diff --git a/webserver/precondition.go b/webserver/precondition.go
new file mode 100644
index 0000000..d13da32
--- /dev/null
+++ b/webserver/precondition.go
@@ -0,0 +1,110 @@
+package webserver
+
+import (
+	"net/http"
+	"strings"
+)
+
+// This is partly based on the Go standard library file net/http/fs.go.
+
+// scanETag determines if a syntactically valid ETag is present at s. If so,
+// the ETag and remaining text after consuming ETag is returned. Otherwise,
+// it returns "", "".
+func scanETag(s string) (etag string, remain string) {
+	s = strings.TrimLeft(s, " \t\n\r")
+	start := 0
+	if strings.HasPrefix(s, "W/") {
+		start = 2
+	}
+	if len(s[start:]) < 2 || s[start] != '"' {
+		return "", ""
+	}
+	// ETag is either W/"text" or "text".
+	// See RFC 7232 2.3.
+	for i := start + 1; i < len(s); i++ {
+		c := s[i]
+		switch {
+		// Character values allowed in ETags.
+		case c == 0x21 || c >= 0x23 && c <= 0x7E || c >= 0x80:
+		case c == '"':
+			return s[:i+1], s[i+1:]
+		default:
+			return "", ""
+		}
+	}
+	return "", ""
+}
+
+// etagMatch returns whether the given eTag matches an IM/INM header value.
+// The empty string in etag is interpreted as a non-existent object.
+func etagMatch(etag, header string) bool {
+	if header == "" {
+		return false
+	}
+	if header == etag {
+		return true
+	}
+
+	for {
+		header = strings.TrimLeft(header, " \t\n\r")
+		if len(header) == 0 {
+			break
+		}
+		if header[0] == ',' {
+			header = header[1:]
+			continue
+		}
+		if header[0] == '*' {
+			return etag != ""
+		}
+		e, remain := scanETag(header)
+		if e == "" {
+			break
+		}
+		if e == etag {
+			return true
+		}
+		header = remain
+	}
+
+	return false
+}
+
+func writeNotModified(w http.ResponseWriter) {
+	// RFC 7232 section 4.1:
+	// a sender SHOULD NOT generate representation metadata other than the
+	// above listed fields unless said metadata exists for the purpose of
+	// guiding cache updates (e.g., Last-Modified might be useful if the
+	// response does not have an ETag field).
+	h := w.Header()
+	delete(h, "Content-Type")
+	delete(h, "Content-Length")
+	delete(h, "Content-Encoding")
+	if h.Get("Etag") != "" {
+		delete(h, "Last-Modified")
+	}
+	w.WriteHeader(http.StatusNotModified)
+}
+
+// checkPreconditions evaluates request preconditions.
+// It interprets an empty etag as a non-existent object.
+func checkPreconditions(w http.ResponseWriter, r *http.Request, etag string) (done bool) {
+	// RFC 7232 section 6.
+	im := r.Header.Get("If-Match")
+	if im != "" && !etagMatch(etag, im) {
+		w.WriteHeader(http.StatusPreconditionFailed)
+		return true
+	}
+	inm := r.Header.Get("If-None-Match")
+	if inm != "" && etagMatch(etag, inm) {
+		if r.Method == "GET" || r.Method == "HEAD" {
+			writeNotModified(w)
+			return true
+		} else {
+			w.WriteHeader(http.StatusPreconditionFailed)
+			return true
+		}
+	}
+
+	return false
+}
diff --git a/webserver/precondition_test.go b/webserver/precondition_test.go
new file mode 100644
index 0000000..5ed61b3
--- /dev/null
+++ b/webserver/precondition_test.go
@@ -0,0 +1,114 @@
+package webserver
+
+import (
+	"net/http"
+	"testing"
+)
+
+type testWriter struct {
+	statusCode int
+}
+
+func (w *testWriter) Header() http.Header {
+	return nil
+}
+
+func (w *testWriter) Write(buf []byte) (int, error) {
+	return len(buf), nil
+}
+
+func (w *testWriter) WriteHeader(statusCode int) {
+	if w.statusCode != 0 {
+		panic("WriteHeader called twice")
+	}
+	w.statusCode = statusCode
+}
+
+func TestEtagMatch(t *testing.T) {
+	type tst struct {
+		etag, header string
+	}
+
+	var match = []tst{
+		{`"foo"`, `"foo"`},
+		{`"foo"`, ` "foo"`},
+		{`"foo"`, `"foo" `},
+		{`"foo"`, ` "foo" `},
+		{`"foo"`, `"foo", "bar"`},
+		{`"foo"`, `"bar", "foo"`},
+		{`W/"foo"`, `W/"foo"`},
+	}
+
+	var mismatch = []tst{
+		{``, ``},
+		{``, `*`},
+		{``, `"foo"`},
+		{`"foo"`, ``},
+		{`"foo"`, `"bar"`},
+		{`"foo"`, `"bar", "baz"`},
+		{`"foo"`, `"baz", "bar"`},
+		{`"foo"`, `W/"foo"`},
+		{`W/"foo"`, `"foo"`},
+	}
+
+	for _, tst := range match {
+		m := etagMatch(tst.etag, tst.header)
+		if !m {
+			t.Errorf("%#v %#v: got %v, expected true",
+				tst.etag, tst.header, m,
+			)
+		}
+	}
+
+	for _, tst := range mismatch {
+		m := etagMatch(tst.etag, tst.header)
+		if m {
+			t.Errorf("%#v %#v: got %v, expected false",
+				tst.etag, tst.header, m,
+			)
+		}
+	}
+}
+
+func TestCheckPreconditions(t *testing.T) {
+	var tests = []struct {
+		method, etag, im, inm string
+		result                int
+	}{
+		{"GET", ``, ``, ``, 0},
+		{"GET", ``, `*`, ``, 412},
+		{"GET", ``, ``, `*`, 0},
+		{"POST", ``, `*`, ``, 412},
+		{"POST", ``, ``, `*`, 0},
+		{"GET", `"123"`, ``, ``, 0},
+		{"GET", `"123"`, `"123"`, ``, 0},
+		{"GET", `"123"`, `"124"`, ``, 412},
+		{"POST", `"123"`, `"124"`, ``, 412},
+		{"GET", `"123"`, `*`, ``, 0},
+		{"GET", `"123"`, ``, `"123"`, 304},
+		{"POST", `"123"`, ``, `"123"`, 412},
+		{"GET", `"123"`, ``, `"124"`, 0},
+		{"GET", `"123"`, ``, `*`, 304},
+	}
+
+	for _, tst := range tests {
+		var w testWriter
+		h := make(http.Header)
+		if tst.im != "" {
+			h.Set("If-Match", tst.im)
+		}
+		if tst.inm != "" {
+			h.Set("If-None-Match", tst.inm)
+		}
+		r := http.Request{
+			Method: tst.method,
+			Header: h,
+		}
+		done := checkPreconditions(&w, &r, tst.etag)
+		if done != (tst.result != 0) || w.statusCode != tst.result {
+			t.Errorf("%#v %#v %#v: got %v, expected %v",
+				tst.etag, tst.im, tst.inm,
+				w.statusCode, tst.result)
+		}
+	}
+}