diff --git a/conn_test.go b/conn_test.go
index 6c89a6a..b5f5a63 100644
--- a/conn_test.go
+++ b/conn_test.go
@@ -1,187 +1,124 @@
 package journal
 
 import (
-	"bytes"
-	"encoding/binary"
-	"encoding/hex"
-	"errors"
-	"fmt"
-	"io"
-	"net"
 	"path/filepath"
-	"strconv"
 	"testing"
+
+	"src.lwithers.me.uk/go/journal/testsink"
 )
 
-type testingCommon interface {
-	TempDir() string
-	Fatalf(string, ...any)
-}
-
-// testConnector spawns a Conn with a local Unix datagram socket that checks
-// incoming datagrams for well-formedness but otherwise discards them.
-func testConnector(t testingCommon) *Conn {
-	sockPath := filepath.Join(t.TempDir(), "test-socket")
-	sock, err := net.ListenUnixgram("unixgram", &net.UnixAddr{
-		Name: sockPath,
-		Net:  "unixgram",
-	})
+// TestConn opens a connection to a test sink and writes one message,
+// ensuring it is received.
+func TestConn(t *testing.T) {
+	sockpath := filepath.Join(t.TempDir(), "socket")
+	sink, err := testsink.New(sockpath)
 	if err != nil {
-		t.Fatalf("testConnector: ListenUnix: %v", err)
-	}
-
-	go func() {
-		buf := make([]byte, 1<<16 /*enough for our tests */)
-		for {
-			n, err := sock.Read(buf)
-			switch err {
-			case nil:
-			case io.EOF:
-				return
-			default:
-				t.Fatalf("testConnector: Read: %v", err)
-			}
-			ok, pos, desc := checkWellFormedProto(buf[:n])
-			if !ok {
-				t.Fatalf("received malformed data at pos 0x%x: %s\n%s", pos, desc, hex.Dump(buf[:n]))
-			}
-		}
-	}()
-
-	conn, err := Connect(sockPath)
-	if err != nil {
-		t.Fatalf("testConnector: DialUnix: %v", err)
-	}
-	conn.ErrHandler = func(err error) {
-		t.Fatalf("testConnector: connection error: %v", err)
-	}
-	return conn
-}
-
-func checkWellFormedProto(buf []byte) (ok bool, pos int, desc string) {
-	for pos < len(buf) {
-		// grab attribute name up to '=' or '\n'
-		off := bytes.IndexAny(buf[pos:], "=\n")
-		if off == -1 {
-			return false, pos, "unterminated key"
-		}
-		key := string(buf[pos : pos+off])
-		if err := AttrKeyValid(key); err != nil {
-			return false, pos, err.Error()
-		}
-		pos += off
-
-		// for KEY=VALUE, the value is terminated by a newline
-		if buf[pos] == '=' {
-			pos++
-			off = bytes.IndexByte(buf[pos:], '\n')
-			if off == -1 {
-				return false, pos, "unterminated value"
-			}
-			pos += off // consume value
-			pos++      // consume trailing newline
-			continue
-		}
-
-		// otherwise, expect an 8-bit little-endian length
-		pos++ // consume newline after key
-		if pos+8 > len(buf) {
-			return false, pos, "value length too short"
-		}
-		vlen := binary.LittleEndian.Uint64(buf[pos:])
-		pos += 8
-		if vlen > uint64(len(buf)) /* protect against overflow */ ||
-			uint64(pos)+vlen+1 > uint64(len(buf)) {
-			return false, pos, "value length too long"
-		}
-		pos += int(vlen)
-		if buf[pos] != '\n' {
-			return false, pos, "value not terminated by newline"
-		}
-		pos++
-	}
-	return true, pos, ""
-}
-
-// TestConcurrentEntries is best run with the race detector, and tries to pick
-// up any faults that might occur when concurrent goroutines write into the same
-// Conn.
-func TestConcurrentEntries(t *testing.T) {
-	c := testConnector(t)
-	const (
-		numGoroutines = 16
-		numEntries    = 100
-	)
-
-	// attributes which will be common to all EntryErr calls
-	attr := make([]Attr, 0, 10 /* enough capacity to avoid realloc on append; might trigger data races */)
-	attr = append(attr, Attr{
-		Key: AttrKey{
-			key: "HELLO",
-		},
-		Value: []byte("world"),
-	})
-
-	// spawn goroutines
-	start := make(chan struct{})
-	result := make(chan error, numGoroutines)
-	for i := range numGoroutines {
-		go func() {
-			var err error
-			<-start
-			for j := range numEntries {
-				err = c.EntryErr(PriDebug, "message "+strconv.Itoa(i)+"."+strconv.Itoa(j), attr)
-				if err != nil {
-					err = fmt.Errorf("message %d.%d error: %w", i, j, err)
-					break
-				}
-			}
-			result <- err
-		}()
-	}
-
-	// try to get all the goroutines to start at roughly the same time
-	close(start)
-
-	// collect results
-	var errs []error
-	for range numGoroutines {
-		if err := <-result; err != nil {
-			errs = append(errs, err)
-		}
-	}
-	if err := errors.Join(errs...); err != nil {
 		t.Fatal(err)
 	}
-}
+	defer sink.Stop()
 
-// BenchmarkEntry is a benchmark for the common Entry function.
-func BenchmarkEntry(b *testing.B) {
-	c := testConnector(b)
+	conn, err := Connect(sockpath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
 
-	// add some common attributes
-	c.Common = make([]Attr, 0, 10)
-	c.Common = append(c.Common, Attr{
-		Key: AttrKey{
-			key: "COMMON_ATTR",
-		},
-		Value: []byte("abc123\n"),
-	})
+	conn.Entry(PriInfo, "hello, world", nil)
 
-	// attributes which will be common to all EntryErr calls
-	attr := make([]Attr, 0, 10)
-	attr = append(attr, Attr{
-		Key: AttrKey{
-			key: "HELLO",
-		},
-		Value: []byte("world"),
-	})
+	msg, err := sink.Message(0)
+	if err != nil {
+		t.Fatal(err)
+	}
 
-	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		err := c.EntryErr(PriDebug, "hello world!", attr)
-		if err != nil {
-			b.Fatalf("message %d: error %v", i, err)
+	msgText, attrs, err := msg.Decode()
+	if err != nil {
+		t.Error(err)
+	}
+	if msgText != "hello, world" {
+		t.Errorf("unexpected message text %q", msgText)
+	}
+	val, ok := testsink.GetAttr(attrs, "PRIORITY")
+	switch {
+	case !ok:
+		t.Error("missing PRIORITY attribute")
+	case val != "6":
+		t.Error("unexpected PRIORITY value")
+	}
+
+	if t.Failed() {
+		for i := range attrs {
+			t.Errorf("attr %q=%q", attrs[i].Key, attrs[i].Val)
+		}
+	}
+}
+
+// TestEntryBinary ensures that we can write a message with an attribute encoded
+// as a binary field.
+func TestEntryBinary(t *testing.T) {
+	sockpath := filepath.Join(t.TempDir(), "socket")
+	sink, err := testsink.New(sockpath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer sink.Stop()
+
+	conn, err := Connect(sockpath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	expAttrs := []Attr{
+		Attr{
+			Key:   MustAttrKey("SHORT"),
+			Value: []byte("short val"),
+		},
+		Attr{
+			Key:   MustAttrKey("BINARY"),
+			Value: []byte("string with\n=embedded newline\nrequires binary protocol\n"),
+		},
+		Attr{
+			Key:   MustAttrKey("LAST"),
+			Value: []byte("last\n"),
+		},
+	}
+	conn.Entry(PriDebug, "hello, binary world", expAttrs)
+
+	msg, err := sink.Message(0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	msgText, attrs, err := msg.Decode()
+	if err != nil {
+		t.Error(err)
+	}
+	if msgText != "hello, binary world" {
+		t.Errorf("unexpected message text %q", msgText)
+	}
+
+	val, ok := testsink.GetAttr(attrs, "PRIORITY")
+	switch {
+	case !ok:
+		t.Error("missing PRIORITY attribute")
+	case val != "7":
+		t.Error("unexpected PRIORITY value")
+	}
+
+	for i := range expAttrs {
+		key, expVal := expAttrs[i].Key.Key(), string(expAttrs[i].Value)
+		val, ok = testsink.GetAttr(attrs, key)
+		switch {
+		case !ok:
+			t.Errorf("missing %s attribute", key)
+		case val != expVal:
+			t.Errorf("unexpected %s value", key)
+		}
+	}
+
+	if t.Failed() {
+		for i := range attrs {
+			t.Errorf("attr %q=%q", attrs[i].Key, attrs[i].Val)
 		}
 	}
 }
diff --git a/testsink/message.go b/testsink/message.go
new file mode 100644
index 0000000..99cae43
--- /dev/null
+++ b/testsink/message.go
@@ -0,0 +1,126 @@
+/*
+Package testsink provides a partial implementation of the systemd-journald
+Unix socket. Datagrams received on its socket are decoded and stored for unit
+tests to examine.
+*/
+package testsink
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+)
+
+// Message returns the Nth message received. It waits for the message to arrive.
+func (sink *Sink) Message(N int) (Message, error) {
+	sink.lock.Lock()
+	defer sink.lock.Unlock()
+
+	for len(sink.msgs) <= N {
+		sink.mcond.Wait()
+		if sink.err != nil {
+			return Message{}, sink.err
+		}
+	}
+	return sink.msgs[N], nil
+}
+
+// Message is recorded for each datagram received.
+type Message struct {
+	Raw []byte
+}
+
+type DecodedAttr struct {
+	Key, Val string
+}
+
+func (m *Message) Decode() (msg string, attr []DecodedAttr, err error) {
+	raw := m.Raw
+	var errs []error
+
+DecodeLoop:
+	for len(raw) > 0 {
+		n, key := decodeAttrKey(raw)
+		raw = raw[n:]
+		switch {
+		case len(raw) == 0:
+			errs = append(errs, fmt.Errorf("unterminated attribute name %q", key))
+			break DecodeLoop
+		case key == "":
+			errs = append(errs, errors.New("empty attribute name"))
+		}
+
+		var val string
+		switch raw[0] {
+		case '=':
+			n, val = decodeAttrValText(raw[1:])
+		case '\n':
+			var err error
+			n, val, err = decodeAttrValLen(raw[1:])
+			if err != nil {
+				errs = append(errs, err)
+			}
+		}
+		raw = raw[1+n:]
+		if len(raw) == 0 {
+			errs = append(errs, fmt.Errorf("unterminated value for attribute %q", key))
+			break DecodeLoop
+		}
+
+		if raw[0] != '\n' {
+			errs = append(errs, errors.New("incorrectly terminated attribute value"))
+		}
+		raw = raw[1:]
+
+		switch key {
+		case "MESSAGE":
+			msg = val
+		default:
+			attr = append(attr, DecodedAttr{Key: key, Val: val})
+		}
+	}
+
+	return msg, attr, errors.Join(errs...)
+}
+
+func decodeAttrKey(raw []byte) (n int, key string) {
+	for i := range raw {
+		switch raw[i] {
+		case '\n', '=':
+			return i, string(raw[:i])
+		}
+	}
+	return len(raw), string(raw)
+}
+
+func decodeAttrValText(raw []byte) (n int, val string) {
+	term := bytes.IndexByte(raw, '\n')
+	if term == -1 {
+		term = len(raw)
+	}
+	return term, string(raw[:term])
+}
+
+func decodeAttrValLen(raw []byte) (n int, val string, err error) {
+	if len(raw) < 8 {
+		return len(raw), "", errors.New("not enough bytes for binary attribute value length")
+	}
+	amt := binary.LittleEndian.Uint64(raw)
+	raw = raw[8:]
+	if uint64(len(raw)) < amt {
+		return 8 + len(raw), string(raw), errors.New("not enough bytes for binary attribute value")
+	}
+	return int(8 + amt), string(raw[:amt]), nil
+}
+
+// GetAttr returns the value of the attribute whose key name matches, and a
+// boolean to indicate if it found a match.
+func GetAttr(attr []DecodedAttr, key string) (value string, ok bool) {
+	for i := range attr {
+		if attr[i].Key == key {
+			return attr[i].Val, true
+		}
+	}
+	return "", false
+}
diff --git a/testsink/testsink.go b/testsink/testsink.go
new file mode 100644
index 0000000..427a775
--- /dev/null
+++ b/testsink/testsink.go
@@ -0,0 +1,82 @@
+/*
+Package testsink provides a partial implementation of the systemd-journald
+Unix socket. Datagrams received on its socket are decoded and stored for unit
+tests to examine.
+*/
+package testsink
+
+import (
+	"net"
+	"slices"
+	"sync"
+)
+
+// Sink provides a Unix socket and captures messages sent to it using the
+// systemd-journald wire protocol.
+type Sink struct {
+	sock *net.UnixConn
+	stop chan struct{}
+
+	lock  sync.Mutex
+	mcond *sync.Cond
+	msgs  []Message
+	err   error
+}
+
+// New returns a new Sink that is listening on the given path.
+func New(sockpath string) (*Sink, error) {
+	sock, err := net.ListenUnixgram("unixgram", &net.UnixAddr{
+		Name: sockpath,
+		Net:  "unixgram",
+	})
+	if err != nil {
+		return nil, err
+	}
+	sink := &Sink{
+		sock: sock,
+		stop: make(chan struct{}, 1),
+	}
+	sink.mcond = sync.NewCond(&sink.lock)
+	go sink.stopper()
+	go sink.recv()
+	return sink, nil
+}
+
+// Stop listening and close the socket.
+func (sink *Sink) Stop() {
+	// non-blocking write onto channel; we only need to read from it once in
+	// order to stop the receiver, but using this rather than close ensures
+	// Stop() can be called multiple times without negative side effects
+	select {
+	case sink.stop <- struct{}{}:
+	default:
+	}
+}
+
+func (sink *Sink) stopper() {
+	<-sink.stop
+	sink.sock.Close()
+}
+
+func (sink *Sink) recv() {
+	buf := make([]byte, 131072)
+	for {
+		n, err := sink.sock.Read(buf)
+
+		sink.lock.Lock()
+		if n > 0 {
+			sink.msgs = append(sink.msgs, Message{
+				Raw: slices.Clone(buf[:n]),
+			})
+		}
+		if err != nil {
+			sink.err = err
+		}
+		sink.lock.Unlock()
+
+		sink.mcond.Broadcast()
+		if err != nil {
+			return
+		}
+	}
+}