Synology

DSM7 system default groups:

administrators is system default admin group
users is system default group. This group includes all users in this DSM and its member list cannot be modified.

Created users (for example):

user-admin, member of groups: administrators and users
user-normal, member of groups: users (no admin)
user root does not exist (and cannot be created) in DSM (for security reasons), but exist in "Linux system" via sudo -i
user admin and user guest are deactivated in DSM7

Enable SSH service via 'Enable SSH service' (Control Panel > Terminal & SNMP > Terminal) which will start /usr/bin/sshd -D.

[pc]$ ssh user-admin@dsm.host
user-admin@dsm.host's password:

Could not chdir to home directory /var/services/homes/user-admin: No such file or directory
user-admin@dsm:/$ pwd
/
user-admin@dsm:/$ sudo -i     # be careful, root user has the highest privileges
Password:
root@dsm:~#

[pc]$ ssh user-normal@dsm.host
user-normal@dsm.host's password:

Permission denied, please try again.
Connection to dsm.host closed.

$ grep -e root -e user-admin -e user-normal /etc/passwd
root:x:0:0::/root:/bin/ash
user-admin:x:1028:100::/var/services/homes/user-admin:/bin/sh
user-normal:x:1029:100:/var/services/homes/user-normal:/sbin/nologin

SSH prihlasenie (by default) je mozne len pre uzivatelov, ktori su clenmi administrators group, resp. len pre uzivatelov, ktori nepouzivaju pri prihlaseni nologin(8) (disabled account). Pridanie uzivatela user-normal do administrators group zameni shell /sbin/nologin na /bin/sh a umozni SSH prihlasenie.

SSH server (Synology DSM7 sshd_config by default) pri autentifikacii pomocou verejneho kluca preveruje subor .ssh/authorized_keys v home directory, ale DSM (by default) nepouziva home directories pre uzivatelov. See also [1].

/var/services/homes -> /volume1/homes             # Enable user home service: yes
/var/services/homes -> /volume1/@fake_home_link   # Enable user home service: no
# after disable user home service, dir /volume1/homes with all subdirs (for each users) are preserved

Tip Working (temporary) without 'Enable user home service'. Example with mc (Midnight Commander) from SynoCli File Tools package.

[pc]$ ssh user-admin@dsm.host

Could not chdir to home directory /var/services/homes/user-admin: No such file or directory
user-admin@dsm:/$ mkdir -p "/tmp/$USER" && HOME="/tmp/$USER" && mc

# DSM7 restart sshd
$ sudo synosystemctl restart sshd.service     # /usr/syno/bin/synosystemctl
[sshd.service] restarted.

Shared Folder permissions

In DSM the access permissions of Shared Folders are based on Windows ACL by default. See also [2], [3] and [4], [5], [6].
NOTE DSM je sice Linux (BSD) operacny system, ale umoznuje pracovat s roznymi file systems, file services, vratane Windows operacne systemy.

DSM rozlisuje permissions: No Access (NA), Read/Write (RW), Read Only (RO) a Custom (Permission Editor/Inspector to manage ACL permissions), pricom plati NA > RW > RO. Access Control Lists (ACL) poskytuje rozsireny a flexibilnejsie mechanizmus povolenia pre file system. Administratorom umoznuje nastavit specificke povolenie pre jednotliveho uzivatela alebo skupinu. V Linux systemoch sa pouzivaju prikazy getfacl(1)^rpm resp. setfacl(1)^rpm. Synology DSM pouziva specificky prikaz synoacltool -get resp. synoacltool -add. ACL permissions nebudeme menit, preto dalej s Custom permissions nebudeme pracovat, uvazovat.

Platia nasledovne pravidla pre pracu with Shared Folder:

Ak nie su nastavene ziadne permissions => No Access permissions.
System default admin group administrators ma pre Shared Folder Read/Write permissions (by default).
System default admin group administrators moze pre Shared Folder zmenit permissions na No Access alebo Read/Write (by default), nie vsak na Read Only, t.j. bud maju plny pristup alebo ziaden (ani citanie).
System default group users nema pre Shared Folder nastavene ziadne permissions, co znamena No Access (by default).
Admin pre system default group users moze pre Shared Folder zmenit permissions na No Access alebo Read/Write alebo Read Only.

Created Shared Folder EXAMPLE_FOLDER. Permissions by default: administrators group Read/Write and users group without any permissions => No Access. Permissions je mozne dalej modifikovat cez 'Control Panel > Shared Folder > EXAMPLE_FOLDER > Edit > Permissions' pre tychto users/groups: System internal user or Local users or Local groups.

Warning Data should only be stored in shared folders. Data stored elsewhere may be deleted when the system is updated/restarted.

$ sudo tune2fs -l /dev/md0 | grep -i acl
Default mount options:    user_xattr acl

$ ls -al -d /volume1/EXAMPLE_FOLDER/
drwxrwxrwx+ 5 root root 4096 Mar 27 16:10 /volume1/EXAMPLE_FOLDER/
#         + (plus sign, after permissions) notice that there is an ACL

$ synoacltool -get /volume1/EXAMPLE_FOLDER/   # /usr/syno/bin/synoacltool
ACL version: 1
Archive: has_ACL,is_support_ACL
Owner: [root(user)]
---------------------
         [0] group:administrators:allow:rwxpdDaARWc--:fd-- (level:0)

rsync

For remote transfers a modern rsync uses remote shell (ssh by default). It is not necessary (nor desirable) to 'Enable rsync service' (Control Panel > File Services > rsync) which will start /usr/bin/rsync --daemon (not recommended, not safety/encrypted).

$ rsync user-admin@dsm.host:/volume1/EXAMPLE_FOLDER/
Permission denied, please try again.
rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: error in rsync protocol data stream (code 12) at io.c(232) [Receiver=3.4.1]

$ rsync --rsync-path='/usr/bin/rsync' user-admin@dsm.host:/volume1/EXAMPLE_FOLDER/
# OK, files listed

Could not chdir to home directory /var/services/homes/user-admin: No such file or directory
# This message (sshd) indicates that the home directory of your account is not available.
# You can simply ignore this message or 'Enable user home service' to prevent showing it

# user-normal not a member of the administrators group
$ rsync --rsync-path='/usr/bin/rsync' user-normal@dsm.host:/volume1/EXAMPLE_FOLDER/
Permission denied, please try again.
rsync: connection unexpectedly closed (0 bytes received so far) [Receiver]
rsync error: error in rsync protocol data stream (code 12) at io.c(232) [Receiver=3.4.1]

`rsync --rsync-path=`	Versions of rsync (DSM 7.2, 2025-04)
/usr/bin/rsync (or /bin/rsync)	original Synology DSM ver 3.1.2 (released 2015-12) of `rsync`
/usr/local/bin/rsync	SynoCli Network Tools ver 3.4.1 (released 2025-01) of `rsync`

Recommended excluded dirs (recursive): @eaDir and '#recycle', @tmp.

file transfer

[musinsky@muke ~]$ truncate --size=12345 test.file
[musinsky@muke ~]$ ls -l --full-time test.file
-rw-r--r-- 1 musinsky musinsky 12345 2025-04-03 14:52:44.326597898 +0200 test.file

# 'test.file' was uploaded via 'File Station' DSM web interface (as user-admin user)
user-admin@dsm:/$ ls -l --full-time /volume1/EXAMPLE_FOLDER/test.file
-rwxrwxrwx+ 1 user-admin users 12345 2025-04-03 14:52:44.326000000 +0200 /volume1/EXAMPLE_FOLDER/test.file
user-admin@dsm:/$ synoacltool -get /volume1/EXAMPLE_FOLDER/test.file
ACL version: 1
Archive: is_inherit,is_support_ACL
Owner: [user-admin(user)]
---------------------
         [0] group:administrators:allow:rwxpdDaARWc--:---- (level:1)

# 'test.file' downloaded (as zip archive) via 'File Station' DSM web interface (as user-admin user)
[musinsky@muke Downloads]$ unzip test.zip   # NOTE zip/unzip drops nanoseconds from the timestamp of file
[musinsky@muke Downloads]$ ls -l --full-time test.file
-rwxrwxrwx 1 musinsky musinsky 12345 2025-04-03 14:52:44.000000000 +0200 test.file

# remove previously uploaded 'test.file' on Synology DSM
[musinsky@muke ~]$ rsync --rsync-path='/usr/local/bin/rsync' --archive test.file \
>                  user-admin@dsm.host:/volume1/EXAMPLE_FOLDER/ --itemize-changes
Could not chdir to home directory /var/services/homes/user-admin: No such file or directory
<f+++++++++ test.file

user-admin@dsm:/$ ls -l --full-time /volume1/EXAMPLE_FOLDER/test.file
-rw-r--r-- 1 user-admin users 12345 2025-04-03 14:52:44.326597898 +0200 /volume1/EXAMPLE_FOLDER/test.file
user-admin@dsm:/$ synoacltool -get /volume1/EXAMPLE_FOLDER/test.file
(synoacltool.c, 596)It's Linux mode

[musinsky@muke ~]$ rm test.file
[musinsky@muke ~]$ rsync --rsync-path='/usr/local/bin/rsync' --archive \
>                  user-admin@dsm.host:/volume1/EXAMPLE_FOLDER/test.file . --itemize-changes
Could not chdir to home directory /var/services/homes/user-admin: No such file or directory
>f+++++++++ test.file
[musinsky@muke ~]$ ls -l --full-time test.file
-rw-r--r-- 1 musinsky musinsky 12345 2025-04-03 14:52:44.326597898 +0200 test.file

notes

Synology alternative (build own NAS)