// -=-=- StartHere -=-=-
//
// # Chapter III: using a custom DNS-over-UDP resolver
//
// In this chapter we learn how to measure sending DNS queries to
// a DNS server speaking the DNS-over-UDP protocol.
//
// Without further ado, let's describe our example `main.go` program
// and let's use it to better understand this flow.
//
// (This file is auto-generated. Do not edit it directly! To apply
// changes you need to modify `./internal/tutorial/measurex/chapter03/main.go`.)
//
// ## main.go
//
// The initial part of the program is pretty much the same as the one
// used in previous chapters, so I will not add further comments.
//
// ```Go
package main

import (
	"context"
	"encoding/json"
	"flag"
	"fmt"
	"time"

	"github.com/ooni/probe-cli/v3/internal/measurex"
	"github.com/ooni/probe-cli/v3/internal/runtimex"
)

func main() {
	query := flag.String("query", "example.com", "domain to resolve")
	address := flag.String("address", "8.8.4.4:53", "DNS-over-UDP server address")
	timeout := flag.Duration("timeout", 60*time.Second, "timeout to use")
	flag.Parse()
	ctx, cancel := context.WithTimeout(context.Background(), *timeout)
	defer cancel()
	mx := measurex.NewMeasurerWithDefaultSettings()
	// ```
	//
	// ### Using a custom UDP resolver
	//
	// We now invoke `LookupHostUDP`. We specify:
	//
	// - a context for timeout information;
	//
	// - the domain to query for;
	//
	// - the address of the DNS-over-UDP server endpoint.
	//
	// ```Go
	m := mx.LookupHostUDP(ctx, *query, *address)
	// ```
	//
	// Also this operation returns a measurement, which
	// we print using the usual three-liner.
	//
	// ```Go
	data, err := json.Marshal(m)
	runtimex.PanicOnError(err, "json.Marshal failed")
	fmt.Printf("%s\n", string(data))
}

// ```
//
// ## Running the example program
//
// As before, let us start off with a vanilla run:
//
// ```bash
// go run -race ./internal/tutorial/measurex/chapter03
// ```
//
// This time we get a much larger JSON, so I will pretend it is
// actually JavaScript and add comments to explain it inline.
//
// (This is the first case in which we see how a single
// method call for measurer causes several events to
// be generated and inserted into a `Measurement`.)
//
// ```JavaScript
// {
//   "domain": "example.com",
//
//   // This block tells us about the UDP connect events
//   // where we bind to the server's endpoint
//   "connect": [
//     {
//       "address": "8.8.4.4:53",
//       "failure": null,
//       "operation": "connect",
//       "proto": "udp",
//       "t": 0.00043175,
//       "started": 0.000191958,
//       "oddity": ""
//     },
//     {
//       "address": "8.8.4.4:53",
//       "failure": null,
//       "operation": "connect",
//       "proto": "udp",
//       "t": 0.042198458,
//       "started": 0.042113208,
//       "oddity": ""
//     }
//   ],
//
//   // This block shows the read and write events
//   // occurred on the sockets (because we control
//   // in full the implementation of this DNS
//   // over UDP resolver, we can see these events)
//   "read_write": [
//     {
//       "address": "8.8.4.4:53",
//       "failure": null,
//       "num_bytes": 29,
//       "operation": "write",
//       "proto": "udp",
//       "t": 0.000459583,
//       "started": 0.00043825,
//       "oddity": ""
//     },
//     {
//       "address": "8.8.4.4:53",
//       "failure": null,
//       "num_bytes": 45,
//       "operation": "read",
//       "proto": "udp",
//       "t": 0.041955792,
//       "started": 0.000471833,
//       "oddity": ""
//     },
//     {
//       "address": "8.8.4.4:53",
//       "failure": null,
//       "num_bytes": 29,
//       "operation": "write",
//       "proto": "udp",
//       "t": 0.042218917,
//       "started": 0.042203,
//       "oddity": ""
//     },
//     {
//       "address": "8.8.4.4:53",
//       "failure": null,
//       "num_bytes": 57,
//       "operation": "read",
//       "proto": "udp",
//       "t": 0.196646583,
//       "started": 0.042233167,
//       "oddity": ""
//     }
//   ],
//
//   // This is the same kind of result as before, we
//   // show the emitted queries and the resolved addrs.
//   //
//   // Also note how here the resolver_address is the
//   // correct endpoint address and the engine tells us
//   // that we're using DNS over UDP.
//   "lookup_host": [
//     {
//       "answers": [
//         {
//           "answer_type": "A",
//           "ipv4": "93.184.216.34"
//         }
//       ],
//       "engine": "udp",
//       "failure": null,
//       "hostname": "example.com",
//       "query_type": "A",
//       "resolver_address": "8.8.4.4:53",
//       "t": 0.196777042,
//       "started": 0.000118542,
//       "oddity": ""
//     },
//     {
//       "answers": [
//         {
//           "answer_type": "AAAA",
//           "ivp6": "2606:2800:220:1:248:1893:25c8:1946"
//         }
//       ],
//       "engine": "udp",
//       "failure": null,
//       "hostname": "example.com",
//       "query_type": "AAAA",
//       "resolver_address": "8.8.4.4:53",
//       "t": 0.196777042,
//       "started": 0.000118542,
//       "oddity": ""
//     }
//   ],
//
//   // This block shows the query we sent (encoded as base64)
//   // and the response we received. Here we clearly see
//   // that we perform two DNS "round trip" (i.e., send request
//   // and receive response) to resolve a domain: one for
//   // A and the other for AAAA.
//   "dns_round_trip": [
//     {
//       "engine": "udp",
//       "resolver_address": "8.8.4.4:53",
//       "raw_query": {
//         "data": "PrcBAAABAAAAAAAAB2V4YW1wbGUDY29tAAABAAE=",
//         "format": "base64"
//       },
//       "started": 0.000191625,
//       "t": 0.041998667,
//       "failure": null,
//       "raw_reply": {
//         "data": "PreBgAABAAEAAAAAB2V4YW1wbGUDY29tAAABAAHADAABAAEAAE8BAARduNgi",
//         "format": "base64"
//       }
//     },
//     {
//       "engine": "udp",
//       "resolver_address": "8.8.4.4:53",
//       "raw_query": {
//         "data": "LAwBAAABAAAAAAAAB2V4YW1wbGUDY29tAAAcAAE=",
//         "format": "base64"
//       },
//       "started": 0.04210775,
//       "t": 0.196701333,
//       "failure": null,
//       "raw_reply": {
//         "data": "LAyBgAABAAEAAAAAB2V4YW1wbGUDY29tAAAcAAHADAAcAAEAAE6nABAmBigAAiAAAQJIGJMlyBlG",
//         "format": "base64"
//       }
//     }
//   ]
// }
// ```
//
// This data format is really an extension of the `LookupHostSystem`
// one. It just adds more fields that clarify what happened at low
// level in terms of socket I/O and queries sent and received.
//
// Let us now try to provoke some errors and see how the
// output JSON changes because of them.
//
// ### Measurement with NXDOMAIN
//
// Let us try to get a NXDOMAIN error.
//
// ```bash
// go run -race ./internal/tutorial/measurex/chapter03 -query antani.ooni.org
// ```
//
// This produces the following JSON:
//
// ```JavaScript
// {
//   "domain": "antani.ooni.org",
//   "connect": [ /* snip */ ],
//   "read_write": [ /* snip */ ],
//   "lookup_host": [
//     {
//       "answers": null,
//       "engine": "udp",
//       "failure": "dns_nxdomain_error",
//       "hostname": "antani.ooni.org",
//       "query_type": "A",
//       "resolver_address": "8.8.4.4:53",
//       "t": 0.098208709,
//       "started": 8.95e-05,
//       "oddity": "dns.lookup.nxdomain"
//     },
//     {
//       "answers": null,
//       "engine": "udp",
//       "failure": "dns_nxdomain_error",
//       "hostname": "antani.ooni.org",
//       "query_type": "AAAA",
//       "resolver_address": "8.8.4.4:53",
//       "t": 0.098208709,
//       "started": 8.95e-05,
//       "oddity": "dns.lookup.nxdomain"
//     }
//   ],
//   "dns_round_trip": [
//     {
//       "engine": "udp",
//       "resolver_address": "8.8.4.4:53",
//       "raw_query": {
//         "data": "jLIBAAABAAAAAAAABmFudGFuaQRvb25pA29yZwAAAQAB",
//         "format": "base64"
//       },
//       "started": 0.000141542,
//       "t": 0.034689417,
//       "failure": null,
//       "raw_reply": {
//         "data": "jLKBgwABAAAAAQAABmFudGFuaQRvb25pA29yZwAAAQABwBMABgABAAAHCAA9BGRuczERcmVnaXN0cmFyLXNlcnZlcnMDY29tAApob3N0bWFzdGVywDJhABz8AACowAAADhAACTqAAAAOEQ==",
//         "format": "base64"
//       }
//     },
//     {
//       "engine": "udp",
//       "resolver_address": "8.8.4.4:53",
//       "raw_query": {
//         "data": "azEBAAABAAAAAAAABmFudGFuaQRvb25pA29yZwAAHAAB",
//         "format": "base64"
//       },
//       "started": 0.034776709,
//       "t": 0.098170542,
//       "failure": null,
//       "raw_reply": {
//         "data": "azGBgwABAAAAAQAABmFudGFuaQRvb25pA29yZwAAHAABwBMABgABAAAHCAA9BGRuczERcmVnaXN0cmFyLXNlcnZlcnMDY29tAApob3N0bWFzdGVywDJhABz8AACowAAADhAACTqAAAAOEQ==",
//         "format": "base64"
//       }
//     }
//   ]
// }
// ```
//
// We indeed get a NXDOMAIN error as the failure in `lookup_host`.
//
// Let us now decode one of the replies by using this program:
//
// ```
// package main
//
// import (
// "fmt"
// "encoding/base64"
//
// "github.com/miekg/dns"
// )
//
// func main() {
//     const query = "azGBgwABAAAAAQAABmFudGFuaQRvb25pA29yZwAAHAABwBMABgABAAAHCAA9BGRuczERcmVnaXN0cmFyLXNlcnZlcnMDY29tAApob3N0bWFzdGVywDJhABz8AACowAAADhAACTqAAAAOEQ=="
//     data, _ := base64.StdEncoding.DecodeString(query)
//     msg := new(dns.Msg)
//     _ = msg.Unpack(data)
//     fmt.Printf("%s\n", msg)
//}
// ```
//
// where `query` is one of the replies. If we run this program
// we get as the output:
//
// ```
// ;; opcode: QUERY, status: NXDOMAIN, id: 27441
// ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
//
// ;; QUESTION SECTION:
// ;antani.ooni.org.	IN	 AAAA
//
// ;; AUTHORITY SECTION:
// ooni.org.	1800	IN	SOA	dns1.registrar-servers.com. hostmaster.registrar-servers.com. 1627397372 43200 3600 604800 3601
// ```
//
// ### Measurement with timeout
//
// Let us now query an IP address known for not responding
// to DNS queries, to get a timeout.
//
// ```bash
// go run -race ./internal/tutorial/measurex/chapter03 -address 182.92.22.222:53
// ```
//
// Here's the corresponding JSON:
//
// ```JavaScript
// {
//   "domain": "example.com",
//   "connect": [ /* snip */ ],
//   "read_write": [
//     {
//       "address": "182.92.22.222:53",
//       "failure": null,
//       "num_bytes": 29,
//       "operation": "write",
//       "proto": "udp",
//       "t": 0.0005275,
//       "started": 0.000500209,
//       "oddity": ""
//     },
//     {
//       "address": "182.92.22.222:53",
//       "failure": "generic_timeout_error",  /* <--- */
//       "operation": "read",
//       "proto": "udp",
//       "t": 5.001140125,
//       "started": 0.000544042,
//       "oddity": ""
//     }
//   ],
//   "lookup_host": [
//     {
//       "answers": null,
//       "engine": "udp",
//       "failure": "generic_timeout_error",  /* <--- */
//       "hostname": "example.com",
//       "query_type": "A",
//       "resolver_address": "182.92.22.222:53",
//       "t": 5.001462084,
//       "started": 0.000127917,
//       "oddity": "dns.lookup.timeout"       /* <--- */
//     },
//     {
//       "answers": null,
//       "engine": "udp",
//       "failure": "generic_timeout_error",
//       "hostname": "example.com",
//       "query_type": "AAAA",
//       "resolver_address": "182.92.22.222:53",
//       "t": 5.001462084,
//       "started": 0.000127917,
//       "oddity": "dns.lookup.timeout"
//     }
//   ],
//   "dns_round_trip": [
//     {
//       "engine": "udp",
//       "resolver_address": "182.92.22.222:53",
//       "raw_query": {
//         "data": "ej8BAAABAAAAAAAAB2V4YW1wbGUDY29tAAABAAE=",
//         "format": "base64"
//       },
//       "started": 0.000220584,
//       "t": 5.001317417,
//       "failure": "generic_timeout_error",
//       "raw_reply": null
//     }
//   ]
// }
// ```
//
// We see that we fail with a timeout (I have marked some of them
// with comments inside the JSON). We see the timeout at three different
// levels of abstractions (from lower to higher abstraction): at the socket layer,
// during the DNS round trip, during the DNS lookup.
//
// What we also see is that `t`'s value is ~5s when the `read` event
// fails, which tells us about the socket's read timeout.
//
// ### Measurement with REFUSED error
//
// Let us now try to get a REFUSED DNS Rcode, again from servers
// that are, let's say, kind enough to easily help.
//
// ```bash
// go run -race ./internal/tutorial/measurex/chapter03 -address 180.97.36.63:53
// ```
//
// Here's the answer I get:
//
// ```JavaScript
// {
//   "domain": "example.com",
//   "connect": [ /* snip */ ],
//
//   // The I/O events look normal this time
//   "read_write": [
//     {
//       "address": "180.97.36.63:53",
//       "failure": null,
//       "num_bytes": 29,
//       "operation": "write",
//       "proto": "udp",
//       "t": 0.000333583,
//       "started": 0.000312125,
//       "oddity": ""
//     },
//     {
//       "address": "180.97.36.63:53",
//       "failure": null,
//       "num_bytes": 29,
//       "operation": "read",
//       "proto": "udp",
//       "t": 0.334948125,
//       "started": 0.000366625,
//       "oddity": ""
//     },
//     {
//       "address": "180.97.36.63:53",
//       "failure": null,
//       "num_bytes": 29,
//       "operation": "write",
//       "proto": "udp",
//       "t": 0.3358025,
//       "started": 0.335725958,
//       "oddity": ""
//     },
//     {
//       "address": "180.97.36.63:53",
//       "failure": null,
//       "num_bytes": 29,
//       "operation": "read",
//       "proto": "udp",
//       "t": 0.739987666,
//       "started": 0.335863875,
//       "oddity": ""
//     }
//   ],
//
//   // But we see both in the error and in the oddity
//   // that the response was "REFUSED"
//   "lookup_host": [
//     {
//       "answers": null,
//       "engine": "udp",
//       "failure": "dns_refused_error",
//       "hostname": "example.com",
//       "query_type": "A",
//       "resolver_address": "180.97.36.63:53",
//       "t": 0.7402975,
//       "started": 7.2291e-05,
//       "oddity": "dns.lookup.refused"
//     },
//     {
//       "answers": null,
//       "engine": "udp",
//       "failure": "dns_refused_error",
//       "hostname": "example.com",
//       "query_type": "AAAA",
//       "resolver_address": "180.97.36.63:53",
//       "t": 0.7402975,
//       "started": 7.2291e-05,
//       "oddity": "dns.lookup.refused"
//     }
//   ],
//
//   // Exercise: do like I did before and decode the messages
//   "dns_round_trip": [
//     {
//       "engine": "udp",
//       "resolver_address": "180.97.36.63:53",
//       "raw_query": {
//         "data": "crkBAAABAAAAAAAAB2V4YW1wbGUDY29tAAABAAE=",
//         "format": "base64"
//       },
//       "started": 0.000130666,
//       "t": 0.33509925,
//       "failure": null,
//       "raw_reply": {
//         "data": "crmBBQABAAAAAAAAB2V4YW1wbGUDY29tAAABAAE=",
//         "format": "base64"
//       }
//     },
//     {
//       "engine": "udp",
//       "resolver_address": "180.97.36.63:53",
//       "raw_query": {
//         "data": "ywcBAAABAAAAAAAAB2V4YW1wbGUDY29tAAAcAAE=",
//         "format": "base64"
//       },
//       "started": 0.335321333,
//       "t": 0.740152375,
//       "failure": null,
//       "raw_reply": {
//         "data": "yweBBQABAAAAAAAAB2V4YW1wbGUDY29tAAAcAAE=",
//         "format": "base64"
//       }
//     }
//   ]
// }
// ```
//
// ## Conclusion
//
// We have seen how to send DNS queries over UDP, measure the
// results, and what happens on common error conditions.
//
// -=-=- StopHere -=-=-