SSH "Host key verification failed"

Same

tux@uml:~$ ssh localhost
Host key verification failed.

Localhost ist the same as 127.0.0.1, I ment the "real" IP address that you also use when you connect from your host. Use the same command that works from host to UML on your UML system (e.g. ssh 192.168.1.2)

Oh sorry. I misunderstood.

host IP = 10.0.1.254
uml IP  = 10.0.1.1

Here are the results
FROM THE UML
==========
* Failed to ssh from uml to uml itself
root@uml:~# ssh tux@10.0.1.1
Host key verification failed.

* Failed to ssh from uml to the host
root@uml:~# ssh tux@10.0.1.254
Host key verification failed.

FROM THE HOST
===========
* Successfull to ssh from host to uml
root@host:~# ssh tux@10.0.1.1    
The authenticity of host '10.0.1.1 (10.0.1.1)' can't be established.
RSA key fingerprint is 91:6e:71:45:3d:61:a4:2e:aa:ce:fd:36:5c:d6:b5:77.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '10.0.1.1' (RSA) to the list of known hosts.
tux@10.0.1.1's password:
tux@uml:~$

* Then successfull to ssh back to the host again ...
tux@uml:~$ ssh 10.0.1.254
The authenticity of host '10.0.1.254 (10.0.1.254)' can't be established.
RSA key fingerprint is ec:80:09:da:90:da:a9:08:14:33:b6:d6:54:10:3d:03.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '10.0.1.254' (RSA) to the list of known hosts.
tux@10.0.1.254's password:
tux@host:~$

I have compiled a new version of openssh (1.3.7p1) in the uml.
Still not working.

ssh can generate debug output. You can add -v flags (up to five) to the ssh command line to get more information about what ssh is doing, and potentially what's wrong:

ssh -v -v -v tux@10.0.1.1

What does this print?

Ughh .... a lot of messages
Here are the last rows.
=============
.....
debug3: check_host_in_hostfile: filename /root/.ssh/known_hosts
debug3: check_host_in_hostfile: filename /etc/ssh/ssh_known_hosts
debug3: check_host_in_hostfile: filename /root/.ssh/known_hosts
debug3: check_host_in_hostfile: filename /etc/ssh/ssh_known_hosts
debug2: no key of type 0 for host 10.0.1.1
debug3: check_host_in_hostfile: filename /root/.ssh/known_hosts2
debug3: check_host_in_hostfile: filename /etc/ssh/ssh_known_hosts2
debug3: check_host_in_hostfile: filename /root/.ssh/known_hosts
debug3: check_host_in_hostfile: filename /etc/ssh/ssh_known_hosts
debug2: no key of type 2 for host 10.0.1.1
error ENOTTY
Host key verification failed.
debug1: Calling cleanup 0x806018c(0x0)
=====================

The "error ENOTTY" was made by me.
I traced the source code, it turned out that
- SSH found that 10.0.0.1 is a new host, so it wanted to confirm
- It called a function named readpassphrase on openbsdcompat/readpassphrase.c
- I found the error is here
=======================
        /*
         * Read and write to /dev/tty if available.  If not, read from
         * stdin and write to stderr unless a tty is required.
         */
        if ((flags & RPP_STDIN) ||
            (input = output = open(_PATH_TTY, O_RDWR)) == -1) {
                if (flags & RPP_REQUIRE_TTY) {
                        errno = ENOTTY;
                        DEBUG("error ENOTTY"); // ERROR HERE
                        return(NULL);
                }
                input = STDIN_FILENO;
                output = STDERR_FILENO;
        }
=======================

So apparently, this is not openssh problem but UML problem.
The mentioned UML's TTY is opened from /etc/inittab like this
=======================
## Open one UML console
0:2345:respawn:/sbin/agetty 115200 console xterm
#1:2345:respawn:/sbin/agetty 38400 tty1 xterm

=======================
 
I'm going to ask this to UML milist.
But please Let me know if you have any ideas.

What is strange is that I did not find any other reports about this problem. Which version of UML are you running? I did find a number of pages that talk about using ssh with UML, so in general it should be possible.

kernel : linux 2.4.22, patched with  uml-patch-2.4.22-8.bz2
rootfs : vector linux 4.0, made by myself (thats the problem LOL).

I found that stock rootfs_slackware8.1 from the UML homepage does not have this problem.

In UML I tried
root:# chmod a+rw /dev/console
still not working.

Just generate the host key:

http://www.ssh.com/support/documentation/online/ssh/adminguide/32/Generating_the_Host_Key.html

then try