Sds-hd nfs

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1 Prerequisites

  • Attention: To access data served by SDS@hd, You need a Service Password. See details Sds-hd_user_access.
  • Additionally the access to SDS@hd is currently only available inside the belwue-Network. This means you have to use the VPN Service of your HomeOrganization, if you want to access SDS@hd from outside the bwHPC-Clusters (e.g. via eduroam or from your personal Laptop)
  • The access via nfs protocol is machine-based, which means new nfs-Clients have to be registered on SDS@hd. During this registration each machine gets a keytab file, which allows mounting SDS@hd.
  • Currently you have to send an email for Clientregistration to SDS@hd Team with the following information:
    • hostname of the new nfs-Client
    • IP address
    • short description
    • location
    • acronym of the Speichervorhabens which should be available on this machine
  • For Kerberos authentication to work, a correctly synchronized system time must be set on each nfs client (e.g. via ntpdate ntp01.urz.uni-heidelberg.de or chrony)

2 Using NFSv4 for UNIX client

The authentication for data access via NFSv4 is performed using Kerberostickets. This requires a functioning Kerberos environment on the client!

The following parameters of kerberos tickets are set on server side:

  • max. Lifetime of a Serviceticket: 10 hours
  • max. Lifetime of a Userticket: 24 hours
  • max. Renewaltime for Usertickets: 10 days

The properties (e.g. lifetimes, encryption, ...) of the kerberos tickets can be changed on client site with different kinit parameters (see manpages of kinit) or via /etc/krb5.conf.

2.1 configure kerberos environment for SDS@hd

2.1.1 kerberized nfs configuration

First you have to install kerberos and nfs packages in your system to provide a working kerberos environment. The exact names of the packages depending on you linux distribution (see examples below).

Example RedHat/CentOS 

yum install krb5-workstation nfs-utils nfs4-acl-tools

Example debian/ubuntu 

apt install krb5-user nfs-common nfs4-acl-tools nfs-server

On ubuntu server: nfs-kernel-server


After installing the packages you have to use the following kerberos parameters for connecting to SDS@hd:

  • Default Realm = BWSERVICES.UNI-HEIDELBERG.DE
  • KDC = bwservices.uni-heidelberg.de

So your kerberos configuration file (/etc/krb5.conf) should contain the following entries: 

[libdefaults]
     default_realm = BWSERVICES.UNI-HEIDELBERG.DE

     [realms]
     BWSERVICES.UNI-HEIDELBERG.DE= {
          kdc = bwservices.uni-heidelberg.de
          admin_server = bwservices.uni-heidelberg.de
      }
      [domain_realm]
         .uni-heidelberg.de = BWSERVICES.UNI-HEIDELBERG.DE
         uni-heidelberg.de = BWSERVICES.UNI-HEIDELBERG.DE

The keytab file of the machine, which you get from the SDS@hd Team, has to be stored as /etc/krb5.keytab in the system.

To enable kerberized NFSv4, you have to set the parameters: 

NEED_IDMAPD=yes
NEED_GSSD=yes

in your nfs configuration. You can find the configuration depending on your linux distribution, e.g.:

  • RedHat/CentOS: 
    /etc/sysconfig/nfs
  • debian/ubuntu: 
    /etc/default/nfs-common

2.1.2 ID Mapping

In order to be able to correctly resolve the ownership of files and folders, an ID mapping must be set up (assignment of uid/gid to uidNumbers/gidNumbers).

Therefor change the file /etc/idmapd.conf with the following lines, to enable id mapping for nfs mounts: 

in /etc/idmapd.conf:
        [General]
        Domain = urz.uni-heidelberg.de
        Local-Realms = BWSERVICES.UNI-HEIDELBERG.DE

The connection to SDS@hd LDAP Server is authenticated with the kerberos keytab of your nfs-Client. You can use any tool which supports GSSAPI with kerberos for authentication with the following parameters:

  • uri: ldap://bwservices.uni-heidelberg.de
  • search_base: dc=bwservices,dc=uni-heidelberg.de,dc=de
  • sasl mech: GSSAPI Authentication
  • krb5 Realm: BWSERVICES.UNI-HEIDELBERG.DE


2.1.2.1 Example sssd

Because sssd is one of the standard tools and it supports more than one ldap/identity provider on a system, we are showing here an example configuration for this tool.

First install the packages:

  • RedHat/CentOS:
> yum install sssd-client sssd-krb5 sssd-ldap
  • debian/ubuntu:
> apt install sssd sssd-krb5 sssd-ldap sssd-tools libnss-sss libsasl2-modules-gssapi-mit

Now create a sssd configuration file (/etc/sssd/sssd.conf) like this: Attention: Don't forget to change "ldap_sasl_authid" to the Hostname in your keytab file! 

[sssd]
            domains = BWSERVICESAD
            config_file_version = 2
            services = nss

[domain/BWSERVICESAD]
            id_provider = ldap
            ldap_uri = ldap://bwservices.uni-heidelberg.de
            ldap_search_base = dc=bwservices,dc=uni-heidelberg,dc=de
            ldap_referrals = false

            ldap_schema = ad
            ldap_id_mapping = true
            min_id = 100000

            ldap_sasl_mech = GSSAPI
            krb5_realm = BWSERVICES.UNI-HEIDELBERG.DE
            ldap_sasl_authid = <HOSTNAME>$
            ldap_krb5_keytab = /etc/krb5.keytab
            krb5_server = bwservices.uni-heidelberg.de
            ldap_sasl_canonicalize = false
            krb5_canonicalize = false

            use_fully_qualified_names = true
            full_name_format = %3$s\%1$s

            re_expression = (((?P<domain>[^\\]+)\\(?P<name>.+$))|((?P<name>[^@]+)@(?P<domain>.+$)))	 
            enumerate = false


If another service for authentication or name resolution is allready be used on the machine, an additional domain block can be set up for this and should be added to the line "domains".

To enable sssd for id mapping in your system the lines "passwd" and "group" in file "/etc/nsswitch.conf" have to be extended by "sss", e.g.: 

        passwd: compat sss
        group: compat sss

Note: If you are using sssd you should not use "nscd" in parallel! Otherwise this could lead to undefined behaviour due to double caching passwd, group and netgroup entries.


Now you should enable and restart the services:

  • systemctl enable sssd.service && systemctl restart sssd.service
  • systemctl restart nfs-idmapd.service

2.2 mount a nfs share

The usual restrictions for mounting drives under Linux apply. Usually this can only be done by the superuser "root". For detailed information, please contact the system administrator of your system.

After successfull configuration (s. 2.1) you can mount your SDS@hd share with the following commands: 

> mkdir <mountpoint>
> mount -t nfs4 -o sec=krb5,vers=4.0 lsdf02.urz.uni-heidelberg.de:/gpfs/lsdf02/ <mountpoint>

To enable the mounting after a restart, you have to add the following line to the file "/etc/fstab" 

   lsdf02.urz.uni-heidelberg.de:/gpfs/lsdf02/   <mountpoint>   nfs4     sec=krb5,vers=4.0     0 0

2.3 access your data

Attention! The access can not be done as root user, because root uses the Kerberosticket of the machine, which does not have data access!

To access your data on SDS@hd you have to fetch a valid kerberos ticket with your SDS@hd user and Servicepassword: 

> kinit hd_xy123
Password for hd_xy123@BWSERVICES.UNI-HEIDELBERG.DE:

You can check afterwards your kerberos ticket with: 

> klist
Ticket cache: FILE:/tmp/krb5cc_1000
Default principal: hd_xy123@BWSERVICES.UNI-HEIDELBERG.DE

Valid starting       Expires              Service principal
20.09.2017 04:00:01  21.09.2017 04:00:01  krbtgt/BWSERVICES.UNI-HEIDELBERG.DE@BWSERVICES.UNI-HEIDELBERG.DE
        renew until 29.09.2017 13:38:49

Afterwards you should be able to access the mountpoint, which contain all Speichervorhaben exported to your machine: 

> ls <mountpoint>
sd16j007  sd17c010  sd17d005

2.4 renew a kerberos ticket

Because a kerberos ticket has a limited lifetime (default: 10 hours, maximum 24 hours) for security reasons, you have to renew your ticket before it expires to prevent access loss. 

> kinit -R

This renewal could only be done for maximum time of 10 Days and as long as the current kerberos ticket is still valid. For renewal of an expired ticket, you have to use again your Servicepassword.

2.5 destroy kerberos ticket

Even if kerberos tickets are only valid for a limited period of time, a ticket should be destroyed as soon as access is no longer needed to prevent misuse on multi-user systems: 

kdestroy

2.6 automated kerberos tickets

Attention! Keep this generated Keytab safe and use it only in trusted environments!

If your workflow needs a permanent access to SDS@hd for longer than 10 Days, you can use ktutil to encrypt your Service Password into a keytab file:

interactive way: 

ktutil
ktutil: addent -password -p hd_xy123@BWSERVICES.UNI-HEIDELBERG.DE -k 1 -e rc4-hmac
    Password for hd_xy123@BWSERVICES.UNI-HEIDELBERG.DE:
ktutil:  addent -password -p hd_xy123@BWSERVICES.UNI-HEIDELBERG.DE -k 1 -e aes256-cts
    Password for hd_xy123@BWSERVICES.UNI-HEIDELBERG.DE:
ktutil:  wkt xy123.keytab
ktuitl: quit

non-interactive way: 

echo -e "addent -password -p hd_xy123@BWSERVICES.UNI-HEIDELBERG.DE -k 1 -e rc4-hmac\n<your_servicepasword>\n
addent -password -p hd_xy123@BWSERVICES.UNI-HEIDELBERG.DE -k 1 -e aes256-cts\n<your_servicepasword>\nwkt xy123.keytab" | ktutil

With this keytab, you can fetch a kerberos ticket without an interactive password: 

kinit -k -t xy123.keytab hd_xy123


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