diff --git a/internal/martian/header/via_modifier.go b/internal/martian/header/via_modifier.go
index 633c3daf..12784652 100644
--- a/internal/martian/header/via_modifier.go
+++ b/internal/martian/header/via_modifier.go
@@ -20,26 +20,32 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"regexp"
 	"strings"
 
 	"github.com/saucelabs/forwarder/internal/martian"
 )
 
-var whitespace = regexp.MustCompile("[\t ]+")
-
 // ViaModifier is a header modifier that checks for proxy redirect loops.
 type ViaModifier struct {
 	requestedBy string
 	boundary    string
+	len         int
 }
 
 // NewViaModifier returns a new Via modifier.
 func NewViaModifier(requestedBy string) *ViaModifier {
-	return &ViaModifier{
+	m := &ViaModifier{
 		requestedBy: requestedBy,
 		boundary:    randomBoundary(),
 	}
+
+	var sb strings.Builder
+	if _, err := m.writeTo(&sb, &http.Request{ProtoMajor: 1, ProtoMinor: 1}); err != nil {
+		panic(err) // fatal programming error
+	}
+	m.len = sb.Len()
+
+	return m
 }
 
 // ModifyRequest sets the Via header and provides loop-detection. If Via is
@@ -49,10 +55,18 @@ func NewViaModifier(requestedBy string) *ViaModifier {
 //
 // http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-14#section-9.9
 func (m *ViaModifier) ModifyRequest(req *http.Request) error {
-	via := fmt.Sprintf("%d.%d %s-%s", req.ProtoMajor, req.ProtoMinor, m.requestedBy, m.boundary)
+	via := req.Header.Get("Via")
+
+	l := m.len
+	if via != "" {
+		l += len(via) + 2
+	}
+
+	var sb strings.Builder
+	sb.Grow(l)
 
-	if v := req.Header.Get("Via"); v != "" {
-		if m.hasLoop(v) {
+	if via != "" {
+		if strings.Contains(via, m.requestedBy+"-"+m.boundary) {
 			req.Close = true
 			return martian.ErrorStatus{
 				Err:    fmt.Errorf("via: detected request loop, header contains %s", via),
@@ -60,31 +74,17 @@ func (m *ViaModifier) ModifyRequest(req *http.Request) error {
 			}
 		}
 
-		via = fmt.Sprintf("%s, %s", v, via)
+		sb.WriteString(via)
+		sb.WriteString(", ")
 	}
 
-	req.Header.Set("Via", via)
-
-	return nil
-}
-
-// hasLoop parses via and attempts to match requestedBy against the contained
-// pseudonyms/host:port pairs.
-func (m *ViaModifier) hasLoop(via string) bool {
-	for _, v := range strings.Split(via, ",") {
-		parts := whitespace.Split(strings.TrimSpace(v), 3)
-
-		// No pseudonym or host:port, assume there is no loop.
-		if len(parts) < 2 {
-			continue
-		}
-
-		if fmt.Sprintf("%s-%s", m.requestedBy, m.boundary) == parts[1] {
-			return true
-		}
+	if _, err := m.writeTo(&sb, req); err != nil {
+		return err
 	}
 
-	return false
+	req.Header.Set("Via", sb.String())
+
+	return nil
 }
 
 // SetBoundary sets the boundary string (random 10 character by default) used to
@@ -105,3 +105,7 @@ func randomBoundary() string {
 	}
 	return hex.EncodeToString(buf[:])
 }
+
+func (m *ViaModifier) writeTo(w io.Writer, req *http.Request) (int, error) {
+	return fmt.Fprintf(w, "%d.%d %s-%s", req.ProtoMajor, req.ProtoMinor, m.requestedBy, m.boundary)
+}