Bidirectional Rsync/Unison based SysVol replication workaround

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Introduction

WORK in Progress

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 rsync and unison.

Information on unison + rsync replication

This HowTo describes a solution for SysVol replication, that is based on rsync and unison. As Compare to the rsync method, it is bidirectional. But this howto only cover two DC setup.

It have the following advantages:

  • setup is fast done
  • configuration is very easy
  • Can work with windows (Please add in)

We will use rsync through a SSH tunnel.

Setup the SysVol replication

Installation script

If you are using apt package manger you can try this script [1]

Please thank L.v. Belle for creating this

Make Change to the following to fix your need: 

SETSAMBASYSVOLBASE=/var/lib/samba
SETSAMBASYSVOLPATH=/var/lib/samba/sysvol
## in Minutes
SETCRONTIME=5
SETCRONFILE=/etc/cron.d/sysvol-sync
SETIPDC1=""
SETIPDC2=""

Run it ./3-setup-sysvol-bidirectional.sh

Manually

Setup on the Domain Controller with the PDC Emulator FSMO role

  • Install rsync by using your package manager or compile from source. Make sure, that you use a version that supports extended ACLs!
  • We don't need to setup rsync server.
  • Install unison by using your package manager or compile from source. (Gentoo need to do eselect unison to create the link)

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

Setup ssh Controle

If the remote system enforces rate limits on incoming ssh connections, unison wil fail if you try to run it this way. So we create the first ssh connection as a controlpath file in the location specified, all subsequent connections will reuse on the first connection. 

mkdir ~/.ssh/ctl
cat << EOF > ~/.ssh/ctl/config
Host *
ControlMaster auto
ControlPath ~/.ssh/ctl/%h_%p_%r
ControlPersist 1
EOF

Setup Sysvolsync Log files

Do the following on DC1 so that we can check what happen on the sync. Please include this files into to log rotate as we didn't control the log size here. 

touch /var/log/sysvol-sync.log
chmod 640 /var/log/sysvol-sync.log

Setup Unison defaults running parameters

Please run the following on DC1 

install -o root -g root -m 0750 -d /root/.unison
cat << EOF > /root/.unison/default.prf
# Unison preferences file
# Roots of the synchronization
#
# copymax & maxthreads params were set to 1 for easier troubleshooting.
# Have to experiment to see if they can be increased again.
root = /var/lib/samba
# Note that 2 x / behind DC2, it is required
root = ssh://root@$DC2//var/lib/samba 
# 
# Paths to synchronize
path = sysvol
#
#ignore = Path stats    ## ignores /var/www/stats
auto=true
batch=true
perms=0
rsync=true
maxthreads=1
retry=3
confirmbigdeletes=false
servercmd=/usr/bin/unison
copythreshold=0
copyprog = /usr/bin/rsync -XAavz --rsh='ssh -p 22' --inplace --compress
copyprogrest = /usr/bin/rsync -XAavz --rsh='ssh -p 22' --partial --inplace --compress
copyquoterem = true
copymax = 1
logfile = ${SETSYNCLOG} 
EOF

Setup SysVol 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!
  • On DC2 Install unison by using your package manager or compile from source.
  • Run the following command on DC1
ssh DC2 "cp -R --preserve=all /var/lib/samba/sysvol /var/lib/samba/sysvol"


Let us 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.

crontab -e

  • /5 * * * * root ${SETRSYNC} -XAavz --log-file ${SETSYNCLOG} --delete-after -f"+ */" -f"- *" ${SETSAMBASYSVOLPATH} root@${SETIPDC2}:${SETSAMBASYSVOLBASE} && ${SETUNISON} &> /dev/null

EOF



Warning: Make sure that the destination folder is really your SysVol folder, because the command will replicate to the given directory and removes everything in it that isn't also on the source! You could damage your system! So check the output carefully if the replication is doing, what you expect!
  • If everything looks sane, run the command without the --dry-run option and let rsync do the replication.
  • To automate synchronisation, you can run the command via cron (e. g. every 5 minutes).
*/5 * * * *          rsync -XAavz --delete-after --password-file=/usr/local/samba/etc/rsync-sysvol.secret rsync://sysvol-replication@{IP-of-you-PDC}/SysVol/ /path/to/your/sysvol/folder/
  • Repeat these steps on every DC (except your PDC!).



FAQ

  • How can I get multi-direction replication ?
    • There are some testing using unsion with rsync to archive this.

Please check this installation script https://secure.bazuin.nl/scripts/3-setup-sysvol-bidirectional.sh


  • 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.
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