Pull request: dhcpd: add ips and text option types

Updates #2385.

Squashed commit of the following:

commit ce8467f1c013c6b3fef59667084e2c6569a7213c
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Mar 15 19:02:17 2021 +0300

    dhcpd: add ips and text option types

internal/dhcpd/options.go

@@ -0,0 +1,138 @@
+package dhcpd
+
+import (
+	"encoding/hex"
+	"fmt"
+	"net"
+	"strconv"
+	"strings"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
+)
+
+// hexDHCPOptionParserHandler parses a DHCP option as a hex-encoded string.
+// For example:
+//
+//   252 hex 736f636b733a2f2f70726f78792e6578616d706c652e6f7267
+//
+func hexDHCPOptionParserHandler(s string) (data []byte, err error) {
+	data, err = hex.DecodeString(s)
+	if err != nil {
+		return nil, fmt.Errorf("decoding hex: %w", err)
+	}
+
+	return data, nil
+}
+
+// ipDHCPOptionParserHandler parses a DHCP option as a single IP address.
+// For example:
+//
+//   6 ip 192.168.1.1
+//
+func ipDHCPOptionParserHandler(s string) (data []byte, err error) {
+	ip := net.ParseIP(s)
+	if ip == nil {
+		return nil, agherr.Error("invalid ip")
+	}
+
+	// Most DHCP options require IPv4, so do not put the 16-byte
+	// version if we can.  Otherwise, the clients will receive weird
+	// data that looks like four IPv4 addresses.
+	//
+	// See https://github.com/AdguardTeam/AdGuardHome/issues/2688.
+	if ip4 := ip.To4(); ip4 != nil {
+		data = ip4
+	} else {
+		data = ip
+	}
+
+	return data, nil
+}
+
+// textDHCPOptionParserHandler parses a DHCP option as a simple UTF-8 encoded
+// text.  For example:
+//
+//   252 text http://192.168.1.1/wpad.dat
+//
+func ipsDHCPOptionParserHandler(s string) (data []byte, err error) {
+	ipStrs := strings.Split(s, ",")
+	for i, ipStr := range ipStrs {
+		var ipData []byte
+		ipData, err = ipDHCPOptionParserHandler(ipStr)
+		if err != nil {
+			return nil, fmt.Errorf("parsing ip at index %d: %w", i, err)
+		}
+
+		data = append(data, ipData...)
+	}
+
+	return data, nil
+}
+
+// ipsDHCPOptionParserHandler parses a DHCP option as a comma-separates list of
+// IP addresses.  For example:
+//
+//   6 ips 192.168.1.1,192.168.1.2
+//
+func textDHCPOptionParserHandler(s string) (data []byte, err error) {
+	return []byte(s), nil
+}
+
+// dhcpOptionParserHandler is a parser for a single dhcp option type.
+type dhcpOptionParserHandler func(s string) (data []byte, err error)
+
+// dhcpOptionParser parses DHCP options.
+type dhcpOptionParser struct {
+	handlers map[string]dhcpOptionParserHandler
+}
+
+// newDHCPOptionParser returns a new dhcpOptionParser.
+func newDHCPOptionParser() (p *dhcpOptionParser) {
+	return &dhcpOptionParser{
+		handlers: map[string]dhcpOptionParserHandler{
+			"hex":  hexDHCPOptionParserHandler,
+			"ip":   ipDHCPOptionParserHandler,
+			"ips":  ipsDHCPOptionParserHandler,
+			"text": textDHCPOptionParserHandler,
+		},
+	}
+}
+
+// parse parses an option.  See the handlers' documentation for more info.
+func (p *dhcpOptionParser) parse(s string) (code uint8, data []byte, err error) {
+	defer func() {
+		if err != nil {
+			err = fmt.Errorf("invalid option string %q: %w", s, err)
+		}
+	}()
+
+	s = strings.TrimSpace(s)
+	parts := strings.SplitN(s, " ", 3)
+	if len(parts) < 3 {
+		return 0, nil, agherr.Error("need at least three fields")
+	}
+
+	codeStr := parts[0]
+	typ := parts[1]
+	val := parts[2]
+
+	var code64 uint64
+	code64, err = strconv.ParseUint(codeStr, 10, 8)
+	if err != nil {
+		return 0, nil, fmt.Errorf("parsing option code: %w", err)
+	}
+
+	code = uint8(code64)
+
+	h, ok := p.handlers[typ]
+	if !ok {
+		return 0, nil, fmt.Errorf("unknown option type %q", typ)
+	}
+
+	data, err = h(val)
+	if err != nil {
+		return 0, nil, err
+	}
+
+	return uint8(code), data, nil
+}