Sun Directory Server, OpenDS, OpenDJ, Port 389 (formerly known as Fedora Directory) at least, do not support Alias dereferencing because the complexity out-weights the benefits, and very few client applications make use of it anyway.

My 2 cents.

Ludovic Poitou

On Mon, Jun 13, 2011 at 4:39 PM, Emmanuel Lecharny <> wrote:
Alias cycle detection

There is an unsolved question about how we should detect Alias cycles. Right now, we check for cycles *before* they are created. The alternative would be to stop any search that could lead to an infinite loop.

A third - but unrealistic - solution would be to don't detect cycle, and process the search until we reach the time or size limit (in other words, it's up to the admin to avoid the creation of such cycle; Highly dangerous…).

The problem with the first approach is that we can't anymore pass the VSLDAP tests. It's a major burden. Also most of the current servers support this feature.

So it seems that everything concurs to get the cycle be detected while searching, not before. Now, how can we do that efficiently ?

Detecting a cycle is a pretty obvious algorithm : every entry we went through should be memorized, and we should check that we aren't going through an entry we already returned.

This is easy, but highly memory consuming, as we have to keep track of *all* the entries during a search. Simply idealistic.

Hopefully, we can use a simplified algorithm : if we hit an Alias, we have to check that we don't come back to the original DN we came from, or one of its descendant. We should also memorize each indirection (each of them is like a new search)

initial search : A.B.C -~-> a(alias)  (A.B.C memorized)
a -~-> B.C… -~-> A.B.C : cycle detected (B.C memorized)


initial search : A.B.C -~-> a(alias)  (A.B.C memorized)
a -~-> D.E… -~-> b(alias) : (D.E memorized)
b -~-> B.C… -~-> A.B.C : cycle detected (B.C memorized)

This way we only have to memorize the roots for all searches, instead of memorize all he entries. As we are very unlikely to have many aliases and many indirection, it should be safe from the memory consumption POV.

One other aspect is that this should be only done if the user has required that we dereference aliases on the server.

Last, not least, if the user has sent the ManageDSAIt control, we should not have to memorize anything either, as we don't dereference aliases in this case.

The DN cache will be stored in the session, and removed as soon as we reached the end of the search. It will be associated with the message ID, so that a user can issue more than one search in parallel.

thoughts ?

Emmanuel Lécharny