I have a static IP address for my Brother HL-3170CDW printer, but can only ever print when connected to my network via ethernet; any suggestions on where I can begin a debug?
Maybe wifi and ethernet are on differend subnets? Some network protocols only work in the same broadcast domain (cannot pass through a router).
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Thanks for the ideas, Alessio; sadly, I already confirmed there aren’t different subnets. 
Please check the result of ip address and ip route so we can see the routing and addressing including netmasks on the interfaces from your pc.
Also check the address and netmask on the printer to confirm all matches.
This almost sounds like the routing is going via the ethernet and not via the wifi so checking all those outputs should assist in finding the issue.
It is also known that having 2 interfaces on the same subnet may at times result in addressing issues. Not as much today but was a major issue in past times.
edit:
One other issue may be at the router.
If the router sees your pc on the ethernet as well as wifi then it may be involved in the traffic misdirection. Possibly a power cycle of the router when this problem is seen could fix the issue.
Check the output:
lpinfo -v; lpstat -t
Does the issue persist if you stop the firewall service?
I have a static ip for both my laptop and the machine, but conflicts between hardline and wifi is an interesting idea; I’ll have to check that.
lpinfo -v; lpstat -t provides exactly the same output:
no system default destination
device for HL-2170W: lpd://Brother/BINARY_P1
device for HL-3170CDW: ipp://Brother%20HL-3170CDW%20series._ipp._tcp.local/
HL-2170W accepting requests since Sun 24 Dec 2023 01:57:56 AM EST
HL-3170CDW accepting requests since Wed 12 Jun 2024 04:41:25 PM EDT
printer HL-2170W is idle. enabled since Sun 24 Dec 2023 01:57:56 AM EST
printer HL-3170CDW is idle. enabled since Wed 12 Jun 2024 04:41:25 PM EDT
You may want to edit your printer URIs, replacing hostnames with IPs:
sudoedit /etc/cups/printers.conf
sudo systemctl restart cups.service
Otherwise, continue troubleshooting:
getent hosts Brother
getent hosts Brother%20HL-3170CDW%20series._ipp._tcp.local
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can only ever print when connected to my network via ethernet
Just to be sure – printing only works when both PC and printer are on ethernet? Are you using IPv4 and/or IPv6? Are you using IPP or a legacy CUPS driver?
Would that be as simple as replacing the URIs with IPs; for example, replacing DeviceURI ipp://Brother%20HL-3170CDW%20series._ipp._tcp.local/ with DeviceURI 192.168.XXX.XXX?
On wifi, neither of those commands results in anything…
The printer is hard-wired, yes, and printing only works when I am also hard-wired. Both IPv4 and IPv6 are enabled on the network and the driver is whatever what automatically set up in CUPS when I added the printer…
That sound like that wifi data aren’t properly bridged to the wired network. If you have a DSL or docsis modem with built-in wifi and ethernet ports I would expect that this box would pass wifi packages to the ethernet ports and vice-versa.
Thanks for all the ideas… One of the two printers is now working on both ethernet and wifi by replacing hostnames with IPs; now to troubleshoot what’s wrong with the other one.
Again, thanks for all the help!