[Babel-users] Send: Cannot assign requested address

[Bobby]

The mystery is partly solved. Most importantly, ipv6 and babeld are both 
now working!

The source of the problem lies in ipv6's DAD (duplicate address 
detection). If I understand correctly, when an ipv6 address is assigned, 
dad is supposed to check the network to see if the newly assigned ipv6 
address is already in use elsewhere. For some unknown reason, this does 
not happen. After manually assigning a link-local address it gets 
flagged as tentative until DAD determines it to be unique, but it never 
makes that determination, and therefore the address stays tentative 
forever. I discovered this after issuing the `ip addr` command and 
noticing that the manually assigned link-local address was listed as 
tentative despite having been assigned over 12 hours prior. While marked 
tentative, the address is unusable on that interface, so even though 
ifconfig lists a link-local address, there really isn't actually a 
link-local address. In my brief googling, I was not able to find a way 
to force it out of a tentative state. I encountered this issue on 
Raspberry Pi's running the latest Raspbian Buster, as well as on Ubuntu 
20.04 (running in a VM with access to a physical usb wifi card), so I 
think this may be a more widespread problem as opposed to merely a 
Raspbian peculiarity. I'm quite surprised nobody else has run into this 
issue.

The only solution I could find is to simply disable DAD on the 
interface. This is done by adding |net.ipv6.conf.wlan1.accept_dad=0 to 
the end of /etc/sysctl.conf. After making the change (and rebooting for 
good measure) DAD no longer held the ipv6 address in a tentative state. 
If an address was assigned, it was accepted immediately.|

|I still do not understand why the link-local address is lost and is not 
automatically regenerated when bringing an interface down and back up. 
It remained necessary to use |`sudo ip -6 addr add 
$(generate-ipv6-address fe80::) dev wlan1` in order to give the 
interface the link-local address after it is brought up. I did also run 
across another way to cause generation of the link-local address 
automatically, but it only happens one time and doesn't seem like the 
proper way to achieve this. It is possible to edit 
/proc/sys/net/ipv6/conf/wlan1/addr_gen_mode and change the value from 1 
to 0. This will cause a link-local address to be created for the wlan1 
interface. However, bringing the interface down and back up will clear 
this link-local address again, and it will not automatically reassign 
one unless you change that value from 0 to 1, save, and then change back 
again from 1 to 0.

It is perhaps not ideal, but it is a very quick and easy solution. 
Babeld runs without any complaints and the Pis discover each other as 
neighbors.

||

Thanks,

Bobby