[Samba] How to disable NTLM authentication on Samba

Gaiseric Vandal gaiseric.vandal at gmail.com
Thu Oct 11 21:14:27 UTC 2018 

How is your sssd settup (sssd.conf) configured?


When someone connects via samba, the underlying linux/unix file system 
routines need to have some what of understanding the windows users and 
groups.   This isn't for authentication  but is instead to make sure 
that the file permissions can be managed and enforced.

My experience - at least when I had classic domain Samba controllers-  
was that the domain controllers did not need winbind added to 
nsswitch.conf but the  member servers did.   If  "getent password" was 
not showing "MYDOMAIN\someuser" then I would likely have problems.





if nsswitch.conf is configured to use sssd, and sssd is configured to 
retrieve account info from the domain server (either via windows or ldap 
) then I would think that "getent passwd" should be listing users.







On 10/11/18 10:13, Reinaldo Souza Gomes wrote:
> There are roughly 20 DC's, spread across multiple different physical 
> locations. It is indeed a replication issue. All of them are windows 
> and we can get authenticated by any of them, randomly. Don't ask me 
> why... they're managed by the "windows' guys"...
>
> I've already tried all sorts of possible combinations for the various 
> NTLM-related parameters and it always fail the same way: if the 
> windows client falls back to NTLM, SSSD can't handle it.
>
> I don't even have wbinfo, as I've not installed winbind.
> "getent passwd" and "getent group" return local users/groups only. 
> "groups domain_user_name" returns the groups for that domain user 
> name. SSH/SFTP logins always succeed.
>
> "net ads testjoin" asks for the machine account's password. Couldn't 
> find a way to use the keytab or change the username for this command. 
> "realm list" show that it's joined. If it wasn't, all of the other 
> logins wouldn't be working.
>
> /etc/nsswitch.conf has 'sss' entries only.
> Em quinta-feira, 11 de outubro de 2018 10:25:38 BRT, Gaiseric Vandal 
> via samba <samba at lists.samba.org> escreveu:
>
>
> Single DC?
>
> If a single DC then there should not be any replication issues - that
> would only be between domain controllers and the event logs would
> indicate that.   I have 2 Windows DC's with a mix of Samba member servers.
>
>
>
> As far as I know, the domain member does not need client NTLM auth to be
> enabled to talk to the DC but I am not 100% sure.  You may want to try
> reenabling it and maybe enabling NTLMv1 for the server auth just to see
> if that makes a difference.  NTLMv1 is not recommended for security
> reasons but it may help identify the problem.
>
>
>
> On my member servers
>
>     # testparm -v | grep -i ntlm
>     ...
>     Loaded services file OK.
>     ...
>     Server role: ROLE_DOMAIN_MEMBER
>
>     Press enter to see a dump of your service definitions
>
>             client NTLMv2 auth = Yes
>             ntlm auth = ntlmv2-only
>             raw NTLMv2 auth = No
>
>
>
> Does "wbinfo -u" and "wbinfo -g" show the domain users and groups?
>
> Does "getent passwd" and "getent group" show the domain users and groups?
>
> Do the  "wbinfo -t" and "net ads testjoin" commands indicate that the
> server has properly joined the domain?   Did you try rejoining the
> server to the domain ?
>
>
> Does your /etc/nsswitch.conf look like
>
>
>
>     passwd: files   winbind
>     group:  files   winbind
>
>
>
> or
>
>     passwd: files   sss
>     group:  files   sss
>
>
>
>
>
> Unless you have ssh, sftp or nfs connections to the samba server from
> either windows or linux clients, there should not be any reason to setup
> SSSD with AD authentication since none of the "unix" type services will
> need it.   Solaris or older linux versions don't even have sssd.
>
>
>
>
> Did you try zapping the winbind and idmap caches ?
>
>
>
> Probably NOT related but I had issues with Windows 10 and SMB3 in the
> past so you may want to try minimizing variables with
>
>         server max protocol = SMB2
>         server min protocol = SMB2
>
>
>
> I haven't had the displeasure of using Windows 8.
>
>
>
>
> On 10/10/18 19:15, Reinaldo Souza Gomes wrote:
> > This issue right here told me exactly what I needed to understand this
> > authentication process:
> > https://pagure.io/SSSD/sssd/issue/3228
> >
> >
> > - The client talks to the DC to try and get a cifs ticket for my samba
> > server's princpal name;
> > - In case the client can't get the ticket for any reason, it falls
> > back to NTLM <- windows client decision, nothing can be done about it
> > by Samba/SSSD;
> >
> > Once I realized this, I investigated the windows machines which
> > couldn't access my Samba server, and I found out that they were
> > authenticating to a DC which didn't receive the replication for the
> > Samba server's machine account (!!!). Therefore the windows client
> > could not get a kerberos tickets for my Samba server, and would
> > eventually fall back to NTLM.
> >
> > Nice. So now I know exactly what's going on. Just have to fix this
> > replication and all will be good.
> >
> > But what if I need to authenticate from a machine where NTLM is the
> > only possibility?
> >
> > I've tried to install the "gssntlmssp.x86_64" package for CentOS, as
> > this post
> > suggests(https://bugzilla.redhat.com/show_bug.cgi?id=963341), but it
> > didn't help.
> >
> > Anyway, as far as Samba goes, I think this is as far as you guys could
> > help me.
> >
> > Thank you for your attention.
> >
> >
> >
> > Em quarta-feira, 10 de outubro de 2018 18:27:50 BRT, Reinaldo Souza
> > Gomes via samba <samba at lists.samba.org 
> <mailto:samba at lists.samba.org>> escreveu:
> >
> >
> >   The domain controler is Windows. The file Server is Linux/Samba. The
> > clients are Windows.
> > I've tested the access on a dozen different windows machines. Three of
> > them used NTLM and failed. All the others used kerberos and succeeded.
> > They're all in the same network, same domain. Maybe it's the windows
> > version? But they're all Window 8 or 10, not a great deal of a
> > difference between them.
> > Those logs are from the Samba server, upon receiving the NTLM
> > authentication attempt.Smbd is version 4.7.1 on CentOS 7.5
> > I've tried a lot of different configurations regarding NTLM on the
> > Samba server. Currently, they're like this:
> > client NTLMv2 auth = noclient lanman auth = nontlm auth =
> > disabledlanman auth = no
> > I thought there could be a way of telling the windows machines
> > something like "Hey, I'm not accepting any kind of NTLM. If you want
> > to access this Samba server, use kerberos!". But I can't find it.
> > Em quarta-feira, 10 de outubro de 2018 18:13:54 BRT, Gaiseric Vandal
> > via samba <samba at lists.samba.org <mailto:samba at lists.samba.org> 
> <mailto:samba at lists.samba.org <mailto:samba at lists.samba.org>>>
> > escreveu:
> >
> > I must be missing something-
> >
> >
> > Are these Windows clients?  Or are these Linux clients authenticating
> > against Samba ?
> >
> >
> > if they were linux clients then yes I could see sssd or other
> > authentication components besides winbind coming into play. And in that
> > case yes you would have sssd work with winbind to enable caching of
> > credentials.
> >
> >
> > Is the event log entry below from the server ?   Is it from the domain
> > controller or a file server?
> >
> >
> > What version of Samba are you running?
> >
> > Are the files servers and domain controllers all Samba or do you have a
> > mix of say Samba file servers with Windows AD servers?
> >
> > The "no logon server" entry looks more relevant. What version of
> > Windows clients.        I think NTLMv2 is supported as far back as NT
> > 4.0 SP6.         Windows 2000 and later should be trying to use kerberos
> > in preference to NTLM.   By chance have you disabled NTLMv2 and only
> > enabled v1?      Are some windows clients failing while others
> > succeeding ?
> >
> >
> >
> >
> >
> >
> >
> >
> > On 10/10/18 16:38, Reinaldo Souza Gomes wrote:
> > > Whenever a client uses kerberos as authentication, it succeeds.
> > >
> > > Whenever a client uses NTLM as authentication, it fails (logs bellow)
> > > since SSSD can't support NTLM. Thus my question: what can I do to
> > > prevent NTLM from being used??
> > >
> > > [2018/10/09 17:49:29.507046,  2]
> > > ../source3/auth/auth.c:332(auth_check_ntlm_password)
> > >   check_ntlm_password:  Authentication for user [MYUSER] -> [MYUSER]
> > > FAILED with error NT_STATUS_NO_LOGON_SERVERS, authoritative=1
> > > [2018/10/09 17:49:29.507074,  2]
> > > ../auth/auth_log.c:760(log_authentication_event_human_readable)
> > >   Auth: [SMB2,(null)] user [MYDOMAIN]\[MYUSER] at [Tue, 09 Oct 2018
> > > 17:49:29.507062 -03] with [NTLMv2] status
> > > [*NT_STATUS_NO_LOGON_SERVERS*] workstation [MACHINENAME] remote host
> > > [ipv4:192.168.1.1:1109] mapped to [MYDOMAIN]\[MYUSER]. local host
> > > [ipv4:10.0.0.1:445]
> > >
> > > Em quarta-feira, 10 de outubro de 2018 17:09:54 BRT, Gaiseric Vandal
> > > via samba <samba at lists.samba.org <mailto:samba at lists.samba.org> 
> <mailto:samba at lists.samba.org <mailto:samba at lists.samba.org>>>
> > escreveu:
> > >
> > >
> > > How would samba forward any requests on to any other service 
> ?       You
> > > can have sssd setup on a server if you also need to support things 
> like
> > > ssh, sftp, and nfs but that is separate from samba's "Windows" 
> services.
> > >
> > > Or do you mean it forwards NTLM requests to a different server ?
> > >
> > >
> > > Disabling NTLM altogether would be a useful feature if you are 
> trying to
> > > minimize the attack surface.
> > >
> > >
> > >
> > >
> > >
> > >
> > > On 10/10/18 15:52, Reinaldo Souza Gomes via samba wrote:
> > > >  Forgive me if I have misundertood your words, but what I want is to
> > > prevent Samba from accepting NTLM(v1, v2, SSP, or whatever) and
> > > forwarding it, since SSSD does not support it. I am not trying to get
> > > SSSD to support any kind of NTLM. So, this would be a Samba issue, not
> > > SSSD's. Isn't that correct?
> > > > Putting it in another words: what can I do (preferrably on the Samba
> > > server) to prevent windows clients from successfully sending NTLM
> > > authentication to my Samba server?    Em quarta-feira, 10 de outubro
> > > de 2018 16:29:28 BRT, Rowland Penny via samba 
> <samba at lists.samba.org <mailto:samba at lists.samba.org>
> > <mailto:samba at lists.samba.org <mailto:samba at lists.samba.org>>
> > > <mailto:samba at lists.samba.org <mailto:samba at lists.samba.org> 
> <mailto:samba at lists.samba.org <mailto:samba at lists.samba.org>>>> escreveu:
> > > >
> > > >  On Wed, 10 Oct 2018 18:50:23 +0000 (UTC)
> > > > Reinaldo Souza Gomes via samba <samba at lists.samba.org 
> <mailto:samba at lists.samba.org>
> > <mailto:samba at lists.samba.org <mailto:samba at lists.samba.org>>
>
> > > <mailto:samba at lists.samba.org <mailto:samba at lists.samba.org> 
> <mailto:samba at lists.samba.org <mailto:samba at lists.samba.org>>>> wrote:
> > > >
> > > >> How can I make sure that NTLM(SSP) will never be used??
> > > >>
> > > >> I’ve set up Samba with SSSD and everything Works fine... except 
> for a
> > > >> few Windows machines which every now and then happen to send NTLM
> > > >> authentication flags to the Samba server, which happily forwards
> > > >> them. And then the authentication fails because SSSD doesn’t 
> support
> > > >> NTLM.
> > > >>
> > > >> I’ve tried all sorts of parameters combination on smb.conf 
> (including
> > > >> "ntlm auth = disabled"), but I didn’t find a way to completely 
> refuse
> > > >> NTLM authentication on the Samba server, and force the client 
> to use
> > > >> another authentication method (kerberos).
> > > > You will have to ask the sssd-users mailing list, you are not using
> > > > Samba for authentication.
> > > >
> > > > sssd isn't a Samba product.
> > > >
> > > > Samba by default no longer uses NTLMv1
> > > >
> > > > Rowland
> > > >
> > >
> > >
> > > --
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> >
> >
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> >
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>
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