DS_FORCE_REDISCOVERY lookup slows ssh logon

Daniel Colascione dancol@dancol.org
Sat Jun 8 06:55:00 GMT 2013

In sec_auth.cc, get_server_groups contains this clause:

  if (get_logon_server (domain, server, false)
      && !get_user_groups (server, grp_list, user, domain)
      && get_logon_server (domain, server, true))
    get_user_groups (server, grp_list, user, domain);

The first call to get_logon_server retrieves cached domain information. We try
to look up user groups based on this information, and if fail to find this group
information (for any reason), we re-query the AD domain, get a new server, and
try again.

get_logon_server is a thin wrapper around DsGetDcName; get_logon_server's third
parameter determines whether we pass the DS_FORCE_REDISCOVERY flag to
DsGetDcName. DsGetDcName's documentation suggests that when doing AD operations,
we first retrieve cached information (by omitting DS_FORCE_REDISCOVERY), try
doing whatever it is that we're going to do, and if we can't reach the domain
controller, ask for another DC name, this time with DS_FORCE_REDISCOVERY, and
having found a better DC, try the operation again.

The problem I'm having is that this rediscover-and-retry step is slowing down my
ssh logons by about three seconds. The DCs on my network (for reasons I don't
understand) reject anonymous connections to PIPE\SAMR, making NetUserGetGroups
fail with ERROR_ACCESS_DENIED. The first call to get_user_groups fails almost
instantaneously, but there's a delay of about three seconds querying the second
server, the one found when we call get_logon_server (domain, server, true), and
this second call also eventually fails with ERROR_ACCESS_DENIED, probably
because the failure is a matter of policy, not of connectivity.

Would it be possible not to make the second call to NetUserGetGroups if the
first fails for a reason that doesn't have anything to do with network
connectivity? The purpose of DS_FORCE_REDISCOVERY seems to be to support
failover, and it doesn't seem useful to try a different server if we
successfully asked the first server and it just happened to say "no".

(By the way: how on earth does logon eventually succeed if group enumeration
fails? I'm using the stored-password authentication method, and when sshd
eventually connects, my user (according to whoami.exe /priv) is a member of the
groups I expect.)

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