In my LAN I have a Windows server and a Synology NAS for storage, directly attached to the server and provided as storage to the clients through a VM. The server provides the domain controller, the DNS server for the main VLAN and the DHCP server functionality. The clients and server are all attached to a switch.

I would like to know that if the server goes offline, which would take the domain, DHCP server and DNS server down, the network would still be available to the clients and the clients can still find the NAS?

If I understand it correctly, devices that already have their IPs assigned would work and find each other, but newly connected devices wouldn't? If so, then the network will continue to work for as long as the IP assignments are still valid?

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    Define available. They have the IP, they have the DNS lookup tables the currencly cache. THAT IS IT. If you enabled broadcast resolution they may find each other - but anything else fails. And the DNS service "times out" when the caching becomes invalid. Which happens FAST. So, no - practically you live on borrowed time. – TomTom Oct 29 '17 at 20:14
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    Since the "server goes offline", so does the VM that provides access to the NAS? If the NAS is not part of the before mentioned LAN, the Client probably won't be able to access it, regardless of DNS and DHCP. But i'm not 100% on what you mean. (VM means Virtual Machine, right?) – sbecker Oct 30 '17 at 12:15
  • @sbecker That's true. But it's my intention of making the NAS storage available as a real NAS, that is by sharing it via SMB directly from the NAS, instead of directly attaching it to the server and have the VM take the responsibility of sharing the network storage to the clients. – EarthMind Oct 31 '17 at 16:37

The network will keep working until your DHCP leases expire. After the leases expire devices may switch to RFC 3927 addresses. But those addresses are not predictable so you'd have to rely on MDNS to find them, and they are unlikely to work between a given pair of devices until both have switched from DHCP assigned addresses to RFC 3927 addresses.

On the DHCP server you can increase the lease time. If this network is behind a NAT you will likely have plenty of addresses and can configure a very high lease time. But it will only work for clients which already had an address.

You probably want to configure the NAS itself with a static IP address. (That will mostly rule out RFC 3927 addresses as fallback, but that wasn't much of a fallback in the first place.)

Unless your clients connect to the NAS by IP address or have the NAS in their hosts file, they will need DNS to find the NAS. The TTL of the DNS record decides for how long they will be able to find the NAS after DNS goes down. But keep in mind that where DHCP is commonly refreshed after half the lease time, DNS records are usually only refreshed once they have already expired. So even with a high TTL the DNS records may expire. And only clients which had cached the name will be able to use it. It is possible that each client machine has its own DNS cache, but it is not guaranteed. You could end up relying on application layer DNS caching, and restarting the application accessing the NAS may lose the cached DNS record.

An alternative I recommend considering if continued operation in case of loss of DHCP and DNS is a must is to use IPv6 link-local addresses. All modern operating system have them turned on by default and devices generally have at least one address that remains static regardless of changes in your network configuration.

  • +1 for your suggestion of using IPv6, most people still only think in IPv4 solutions these days, while IPv6 basically fixes most of the issues present in today's IPv4 networking – Ferrybig Oct 30 '17 at 9:49
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    It's worth noting that "static IP" could mean two things, either IP address is directly assigned to device, or fixed IP address is assigned by DHCP. The latter requires, obviously, DHCP server to work, whereas the former doesn't. – el.pescado Oct 30 '17 at 10:00
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    @el.pescado True. However if the lease time is high it makes less of a difference. Of course if you rely on DHCP to be assigning the static IP address it might give you trouble if you restart the NAS while DHCP is down. – kasperd Oct 30 '17 at 16:42

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