AUTHOR: Stuart Foster <smf.account1@ntlworld.com>

DATE: 2023-08-10

LICENSE: GNU Free Documentation License Version 1.3

SYNOPSIS: Enable SysV BLFS systems to access a networked CUPS server.

DESCRIPTION:

This hint describes changes to BLFS configurations and the addition of new
software to allow location on a network of a LFS/BLFS CUPS computer running as
a print server. The core mechanism used is mDNS which is the "multicast Domain
Name System" or "Bonjour". 

ATTACHMENTS:

PREREQUISITES:

- Basic knowledge and experience of building LFS/BLFS packages.  
- A preconfigured CUPS server that has nominated printers to share network wide.  
- An existing LFS/BLFS client system or a new LFS/BLFS client system in construction.  
- Some familiarity with GIT for downloading software.  

This hint is intended only for users that follow the LFS/BLFS development
process.  LFS/BLFS version 11.3 was used to investigate these changes, but it
should be applicable to other recent SysV LFS/BLFS builds.

HINT:

This hint describes a mechanism for the creation of temporary CUPS
print queues (as apposed to the normal permanent printer queues created
by the standard BLFS installation of CUPS). These temporary print
queues track the permanent print queues present on the main CUPS print
server. The changes will enable network wide access to these printers
on LFS/BLF client computers via these queues.

To implement the new configuration the following BLFS components will
be required to be installed on client systems with specific
configuration changes which are detailed below,

* AVAHI (service discovery)
* CUPS
* CUPS-FILTERS
* GIT
* /etc/nsswitch.conf

For reference the test system described here also had installed:

* CUPS-PK-HELPER

This package probably does not contribute much to the final solution
but was present through out the whole process so it is included here
for completeness.

The core addition to the BLFS system is the NSS-MDNS software package:

NSS-MDNS is from https://github.com/lathiat/nss-mdns/

Use GIT to clone this package into a suitable location (/sources).
NSS-MDNS is a plugin for the GNU NSS (Name Service Switch)
functionality which is part of the GNU C Library (GLIBC) it allows host
name resolution via Multicast DNS.

For consistency I suggest these changes be included on all the LFS/BLFS
systems on your local network.

Start by installing AVAHI include the required, recommended and
optional dependencies.

Once installed run the following tests:

avahi-resolve-host-name <cupsserver>.<yourdomain>
Only a fully qualified address will work for this test.

e.g.
From the example test system:

avahi-resolve-host-name Moira.delta

The example system returned:

Moira.delta 192.168.0.1

Then run:

avahi-resolve-host-name <cupsserver>.local
At this stage this should fail.

e.g.
From an example system:

avahi-resolve-host-name Moira.local

This time the example system returned:

Failed to resolve host name 'Moira.local': DNS failure: NXDOMAIN

This is the correct behaviour prior to the installation of NSS-MDNS.

Next proceed to install CUPS, If CUPS is already present uninstall it
and remove the /etc/cups directory and contents, a new clean directory
will be created during the building of CUPS. Again install the
required, recommended and optional dependencies. Note specifically the
AVAHI option and the installation of the boot script.

The next package to install is CUPS-FILTERS.
Include the required and recommend dependencies.
Also include any optional printer drivers (for runtime) as appropriate
for your configuration.
Configure and install the package as follows to allow interfacing with
AVAHI (see the package notes):

...
CXXFLAGS="$CXXFLAGS -std=c++17" \
./configure --prefix=/usr \
 --sysconfdir=/etc \
 --localstatedir=/var \
 --without-rcdir \
 --disable-static \
 --docdir=/usr/share/doc/cups-filters-1.28.16 &&
make
...

The LFS book constructs the /etc/nsswitch as part of the LFS GLIBC
configuration this now needs to be modified to enable multicast dns
lookups.

In "/etc/nsswitch.conf" change the hosts line from:

hosts: files dns

to:

hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4

The final stage in the modified CUPS BLFS system build is to include
the NSS-MDNS package.

First using "ls" make a note of the AVAHI communication sockets
location, it should be:

 /run/avahi-daemon/socket

Next go to the location of the GIT downloaded NSS_MDNS package and read
the notes (README.md). Then build the package as follows:

./configure --prefix=/usr/ --localstatedir=/
make

When built check that the package has the right interface to AVAHI run:

strings ./.libs/libnss_mdns.so.2|grep avahi-daemon

This should return:

//run/avahi-daemon/socket

This must resolve to the same location expected by AVAHI.

If all is well install the package:

make install

At this point I suggest a reboot is in order to ensure a consistent
system.

Now if we revisit the failed AVAHI test we did earlier:

avahi-resolve-host-name <cupsserver>.local

This should now return the address of the CUPS server:

e.g.
From the example system:

avahi-resolve-host-name Moira.local

now returns the correct IP address:

Moira.local 192.168.0.1

We are now in a position to interogate the CUPS server for any printers
it is advertising.

Run the following command:

lpstat -l -e

This must return at least one network available printer queue before
proceeding.
Assuming at least one printer is adversised run the following test to
check that all is well and the printer is functional.

lp -d <network printer queue> /etc/fstab

A printed copy of your /etc/fstab should result.

e.g.
Using the example system (There are 3 printers on the example test CUPS
server):

lpstat -l -e

returned:

Hp5550_Inkjet_Printer_Moira network none ipps://Hp5550%20Inkjet%20Printer%20%40%20Moira._ipps._tcp.local/cups
HP6020e_Moira network none ipps://HP6020e%20%40%20Moira._ipps._tcp.local/cups
hp_photosmart_7700_series_Moira network none ipps://hp%20photosmart%207700%20series%20%40%20Moira._ipps._tcp.local/cups

Selecting the HP7700 series printer I can print an example file with:

lp -d hp_photosmart_7700_series_Moira  /etc/fstab

The file is printed and the following is typically returned:

request id is hp_photosmart_7700_series_Moira-1 (1 file(s))

Applications (GIMP, FIREFOX etc) should now be able to print using
these temporary printer queues.

That concludes this hint, for more information refer to the
documentation on the referenced web sites.

Finally it is worth stating that the core of this hint is the
introduction of NSS-MDNS. The inclusion of ALL the dependencies around
AVAHI and CUPS may be considered overkill by some, but I was looking
for a consistent approach going forward so I took the approach
described here, I was also influenced by the approach used in AFLS
which I have recently been investigating. Please feel free to adjust
these dependencies if you feel an alternate arrangement is more
suitable for your needs, you can always fallback on the complete
solution described here.

ACKNOWLEDGEMENTS:
 * OpenPrinting CUPS (documentation
https://openprinting.github.io/cups)
 * nss-mdns https://github.com/lathiat/nss-mdns/
 * Bonjour https://developer.apple.com/bonjour/


CHANGELOG:
[2023-08-10]
 * Initial release

