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The prologue
Most of the articles, descriptions and instructions written here are applicable to the most common Debian-based Linux derivatives. Depending on the respective operating system, there may be minor or major discrepancies. 
This website is for educational purposes only. Please do not deploy anything in manufacturing plants.
No warranty or compensation is given for loss of data or hardware.
It should be also mentioned that this modest web server is hosted on a Raspberry Pi type 4B at home.
The Raspberry Pi mini-computer board as multi-purpose server deployed
A competent allrounder for domestic purposes and micro-enterprises
Raspberry Pi is a series of small single-board computers (SBCs) developed in the United Kingdom by the Raspberry Pi Foundation in association with Broadcom. The mini-computer with its armv7l processor has quickly become the favourite of hobbyists. Projects can be started with suitable Linux distributions. Even an aged RasPi e.g. the models 2B and 2B+ can definitely serve to simple tasks quite well.
Caution Proprietary and untested third-party software sources (Linuxes) can threaten the system.
Prologue
This description is valid for any Raspberry Pi with Debian, Raspbian operating system. It is also possible to use an old and small netbook or similar. I am mainly concerned in this manual / description about the use of computers with low power consumption in residential home networks and micro-enterprises.
If the main scan device is not yet connected to the local network.
SANE | Get the scanner device from the All-In-One printer working
... on your local network server
Raspberry Pi ←→ Ethernet ←→ Hub / Modem / Router
Raspberry Pi ←→ USB ←→ All-In-One printer (with scanner)
Raspberry Pi operates as »headless« server : no monitor, no mouse device and no keyboard plugged.
SANE project | Prologue
The SANE-project : Scanner Access Now Easy
Scanners are available in many versions. To use the basic functions of a scanner, such as resolution, black and white / grayscale / color scans or special keys, a device driver is required. Debian, Linux and Ubuntu in general uses its own open interface for scanners, which is independent from the manufacturers: the SANE interface. This interface was developed as a standard not only for flatbed scanners, but also for All-In-One printers, hand scanners, video and still cameras or frame grabbers.
Server | Preparatory work
Step-By-Step : connect your Raspberry Pi via LAN-cable (ethernet) to your local Hub, modem or router. Connect your Raspberry Pi via USB-cable to your printer. Switch on the printer then switch on (boot) the Raspberry Pi. Login via the terminal program.
pi@NetworkPrinter:~ $ sudo su
root@NetworkPrinter:# apt update && apt upgrade
Server | Rpi-update
Not needed, just in case. »rpi-update« is a command line application that will update your Raspberry Pi OS kernel and VideoCore firmware to the latest pre-release versions. WARNING: Pre-release versions of software are not guaranteed to work. Execute the command only as a last resort and be sure to make a backup of your important data.
root@NetworkPrinter:# rpi-update
Client | Preparatory work
Login via terminal the program.
client:~ $ sudo su
root@client:# apt update && apt upgrade
SANE | Server (Raspberry Pi) setup and configuration
Merely install the SANE interface and not more.
root@NetworkPrinter:# apt install sane --no-install-recommends
SANE comes with useful tools for checking the configuration.
Find out if it can find the scan device in your All-In-One printer.
For our case we have it to deal with my antique 
HP Deskjet All-In-One printer F4180.
root@NetworkPrinter:# sane-find-scanner
# sane-find-scanner will now attempt to detect your scanner. If the
# result is different from what you expected, first make sure your
# scanner is powered up and properly connected to your computer.
# No SCSI scanners found. If you expected something different, make sure that
# you have loaded a kernel SCSI driver for your SCSI adapter.
found USB scanner (vendor=0x03f0 [HP], product=0x7e04 [Deskjet F4100 series])
at libusb:001:005
found USB scanner (vendor=0x0424, product=0xec00)
at libusb:001:003
# Your USB scanner was (probably) detected. It may or may not be supported by
# SANE. Try scanimage -L and read the backend's manpage.
# Not checking for parallel port scanners.
# Most Scanners connected to the parallel port or other proprietary ports
# can't be detected by this program.
root@NetworkPrinter:# scanimage -L
device `hpaio:/usb/Deskjet_F4100_series?serial=CN7CQ6V11X04TJ' is a
Hewlett-Packard Deskjet_F4100_series all-in-one
Try scanning something to confirm the functionality. Be patient. Your little Raspberry Pi needs a while.
Put a printed A4 sample sheet inside the scanner and use the command.
root@NetworkPrinter:# scanimage > ~/test-scanner.pnm
Details about the test file scanned. Also you can open the *.prm file with a program like GIMP.
root@NetworkPrinter:# file ~/test-scanner.pnm
/root/test-scanner.pnm: Netpbm image data, size = 638 x 877, rawbits, pixmap
Scanner works now locally. Now set up the sane daemon.
Alter the configuration file /etc/default/saned and to turn the sane daemon/server on
root@NetworkPrinter:# nano /etc/default/saned
# Defaults for the saned initscript, from sane-utils
# Set to the user saned should run as
RUN_AS_USER=saned
RUN=yes
Alter the file /etc/sane.d/saned.conf and tell the server to accept connections from all IP addresses in the range 192.168.1.1 to 192.168.1.255 (IP ranges varies, use hostname -I to identify).
Uncomment the line : #data_portrange = 10000 - 10100.
root@NetworkPrinter:# nano /etc/sane.d/saned.conf
...
## Daemon options
# Port range for the data connection. Choose a range inside [1024 - 65535].
# Avoid specifying too large a range, for performance reasons.
#
# ONLY use this if your saned server is sitting behind a firewall. If your
# firewall is a Linux machine, we strongly recommend using the
# Netfilter nf_conntrack_sane connection tracking module instead.
#
data_portrange = 10000 - 10100
...
...
#scan-client.somedomain.firm
#192.168.0.1
#192.168.0.1/29
#[2001:db8:185e::42:12]
#[2001:db8:185e::42:12]/64
192.168.1.0/24
...
The SANE daemon has to access the scanner without »root« permissions.
"Found USB scanner" ... "at libusb:001:005" tells, scanner is located at /dev/bus/usb/001/005.
root@NetworkPrinter:# ls -l /dev/bus/usb/001
total 0
crw-rw-r-- 1 root root 189, 0 Oct 11 06:14 001
crw-rw-r-- 1 root root 189, 1 Oct 11 01:37 002
crw-rw-r-- 1 root root 189, 2 Oct 11 01:37 003
crw-rw-r-- 1 root root 189, 3 Oct 11 06:14 004
crw-rw-r--+ 1 root lp 189, 4 Oct 11 06:14 005
The scanner is owned by »root« within the group »lp« - »linux printing«.
Add the SANE daemon to the group »lp« for the essential permissions.
root@NetworkPrinter:# adduser saned lp
Adding user `saned' to group `lp' ...
Adding user saned to group lp
Done.
Restart SANE daemon to overtake the changes.
root@NetworkPrinter:# systemctl start saned.socket
root@NetworkPrinter:# systemctl enable saned.socket
root@NetworkPrinter:# systemctl status saned.socket
● saned.socket - saned incoming socket
Loaded: loaded (/lib/systemd/system/saned.socket; enabled;
vendor preset: enabled)
Active: active (listening) since Sun 2020-10-11 21:55:06 CEST; 5min ago
Listen: [::]:6566 (Stream)
Accepted: 0; Connected: 0;
Tasks: 0 (limit: 995)
CGroup: /system.slice/saned.socket
Oct 11 21:55:06 NetworkPrinter systemd[1]: Listening on saned incoming socket.
Perform a »nmap« port scan. Use either hostname or the IP-address of your Raspberry Pi.
root@NetworkPrinter:# apt install nmap
root@NetworkPrinter:# nmap networkprinter
Starting Nmap 7.70 ( https://nmap.org ) at 2020-10-11 22:17 CEST
Nmap scan report for NetworkPrinter (192.168.1.27)
Host is up (0.00044s latency).
rDNS record for 192.168.1.27: Pi-PrintServer.fritz.box
Not shown: 994 closed ports
PORT STATE SERVICE
22/tcp open ssh
...
...
631/tcp open ipp
6566/tcp open sane-port
Nmap done: 1 IP address (1 host up) scanned in 9.17 seconds
Now everything is fine. The Raspberry Pi has services linked to port 22 for SSH, port 631 for printing via CUPS and port 6566 for scanning via SANE.
It's coffeetime now !
SANE | Client (laptop, notebook, PC workstation) setup and configuration
XSane is an open source and freely distributed graphical application implemented in GTK+ and designed from the offset to act as a GUI (Graphical User Interface) for the SANE (Scanner Access Now Easy) image scanning command-line program.
Skanlite is a simple image scanning application to scan and save images. Skanlite is based on libksane, an interface for SANE library to control image scanners.
Simple Scan is a PDF document scanner application. You can scan documents, photos, receipts, reports, or just about anything. The scan will be saved to the device in image or PDF format.
root@client:# apt install xsane skanlite simple-scan
Simple Scan is my favourite because it is so simple.
Tell your client computer where to search for the SANE services.
Uncomment one line in /etc/sane.d/net.conf and enter the IP-address of your Raspberry Pi.
root@client:# nano /etc/sane.d/net.conf
# This is the net backend config file.
## net backend options
# Timeout for the initial connection to saned. This will prevent the backend
# from blocking for several minutes trying to connect to an unresponsive
# saned host (network outage, host down, ...). Value in seconds.
connect_timeout = 60
## saned hosts
# Each line names a host to attach to.
# If you list "localhost" then your backends can be accessed either
# directly or through the net backend. Going through the net backend
# may be necessary to access devices that need special privileges.
# localhost
192.168.1.27
Client and server | Complementary work
To be on the safe side reboot client and server. Clientside close all open applications.
root@client:# shutdown -r now
root@NetworkPrinter:# shutdown -r now
Work done. Scan a test document after reboot.
11-Oct 2020