Where is the cache of known MAC addresses stored?

[bibikalka]

Sorry if trivial question - could not find it with a few minutes of search.

If I plug in a rare device that was offline for a while, DHCP will still assign the same IP address to it as last time - many reboots ago.

It sort of seems this info has to be cached somewhere. Any idea?

 

[alan6854321]

It doesn't work like that.
dnsmasq uses a hash of the client's MAC address to choose an IP address for it.
So, unless there's a clash, a client will get the same IP address each time.

See here ...

[OpenWrt Wiki] DHCP and DNS configuration /etc/config/dhcp

openwrt.org

 

[bibikalka]

It doesn't work like that.
dnsmasq uses a hash of the client's MAC address to choose an IP address for it.
So, unless there's a clash, a client will get the same IP address each time.

See here ...

[OpenWrt Wiki] DHCP and DNS configuration /etc/config/dhcp

openwrt.org

OK, this makes sense.

I was chasing a warm reboot bug for Diversion, and changing the IP of the DNS server that did not filter anything fixed the issue:

Diversion - Diversion 5.1.2 - the Router Ad-Blocker, April 21, 2024

I don't know. I was assuming with custom DNS you mean a third party DNS resolver. But you are using a Diversion service which is not custom. Anyway, the excerpt above is the log of Diversion trying to start a separate Dnsmasq instance for the service "Exclude devices from ad-blocking" on IP...

www.snbforums.com

I guess it could be some memory leftover that does not get purged between reboots or something ...

 

[Morris]

It doesn't work like that.
dnsmasq uses a hash of the client's MAC address to choose an IP address for it.
So, unless there's a clash, a client will get the same IP address each time.

See here ...

[OpenWrt Wiki] DHCP and DNS configuration /etc/config/dhcp

openwrt.org

Not only that, some clines request the same IP they previously used.

The answer to your question if there is a list of mac addresses stored, every device has an ARP table has a list of MAC addresses and there associated IP. You can see it using arp -a on many systems.