Samba 3.6.x changed the security defaults, which affects the smbclient as well. The root cause is that a current ubuntu-like 3.6.3 revision cannot login onto a 3.4.x based samba server.
It always hits an error like this (pretty misleading, as the user exists, and the password is entered correctly).
$ smbclient //share.host/sharename -U youruser Enter youruser's password: session setup failed: NT_STATUS_LOGON_FAILURE $ smbclient -V Version 3.6.3
The source of that can be read on the Samba Changelog
Changed security defaults ------------------------- Samba 3.6 has adopted a number of improved security defaults that will impact on existing users of Samba. client ntlmv2 auth = yes client use spnego principal = no send spnego principal = no The impact of 'client ntlmv2 auth = yes' is that by default we will not use NTLM authentication as a client. This applies to the Samba client tools such as smbclient and winbind, but does not change the separately released in-kernel CIFS client. To re-enable the poorer NTLM encryption set '--option=clientusentlmv2auth=no' on your smbclient command line, or set 'client ntlmv2 auth = no' in your smb.conf
Fixing this for the smbclient is not using the commandline option (does not work for me), but generally within the smb.conf (not within any section like [global], but really global in the first place!)
$ sudo vim /etc/samba/smb.conf client ntlmv2 auth = no
Then we are lucky again.
$ smbclient //share.host/sharename -U youruser
Enter youruser’s password:
Domain=[SHARE] OS=[Unix] Server=[Samba 3.4.x]
Update 2012-08-06: You can also put the complete string “domainusername” in order to compete with the changed auth.