Report the hex-code of the found invalid character

[DL6ER]

What does this implement/fix?

Make the Pi-hole diagnosis message a bit clearer.

Before:

Host name of client "192.168.1.2" => "äbc" contains (at least) one invalid character at position 0

Now:

Host name of client "192.168.1.2" => "äbc" contains (at least) one invalid character (hex e4) at position 0

Knowing the hex code may be useful in trying to debug this: https://www.rapidtables.com/code/text/ascii-table.html

This may not be the very best example but think of the invalid character being something unprintable like <EOF>.

We also add some safety measure so malicious modifications to the Pi-hole diagnosis entries in the database cannot trigger a buffer overflow.

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Related issue or feature (if applicable): See discourse thread linked below.

Pull request in docs with documentation (if applicable): N/A

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2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• [x] The code change is tested and works locally.
• [x] I based my code and PRs against the repositories developmental branch.
• [x] I signed off all commits. Pi-hole enforces the DCO for all contributions
• [x] I signed all my commits. Pi-hole requires signatures to verify authorship
• [x] I have read the above and my PR is ready for review.

[pralor-bot]

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

https://discourse.pi-hole.net/t/host-name-of-client-xxx-contains-at-least-one-invalid-character-at-position-0/69132/11

[DL6ER]

079c66c adds control character escaping to the output. To reduce code-duplication, we simply reuse the already existing JSON string encoder acknowledging that the log file is UTF-8 so we only really need to escape control characters:

Another example containing a control character:

WARNING: Host name of client "127.0.0.1" => "xyz\nabc" contains (at least) one invalid character (hex 0a) at position 3

if a client advertises host name containing a newline as in this new example:

The "one-character sequences" will be escaped in the following, well-known, way:

• \"
• \b
• \f
• \n
• \r
• \t, and also
• \\ because this is needed to make the above useful

all others will be escaped following the JSON syntax as, e.g. \u001B for "escape" also known but not standardized as "\^[".