spb/internetpressure

1// Package dns implements the "dns" check: one A query per configured resolver, run sequentially.2//3// Resolver "system" (empty address) uses the OS resolver through net.DefaultResolver; every other resolver is4// queried directly over UDP with github.com/miekg/dns (EDNS0, 3 s timeout), retried once over TCP only when the5// UDP answer is truncated. Exactly one query per resolver per check.6package dns78import (9	"context"10	"errors"11	"net"12	"sort"13	"strings"14	"time"1516	mdns "github.com/miekg/dns"1718	"internetpressure.io/probe-agent/internal/protocol"19)2021// Timeout per query.22const Timeout = 3 * time.Second2324// DefaultResolvers is used by `once` and when the server sends none.25var DefaultResolvers = []protocol.Resolver{26	{ID: "system", Address: ""},27	{ID: "google", Address: "8.8.8.8:53"},28	{ID: "cloudflare", Address: "1.1.1.1:53"},29	{ID: "quad9", Address: "9.9.9.9:53"},30}3132// Checker runs dns checks.33type Checker struct{}3435// Run queries every resolver for the A record of t.Hostname and returns one measurement per resolver.36func (c *Checker) Run(ctx context.Context, t protocol.Target, resolvers []protocol.Resolver) []protocol.Measurement {37	if len(resolvers) == 0 {38		resolvers = DefaultResolvers39	}40	out := make([]protocol.Measurement, 0, len(resolvers))41	for _, r := range resolvers {42		if ctx.Err() != nil {43			break44		}45		out = append(out, c.query(ctx, t, r))46	}47	return out48}4950func (c *Checker) query(ctx context.Context, t protocol.Target, r protocol.Resolver) protocol.Measurement {51	m := protocol.Measurement{TS: protocol.FormatTime(time.Now()), TargetID: t.TargetID, Kind: "dns", Resolver: r.ID}52	ctx, cancel := context.WithTimeout(ctx, Timeout)53	defer cancel()5455	start := time.Now()56	var answers []string57	var rcode string58	var err error59	if r.Address == "" || r.ID == "system" {60		answers, rcode, err = querySystem(ctx, t.Hostname)61	} else {62		answers, rcode, err = queryServer(ctx, t.Hostname, r.Address)63	}64	m.DNSMs = protocol.F(protocol.Ms(time.Since(start)))65	m.DNSRcode = rcode66	sort.Strings(answers)67	m.DNSAnswers = answers6869	switch {70	case err != nil:71		m.Error = classify(err, rcode)72	case rcode == "NOERROR" && len(answers) > 0:73		m.OK = true74	case rcode == "NOERROR":75		m.Error = protocol.ErrDNSFail // NOERROR but no A record (e.g. AAAA-only or CNAME chain without A)76	default:77		m.Error = classify(nil, rcode)78	}79	return m80}8182// querySystem resolves through the OS stub resolver. The rcode is inferred from the error class.83func querySystem(ctx context.Context, host string) ([]string, string, error) {84	ips, err := net.DefaultResolver.LookupIP(ctx, "ip4", host)85	if err != nil {86		var dnsErr *net.DNSError87		if errors.As(err, &dnsErr) {88			switch {89			case dnsErr.IsNotFound:90				return nil, "NXDOMAIN", err91			case dnsErr.IsTimeout || errors.Is(err, context.DeadlineExceeded):92				return nil, "TIMEOUT", err93			default:94				return nil, "SERVFAIL", err95			}96		}97		if errors.Is(err, context.DeadlineExceeded) {98			return nil, "TIMEOUT", err99		}100		return nil, "ERROR", err101	}102	out := make([]string, 0, len(ips))103	for _, ip := range ips {104		if v4 := ip.To4(); v4 != nil {105			out = append(out, v4.String())106		}107	}108	return out, "NOERROR", nil109}110111// queryServer sends one A query over UDP (EDNS0 1232) and retries once over TCP if truncated.112func queryServer(ctx context.Context, host, server string) ([]string, string, error) {113	msg := new(mdns.Msg)114	msg.SetQuestion(mdns.Fqdn(host), mdns.TypeA)115	msg.RecursionDesired = true116	msg.SetEdns0(1232, false)117118	client := &mdns.Client{Net: "udp", Timeout: Timeout, UDPSize: 1232}119	resp, _, err := client.ExchangeContext(ctx, msg, server)120	if err == nil && resp != nil && resp.Truncated {121		tcp := &mdns.Client{Net: "tcp", Timeout: Timeout}122		resp, _, err = tcp.ExchangeContext(ctx, msg, server)123	}124	if err != nil {125		var ne net.Error126		if errors.Is(err, context.DeadlineExceeded) || (errors.As(err, &ne) && ne.Timeout()) ||127			strings.Contains(err.Error(), "i/o timeout") {128			return nil, "TIMEOUT", err129		}130		return nil, "ERROR", err131	}132	rcode := mdns.RcodeToString[resp.Rcode]133	if rcode == "" {134		rcode = "RCODE" + itoa(resp.Rcode)135	}136	var out []string137	for _, rr := range resp.Answer {138		if a, ok := rr.(*mdns.A); ok {139			out = append(out, a.A.String())140		}141	}142	return out, rcode, nil143}144145// classify maps (transport error, rcode) to a protocol error code.146func classify(err error, rcode string) string {147	switch rcode {148	case "TIMEOUT":149		return protocol.ErrDNSTimeout150	case "SERVFAIL":151		return protocol.ErrDNSServfail152	case "NXDOMAIN":153		return protocol.ErrDNSNxdomain154	}155	if err != nil {156		var ne net.Error157		if errors.Is(err, context.DeadlineExceeded) || (errors.As(err, &ne) && ne.Timeout()) {158			return protocol.ErrDNSTimeout159		}160	}161	return protocol.ErrDNSFail162}163164func itoa(n int) string {165	if n == 0 {166		return "0"167	}168	var b [20]byte169	i := len(b)170	for n > 0 {171		i--172		b[i] = byte('0' + n%10)173		n /= 10174	}175	return string(b[i:])176}177