Cannot mount to DFS namespase

[Mimiko <vbvbrj@gmail.com>]

Hello.

I want to mount to a DFS namespase but got error.

In /etc/fstab I use:
//domain.com/op/dir1 /mnt/dir1 cifs 
username=username_cifs,domain=domain.com,password=pass,iocharset=utf8,sec=ntlmi,rw,soft,nocase 
0 0

But I got this error:
mount.cifs kernel mount options: 
ip=IP,unc=\\domain.com\op,iocharset=utf8,sec=ntlmi,soft,nocase,user=username_cifs,,domain=domain.com,prefixpath=dir1,pass=********
mount error(2): No such file or directory

This is one error, removing `prefixpath` resolves the problem. Why 
prefixpath cannot be used even that the user has read access from top?

Another problem is that //domain.com resolves to some IPs related to 
domain controllers, while //domain.com/op resolves to another IPs 
related to /op namespase. Why cifs resolves to domain controllers?

Linux 3.2.0-4-amd64 #1 SMP Debian 3.2.63-2+deb7u1 x86_64 GNU/Linux

Thank you.
--
Mimiko desu.