l4proxy: add proxy_protocol matcher (#30)

* feat(PROXY): add matcher

* refactor(ProxyProtocol): drop the dependency

* fix(PROXY): handle null in v2 header

* renaming struct and ID to match convention

modules/l4proxy/matcher.go

@@ -0,0 +1,90 @@
+// Copyright 2020 Matthew Holt
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package l4proxy
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"io"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/mholt/caddy-l4/layer4"
+)
+
+// https://www.haproxy.org/download/1.8/doc/proxy-protocol.txt
+var (
+	headerV1Prefix = []byte("PROX") // intentional to not include "Y", see match() function for details
+	headerV2Prefix = []byte{0x0D, 0x0A, 0x0D, 0x0A, 0x00, 0x0D, 0x0A, 0x51, 0x55, 0x49, 0x54, 0x0A}
+)
+
+func init() {
+	caddy.RegisterModule(MatchProxyProtocol{})
+}
+
+type MatchProxyProtocol struct{}
+
+// CaddyModule returns the Caddy module information.
+func (MatchProxyProtocol) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "layer4.matchers.proxy_protocol",
+		New: func() caddy.Module { return new(MatchProxyProtocol) },
+	}
+}
+
+// Match returns true if the connection looks like it is using the Proxy Protocol.
+func (m MatchProxyProtocol) Match(cx *layer4.Connection) (bool, error) {
+	p := make([]byte, 4)
+	_, err := io.ReadFull(cx, p)
+	if err != nil {
+		return false, err
+	}
+	if bytes.Equal(p, headerV1Prefix) {
+		return true, nil
+	}
+
+	buf := p[:]
+	bufReader := bufio.NewReader(cx)
+	for i := 1; i <= 8; i++ {
+		// read the next 8 bytes, one byte at a time
+		// since 5th byte being null is valid in v2 header
+		// but is considered EOF by readers
+		b, err := bufReader.ReadByte()
+		if err == nil {
+			buf = append(buf, b)
+			continue
+		}
+
+		if !errors.Is(err, io.ErrUnexpectedEOF) {
+			return false, err
+		}
+
+		// 4 bytes were already read, so i == 1 is the 5th byte
+		if i != 1 {
+			return false, err
+		}
+
+		buf = append(buf, 0x00)
+	}
+
+	if bytes.Equal(buf, headerV2Prefix) {
+		return true, nil
+	}
+
+	return false, nil
+}
+
+// Interface guard
+var _ layer4.ConnMatcher = (*MatchProxyProtocol)(nil)