4.5 -> 5.1, rlogin works but rsh fails

[Jim Hatfield]

I'm trying to use rdist to mirror a tree from a 4.5 machine
("speyburn") to a 5.1 machine ("banff"). So firstoff rsh has to work.
This is root-to-root.

I've set up the /root/.rhosts on the target 5.1 machine and also
on the 4.5 machine so I can check it both ways round.

Logged on as root on the 5.1 machine I can do "rsh speyburn ls"
and get a listing with no problems. But the other way round I
get "rshd: Login incorrect" and the auth.log file on the server
shows:

>Dec 19 14:36:29 banff rshd[74139]: root at speyburn as root: permission denied (authentication error). cmd='ls'

But I can use "rsh banff" and I get logged on with no password
prompt.

It's not tcp wrappers since the hosts.allow is unchanged from
the as-installed version. Both .rhosts files are rw-r--r--.

It's got to be something simple but I can't see it.