[jcifs] JCIFS and DFS

[Michael B Allen]

On Wed, Mar 30, 2011 at 4:55 PM, Brent Lowe <brent.lowe at strsoftware.com> wrote:
> A long winded question(s) on DFS and JCIFS.  Essentially, I am looking to
> understand how JCIFS works in a Domain-based DFS system.  I am new to both
> JCIFS and DFS, so please excuse my ignorance if I completely miss the boat.
>
>
>
> Setup:
>
>
>
> Domain:  company.com
>
> DNS servers that resolve to company.com when doing a nslookup on the Linux
> side of the house:
>
>                 server1
>
>                 server2
>
> DFS Namespace server: Server3
>
> DFS Root: \\company.com\dfs
>
> File I would like to retrieve via JCIFS: \\company.com\dfs\test\test.txt
>
>
>
> In Windows:
>
> Going to \\company.com\dfs\test.txt results in the file being accessible.
>
> Going to \\server1\dfs\test.txt results in the file NOT being accessible
>
> Going to \\server2\dfs\test.txt results in the file NOT being accessible
>
> Going to \\server3\dfs\test.txt results in the file being accessible

Hi Brent,

If you use domain based DFS, you're really only supposed to access it
through the domain name. So technicallly I don't think it is an error
that the file is not accessible through the specific file server. But
I also think that it is unusual that it is not accessible. Usually in
this scenario there is a file replication service that makes sure all
servers have the same files. With your setup, it appears that you do
not have replication setup or it's broken.

>
> Utilizing JCIFS to retrieve this file with the example Get.java, is the
> following (loosely) true?   I say loosely because I’m sure there is plenty
> more going behind the scenes, I’m just generalizing.
>
>
>
> JCIFS will resolve strsoftware.com and determine that it’s made up of
> server1 and server2.

A DFS referral for "company.com" will return servers that are
responsible for handling referrals for domain-based DFS in the domain.

A DFS referral to one of those servers for a DFS root will return
servers and paths that makeup the DFS filesystem.

> JCIFS will try to retrieve the file from server1 and fail, return Network
> name not found exception because the file is truly not accessible on that
> server because it is not a namespace server.

Not really. JCIFS will retry if it cannot contact the target server or
if an exception occurs trying to access the share. But otherwise, if
the file is actually not there, JCIFS will not retry another server.

> JCIFS will try to retrieve the file from server2 and fail, return Network
> name not found exception because the file is truly not accessible on that
> server because it is not a namespace server.
>
>
>
> Setup 2:
>
> Domain:  company.com
>
> DNS servers that resolve to company.com when doing a nslookup on the Linux
> side of the house:
>
>                 server1
>
>                 server2
>
> DFS Namespace server: Server3 AND Server2
>
> DFS Root: \\company.com\dfs
>
> File I would like to retrieve via JCIFS: \\company.com\dfs\test\test.txt
>
>
>
> In Windows:
>
> Going to \\company.com\dfs\test.txt results in the file being accessible.
>
> Going to \\server1\dfs\test.txt results in the file NOT being accessible
>
> Going to \\server2\dfs\test.txt results in the file being accessible
>
> Going to \\server3\dfs\test.txt results in the file being accessible
>
>
>
> Utilizing JCIFS to retrieve this file with the example Get.java, is the
> following (loosely) true?  I say loosely because I’m sure there is plenty
> more going behind the scenes, I’m just generalizing.
>
>
>
> JCIFS will resolve strsoftware.com and determine that it’s made up of
> server1 and server2.
>
> JCIFS will try to retrieve the file from Server1 and fail, return Network
> name not found exception because the file is truly not accessible on that
> server because it is not a namespace server.
>
> JCIFS will try to retrieve the file from Server2 and succeed because the
> file is truly accessible from the server as it is a namespace server.
>
>
>
>
>
> The above examples have been my experience and I just want to make sure that
> I’m not missing something.  In the end, for a file to be accessible in this
> situation, at least one of the hosts that are resolved via nslookup or dig
> must be a namespace server.  Correct?

Not sure. If by "namespace server" you mean a server that is returned
in a referral for some DFS point, then JCIFS should try to locate the
file there. But if it's not actually there, JCIFS will not failover
and try the next one in the list. It will just fail to find the file.
It would be nice if it did. But currently I don't think it does. And
again, technically I think all targets should have the same files. I
don't think it's acceptable practice to use DFS to overlay parts of
filesystems into the same namespace. At the very least that would be
rather inefficient and worse it could easily yield inconsistent
results. Without file replication you could have two files with the
same path that actually contained totally different content. That
could lead to some bazaar failures in applications.

Mike

-- 
Michael B Allen
Java Active Directory Integration
http://www.ioplex.com/