A DDNS service provider script for Synology NAS (based off https://github.com/cpascal/syno-ddns-linode/blob/master/linode.php)

synology-ddns.linode.php

#!/usr/bin/php -d open_basedir=/usr/syno/bin/ddns
<?php

if ($argc !== 5) {
    echo 'badparam';
    exit();
}

// Entered into DSM (Control Panel > External Access > DDNS)
$username = (string) $argv[1];  // Resource ID
$password = (string) $argv[2];  // API Key
$hostname = (string) $argv[3];  // DomainID.domain (e.g. 1234567.henrik.io)
$ip = (string) $argv[4];        // External address

$token      = $password;
$domainId   = explode('.', $hostname)[0];
$resourceId = $username;

// only for IPv4 format
if (!filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4)) {
    echo "badparam";
    exit();
}

function linode_request($curl, $token, $uri, $fields = [], $type = 'GET')
{
    $BASE_URL = 'https://api.linode.com/v4';
    $HEADERS = [
        'Authorization: Bearer '.$token,
        'Content-type: application/json',
    ];

    curl_setopt($curl, CURLOPT_URL, $BASE_URL.$uri);
    curl_setopt($curl, CURLOPT_CUSTOMREQUEST, $type);
    curl_setopt($curl, CURLOPT_HTTPHEADER, $HEADERS);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    if (count($fields) > 0) {
        curl_setopt($curl, CURLOPT_POST, true);
        curl_setopt($curl, CURLOPT_POSTFIELDS, json_encode($fields));
        array_push($HEADERS, 'Accept: application/json');
    } else {
        curl_setopt($curl, CURLOPT_POST, false);
    }
    $res = curl_exec($curl);
    $http_status = curl_getinfo($curl, CURLINFO_HTTP_CODE);

    $error = null;
    switch ($http_status) {
        case 200: break;
        case 401: $error = 'badauth'; break;
        default : $error = '911'; break;
    }
    if ($error !== null) {
        echo $error;
        curl_close($curl):
        exit(-1);
    }
    return json_decode($res, true);
}


$req = curl_init();

//
$json = linode_request($req, $token, 'GET', "/domains/$domainId/records");
if ($json['results'] < 1) {
    echo 'nohost';
    curl_close($req);
    exit();
}

$record = null;
foreach($json['data'] as $row) {
    if ($row['type'] == 'A' && (string) $row['id'] == $resourceId) {
        $record = $row['target'];
    }
}
if ($record === null) {
    echo 'nohost';
    curl_close($req);
    exit();
}
if ($record == $ip) {
    echo 'nochg';
    curl_close($req);
    exit();
}

//
linode_request($req, $token, 'PUT', "/domains/$domainId/records/$resourceId", [ 'target' => $ip ]);
curl_close($req);
echo 'good';