all: rework bootstrap IP discovering

At startup, ctrld gathers bootstrap IP information and use this
bootstrap IP for connecting to upstream. However, in case the network
stack changed, for example, dues to VPN connection, ctrld will still use
this old (maybe invalid) bootstrap IP for the current network stack.

This commit rework the discovering process, and re-initializing the
bootstrap IP if connecting to upstream failed.

cmd/ctrld/dns_proxy.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/rand"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"net"
 	"runtime"
@@ -182,6 +183,15 @@ func (p *prog) proxy(ctx context.Context, upstreams []string, failoverRcodes []i
 			resolveCtx = timeoutCtx
 		}
 		answer, err := dnsResolver.Resolve(resolveCtx, msg)
+		if errors.Is(err, ctrld.ErrUpstreamFailed) {
+			ctrldnet.Reset()
+			if err := upstreamConfig.SetupBootstrapIP(); err != nil {
+				mainLog.Error().Err(err).Msg("could not re-initialize bootstrap IP")
+			} else {
+				mainLog.Debug().Msg("re-initialize bootstrap IP done")
+			}
+			return nil
+		}
 		if err != nil {
 			ctrld.Log(ctx, mainLog.Error().Err(err), "failed to resolve query")
 			return nil

cmd/ctrld/prog.go

@@ -11,11 +11,9 @@ import (
 	"syscall"
 
 	"github.com/kardianos/service"
-	"github.com/miekg/dns"
 
 	"github.com/Control-D-Inc/ctrld"
 	"github.com/Control-D-Inc/ctrld/internal/dnscache"
-	ctrldnet "github.com/Control-D-Inc/ctrld/internal/net"
 )
 
 var logf = func(format string, args ...any) {
@@ -37,7 +35,6 @@ type prog struct {
 func (p *prog) Start(s service.Service) error {
 	p.cfg = &cfg
 	go p.run()
-	mainLog.Info().Msg("Service started")
 	return nil
 }
 
@@ -67,45 +64,10 @@ func (p *prog) run() {
 	for n := range p.cfg.Upstream {
 		uc := p.cfg.Upstream[n]
 		uc.Init()
-		if uc.BootstrapIP == "" {
-			// resolve it manually and set the bootstrap ip
-			c := new(dns.Client)
-			for _, dnsType := range []uint16{dns.TypeAAAA, dns.TypeA} {
-				if !ctrldnet.SupportsIPv6() && dnsType == dns.TypeAAAA {
-					continue
-				}
-				m := new(dns.Msg)
-				m.SetQuestion(uc.Domain+".", dnsType)
-				m.RecursionDesired = true
-				r, _, err := c.Exchange(m, net.JoinHostPort(bootstrapDNS, "53"))
-				if err != nil {
-					mainLog.Error().Err(err).Msgf("could not resolve domain %s for upstream.%s", uc.Domain, n)
-					continue
-				}
-				if r.Rcode != dns.RcodeSuccess {
-					mainLog.Error().Msgf("could not resolve domain return code: %d, upstream.%s", r.Rcode, n)
-					continue
-				}
-				if len(r.Answer) == 0 {
-					continue
-				}
-				for _, a := range r.Answer {
-					switch ar := a.(type) {
-					case *dns.A:
-						uc.BootstrapIP = ar.A.String()
-					case *dns.AAAA:
-						uc.BootstrapIP = ar.AAAA.String()
-					default:
-						continue
-					}
-					mainLog.Info().Str("bootstrap_ip", uc.BootstrapIP).Msgf("Setting bootstrap IP for upstream.%s", n)
-					// Stop if we reached here, because we got the bootstrap IP from r.Answer.
-					break
-				}
-				// If we reached here, uc.BootstrapIP was set, nothing to do anymore.
-				break
-			}
+		if err := uc.SetupBootstrapIP(); err != nil {
+			mainLog.Fatal().Err(err).Msgf("failed to setup bootstrap IP for upstream.%s", n)
 		}
+		mainLog.Info().Str("bootstrap_ip", uc.BootstrapIP).Msgf("Setting bootstrap IP for upstream.%s", n)
 		uc.SetupTransport()
 	}
 

config.go

@@ -2,19 +2,26 @@ package ctrld
 
 import (
 	"context"
+	"errors"
 	"net"
 	"net/http"
 	"net/url"
 	"os"
 	"strings"
+	"sync"
 	"time"
 
-	"github.com/Control-D-Inc/ctrld/internal/dnsrcode"
 	"github.com/go-playground/validator/v10"
 	"github.com/miekg/dns"
 	"github.com/spf13/viper"
+
+	"github.com/Control-D-Inc/ctrld/internal/dnsrcode"
+	ctrldnet "github.com/Control-D-Inc/ctrld/internal/net"
 )
 
+// ErrUpstreamFailed indicates that ctrld failed to connect to an upstream.
+var ErrUpstreamFailed = errors.New("could not connect to upstream")
+
 // SetConfigName set the config name that ctrld will look for.
 func SetConfigName(v *viper.Viper, name string) {
 	v.SetConfigName(name)
@@ -100,6 +107,9 @@ type UpstreamConfig struct {
 	Timeout           int               `mapstructure:"timeout" toml:"timeout,omitempty" validate:"gte=0"`
 	transport         *http.Transport   `mapstructure:"-" toml:"-"`
 	http3RoundTripper http.RoundTripper `mapstructure:"-" toml:"-"`
+
+	// guard BootstrapIP
+	mu sync.Mutex
 }
 
 // ListenerConfig specifies the networks configuration that ctrld will run on.
@@ -155,13 +165,51 @@ func (uc *UpstreamConfig) SetupTransport() {
 	}
 }
 
+// SetupBootstrapIP manually find all available IPs of the upstream.
+func (uc *UpstreamConfig) SetupBootstrapIP() error {
+	uc.mu.Lock()
+	defer uc.mu.Unlock()
+
+	c := new(dns.Client)
+	m := new(dns.Msg)
+	dnsType := dns.TypeA
+	if ctrldnet.SupportsIPv6() {
+		dnsType = dns.TypeAAAA
+	}
+	m.SetQuestion(uc.Domain+".", dnsType)
+	m.RecursionDesired = true
+	r, _, err := c.Exchange(m, net.JoinHostPort(bootstrapDNS, "53"))
+	if err != nil {
+		ProxyLog.Error().Err(err).Msgf("could not resolve domain %s for upstream", uc.Domain)
+		return err
+	}
+	if r.Rcode != dns.RcodeSuccess {
+		ProxyLog.Error().Msgf("could not resolve domain return code: %d, upstream", r.Rcode)
+		return errors.New(dns.RcodeToString[r.Rcode])
+	}
+	if len(r.Answer) == 0 {
+		return errors.New("no answer from bootstrap DNS server")
+	}
+	for _, a := range r.Answer {
+		switch ar := a.(type) {
+		case *dns.A:
+			uc.BootstrapIP = ar.A.String()
+			break
+		case *dns.AAAA:
+			uc.BootstrapIP = ar.AAAA.String()
+			break
+		}
+	}
+	return nil
+}
+
 func (uc *UpstreamConfig) setupDOHTransport() {
 	uc.transport = http.DefaultTransport.(*http.Transport).Clone()
 	uc.transport.IdleConnTimeout = 5 * time.Second
 	uc.transport.DialContext = func(ctx context.Context, network, addr string) (net.Conn, error) {
 		dialer := &net.Dialer{
-			Timeout:   10 * time.Second,
-			KeepAlive: 10 * time.Second,
+			Timeout:   5 * time.Second,
+			KeepAlive: 5 * time.Second,
 		}
 		Log(ctx, ProxyLog.Debug(), "debug dial context %s - %s - %s", addr, network, bootstrapDNS)
 		// if we have a bootstrap ip set, use it to avoid DNS lookup
@@ -169,9 +217,14 @@ func (uc *UpstreamConfig) setupDOHTransport() {
 			if _, port, _ := net.SplitHostPort(addr); port != "" {
 				addr = net.JoinHostPort(uc.BootstrapIP, port)
 			}
-			Log(ctx, ProxyLog.Debug(), "sending doh request to: %s", addr)
 		}
-		return dialer.DialContext(ctx, network, addr)
+		Log(ctx, ProxyLog.Debug(), "sending doh request to: %s", addr)
+		conn, err := dialer.DialContext(ctx, network, addr)
+		if err != nil {
+			Log(ctx, ProxyLog.Debug().Err(err), "could not dial to upstream")
+			return nil, ErrUpstreamFailed
+		}
+		return conn, nil
 	}
 
 	uc.pingUpstream()

config_quic.go

@@ -32,7 +32,12 @@ func (uc *UpstreamConfig) setupDOH3Transport() {
 		if err != nil {
 			return nil, err
 		}
-		return quic.DialEarlyContext(ctx, udpConn, remoteAddr, host, tlsCfg, cfg)
+		conn, err := quic.DialEarlyContext(ctx, udpConn, remoteAddr, host, tlsCfg, cfg)
+		if err != nil {
+			Log(ctx, ProxyLog.Debug().Err(err), "could not dial early to upstream")
+			return nil, ErrUpstreamFailed
+		}
+		return conn, nil
 	}
 
 	uc.http3RoundTripper = rt

doq.go

@@ -47,7 +47,7 @@ func resolve(ctx context.Context, msg *dns.Msg, endpoint string, tlsConfig *tls.
 func doResolve(ctx context.Context, msg *dns.Msg, endpoint string, tlsConfig *tls.Config) (*dns.Msg, error) {
 	session, err := quic.DialAddr(endpoint, tlsConfig, nil)
 	if err != nil {
-		return nil, err
+		return nil, ErrUpstreamFailed
 	}
 	defer session.CloseWithError(quic.ApplicationErrorCode(quic.NoError), "")
 

internal/net/net.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"net"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"tailscale.com/logtail/backoff"
@@ -28,13 +29,17 @@ var Dialer = &net.Dialer{
 }
 
 var (
-	stackOnce          sync.Once
+	stackOnce          atomic.Pointer[sync.Once]
 	ipv4Enabled        bool
 	ipv6Enabled        bool
 	canListenIPv6Local bool
 	hasNetworkUp       bool
 )
 
+func init() {
+	stackOnce.Store(new(sync.Once))
+}
+
 func probeStack() {
 	b := backoff.NewBackoff("probeStack", func(format string, args ...any) {}, time.Minute)
 	for {
@@ -57,23 +62,27 @@ func probeStack() {
 	}
 }
 
+func Reset() {
+	stackOnce.Store(new(sync.Once))
+}
+
 func Up() bool {
-	stackOnce.Do(probeStack)
+	stackOnce.Load().Do(probeStack)
 	return hasNetworkUp
 }
 
 func SupportsIPv4() bool {
-	stackOnce.Do(probeStack)
+	stackOnce.Load().Do(probeStack)
 	return ipv4Enabled
 }
 
 func SupportsIPv6() bool {
-	stackOnce.Do(probeStack)
+	stackOnce.Load().Do(probeStack)
 	return ipv6Enabled
 }
 
 func SupportsIPv6ListenLocal() bool {
-	stackOnce.Do(probeStack)
+	stackOnce.Load().Do(probeStack)
 	return canListenIPv6Local
 }
 

resolver.go

@@ -93,5 +93,8 @@ func (r *legacyResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, e
 		Dialer: dialer,
 	}
 	answer, _, err := dnsClient.ExchangeContext(ctx, msg, r.endpoint)
+	if _, ok := err.(*net.OpError); ok {
+		return answer, ErrUpstreamFailed
+	}
 	return answer, err
 }