Haven't tried but ifconfig interface down up might (should imho) do the trick..

/me is a linux n00b, but for reference, in windoze you can simply do a: arp -d *

yeah, i tried this in the first place but unfortunately under linux it doesn't work, the manpage to ARP says nothing about any possibility to flush the whole cache, a pity...
but i am happy that i know now what to do to get things working as they should

This is why apps shouldn't set the MAC address if one is already set, like I discussed in the other thread... :)

yes.. read MAC, if <> lotsa 0s, set it, otherwise stay away, noted, got it!

..so an init routine that resets the nic, should read and store the mac first.. hmm but if that succeeds, there's probably no need for resetting the nic... which there probably isn't anyway, but might be good practice nonetheless... hmmmmm

Could someone verify if RR-net's MAC remain set after hard-resetting the computer? (I currently don't have any monitor so I can't try myself). My guessing is that it will be reset...

Hehe... apparently it remembers the MAC even after a power cycle:

Resetting the C64 has no effect on the MAC stored in RR-Net.
Resetting the RR-Net has no effect on the MAC stored in RR-Net.
Powering off for less than 60 seconds while keeping the cart inserted has no effect on the MAC stored in RR-Net on my test system.
Powering off for longer periods WILL reset the MAC to the default broadcast address FF-FF-FF-FF-FF-FF

For how long can YOUR RR-Net hold it's breath?

btw: is there a particular reason why the rr-net's MAC address is changeable at all??! I always thought that each network device has its own absolutely unique hardcoded MAC.

The MAC is usually (practically always) stored in a ROM of some sort and then read into the chip upon init. Some chips, e.g. RTL8019AS iirc, require you to actually emulate an EPROM in order for you to "upload" the MAC address to it. It has no writeable register for the MAC. I'd suspect that many ethernet controllers are designed this way, to simply read the MAC from a ROM.
An ethernet controller with a 6 byte EEPROM or FlashMem would be a nice thing

That's why RR has a 128K MAC prom

hehe.. yeah but unfortunately the interface between RR and the ethernet controller is limited to the Clock Port and thus not able to provide for the ROM functionality