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Network landscape

My network is made through Internet Sharing from a Mac: Bob hosting an USB printer: Clara. On this home network is a Mac: Alice which is usually printing on Clara through Bob.

Alice    --AirPort--    Bob    --Ethernet--    Internet
Mac                     Mac
10.0.2.23       10.0.2.1    192.168.1.1
                         |
                        USB
                         |
                        Clara
                        printer

Of course, on Bob the Firewall is on, and the sharing of Clara is on.

One key detail: this problem of shared printing failing occured when I upgraded Bob from Mountain Lion to Yosemite.

Bug

For security reasons (don't let open what you don't fully control), I stop any IPv6 on the internal side of my network.

On both Alice, Bob I ran:

networksetup -setv6off Wi-Fi

Since this modification my network is working perfectly (and in fact much better). Alice can access the Internet with ssh, http, IMAP, smtp… without a glitch, but for printing.

Alice does still see Clara as a shared printer, but printing dosen't work, and the printer window stays on the message:

Printing - Looking for printer

forever. The CUPS error log does says:

E [23/May/2015:11:05:50 +0200] [Job 621] Unable to find printer.

If on Alice I check the reachability of CUPS on Bob I get:

alice$ telnet 10.0.2.1 631
Trying 10.0.2.1...
Connected to bob.
Escape character is '^]'.

Hence CUPS is running on Bob and perfectly reachable through IPv4.

If I turn IPv6 back on, shared printing is working again. The firewall isn't involved in this network problem.

Question

How may I get shared printing to work on an IPv4 only network?

I already found this correct analysis of a similar problem on Linux: Disabling IPv6 seems to break CUPS

But the suggested modification to the cupsd.conf framed with the relevant launchctl stop... launchctl start doesn't fix it.

Moreover I discovered that I have to be cautious on tuning:

/etc/cups/cupsd.conf

because Printer Sharing activation is overwriting this configuration file.

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