I am experimenting with NPR access to AREDN. The NPR master has a fixed IP address that I can reserve in the LAN-side DHCP server. This master has its own DHCP server providing IP addresses to the slave(s) that are in the address range of the LAN. Since I cannot reserve IP addresses without a MAC address it can happen that the AREDN-node DHCP server assigns an IP address which is already used by NPR.
I am, therefore, looking for a feature that lets me just block certain IP addresses so they won't be assigned by the AREDN node. I believe this could simply be achieved by allowing IP address reservation without specifying a MAC address.
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Reserving LAN-side DHCP server addresses
Sat, 12/16/2023 - 09:10
#1
Reserving LAN-side DHCP server addresses
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Normally not, but in this case I don't have a choice. I cannot change the implementation of the NPR modem. Other DHCP servers have the option of blocking a range of IP addresses. This is what I would like to see here as well.
The NPR slaves don't request addresses from the AREDN DHCP server but only from the NPR master which uses a reserved address range.
That's not the way DHCP works. A client device requesting a DHCP address makes a broadcast requesting a DHCP assignment. ANY DHCP server on the network can and will respond. If multiple DHCP servers respond, it's essentially a race to to see who gets to the client first. Slightly simplified answer.
The only exception to this is if a DHCP client already has a DHCP assigned address that needs to be renewed (normally that starts at 50% of the DHCP lease time), the DHCP client MAY (usually, not always) specifically request a DHCP renewal from the DHCP server that initially issued the DHCP assignment.
BTW, this discussion does not cover redundant DHCP servers that operate as a redundant pair - as this does not apply in this case.
This description is correct when you have multiple DHCP servers in the same broadcast domain. But this is not the case in this scenario. The NPR master acts as a kind of DHCP proxy server that only serves the requests from the slaves through its PtMP radio network. It is not a DHCP relay, i.e., it won't forward the requests to the AREDN node's DHCP server. This way there is always exactly one DHCP server serving a particular request. The only difficulty is the fact that they use overlapping address ranges without knowing of each other.
This is a design choice the developer of NPR has made, probably due to the limited processing capabilities of the NPR hardware (basically an Arduino). I am not here to defend this choice but to find a clean way of dealing with this scenario. I am sure that I am not the only one considering NPR as an additional way of providing access to AREDN in NLOS situations so there may be merit to find a generic solution.
73, Wolfgang
In my specific example the AREDN node's LAN address is 10.221.19.129/27. It assigns IP addresses from ...130 to ...158 via DHCP. The NPR master has a fixed reserved address of 10.221.19.130 and assigns addresses ...131 to ...146 via its own DHCP server to its slaves. The slaves' IP addresses have to be part of the AREDN node's LAN network and, thus, in its IP address range in order to communicate with it, but they cannot use the AREDN node's DHCP server. So it has no knowledge about these NPR slave nodes. The node only learns the IP / MAC binding through ARP but this does not affect DHCP.
In order to prevent the AREDN node's DHCP server to assign addresses below ...147 I want to block these without prior knowledge of their MAC addresses, so that the case of duplicate IP addresses on the interface will be avoided.
I hope that this makes the problem slightly clearer.
vy73, Wolfgang
Thanks!
73, Wolfgang