Support $DHCP and $BOOTSTRAP keywords in forwarding rules

Ideally, that should also be supported by the captive portal
handler.

Great work by @lifenjoiner

Fixes #2460

dnscrypt-proxy/example-forwarding-rules.txt

@@ -7,21 +7,31 @@
 ## <domain> <server address>[:port] [, <server address>[:port]...]
 ## IPv6 addresses can be specified by enclosing the address in square brackets.
 
+## The following keywords can also be used instead of a server address:
+## $BOOTSTRAP to use the default bootstrap resolvers
+## $DHCP to use the default DNS resolvers provided by the DHCP server
+
 ## In order to enable this feature, the "forwarding_rules" property needs to
 ## be set to this file name inside the main configuration file.
 
 ## Blocking IPv6 may prevent local devices from being discovered.
 ## If this happens, set `block_ipv6` to `false` in the main config file.
 
-## Forward *.lan, *.local, *.home, *.home.arpa, *.internal and *.localdomain to 192.168.1.1
+## Forward *.lan, *.home, *.home.arpa, and *.localdomain to 192.168.1.1
 # lan              192.168.1.1
-# local            192.168.1.1
 # home             192.168.1.1
 # home.arpa        192.168.1.1
-# internal         192.168.1.1
 # localdomain      192.168.1.1
 # 192.in-addr.arpa 192.168.1.1
 
+## Forward *.local to the resolvers provided by the DHCP server
+# local            $DHCP
+
+## Forward *.internal to 192.168.1.1, and if it doesn't work, to the
+## DNS from the local DHCP server, and if it still doesn't work, to the
+## bootstrap resolvers
+# internal         192.168.1.1,$DHCP,$BOOTSTRAP
+
 ## Forward queries for example.com and *.example.com to 9.9.9.9 and 8.8.8.8
 # example.com      9.9.9.9,8.8.8.8
 

dnscrypt-proxy/plugin_forward.go

@@ -7,16 +7,32 @@ import (
 	"strings"
 
 	"github.com/jedisct1/dlog"
+	"github.com/lifenjoiner/dhcpdns"
 	"github.com/miekg/dns"
 )
 
+type SearchSequenceItemType int
+
+const (
+	Explicit SearchSequenceItemType = iota
+	Bootstrap
+	DHCP
+)
+
+type SearchSequenceItem struct {
+	typ     SearchSequenceItemType
+	servers []string
+}
+
 type PluginForwardEntry struct {
 	domain   string
-	servers []string
+	sequence []SearchSequenceItem
 }
 
 type PluginForward struct {
 	forwardMap         []PluginForwardEntry
+	bootstrapResolvers []string
+	dhcpdns            []*dhcpdns.Detector
 }
 
 func (plugin *PluginForward) Name() string {
@@ -29,6 +45,11 @@ func (plugin *PluginForward) Description() string {
 
 func (plugin *PluginForward) Init(proxy *Proxy) error {
 	dlog.Noticef("Loading the set of forwarding rules from [%s]", proxy.forwardFile)
+
+	if proxy.xTransport != nil {
+		plugin.bootstrapResolvers = proxy.xTransport.bootstrapResolvers
+	}
+
 	lines, err := ReadTextFile(proxy.forwardFile)
 	if err != nil {
 		return err
@@ -46,9 +67,37 @@ func (plugin *PluginForward) Init(proxy *Proxy) error {
 			)
 		}
 		domain = strings.ToLower(domain)
-		var servers []string
+		requiresDHCP := false
+		var sequence []SearchSequenceItem
 		for _, server := range strings.Split(serversStr, ",") {
 			server = strings.TrimSpace(server)
+			switch server {
+			case "$BOOTSTRAP":
+				if len(plugin.bootstrapResolvers) == 0 {
+					return fmt.Errorf(
+						"Syntax error for a forwarding rule at line %d. No bootstrap resolvers available",
+						1+lineNo,
+					)
+				}
+				if len(sequence) > 0 && sequence[len(sequence)-1].typ == Bootstrap {
+					// Ignore repetitions
+				} else {
+					sequence = append(sequence, SearchSequenceItem{typ: Bootstrap})
+					dlog.Infof("Forwarding [%s] to the bootstrap servers", domain)
+				}
+			case "$DHCP":
+				if len(sequence) > 0 && sequence[len(sequence)-1].typ == DHCP {
+					// Ignore repetitions
+				} else {
+					sequence = append(sequence, SearchSequenceItem{typ: Bootstrap})
+					dlog.Infof("Forwarding [%s] to the DHCP servers", domain)
+				}
+				requiresDHCP = true
+			default:
+				if strings.HasPrefix(server, "$") {
+					dlog.Criticalf("Unknown keyword [%s] at line %d", server, 1+lineNo)
+					continue
+				}
 				server = strings.TrimPrefix(server, "[")
 				server = strings.TrimSuffix(server, "]")
 				if ip := net.ParseIP(server); ip != nil {
@@ -58,15 +107,37 @@ func (plugin *PluginForward) Init(proxy *Proxy) error {
 						server = fmt.Sprintf("[%s]:%d", server, 53)
 					}
 				}
-			dlog.Infof("Forwarding [%s] to %s", domain, server)
+				idxServers := -1
-			servers = append(servers, server)
+				for i, item := range sequence {
+					if item.typ == Explicit {
+						idxServers = i
+					}
+				}
+				if idxServers == -1 {
+					sequence = append(sequence, SearchSequenceItem{typ: Explicit, servers: []string{server}})
+				} else {
+					sequence[idxServers].servers = append(sequence[idxServers].servers, server)
+				}
+				dlog.Infof("Forwarding [%s] to [%s]", domain, server)
+			}
+		}
+		if requiresDHCP {
+			if proxy.SourceIPv6 {
+				dlog.Info("Starting a DHCP/DNS detector for IPv6")
+				d6 := &dhcpdns.Detector{RemoteIPPort: "[2001:DB8::53]:80"}
+				go d6.Serve(9, 10)
+				plugin.dhcpdns = append(plugin.dhcpdns, d6)
+			}
+			if proxy.SourceIPv4 {
+				dlog.Info("Starting a DHCP/DNS detector for IPv4")
+				d4 := &dhcpdns.Detector{RemoteIPPort: "192.0.2.53:80"}
+				go d4.Serve(9, 10)
+				plugin.dhcpdns = append(plugin.dhcpdns, d4)
 			}
-		if len(servers) == 0 {
-			continue
 		}
 		plugin.forwardMap = append(plugin.forwardMap, PluginForwardEntry{
 			domain:   domain,
-			servers: servers,
+			sequence: sequence,
 		})
 	}
 	return nil
@@ -83,7 +154,7 @@ func (plugin *PluginForward) Reload() error {
 func (plugin *PluginForward) Eval(pluginsState *PluginsState, msg *dns.Msg) error {
 	qName := pluginsState.qName
 	qNameLen := len(qName)
-	var servers []string
+	var sequence []SearchSequenceItem
 	for _, candidate := range plugin.forwardMap {
 		candidateLen := len(candidate.domain)
 		if candidateLen > qNameLen {
@@ -92,25 +163,68 @@ func (plugin *PluginForward) Eval(pluginsState *PluginsState, msg *dns.Msg) erro
 		if (qName[qNameLen-candidateLen:] == candidate.domain &&
 			(candidateLen == qNameLen || (qName[qNameLen-candidateLen-1] == '.'))) ||
 			(candidate.domain == ".") {
-			servers = candidate.servers
+			sequence = candidate.sequence
 			break
 		}
 	}
-	if len(servers) == 0 {
+	if len(sequence) == 0 {
 		return nil
 	}
-	server := servers[rand.Intn(len(servers))]
+	var err error
+	var respMsg *dns.Msg
+	var tries = 4
+	for _, item := range sequence {
+		var server string
+		switch item.typ {
+		case Explicit:
+			server = item.servers[rand.Intn(len(item.servers))]
 			pluginsState.serverName = server
+		case Bootstrap:
+			server = plugin.bootstrapResolvers[rand.Intn(len(plugin.bootstrapResolvers))]
+			pluginsState.serverName = "[BOOTSTRAP]"
+		case DHCP:
+			const maxInconsistency = 9
+			for _, dhcpdns := range plugin.dhcpdns {
+				inconsistency, ip, dhcpDNS, err := dhcpdns.Status()
+				if err != nil && ip != "" && inconsistency > maxInconsistency {
+					dhcpDNS = nil
+				}
+				if len(dhcpDNS) > 0 {
+					server = net.JoinHostPort(dhcpDNS[rand.Intn(len(dhcpDNS))].String(), "53")
+					break
+				}
+			}
+			if len(server) == 0 {
+				dlog.Warn("DHCP didn't provide any DNS server")
+				continue
+			}
+			pluginsState.serverName = "[DHCP]"
+		}
+		if len(server) == 0 {
+			continue
+		}
+
+		if tries == 0 {
+			break
+		}
+		tries--
+		dlog.Debugf("Forwarding [%s] to [%s]", qName, server)
 		client := dns.Client{Net: pluginsState.serverProto, Timeout: pluginsState.timeout}
-	respMsg, _, err := client.Exchange(msg, server)
+		respMsg, _, err = client.Exchange(msg, server)
 		if err != nil {
-		return err
+			continue
 		}
 		if respMsg.Truncated {
 			client.Net = "tcp"
 			respMsg, _, err = client.Exchange(msg, server)
 			if err != nil {
-			return err
+				continue
+			}
+		}
+		if len(sequence) > 0 {
+			switch respMsg.Rcode {
+			case dns.RcodeNameError, dns.RcodeRefused, dns.RcodeNotAuth:
+				continue
 			}
 		}
 		if edns0 := respMsg.IsEdns0(); edns0 == nil || !edns0.Do() {
@@ -122,3 +236,5 @@ func (plugin *PluginForward) Eval(pluginsState *PluginsState, msg *dns.Msg) erro
 		pluginsState.returnCode = PluginsReturnCodeForward
 		return nil
 	}
+	return err
+}

go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/jedisct1/xsecretbox v0.0.0-20241212092125-3afc4917ac41
 	github.com/k-sone/critbitgo v1.4.0
 	github.com/kardianos/service v1.2.2
+	github.com/lifenjoiner/dhcpdns v0.0.6
 	github.com/miekg/dns v1.1.62
 	github.com/opencoff/go-sieve v0.2.1
 	github.com/powerman/check v1.8.0

go.sum

@@ -51,6 +51,8 @@ github.com/k-sone/critbitgo v1.4.0 h1:l71cTyBGeh6X5ATh6Fibgw3+rtNT80BA0uNNWgkPrb
 github.com/k-sone/critbitgo v1.4.0/go.mod h1:7E6pyoyADnFxlUBEKcnfS49b7SUAQGMK+OAp/UQvo0s=
 github.com/kardianos/service v1.2.2 h1:ZvePhAHfvo0A7Mftk/tEzqEZ7Q4lgnR8sGz4xu1YX60=
 github.com/kardianos/service v1.2.2/go.mod h1:CIMRFEJVL+0DS1a3Nx06NaMn4Dz63Ng6O7dl0qH0zVM=
+github.com/lifenjoiner/dhcpdns v0.0.6 h1:rn4Y5RRR5sgQ6RjWenwhA7i/uHzHW9hbZpCobA4CAJs=
+github.com/lifenjoiner/dhcpdns v0.0.6/go.mod h1:BixeaGeafYzDIuDCYIUbSOdi4m+TScpzI9cZGYgzgSk=
 github.com/miekg/dns v1.1.62 h1:cN8OuEF1/x5Rq6Np+h1epln8OiyPWV+lROx9LxcGgIQ=
 github.com/miekg/dns v1.1.62/go.mod h1:mvDlcItzm+br7MToIKqkglaGhlFMHJ9DTNNWONWXbNQ=
 github.com/onsi/ginkgo/v2 v2.22.2 h1:/3X8Panh8/WwhU/3Ssa6rCKqPLuAkVY2I0RoyDLySlU=

vendor/modules.txt

@@ -65,6 +65,9 @@ github.com/k-sone/critbitgo
 # github.com/kardianos/service v1.2.2
 ## explicit; go 1.12
 github.com/kardianos/service
+# github.com/lifenjoiner/dhcpdns v0.0.6
+## explicit; go 1.20
+github.com/lifenjoiner/dhcpdns
 # github.com/miekg/dns v1.1.62
 ## explicit; go 1.19
 github.com/miekg/dns