diff --git a/conn_test.go b/conn_test.go
new file mode 100644
index 0000000..34bdffa
--- /dev/null
+++ b/conn_test.go
@@ -0,0 +1,166 @@
+package journal
+
+import (
+	"bytes"
+	"encoding/binary"
+	"encoding/hex"
+	"io"
+	"net"
+	"path/filepath"
+	"strconv"
+	"testing"
+)
+
+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",
+	})
+	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)
+
+	// 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{})
+	for i := 0; i < 16; i++ {
+		go func() {
+			<-start
+			for j := 0; j < 100; j++ {
+				err := c.EntryErr(PriDebug, "message "+strconv.Itoa(i)+"."+strconv.Itoa(j), attr)
+				if err != nil {
+					t.Fatalf("message %d.%d error: %v", i, j, err)
+				}
+			}
+		}()
+	}
+
+	// try to get all the goroutines to start at roughly the same time
+	close(start)
+}
+
+// BenchmarkEntry is a benchmark for the common Entry function.
+func BenchmarkEntry(b *testing.B) {
+	c := testConnector(b)
+
+	// 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"),
+	})
+
+	// 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"),
+	})
+
+	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)
+		}
+	}
+}