Display more meaningful exit message if dig command fails during os_check

[PromoFaux]

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• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

---

What does this PR aim to accomplish?:

Fixes #3694

Alternative to #3695

Outputs as following:

Note: I couldn't be bothered with the rigmaroll of blocking outbound port 53 on my network so I changed the @ns1.pi-hole.net to @ns141@pi-hole.net to simulate being unable to resolve @ns1.pi-hole.net

[PromoFaux]

Got some more stuff to do here...

[DL6ER]

This runs the identical dig two times and wouldn't catch it the second fails. I wonder if we should, instead, rather check for an invalid or empty response from

output="$(dig +short -t txt versions.pi-hole.net @ns1.pi-hole.net)"

rather than trying to extract the exact exit code?

In this case, we could show what was returned ("expired", empty or whatever) and still catch the error.

[DL6ER]

In the case of the first failure (ns1.pi-hole.net = NXDOMAIN), the supportedOS array contains one element of FAILED

In the case of the second (a timeout - hence random IP) the supportedOS array contains the entire ;; connection timed out... message, but not the FAILED

We could maybe simply redirect stderr into stdout to resolve this.

Example:

$ out="$(dig +short -t txt versions.pi-hole.net @use-application-dns.net || echo "FAILED")"
dig: couldn't get address for 'use-application-dns.net': not found

$ echo "$out"
FAILED

With 2>&1:

$ out="$(dig +short -t txt versions.pi-hole.net @use-application-dns.net 2>&1 || echo "FAILED")"
[no output]

$ echo "$out"
dig: couldn't get address for 'use-application-dns.net': not found
FAILED

Also:

$ out="$(dig +short -t txt versions.pi-hole.net @1.2.3.4 2>&1 || echo "FAILED")"
[no output]

$ echo "$out"
;; connection timed out; no servers could be reached
FAILED

Note: use-application-dns.net is always NXDOMAIN in Pi-hole environments to prevent Firefox's inbuilt DoH from taking over.

[PromoFaux]

In the case of a failure, we only want the "failed" part (or more helpfully, just the exit code). Hence the double-hit of the function. With the 2>&1 it is nice because it hides the error message from immediate display, but we don't want the error message in the variable :)

Otherwise we are in the territory of string comparison for actual error messages, which obviously will cause issues internationally

[DL6ER]

I was thinking more along the lines of doing exactly this || echo "FAILED" and then check only for this FAILED being the last line - or not. Then no international stuff comes into play. For this, we'd obviously have to do the dig before the loop, then do some checking and then iterate over the variable instead of the immediate dig output.
More like

out="$(dig +short -t txt versions.pi-hole.net || echo "FAILED")"

# last line should be
echo "${out##*$'\n'}"
# or
echo "$out" | tail -n1

# Test based on these lines being == "FAILED"
# Only run loop if this is not the case!

while IFS= read -r line; do
    # Check if $line is empty, in this case, break out of the loop and print some error?
    echo "... $line ..."
done <<< "$out"

Sorry, I don't have a Pi-hole with me right now or I'd propose a more working code for you. I'm just thinking out aloud here, trusting you will do something amazing with it :-)

[PromoFaux]

Hold my beer...

[PromoFaux]

out="$(dig +short -t txt +timeout=1 versions.pi-hole.net @ns1.pi-hole.net 2>&1|| echo $?)"
echo "ResultFromWorking:"
echo "${out##*$'\n'}"
out="$(dig +short -t txt +timeout=1 versions.pi-hole.net @10.1.2.3 2>&1 || echo $?)"
echo "ResultFromTimeout:"
echo "${out##*$'\n'}"
out="$(dig +short -t txt +timeout=1 versions.pi-hole.net @use-application-dns.net 2>&1 || echo $?)"
echo "ResultFromNXDomain:"
echo "${out##*$'\n'}"

results:

ResultFromWorking:
"Raspbian=9,10 Ubuntu=16,18,20 Debian=9,10 Fedora=31,32 CentOS=7,8"
ResultFromTimeout:
9
ResultFromNXDomain:
10

OK, I should be able to work with this, just need to check if the result is a number. According to this page it should only be one of 5 numbers:

Dig return codes are:

• 0: Everything went well, including things like NXDOMAIN
• 1: Usage error
• 8: Couldn't open batch file
• 9: No reply from server
• 10: Internal error

[PromoFaux]

Right, totally redid it (force pushed changes), and made some tweaks to the copy

Example outputs:

Blank response

dig returns non-zero

Debug info:

Full error output:

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pihole-fails-to-update-on-ubuntu-18-04-lts-says-unsupported-os/37454/15

[DL6ER]

This comment is wrong here (copy-paste from the installer)

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-core-web-v5-2-and-ftl-v5-3-released/40909/1