Wait until bootstrap IPs resolved

When bootstrapping, if the network changed, for example, firewall rules
changed during VPN connection, the bootstrap IPs may not be resolved, so
ctrld won't work. Since bootstrap IPs is necessary for ctrld to work
properly, we should wait until we can resolve upstream IP before we can
start serving requests.

config.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/tls"
 	"crypto/x509"
+	"errors"
 	"math/rand"
 	"net"
 	"net/http"
@@ -17,6 +18,7 @@ import (
 	"github.com/miekg/dns"
 	"github.com/spf13/viper"
 	"golang.org/x/sync/singleflight"
+	"tailscale.com/logtail/backoff"
 
 	"github.com/Control-D-Inc/ctrld/internal/dnsrcode"
 	ctrldnet "github.com/Control-D-Inc/ctrld/internal/net"
@@ -240,7 +242,15 @@ func (uc *UpstreamConfig) SetupBootstrapIP() {
 // SetupBootstrapIP manually find all available IPs of the upstream.
 // The first usable IP will be used as bootstrap IP of the upstream.
 func (uc *UpstreamConfig) setupBootstrapIP(withBootstrapDNS bool) {
-	uc.bootstrapIPs = lookupIP(uc.Domain, uc.Timeout, withBootstrapDNS)
+	b := backoff.NewBackoff("setupBootstrapIP", func(format string, args ...any) {}, 2*time.Second)
+	for {
+		uc.bootstrapIPs = lookupIP(uc.Domain, uc.Timeout, withBootstrapDNS)
+		if len(uc.bootstrapIPs) > 0 {
+			break
+		}
+		ProxyLog.Warn().Msg("could not resolve bootstrap IPs, retrying...")
+		b.BackOff(context.Background(), errors.New("no bootstrap IPs"))
+	}
 	for _, ip := range uc.bootstrapIPs {
 		if ctrldnet.IsIPv6(ip) {
 			uc.bootstrapIPs6 = append(uc.bootstrapIPs6, ip)