New Linux distributions are enabling local DNS caching. For most users this is a sane default, but as a system administrator this can sometimes get in the way. You can check if your local cache is running and see the statistics with:

resolvectl statistics

Which will tell you how active your cache is, what the hit/miss ratio is, and information about DNSSEC. If you need to clear your local cache you can run:

resolvectl flush-caches

Doing this repeatedly can get cumbersome if you are testing remote DNS frequently. If you want to disable local caching all together you can run:

systemctl disable systemd-resolved.service --now

Remember to update your /etc/resolv.conf to point at the correct DNS server after you're done.

I need to read the RPMs of some 12v PC fans so I wrote up a quick sketch to have an ESP32 monitor and log the RPM for me. You need to use an external interrupt that triggers every time the fan rotates (it triggers 2x per full rotation). I was having crashes before I added the ICACHE_RAM_ATTR parameter to the function. ICACHE_RAM_ATTR tell Arduino to store this function in RAM instead of on the flash. The logic here is that your interrupt function will probably be getting called a lot so it needs to be fast.

Connect the yellow RPM wire from your fan to pin 4, and the ground wire from the fan to the ground of the Arduino. Without the ground wire connected you will now have a shared ground between the Arduino and the fan and the interrupt will not fire.

const uint8_t tachoPin    = 4;
volatile uint32_t counter = 0;

ICACHE_RAM_ATTR void countPulses() {
    counter++;
}

void setup() {
    pinMode(tachoPin, INPUT_PULLUP);
    attachInterrupt(digitalPinToInterrupt(tachoPin), countPulses, FALLING);
    Serial.begin(115200);
}

void loop() {
    delay(1000);
    Serial.printf("%u fan revolutions in the last second\n", counter / 2);

    counter = 0;
}

Note: This code will work on an Arduino Uno without the ICACHE_RAM_ATTR which can be confusing.

Note: Using the internal pull-up resistor on the ESP32 means you can connect the yellow fan wire directly to the MCU. On an Arduino Uno an external pull-up resistor is required.

I'm working with a 32bit PRNG and I'd like to get a 64bit integer out of it. You can take two random 32bit integers and combine them into a 64bit one this way:

uint64_t high = rand32();
uint64_t low  = rand32();

uint64_t num64 = (high << 32) | low;

Someone on Reddit suggested using this cool hash to perform a similar task to "upgrade" a 32bit number into a 64bit one by hashing it:

// Borrowed from: https://elixir.bootlin.com/linux/v6.11.5/source/include/linux/hash.h
static inline uint64_t hash64(uint64_t val, unsigned bits) {
        return (val * 0x61c8864680b583ebull) >> (64 - bits);
}

Once you find a Unicode code point you can put it into a Perl string in several ways:

my $thumbs_up = "";

$thumbs_up = "\x{1F44D}";
$thumbs_up = "\N{U+1F44D}";
$thumbs_up = chr(0x1F44D);
$thumbs_up = pack("U", 0x1F44D);

# Make sure STDOUT is set to accept UTF8
binmode(STDOUT, ":utf8");

print $thumbs_up x 2 . "\n";

The French author Georges Perec wrote a novel titled A Void without using the letter E. Gilbert Adair translated it into English without using the letter E. Perec wrote a follow up novel, Les revenentes, in which E is the only vowel used.

The Wikipedia summary also excludes the letter E, and it's amazing.

If you need to see if a file is world readable you'll need to be able to break out file permissions.

my @p     = get_file_permissions("/tmp/foo.txt");
my $other = $p[2];

# 4 = readable, 2 = writeable, 1 = executable
if ($other & 4) { print "File is world readable\n"; }
if ($other & 2) { print "File is world writeable\n"; }
if ($other & 1) { print "File is world executable\n"; }

sub get_file_permissions {
    my $file = shift();

    my @x    = stat($file);
    my $mode = $x[2];

    my $user  = ($mode & 0700) >> 6;
    my $group = ($mode & 0070) >> 3;
    my $other = ($mode & 0007);

    my @ret = ($user, $group, $other);

    return @ret;
}

If you need to calculate the average volume or loudness of a file you can use FFmpeg with the volumedetect filter:

ffmpeg -i input.mp3 -af volumedetect -f null /dev/null 2>&1 | grep -P '(mean|max)_volume'

This will report the average (mean) and the maximum volume. This information is useful when comparing two files to see if their "loudness" (or quietness) is comparable.

[Parsed_volumedetect_0 @ 0x698a300] mean_volume: -19.7 dB
[Parsed_volumedetect_0 @ 0x698a300] max_volume: -1.0 dB

Meta released an update of their Llama AI today and it has a mostly open source license. It's 100% free as long as you have less than "700 million monthly active users in the preceding calendar month". That's cool, but it seems like a pretty arbitrary number. The current world population is 8.1 billion people. As long as no more than 11.5% of the world's population uses your instance of Llama, it's free to you.

Reddit had a mini challenge about implementing UUIDv7 in various languages. I whipped up a Perl implementation that turned out pretty well. I submitted it to the official GitHub repo and it was accepted.

See also: UUIDv4 in Perl.

Newer versions of OpenSSH have deprecated small key sizes for security reasons. We still have some older equipment that uses these types of keys that we need to access. You can work around this with:

ssh -o RSAMinSize=1024 user@domain.com

Perl has regular expressions built in to the core of the language and they are very powerful. It's easy enough to find a single match with a regexp:

# Find the *first* three letter word in the string
my $str = "one two three four five six seven eight nine ten";
my @x   = $str =~ m/\b(\w{3})\b/; # ("one")

If you want to find all of the three letter words you can add the g modifier to the end of your regex to tell it to match "globally".

# Find *all* the three letter words
my $str = "one two three four five six seven eight nine ten";
my @x   = $str =~ m/\b(\w{3})\b/g; # ("one", "two", "six", "ten")

You can also iterate on your global regexp if you want to get the matches one at a time:

my $str = "one two three four five six seven eight nine ten";
my @x   = ();
while ($str =~ m/\b(\w{3})\b/g) {
    push(@x, $1);
}

print join(",", @x); # "one,two,six,ten"

Rocky Linux uses PrivateTmp to give each process it's own walled off section of /tmp/. This is good for security sometimes, but it can also lead to frustration because files written to /tmp/ or /var/tmp/ will not show up when you need to debug. To disable PrivateTmp for php-fpm you need to modify the systemd configuration for php-fpm. This is done by running systemctl edit php-fpm and adding a section that says:

[Service]
PrivateTmp=false

Once the change is in place you will need to systemctl daemon-reload and then restart systemctl restart php-fpm.

Note: Borrowed from Stack Overflow.

I'm using Kea as a DHCP server and I need the service to start on boot. I'm not sure why it doesn't ship with a systemd .service file. Here is the file I ended up using:

# Put this in /etc/systemd/system/kea-dhcp4.service
# Reload systemd so it picks it up `systemctl daemon-reload`
# Enable it on boot: `systemctl enable kea-dhcp4`

[Unit]
Description=ISC KEA IPv4 DHCP daemon
Documentation=man:kea-dhcp4(8)
Wants=network-online.target mariadb.service
Requires=kea-ctrl-agent.service
After=network-online.target mariadb.service mysql.service

[Service]
ExecStart=/usr/sbin/kea-dhcp4 -c /etc/kea/kea-dhcp4.conf

[Install]
WantedBy=multi-user.target

This is borrowed from Skywalker-11.

I need to monitor the Ethernet interfaces of a Linux VM. This is the perfect job for snmpd which you can get by installing net-snmp and then applying a basic config. Here is a simplified config that will get you basic read-only access for a community and subnet.

# /etc/snmp/snmpd.conf
rocommunity snmp-read 165.92.231.0/24
rocommunity snmp-read 10.3.1.0/24
rocommunity snmp-read localhost
syslocation "City, State"
syscontact  "Admin <user@domain.com>"

You can test your new SNMP configuration with snmpwalk

snmpwalk -v 2c -c snmp-read 127.0.0.1

List of books I read in 2024. Also see the list of 2023. The date indicated denotes the date I started reading the book.

Drizzt novels

2024-01-08 - Homeland - Drizzt #1 by R. A. Salvatore - 343 pages
2024-02-06 - Exile - Drizzt #2 by R. A. Salvatore - 343 pages
2024-03-02 - Sojourn - Drizzt #3 by R. A. Salvatore - 346 pages
2024-04-01 - The Crystal Shard - Drizzt #4 by R. A. Salvatore - 344 pages
2024-05-09 - Streams Of Silver - Drizzt #5 by R. A. Salvatore - 377 pages
2024-06-10 - The Halfling's Gem - Drizzt #6 by R. A. Salvatore - 378 pages

Dark Tower novels (Paperback collection)

2024-01-13 - The Gunslinger: Revised - Dark Tower #1 by King, Stephen - 253 pages
2024-01-28 - The Gunslinger: Original - Dark Tower #1 by King, Stephen - 224 pages
2024-02-12 - The Drawing Of The Three - Dark Tower #2 by Stephen King - 459 pages
2024-03-14 - The Waste Lands - Dark Tower #3 by Stephen King - 606 pages
2024-04-12 - Wizard and Glass - Dark Tower #4 by Stephen King - 893 pages
2024-05-04 - The Wind Through The Keyhole - Dark Tower #4.5 by Stephen King - 309 pages
2024-06-15 - Wolves Of The Calla - Dark Tower #5 by Stephen King - 709 pages
2024-07-26 - Song Of Susannah - Dark Tower #6 by Stephen King - 411 pages
2024-08-15 - The Dark Tower - Dark Tower #7 by Stephen King - 845 pages

Redwall novels

2024-01-16 - Redwall by Brian Jacques - 351 pages
2024-02-19 - Mossflower by Brian Jacques - 432 pages
2024-03-08 - Mattimeo by Brian Jacques - 446 pages
2024-04-06 - Mariel of Redwall by Brian Jacques - 387 pages
2024-05-14 - Salamandastron by Brian Jacques - 391 pages
2024-06-03 - Martin The Warrior by Brian Jacques - 376 pages

J.R.R. Tolkien

2024-09-23 - The Hobbit by J.R.R. Tolkien - 303 pages
2024-10-18 - The Fellowship of the Ring by J.R.R. Tolkien - 527 pages
2024-11-24 - The Two Towers by J.R.R. Tolkien - xxx pages
2024-xx-xx - Return of the King by J.R.R. Tolkien - xxx pages

Various novels

2024-01-01 - Revival by Stephen King - 466 pages
2024-01-21 - White Death by Clive Cussler - 419 pages
2024-01-31 - Red Rising by Pierce Brown - 382 pages
2024-02-25 - Star Wars: Exile by Aaron Allston - 337 pages
2024-03-23 - Empire Of Pain by Patrick Radden Keefe - 452 pages
2024-03-28 - Atlas Shrugged by Ayn Rand - 1067 pages
2024-05-28 - The Cat Who Played Post Office by Lilian Jackson Braun - 262 pages
2024-05-31 - The Westing Game by Ellen Raskin - 216 pages
2024-06-27 - The Martian by Andy Weir - 435 pages
2024-07-04 - The Silence Of The Lambs by Thomas Harris - 367 pages
2024-07-09 - Test Of The Twins by Margaret Weis - 330 pages
2024-07-15 - Piranesi by Susanna Clarke - 245 pages
2024-07-19 - You Like It Darker: Stories by Stephen King - 502 pages
2024-07-29 - Dark Matter by Blake Crouch - 342 pages
2024-08-06 - Hannibal by Thomas Harris - 546 pages
2024-08-25 - The Kaiju Preservation Society by John Scalzi - 264 pages
2024-08-28 - Time Of The Twins by Margaret Weis - 432 pages
2024-09-04 - Saint Of Bright Doors by Vajra Chandrasekera - 353 pages
2024-09-10 - Hannibal Rising by Thomas Harris - 368 pages
2024-09-16 - Witch King by Martha Wells - 411 pages
2024-09-27 - Adventures Of Amina Al-sirafi by S. A. Chakraborty - 482 pages
2024-10-10 - Presumed Innocent by Scott Turow - 461 pages
2024-10-26 - Some Desperate Glory by Emily Tesh - 436 pages
2024-11-01 - War Of The Twins by Margaret Weis - 432 pages
2024-11-08 - Mr. Mercedes by Stephen King - 436 pages
2024-12-01 - Test Of The Twins by Margaret Weis - 330 pages
2024-12-06 - Starter Villain by John Scalzi - 264 pages

Anne Rice - The Vampire Chronicles

2024-10-05 - Interview With The Vampire by Anne Rice - 342 pages
2024-11-15 - The Vampire Lestat by Anne Rice - 550 pages
2024-xx-xx - The Queen Of The Damned by Anne Rice - 491 pages