Adding the support of reusing machine-wide credentials in the case where the machine is already domain-joined to the LDAP Server.

[joperezr]

Contributes to #23944

This PR will add support of reusing the credentials a machine has stored when it has already domain-joined to an ActiveDirectory from a Linux machine. In order for this to work, you will need to:

• Make sure you have installed package libsasl2-modules-gssapi-mit which will make the sasl interaction to use GSSAPI as the connection mechanism if machine-wide credentials are present.
• Be able to run succesfully ldap tools commands that reuse machine-wide credentials like ldapwhoami -h REALM for example. This will confirm that you have correctly joined the domain with your domain controller and Active Directory.
  After validating that, then people should now simply be able to write something like:

// LdapDirectoryIdentifier directoryIdentifier = ...
using (LdapConnection connection = new LdapConnection(directoryIdentifier))
{
  connection.AuthType = AuthType.Kerberos;
  connection.SessionOptions.ProtocolVersion = 3;
  connection.Bind(); //This will now use GSSAPI to bind to the LDAP Server.

  // Use your ldap connection object
}

I want to add special thanks to @flamencist (co-author of the commit) whose help with both the implementation and ensuring we were able to setup a test environment correctly was vital to getting this PR ready. @flamencist developed a library called ldap4net which is a library that provides ldap functionalities that works cross platform.

cc: @Tratcher @JunTaoLuo @davidsh @bartonjs @ericstj

Assuming this question will be asked on the PR: I haven't found a good way of adding testing for this as it requires a very specific setup which is hard to achieve with our current testing infrastructure. I'm evaluating options to use environments similar to @davidsh enterprise scenarios to add some coverage for this.

[tarekgh]

CC @tquerec

[tarekgh]

why do you need to copy this to a local variable? can't you use interactPtr directly?

[joperezr]

interact structure actually contains several interactchallenges which we loop in the while loop bellow. I guess we could just reuse interctPtr and reassign it but I opted to keep a current in case we ever needed to diagnose some issue with this and needed to know what was the original interactPtr that got passed in

[tarekgh]

how big usually the size of defaults? I am just wondering if we need to use the stackalloc for small sizes to avoid the allocations?

[joperezr]

not big at all, it is just one of these

    [StructLayout(LayoutKind.Sequential)]
    internal struct SaslDefaultCredentials
    {
        public string mech;
        public string realm;
        public string authcid;
        public string passwd;
        public string authzid;
    }

which will usually only have mech and authcid set, with all the other strings set to NULL.

[joperezr]

Any objections with merging this? I'm trying to see if we can move this into preview 5, so it would be good to merge sooner rather than later. If I don't get additional feedback by EOD I'll go ahead and merge and will address anything that comes after that in a follow-up PR.