivity: Check the connectivity of the client machine to the network and ensure it can reach the DHCP server.

Check DHCP server status: Verify that the DHCP server is running and responding to requests. You can check DHCP server logs to see if there are any errors or warnings.

Check for IP address conflicts: Verify that the IP address range configured on the DHCP server does not overlap with other devices' IP addresses.

Verify DHCP server configuration: Check the DHCP server configuration to ensure that it is correctly configured, including IP address ranges, subnet masks, and DNS server settings.

Check firewall rules: Verify that there are no firewall rules or policies blocking DHCP traffic.

By performing these checks, you should be able to identify the cause of the issue and take corrective actions to resolve it.

it's difficult to determine the exact reason why the machine was unable to obtain an IP address from the DHCP server after a restart. However, flushing the DNS cache may have resolved the issue due to a DNS-related problem that was preventing the machine from communicating with the DHCP server.

When a machine is configured to obtain its IP address from a DHCP server, it sends out a broadcast request on the network to obtain an IP address lease from an available DHCP server. The DHCP server then responds with an offer of an IP address lease, which the machine can accept and use.

One possible reason for the issue you experienced is that the machine was not able to communicate with the DHCP server due to a network connectivity problem or a misconfiguration. This could have prevented the machine from receiving an IP address lease from the DHCP server.

Another possible reason is a DNS-related issue. When a machine attempts to obtain an IP address lease from a DHCP server, it needs to resolve the hostname of the DHCP server to its IP address using DNS. If there was a problem with the DNS resolution, such as an incorrect or outdated DNS cache entry, this could have prevented the machine from communicating with the DHCP server and obtaining an IP address lease.

By flushing the DNS cache, any incorrect or outdated DNS cache entries were cleared, which may have allowed the machine to properly resolve the hostname of the DHCP server and communicate with it to obtain an IP address lease.

If the issue persists, further troubleshooting steps may be necessary to determine the root cause.