Difference between revisions of "Bidirectional Rsync/Unison based SysVol replication workaround"

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Revision as of 20:08, 25 August 2015

Introduction

Samba AD currently doesn't provide support for SysVol replication. To achive this important feature in a Multi-DC environment, until it's implemented, workarounds are necessary to keep it in sync. This HowTo provides a basic workaround solution based on Osync.

Information on Osync replication

This HowTo describes a solution for SysVol replication, that is based on Osync, which is two way sync system based on rsync. This howto covers the bidirectional synchronization of two DC servers.

This solution has the following advantages:

  • setup is fast
  • configuration is very easy
  • Can work with windows (Windows can be used as master using the MinGW environment)

Osnyc uses rsync through a SSH tunnel.

Setup the SysVol replication

Installation

There aren't any deb or rpm packages as osync is a script, but you can get a copy here: [1]http://www.netpower.fr/projects/osync/osync.v1.00a.tar.gz

You can then install osync with the following commands

tar xvf osync.v1.00a.tar.gz
cd ./osync
sh ./install.sh

This will copy osync to /usr/local/bin and create an example configuration file at /etc/osync/sync.conf

Setup on the Domain Controller with the PDC Emulator FSMO role

Some assumption:

  • You are running all command as root.
  • rsync is installed on both servers
  • sysvol is located /var/lib/samba/sysvol on both DC1 and DC2
  • osync is located /usr/local/bin/osync
  • sync.conf is located /etc/osync/sync.conf
  • DC1 is at DC1
  • DC2 is at DC2 (And already join DC1)
  • sysvolsync log is located /var/log/osync_*.log
  • rsync must support extended ACLs
  • Install rsync by using your package manager (eg: yum install rsync or apt-get install rsync) or compile from source. Make sure, that you use a version that supports extended ACLs (most versions do)!
  • sysvol must be on disk which is mounted to support acl and also xattr
  • We don't need to setup rsync server.

Change the path if that don't fit your setup.

Creating SSH Public Key and ssh-copy to DC2

ssh-keygen -t dsa
ssh-copy-id -i ~/.ssh/id_dsa.pub root@DC2

You can try to access DC2 via ssh

ssh DC2

Osync Configuration Setup on DC1

Edit the /etc/osync/sync.conf and make some changes

#!/usr/bin/env bash
SYNC_ID="sysvol_sync"
MASTER_SYNC_DIR="/var/lib/samba/sysvol"
SLAVE_SYNC_DIR="ssh://root@DC2:22//var/lib/samba/sysvol"
SSH_RSA_PRIVATE_KEY="/root/.ssh/id_rsa"
PRESERVE_ACL=yes
PRESERVE_XATTR=yes
SOFT_DELETE=yes
DESTINATION_MAILS="your@email.com"

Osync also does backups and soft deletes which will keep a copy of deleted files and folders in a subfolder named .osync_workdir. Unless you have specific reasons to do so, leave these options enabled. Osync has one benefit as it will only send email alert if there is problem.

Setup on DC2

  • On DC2 Install rsync by using your package manager or compile from source. Make sure, that you use a version that supports extended ACLs!
  • Shutdown DC2 Samba AD DC
mv /var/lib/samba/private/idmap.ldb /var/lib/samba/private/idmap.ldb.backup"
rm /var/cache/samba/gencache.tdb"
  • Copy idmap.ldb from DC1 to sync the idmap.
  • Run the following command on DC1
scp /var/lib/samba/private/idmap.ldb root@DC2:/var/lib/samba/private/

1st Try

What happen is we use rsync to create the directory structure with extended attributes Than unison setup copies only the extened attributes on files.


Before starting, make a backup of your sysvol just in case.

/usr/local/bin/osync.sh /etc/osync/sync.conf --dry --verbose

If this runs successfully, you may remove the --dry option and run again.

/usr/local/bin/osync.sh /etc/osync/sync.conf --verbose

Add to Crontab on DC1

On DC1 run the following:

crontab -e 
*/5 * * * * root  /usr/local/bin/osync.sh /etc/osync/sync.conf --silent
Warning: Make sure that the destination folder is really your SysVol folder, because the command will replicate to the given directory and sync everything in it that isn't also on the source! You could damage your system! So check the output carefully to see if the replication is doing, what you expect!

When you try to resync the folder

Warning: Please follow the steps below OR you can end up with an empty sysvol folder.
  1. Disable Cron on DC1, like Add a "#" on the line with crontab -e
  2. Check is any rsync or osync are currently running in ps -aux | grep sync if yes, wait for it to finished OR kill it (if it is zombie)
  3. Remove the .osync_workdir hash files on both DC1 and DC2 on MASTER/SLAVE_SYNC_DIR "/var/lib/samba/sysvol"
  4. Now check your sysvol and resync
  5. Confirm that everything is working again
  6. Re-enable the Cron on DC1 again

FAQ

  • How can I do this on windows?
    • Install the MSYS bash environment (read the osync documentation), then you may run osync as master from Windows.
  • What to do if I've more than one DC?
    • By Theory, We would just make more cron jobs on DC1 and the complete sync will be perform next sync to all server. (Not tested)
    • Something like:
    • DC1 <> DC2
    • DC1 <> DC3
    • DC1 <> DC2


  • Why can't I simply use a distributed filesystem like GlusterFS, Lustre, etc. for SysVol?
    • A cluster file system with Samba requires CTDB to be able to do it safely. And CTDB and AD DC are incompatible. Please check on the reply from Samba Developer on the mailing list .