I created an Alpine container, mounted an external folder located on a FAT32 partition, and connected to the container's command line.
RouterOS commands
[admin@MikroTik] > /system/package print detail
Flags: X - disabled; A - available
0 name="routeros" version="7.19.2" build-time=2025-06-20 07:55:02 scheduled="" size=18.6MiB
1 name="container" version="7.19.2" build-time=2025-06-20 07:55:02 scheduled="" size=152.1KiB
[admin@MikroTik] > /disk/print detail where slot=sata1-part1
Flags: X - disabled; E - empty, B - block-device; M - mounted, F - formatting, S - swap-enabled; f - raid-member-failed; r - raid-member, c - encrypted; g - guid-partition-table, p - partition; t - nvme-tcp-export, i - iscsi-export; s - smb-sharing; n - nfs-sharing; m - media-sharing; O - tcg-opal-self-encryption-enabled, o - tcg-opal-self-encryption-supported
0 BM p type=partition slot="sata1-part1" slot-default="sata1-part1" parent=sata1 fs-label="NO NAME" fs-uuid="68d38280" fs=fat32 model="VMware Virtual S" serial="@65'536-500'065'792" size=500 000 256 free=492 175 360 use=0% mount-point="sata1-part1" mount-filesystem=yes mount-read-only=no compress=no partition-number=1 partition-offset=65 536 partition-size=500 000 256 sector-size=512 smb-sharing=no media-sharing=no media-interface=none swap=no
[admin@MikroTik] > /file print detail where name~"sata1-part1"
0 name=sata1-part1 type=disk last-modified=1970-01-01 03:00:00
1 name=sata1-part1/var_log type=directory last-modified=2025-09-24 09:26:00
[admin@MikroTik] > /container/mounts add name=var_log src=/sata1-part1/var_log dst=/var/log
[admin@MikroTik] > /container add interface=veth1 remote-image=library/alpine root-dir=alpine mounts=var_log cmd="tail -f /dev/null"
[admin@MikroTik] > /container print detail
0 name="alpine" repo="registry-1.docker.io/library/alpine:latest" os="linux" arch="amd64" interface=veth1 cmd="tail -f /dev/null" root-dir=alpine mounts=var_log workdir="/" status=stopped
[admin@MikroTik] > /container start 0
[admin@MikroTik] > /container shell 0
/ #
From the container's command line, I ran the following commands:
/ # whoami
root
/ # cat /etc/alpine-release
3.22.1
/ # mount | grep sda1
/dev/sda1 on /var/log type vfat (rw,nosuid,nodev,noexec,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=utf8,shortname=mixed,utf8,errors=remount-ro)
/ # ls -al /var/log
total 8
drwxr-xr-x 2 nobody nobody 4096 Sep 24 09:26 .
drwxr-xr-x 11 root root 4096 Sep 24 09:41 ..
/ # chown root:root /var/log
chown: /var/log: Operation not permitted
/ # chmod a+w /var/log
chmod: /var/log: Operation not permitted
/ # ls -al /var/log
total 8
drwxr-xr-x 2 nobody nobody 4096 Sep 24 09:26 .
drwxr-xr-x 11 root root 4096 Sep 24 09:41 ..
/ # mkdir /var/log/test
mkdir: can't create directory '/var/log/test': Permission denied
/ # ls -al /var/log
total 8
drwxr-xr-x 2 nobody nobody 4096 Sep 24 09:26 .
drwxr-xr-x 11 root root 4096 Sep 24 09:41 ..
The result shows that the root user doesn't have write access to the mounted folder (mkdir: can't create directory '/var/log/test': Permission denied). The same result with the exFAT partition.
When I do the same thing using Alpine-Docker, there are no issues writing to the mounted folder.
Alpine-Docker commands
alpine-docker:~# whoami
root
alpine-docker:~# cat /etc/alpine-release
3.11.5
alpine-docker:~# mount | grep sata1-part1
/dev/sda1 on /sata1-part1 type vfat (rw,nosuid,nodev,noexec,relatime,uid=65534,gid=65534,fmask=0022,dmask=0022,codepage=437,iocharset=utf8,shortname=mixed,errors=remount-ro)
alpine-docker:~# ls -al /sata1-part1/var_log
total 8
drwxr-xr-x 2 nobody nobody 4096 Sep 24 13:11 .
drwxr-xr-x 3 nobody nobody 4096 Jan 1 1970 ..
alpine-docker:~# docker run -it -v /sata1-part1/var_log:/var/log library/alpine
/ #
From the container's command line:
/ # whoami
root
/ # cat /etc/alpine-release
3.22.1
/ # mount | grep sda1
/dev/sda1 on /var/log type vfat (rw,nosuid,nodev,noexec,relatime,uid=65534,gid=65534,fmask=0022,dmask=0022,codepage=437,iocharset=utf8,shortname=mixed,errors=remount-ro)
/ # ls -al /var/log
total 8
drwxr-xr-x 2 nobody nobody 4096 Sep 24 13:11 .
drwxr-xr-x 11 root root 4096 Jul 15 10:42 ..
/ # chown root:root /var/log
chown: /var/log: Operation not permitted
/ # chmod a+w /var/log
/ # ls -al /var/log
total 8
drwxr-xr-x 2 nobody nobody 4096 Sep 24 13:11 .
drwxr-xr-x 11 root root 4096 Jul 15 10:42 ..
/ # mkdir /var/log/test
/ # ls -al /var/log
total 12
drwxr-xr-x 3 nobody nobody 4096 Sep 24 13:28 .
drwxr-xr-x 11 root root 4096 Jul 15 10:42 ..
drwxr-xr-x 2 nobody nobody 4096 Sep 24 13:28 test
/ # ls -al /var/log/test
total 8
drwxr-xr-x 2 nobody nobody 4096 Sep 24 13:28 .
drwxr-xr-x 3 nobody nobody 4096 Sep 24 13:28 ..
How can I grant everyone in the container write access to a mounted folder on a FAT32 partition in RouterOS (I assume I need to change the fmask and dmask during mounting to fmask=0000, dmask=0000, but I don't know how to do this between RouterOS and the container, if it's even possible)?