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I have some systems connected via a simple stupid ethernet switch. Under usual conditions, the switch is connected to a large network providing DHCP service and Internet gateway. However, if the group gets disconnected from the large network, or the DHCP is down, no adresses are assigned and the subnetwork stays unconfigured. I like to have the subnetwork usable in any case.

Can this be done by a proper network-manager config? Like defining additional manual assigne IPs, that are overridden if DHCP comes available?

Is this a use case for avahi / zeroconf?

Is it possible to keep the DHCP assigned IPs as long as the DHCP is unavailable, even if a system reboots?

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Why don't you work on fixing the conditions where the subnet becomes disconnected. That doesn't seem like a condition that you should be designing around. – MDMarra Feb 16 '13 at 23:05
The large network is not on my responsibility. But the subnetwork is, and it's operation can work without Internet for some time. One of the systems is a server needed by the other stations, so a working subnetwork will enable the use of the stations. So it should not need to depend on the large network. – dronus Feb 17 '13 at 1:01
If the network is small enough that setting manual IP addresses as secondary addresses is viable, why not just set manual IP addresses instead of DHCP completely and have done with it. Other than that your options are pretty much set up your own DHCP server that can serve the local subnet when the main one is unavailable, or as MDMarra says, fix the network issue (while it's not your responsibility, it's someone's right? Make it their problem to get it fixed). – RobM Feb 17 '13 at 12:44

The best solution is to carve up a range of addresses for local use (a subnet), and set up a local DHCP server handling those. As you can get cut off at any time, this is just recognizing the existing situation. Unless the load is large, a machine configured as firewall/router/DHCP server could even be a recycled workstation.

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Instead of a recycled workstation I could replace the switch with a router then.. consumer routers could be configured to obtain internet on one of it's ethernet ports and gateway it I think. Thus the router serves as DHCP server, and as DHCP client and gateway to the large network. – dronus Feb 17 '13 at 13:01
@dronus, the "consumer routers" I've seen tend to lack configurability. – vonbrand Feb 17 '13 at 13:14
Maybe, but their vanilla function is to provide a DHCP server, and many of them have a special ethernet port called 'uplink' or 'WAN' which would be used as gateway to the internet if connected to a DSL modem for example. Those routers would serve this purpose out-of-the-box... – dronus Apr 13 '13 at 10:30

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