samba - Oct 2025 - 'LDAP_PROTOCOL_ERROR' when NTLMSSP_NEGOTIATE bind request

On Fri, 24 Oct 2025 11:44:49 +0000
Nicolas Martinussen <nicolas.martinussen at joskin.com> wrote:
> I've found a way to reproduce the issue I have with a small C# code
> executed on Windows. It gets me almost the same packets when I do a
> capture, I also get the 'LDAP_PROTOCOL_ERROR' in the Samba logs,
and
> when I try it against a Windows DC, it works.
> 
> Here is the code:
> 
> using System.DirectoryServices.Protocols;
> using System.Net;
> 
> class Program
> {
>     static void Main()
>     {
>         string ldapServer = "dc-01.ad.mydomain.com";
>         
>         LdapConnection ldap = new LdapConnection(ldapServer);
>         ldap.AuthType = AuthType.Ntlm;
>         ldap.Credential = new NetworkCredential("ldap",
"PASSWORD",
> "MYDOMAIN");
> 
>         ldap.Bind();
>     }
> }
> 
> It seems like AuthType.Ntlm isn't supported by the Samba code. When I
> try the same code with AuthType.Basic, in that case, it works (I get
> the "Strong authentication is required for this operation.", but
> that's normal as I use LDAP and not LDAPS).
> 
I am also fairly certain it is supported, but I am also running out of
ideas. My last idea is, you are using 'ldap at ad.mydomain.com' for the
username, what if it expects the samaccountname ? try it with just
'ldap' (which I presume is an AD username).

Rowland

> > I've found a way to reproduce the issue I have with a small C#
code
> > executed on Windows. It gets me almost the same packets when I do a
> > capture, I also get the 'LDAP_PROTOCOL_ERROR' in the Samba
logs, and
> > when I try it against a Windows DC, it works.
> >
> > Here is the code:
> >
> > using System.DirectoryServices.Protocols;
> > using System.Net;
> >
> > class Program
> > {
> >???? static void Main()
> >???? {
> >???????? string ldapServer = "dc-01.ad.mydomain.com";
> >
> >???????? LdapConnection ldap = new LdapConnection(ldapServer);
> >???????? ldap.AuthType = AuthType.Ntlm;
> >???????? ldap.Credential = new NetworkCredential("ldap",
"PASSWORD",
> > "MYDOMAIN");
> >
> >???????? ldap.Bind();
> >???? }
> > }
> >
> > It seems like AuthType.Ntlm isn't supported by the Samba code.
When I
> > try the same code with AuthType.Basic, in that case, it works (I get
> > the "Strong authentication is required for this operation.",
but
> > that's normal as I use LDAP and not LDAPS).
> >
> 
> I am also fairly certain it is supported, but I am also running out of
> ideas. My last idea is, you are using 'ldap at ad.mydomain.com' for
the
> username, what if it expects the samaccountname ? try it with just
> 'ldap' (which I presume is an AD username).
> 
> Rowland
Yes, 'ldap' is in fact the username I try. I've already tried
'CN=LDAP,CN=Users,CN=AD,CN=MYDOMAIN,CN=com', 'ldap at
ad.mydomain.com', 'MYDOMAIN\ldap' and just 'ldap'.

When I check the packet trace, I don't even see the username in the packet
that result in a TCP RESET from the DC. The username is sent later when I check
the same traffic with a Windows DC. So, what I'm guessing is that for some
reason, this specific bind packet isn't correctly recognized by Samba.
I'm willing to try to do a code debugging to see exactly where there is an
issue, but I didn't find any info about how to debug the Samba code in the
wiki and I have never really used GDB, so I have no idea where to start.

Nicolas