feat: log local TCP address

When troubleshooting issues related to keepalive it is often useful to
be able to correlate multiple requests made over the same connection. A
first step was to introduce the information on whether the request was
made on a re-used connection or not which was done with a7dbf84. This
commit adds the local address of the TCP connection which adds the
capability to distinguish between multiple, parallel keep-alive
connections by looking at the client port of gorouter for the backend
connection.

accesslog/schema/access_log_record.go

@@ -110,6 +110,7 @@ type AccessLogRecord struct {
 	LogAttemptsDetails     bool
 	FailedAttempts         int
 	RoundTripSuccessful    bool
+	ExtraFields            []string
 	record                 []byte
 
 	// See the handlers.RequestInfo struct for details on these timings.
@@ -124,6 +125,8 @@ type AccessLogRecord struct {
 	TlsHandshakeFinishedAt      time.Time
 	AppRequestFinishedAt        time.Time
 	FinishedAt                  time.Time
+
+	LocalAddress string
 }
 
 func (r *AccessLogRecord) formatStartedAt() string {
@@ -319,6 +322,17 @@ func (r *AccessLogRecord) makeRecord(performTruncate bool) []byte {
 		b.WriteDashOrFloatValue(r.successfulAttemptTime())
 	}
 
+	// We have to consider the impact of iterating over a list. This technically allows to repeat
+	// some of the fields but it allows us to iterate over the list only once instead of once per
+	// field when we perform a [slices.Contains] check.
+	for _, field := range r.ExtraFields {
+		switch field {
+		case "local_address":
+			b.WriteString(`local_address:`)
+			b.WriteDashOrStringValue(r.LocalAddress)
+		}
+	}
+
 	b.AppendSpaces(false)
 	// #nosec  G104 - ignore errors from writing the access log as it will only cause more errors to log this error
 	b.WriteString(`x_cf_routererror:`)

accesslog/schema/access_log_record_test.go

@@ -314,6 +314,43 @@ var _ = Describe("AccessLogRecord", func() {
 				Eventually(r).Should(Say(`x_cf_routererror:"-"`))
 			})
 		})
+
+		Context("when extra_fields is set to [local_address]", func() {
+			Context("and the local address is empty", func() {
+				It("makes a record with the local address set to -", func() {
+					record.ExtraFields = []string{"local_address"}
+
+					r := BufferReader(bytes.NewBufferString(record.LogMessage()))
+					Eventually(r).Should(Say(`local_address:"-"`))
+				})
+			})
+			Context("and the local address contains an address", func() {
+				It("makes a record with the local address set to that address", func() {
+					record.ExtraFields = []string{"local_address"}
+					record.LocalAddress = "10.0.0.1:34823"
+
+					r := BufferReader(bytes.NewBufferString(record.LogMessage()))
+					Eventually(r).Should(Say(`local_address:"10.0.0.1:34823"`))
+				})
+			})
+		})
+
+		Context("when extra_fields is set to [foobarbazz]", func() {
+			It("ignores it", func() {
+				record.ExtraFields = []string{"foobarbazz"}
+				record.LocalAddress = "10.0.0.1:34823"
+
+				r := BufferReader(bytes.NewBufferString(record.LogMessage()))
+				Consistently(r).ShouldNot(Say("foobarbazz"))
+			})
+			It("does not log local_address", func() {
+				record.ExtraFields = []string{"foobarbazz"}
+				record.LocalAddress = "10.0.0.1:34823"
+
+				r := BufferReader(bytes.NewBufferString(record.LogMessage()))
+				Consistently(r).ShouldNot(Say(`local_address:"10.0.0.1:34823"`))
+			})
+		})
 	})
 
 	Describe("WriteTo", func() {

config/config.go

@@ -186,6 +186,7 @@ type LoggingConfig struct {
 	RedactQueryParams      string       `yaml:"redact_query_params"`
 	EnableAttemptsDetails  bool         `yaml:"enable_attempts_details"`
 	Format                 FormatConfig `yaml:"format"`
+	ExtraFields            []string     `yaml:"extra_fields"`
 
 	// This field is populated by the `Process` function.
 	JobName string `yaml:"-"`

handlers/access_log.go

@@ -20,6 +20,7 @@ type accessLog struct {
 	accessLogger       accesslog.AccessLogger
 	extraHeadersToLog  []string
 	logAttemptsDetails bool
+	extraFields        []string
 	logger             *slog.Logger
 }
 
@@ -46,6 +47,7 @@ func (a *accessLog) ServeHTTP(rw http.ResponseWriter, r *http.Request, next http
 		Request:            r,
 		ExtraHeadersToLog:  a.extraHeadersToLog,
 		LogAttemptsDetails: a.logAttemptsDetails,
+		ExtraFields:        a.extraFields,
 	}
 
 	requestBodyCounter := &countingReadCloser{delegate: r.Body}
@@ -82,6 +84,8 @@ func (a *accessLog) ServeHTTP(rw http.ResponseWriter, r *http.Request, next http
 	alr.AppRequestFinishedAt = reqInfo.AppRequestFinishedAt
 	alr.FinishedAt = reqInfo.FinishedAt
 
+	alr.LocalAddress = reqInfo.LocalAddress
+
 	a.accessLogger.Log(*alr)
 }
 

handlers/requestinfo.go

@@ -71,6 +71,8 @@ type RequestInfo struct {
 	ShouldRouteToInternalRouteService bool
 	FailedAttempts                    int
 
+	LocalAddress string
+
 	// RoundTripSuccessful will be set once a request has successfully reached a backend instance.
 	RoundTripSuccessful bool
 

proxy/round_tripper/proxy_round_tripper.go

@@ -191,6 +191,7 @@ func (rt *roundTripper) RoundTrip(originalRequest *http.Request) (*http.Response
 				slog.Float64("dns-lookup-time", trace.DnsTime()),
 				slog.Float64("dial-time", trace.DialTime()),
 				slog.Float64("tls-handshake-time", trace.TlsTime()),
+				slog.String("local-addr", trace.LocalAddr()),
 			)
 
 			if err != nil {
@@ -255,6 +256,7 @@ func (rt *roundTripper) RoundTrip(originalRequest *http.Request) (*http.Response
 				slog.Float64("dns-lookup-time", trace.DnsTime()),
 				slog.Float64("dial-time", trace.DialTime()),
 				slog.Float64("tls-handshake-time", trace.TlsTime()),
+				slog.String("local-addr", trace.LocalAddr()),
 			)
 
 			if err != nil {
@@ -347,6 +349,7 @@ func (rt *roundTripper) RoundTrip(originalRequest *http.Request) (*http.Response
 	reqInfo.DialFinishedAt = trace.DialDone()
 	reqInfo.TlsHandshakeStartedAt = trace.TlsStart()
 	reqInfo.TlsHandshakeFinishedAt = trace.TlsDone()
+	reqInfo.LocalAddress = trace.LocalAddr()
 
 	if res != nil && endpoint.PrivateInstanceId != "" && !requestSentToRouteService(request) {
 		setupStickySession(

proxy/round_tripper/trace.go

@@ -11,8 +11,9 @@ import (
 // requestTracer holds trace data of a single request.
 type requestTracer struct {
 	gotConn      atomic.Bool
-	connInfo     atomic.Pointer[httptrace.GotConnInfo]
+	connReused   atomic.Bool
 	wroteHeaders atomic.Bool
+	localAddr    atomic.Pointer[string]
 
 	// all times are stored as returned by time.Time{}.UnixNano()
 	dnsStart  atomic.Int64
@@ -26,8 +27,9 @@ type requestTracer struct {
 // Reset the trace data. Helpful when performing the same request again.
 func (t *requestTracer) Reset() {
 	t.gotConn.Store(false)
-	t.connInfo.Store(nil)
+	t.connReused.Store(false)
 	t.wroteHeaders.Store(false)
+	t.localAddr.Store(nil)
 	t.dnsStart.Store(0)
 	t.dnsDone.Store(0)
 	t.dialStart.Store(0)
@@ -49,11 +51,15 @@ func (t *requestTracer) WroteHeaders() bool {
 // ConnReused returns true if the traced request used an idle connection.
 // it returns false if no idle connection was used or if the information was unavailable.
 func (t *requestTracer) ConnReused() bool {
-	info := t.connInfo.Load()
-	if info != nil {
-		return info.Reused
+	return t.connReused.Load()
+}
+
+func (t *requestTracer) LocalAddr() string {
+	p := t.localAddr.Load()
+	if p == nil {
+		return ""
 	}
-	return false
+	return *p
 }
 
 func (t *requestTracer) DnsStart() time.Time {
@@ -121,7 +127,10 @@ func traceRequest(req *http.Request) (*http.Request, *requestTracer) {
 	r2 := req.WithContext(httptrace.WithClientTrace(req.Context(), &httptrace.ClientTrace{
 		GotConn: func(info httptrace.GotConnInfo) {
 			t.gotConn.Store(true)
-			t.connInfo.Store(&info)
+			t.connReused.Store(info.Reused)
+			la := info.Conn.LocalAddr().String()
+			t.localAddr.Store(&la)
+
 			// FIXME: due to https://github.com/golang/go/issues/31259 this breaks our acceptance tests and is dangerous
 			//        disabled for now even though this will reduce the number of requests we can retry
 			// if !info.Reused {