Pull request: all: support more $dnsrewrite rr types

Merge in DNS/adguard-home from 2102-dnsrewrite-2 to master

Updates #2102.
Updates #2452.

Squashed commit of the following:

commit b41e57731b4f276e97202e172fa400067174653d
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Dec 22 20:40:56 2020 +0300

    dnsforward: improve naming

commit 70b832ce969d8cdcf4224d221e5e1e2057fba6ee
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Dec 22 19:36:21 2020 +0300

    all: support more $dnsrewrite rr types

internal/dnsforward/svcbmsg.go

@@ -0,0 +1,168 @@
+package dnsforward
+
+import (
+	"encoding/base64"
+	"net"
+	"strconv"
+
+	"github.com/AdguardTeam/golibs/log"
+	"github.com/AdguardTeam/urlfilter/rules"
+	"github.com/miekg/dns"
+)
+
+// genAnswerHTTPS returns a properly initialized HTTPS resource record.
+//
+// See the comment on genAnswerSVCB for a list of current restrictions on
+// parameter values.
+func (s *Server) genAnswerHTTPS(req *dns.Msg, svcb *rules.DNSSVCB) (ans *dns.HTTPS) {
+	ans = &dns.HTTPS{
+		SVCB: *s.genAnswerSVCB(req, svcb),
+	}
+
+	ans.Hdr.Rrtype = dns.TypeHTTPS
+
+	return ans
+}
+
+// strToSVCBKey is the string-to-svcb-key mapping.
+//
+// See https://github.com/miekg/dns/blob/23c4faca9d32b0abbb6e179aa1aadc45ac53a916/svcb.go#L27.
+//
+// TODO(a.garipov): Propose exporting this API or something similar in the
+// github.com/miekg/dns module.
+var strToSVCBKey = map[string]dns.SVCBKey{
+	"alpn":            dns.SVCB_ALPN,
+	"echconfig":       dns.SVCB_ECHCONFIG,
+	"ipv4hint":        dns.SVCB_IPV4HINT,
+	"ipv6hint":        dns.SVCB_IPV6HINT,
+	"mandatory":       dns.SVCB_MANDATORY,
+	"no-default-alpn": dns.SVCB_NO_DEFAULT_ALPN,
+	"port":            dns.SVCB_PORT,
+}
+
+// svcbKeyHandler is a handler for one SVCB parameter key.
+type svcbKeyHandler func(valStr string) (val dns.SVCBKeyValue)
+
+// svcbKeyHandlers are the supported SVCB parameters handlers.
+var svcbKeyHandlers = map[string]svcbKeyHandler{
+	"alpn": func(valStr string) (val dns.SVCBKeyValue) {
+		return &dns.SVCBAlpn{
+			Alpn: []string{valStr},
+		}
+	},
+
+	"echconfig": func(valStr string) (val dns.SVCBKeyValue) {
+		ech, err := base64.StdEncoding.DecodeString(valStr)
+		if err != nil {
+			log.Debug("can't parse svcb/https echconfig: %s; ignoring", err)
+
+			return nil
+		}
+
+		return &dns.SVCBECHConfig{
+			ECH: ech,
+		}
+	},
+
+	"ipv4hint": func(valStr string) (val dns.SVCBKeyValue) {
+		ip := net.ParseIP(valStr)
+		if ip4 := ip.To4(); ip == nil || ip4 == nil {
+			log.Debug("can't parse svcb/https ipv4 hint %q; ignoring", valStr)
+
+			return nil
+		}
+
+		return &dns.SVCBIPv4Hint{
+			Hint: []net.IP{ip},
+		}
+	},
+
+	"ipv6hint": func(valStr string) (val dns.SVCBKeyValue) {
+		ip := net.ParseIP(valStr)
+		if ip == nil {
+			log.Debug("can't parse svcb/https ipv6 hint %q; ignoring", valStr)
+
+			return nil
+		}
+
+		return &dns.SVCBIPv6Hint{
+			Hint: []net.IP{ip},
+		}
+	},
+
+	"mandatory": func(valStr string) (val dns.SVCBKeyValue) {
+		code, ok := strToSVCBKey[valStr]
+		if !ok {
+			log.Debug("unknown svcb/https mandatory key %q, ignoring", valStr)
+
+			return nil
+		}
+
+		return &dns.SVCBMandatory{
+			Code: []dns.SVCBKey{code},
+		}
+	},
+
+	"no-default-alpn": func(_ string) (val dns.SVCBKeyValue) {
+		return &dns.SVCBNoDefaultAlpn{}
+	},
+
+	"port": func(valStr string) (val dns.SVCBKeyValue) {
+		port64, err := strconv.ParseUint(valStr, 10, 16)
+		if err != nil {
+			log.Debug("can't parse svcb/https port: %s; ignoring", err)
+
+			return nil
+		}
+
+		return &dns.SVCBPort{
+			Port: uint16(port64),
+		}
+	},
+}
+
+// genAnswerSVCB returns a properly initialized SVCB resource record.
+//
+// Currently, there are several restrictions on how the parameters are parsed.
+// Firstly, the parsing of non-contiguous values isn't supported.  Secondly, the
+// parsing of value-lists is not supported either.
+//
+//   ipv4hint=127.0.0.1             // Supported.
+//   ipv4hint="127.0.0.1"           // Unsupported.
+//   ipv4hint=127.0.0.1,127.0.0.2   // Unsupported.
+//   ipv4hint="127.0.0.1,127.0.0.2" // Unsupported.
+//
+// TODO(a.garipov): Support all of these.
+func (s *Server) genAnswerSVCB(req *dns.Msg, svcb *rules.DNSSVCB) (ans *dns.SVCB) {
+	ans = &dns.SVCB{
+		Hdr:      s.hdr(req, dns.TypeSVCB),
+		Priority: svcb.Priority,
+		Target:   svcb.Target,
+	}
+	if len(svcb.Params) == 0 {
+		return ans
+	}
+
+	values := make([]dns.SVCBKeyValue, 0, len(svcb.Params))
+	for k, valStr := range svcb.Params {
+		handler, ok := svcbKeyHandlers[k]
+		if !ok {
+			log.Debug("unknown svcb/https key %q, ignoring", k)
+
+			continue
+		}
+
+		val := handler(valStr)
+		if val == nil {
+			continue
+		}
+
+		values = append(values, val)
+	}
+
+	if len(values) > 0 {
+		ans.Value = values
+	}
+
+	return ans
+}