Pi-hole install script and dhcpcd.conf

[deHakkelaar]

@Roo

my pi-hole host OS cannot do local name resolution of other devices on my network. The pi-hole itself is correctly resolving these.

Above is bit confusing?

@Roo

The hostname of my pi-hole is "myhost".

@Roo

sudo gives a non-fatal error: myhost : Jul 4 08:14:10 : www-data : unable to resolve host myhost

Sounds as if something is/was missconfigured in below files which the Pi-hole installer doesnt touch:

/etc/hostname

/etc/hosts

@Roo

nslookup myhost will fail

The nslookup tool doesnt read/resolve names in the /etc/hosts file.
Instead it default queries the nameserver(s) defined in below file (which is usualy populated by your installed network manager) if dont specify a DNS server as an extra argument:

/etc/resolv.conf

@Roo

is doing a few things

        # we can append these lines to dhcpcd.conf to enable a static IP
        echo "interface ${PIHOLE_INTERFACE}
        static ip_address=${IPV4_ADDRESS}

Above code only applies if the network manager called dhcpcd5 is installed and active (like for the Pi-OS distro).
My Debian laptop uses an entirely different network manager called NetworkManager thus above code would not apply.
And most likely, your Ubuntu distro uses netplan as a network manager.

@Roo

Many base OS installs will have a default configuration which gets the IP address from the network DHCP server. I would expect that most pi-hole installs are running down this logic path and updating the /etc/dhcpcd.conf file to change to a static address.

Not everyone boots up his/her Pi-hole instance as a DHCP client OOTB.
When I setup a Pi-hole host on a Raspi, my Pi will not query DHCP at any stage.
Instead once I've written the Pi-OS image for my Raspi, I mount the SD card partitions on another host and set a static IP manually plus activate sshd before I insert the SD card in the Pi and boot.
Many hypervisors allow to boot up a VM guest already configured with a static IP.

@Roo

In this change - the DNS server is being set to what was configured as the PI_HOLE_DNS1 / PI_HOLE_DNS1. This seems wrong - as it will immediately break local resolution.

Maybe this is because I specified custom DNS servers? (vs using google/quad9/etc)

That code snippet clearly states "Do you want to use your current network settings as a static address?".
And "Otherwise, we need to ask the user to input their desired settings.".
So the PI_HOLE_DNS1 & PI_HOLE_DNS2 IP's is/are derived from what you've chosen it to be.
These settings/DNS IP's are unrelated to what you choose as upstream DNS servers for the pihole-FTL daemon during install (or via the GUI).
These are settings that also need to be in place if you were not running Pi-hole on the host just like any OS thats connected and needs DNS settings.

You have to keep in mind that at first, the host is only a DNS client and depends on an external DNS server for resolution.
Once you've installed Pi-hole, the host functions as a DNS client AND server.
And the host can be a client to its own services if you choose so.
But this is not necessary for your other network clients to function properly.
I for example have configured an external PI_HOLE_DNS1 IP (my router) because when tinkering (which I do allot) and break Pi-hole again, I still have DNS available for local running software.

@Roo

My core question here is why is the install script like this?

As it feels like your question was based on wrong assumptions, could you rephrase your question again pls?