Access control for DPM storage elements

This page is part of an investigation on How to control access rights for LFC/SRM files .

Finding out how storage is organized

To list the storage systems to which you have access use

 lcg-infosites --vo <YOUR-VO> se

which results in

Avail Space(Kb) Used Space(Kb)  Type   SEs
----------------------------------------------------------
12078           108             n.a    srm.grid.rug.nl
12078           108             n.a    srm.grid.rug.nl
730582644       681194097       n.a    gb-se-amc.amc.nl
8226695519985   23304480014     n.a    srm.grid.sara.nl
605355546       806421195       n.a    gb-se-nki.els.sara.nl
6575746866      20920246        n.a    carme.htc.biggrid.nl
152913518       115521938       n.a    se.grid.rug.nl
248345185       1166074827      n.a    gb-se-ams.els.sara.nl
355230761       1056545980      n.a    gb-se-uu.science.uu.nl
1266740857      145035883       n.a    gb-se-wur.els.sara.nl
337812899       1076607113      n.a    gb-se-kun.els.sara.nl
2195706454      3048365         n.a    tbn18.nikhef.nl
771834491       620488567       n.a    gb-se-lumc.lumc.nl

Note the current version of the lcg-infosites command does not use your grid proxy at all !

You can then use the srmls command to figure out how the storage is organized:

$ srmls srm://tbn18.nikhef.nl
SRMClientV2 : srmLs: try # 0 failed with error
SRMClientV2 : Method 'ns1:srmLs' not implemented: method name or namespace not recognized
SRMClientV2 : srmLs: try again

Ah... this is a SRMv1 interface, which does not support the srmLs command. The SRMv2 interface for DPM installations is on port 8446 by default.

$ srmls srm://tbn18.nikhef.nl:8446/
 0 //
     0 //dpm/

Note the extra slash ('/') at the end of the command!

So yes, we are talking to a DPM server. Subsequent srmls commands then give us:

$ srmls srm://tbn18.nikhef.nl:8446/dpm
 0 /dpm/
     0 /dpm/nikhef.nl/

$ srmls srm://tbn18.nikhef.nl:8446/dpm/nikhef.nl
0 /dpm/nikhef.nl/
    0 /dpm/nikhef.nl/home/

$ srmls srm://tbn18.nikhef.nl:8446/dpm/nikhef.nl/home
 0 /dpm/nikhef.nl/home/
     [SNIP]
     0 /dpm/nikhef.nl/home/pvier/
     0 /dpm/nikhef.nl/home/vlemed/
     [SNIP]

Hey, we are at the VO level now. Here I've listed the two VOs which will be used throughout this page.

Creating your own directory in SRM-space

Before we copy a file to the DPM SRM we first create our own directory. If we do not do this then SRM will store the files in generated directories, over which we have little or no control.

srmmkdir srm://tbn18.nikhef.nl:8446/dpm/nikhef.nl/home/pvier/janjust

The URL for this directory will be used throughout the rest of this page, hence we abbreviate it to

SRM=srm://tbn18.nikhef.nl:8446/dpm/nikhef.nl/home/pvier/janjust

Copying and registering your file

Next we will copy a file to our SRM directory and register it in the LFC in one go:

$ lcg-cr -d $SRM/myfile -l lfn:/grid/pvier/janjust/my-dpm-file file://$PWD/myfile
guid:bbdad839-b2d1-46f6-95ab-5b6561f7e72f

which returns the LFC GUID for the file upon success.

(For details on how to find out how the LFC directory space is organized see Access control for the LFC)

We could also have copied the file to the SRM only, bypassing the LFC registration, using the command

srmcp file://$PWD/myfile $SRM/myfile

but as we needed the LFC entry as well we used the (preferred) lcg-cr command.

Looking at the permissions

For a file that is copied to SRM and that is registered in the LFC there are 2 sets of permissions:

1. SRM-level
2. LFC-level

These permissions are not directly related to each other and need to be modified separately. In this section we explain how to modify the SRM-level permissions. The LFC-level permissions are explained in Access control for the LFC.

First we try the "standard" srm-get-permissions command:

$ srm-get-permissions $SRM/myfile
srm client error:
java.lang.NullPointerException

Eh? It turns out that this is a bug: https://gus.fzk.de/ws/ticket_info.php?ticket=50779

Luckily there is another way of looking at DPM permissions:

$ export DPNS_HOST=tbn18.nikhef.nl
$ dpns-getacl /dpm/nikhef.nl/home/pvier/janjust/myfile
# file: /dpm/nikhef.nl/home/pvier/janjust/myfile
# owner: /O=dutchgrid/O=users/O=nikhef/CN=Jan Just Keijser
# group: pvier
user::rw-
group::rw-              #effective:rw-
group:pvier/Role=production:rwx         #effective:rw-
group:pvier/Role=lcgadmin:rwx           #effective:rw-
mask::rw-
other::r--

Notes

• the dpns-* commands look at the DPNS_HOST environment variable to determine which server to talk to
• the dpns-* commands do not use srm:// URLs so we need to strip off the srm://tbn18.nikhef.nl part

Just for the fun of it, let's list a file which we did not create ourselves:

$ dpns-getacl /dpm/nikhef.nl/home/vlemed/foo.txt
# file: /dpm/nikhef.nl/home/vlemed/foo.txt
# owner: /O=dutchgrid/O=users/O=uva/OU=wins/CN=Tristan Glatard
# group: vlemed
user::rw-
group::rw-              #effective:rw-
group:vlemed/Role=production:rwx                #effective:rw-
group:vlemed/Role=lcgadmin:rwx          #effective:rw-
mask::rw-
other::r--

When you compare it to dCache (see Access control for dCache storage elements) this looks more like "regular" UNIX permissions:

• the file is owned by a single user
• there are 'group' and 'other' permissions

but there is a difference

• there are multiple group permissions

We will have to keep this in mind when we want to limit access to our files.

Modifying the permissions

We would want to use the srm-set-permissions command to modify the permissions, but similar to srm-get-permissions this command does not work on DPM servers.

Instead we use dpns-setacl. Use

$ man dpns-setacl

for a full manual page, as there are many options for this command.

For our example we wish to

• remove the default group bits
• remove the mask (to make sure other groups do not inherit permissions)
• remove the otherbits

dpns-setacl -m g::0,m:0,o::0 /dpm/nikhef.nl/home/pvier/janjust/myfile

DPM and the dpns-setacl command can do very fine-grained access control on files and directories, yet that falls outside the scope of this investigation.

Verifying access control

As the original user:

$ dpns-getacl /dpm/nikhef.nl/home/pvier/janjust/myfile
# file: /dpm/nikhef.nl/home/pvier/janjust/myfile
# owner: /O=dutchgrid/O=users/O=nikhef/CN=Jan Just Keijser
# group: pvier
user::rw-
group::---              #effective:---
group:pvier/Role=production:rwx         #effective:---
group:pvier/Role=lcgadmin:rwx           #effective:---
mask::---
other::---

so this file should now be accessible only to the user.

Now if we switch to another VO:

$ voms-proxy-init --voms vlemed
Enter GRID pass phrase:
Your identity: /O=dutchgrid/O=users/O=nikhef/CN=Jan Just Keijser
Creating temporary proxy .......................................... Done
Contacting  voms.grid.sara.nl:30003 [/O=dutchgrid/O=hosts/OU=sara.nl/CN=voms.grid.sara.nl] "vlemed" Done
Creating proxy ............................. Done
Your proxy is valid until Thu Aug  6 23:45:56 2009

and then try to copy the file

$ srmcp -2 $SRM/myfile file://$PWD/blah

we get NO error message: DPM behaves much more like regular UNIX. The file is still owned by the original user (/CN=Jan Just Keijser) and thus that user is still allowed to retrieve the file, even though the voms proxy belongs to another VO.

Note the srmcp -2 flag: without it the srmcp command tries to retrieve the file using an SRMv1 protocol, even though we have added the SRMv2 port 8446 to the SRM URL.

If we ask another member of the pvier VO to retrieve the file we see:

[tbalint] $ srmcp -2 $SRM/myfile file://$PWD/blah
Thu Aug 06 15:00:37 CEST 2009: srmPrepareToGet update failed, status : SRM_FAILURE explanation=Failed for all SURLs
Thu Aug 06 15:00:37 CEST 2009: GetFileRequest[srm://tbn18.nikhef.nl:8446/dpm/nikhef.nl/home/pvier/janjust/myfile]
status=SRM_AUTHORIZATION_FAILURE explanation=null
srm client error:
java.lang.Exception:  stopped 

i.e. other users can no longer access the file: success!