I'm trying to set up radvd on a 6RD configuration (on Linux), where I have a /64 subnet, and I'm trying to use it on two distinct physical networks, so I'm trying to give each of them a /80 subnet. However, when I start radvd on these interfaces, it says this:

radvd[3987]: prefix length should be 64 for int

(int being the name of one of the interfaces.)

I know that stateless autoconfiguration doesn't work on subnets that are larger than /64, like a /48 subnet, but AFAIK it's supposed to work on anything that is smaller than /64, so /80 shouldn't be a problem. The final effect, now, is that radvd simply advertises the wrong subnet prefix, effectively "removing" the first 16 bits of it.

What is this? Am I all wrong on that, is radvd buggy, or is there some configuration option that I'm missing?