Client address is the right-most untrusted X-Forwarded-For entry (closes #144)

From a peer in TRUSTED_PROXIES, clientIP took the left-most
X-Forwarded-For entry, which the client writes itself behind a proxy
that appends to the header, so a client could choose the address the
rate limit counts and the request log records. It now walks the entries
from the right past trusted proxies and takes the first that is not
one; with only trusted entries, or one that is not an address, it uses
the peer. Every X-Forwarded-For header line is read. X-Real-IP is read
only when there is no X-Forwarded-For entry. README.md Settings says
which entry is taken.

Model: opus-5-5

internal/middleware/middleware_test.go

@@ -25,6 +25,9 @@ import (
 const (
 	// loopbackPeer is a remote address inside the trusted-proxy allowlist.
 	loopbackPeer = "127.0.0.1:5000"
+	// loopbackIP is the address of loopbackPeer, which ClientIP returns
+	// when it uses the peer.
+	loopbackIP = "127.0.0.1"
 	// forwardedIP is the client address presented via X-Forwarded-For.
 	forwardedIP = "203.0.113.7"
 	// realIP is the client address presented via X-Real-IP.
@@ -85,10 +88,35 @@ func clientIPCases() []clientIPCase {
 			want:       forwardedIP,
 		},
 		{
-			name:       "trusted proxy uses left-most of chain",
-			remoteAddr: "10.1.2.3:5000",
-			xff:        forwardedIP + ", 10.1.2.3",
-			want:       forwardedIP,
+			name:       "trusted proxy uses right-most of chain",
+			remoteAddr: loopbackPeer,
+			xff:        "203.0.113.9, 198.51.100.7",
+			want:       "198.51.100.7",
+		},
+		{
+			name:       "trusted proxy entries are skipped from the right",
+			remoteAddr: loopbackPeer,
+			xff:        "203.0.113.9, 10.0.0.5",
+			want:       "203.0.113.9",
+		},
+		{
+			name:       "chain of only trusted proxies uses peer",
+			remoteAddr: loopbackPeer,
+			xff:        "10.0.0.5, 10.1.2.3",
+			want:       loopbackIP,
+		},
+		{
+			name:       "entry that is not an address uses peer",
+			remoteAddr: loopbackPeer,
+			xff:        "203.0.113.9, 198.51.100.7:5000",
+			want:       loopbackIP,
+		},
+		{
+			name:       "x-real-ip ignored when forwarded-for has entries",
+			remoteAddr: loopbackPeer,
+			xff:        "10.0.0.5",
+			xRealIP:    realIP,
+			want:       loopbackIP,
 		},
 		{
 			name:       "trusted proxy falls back to x-real-ip",
@@ -117,7 +145,7 @@ func clientIPCases() []clientIPCase {
 			name:       "trusted proxy with garbage header uses peer",
 			remoteAddr: loopbackPeer,
 			xff:        "not-an-ip",
-			want:       "127.0.0.1",
+			want:       loopbackIP,
 		},
 	}
 }
@@ -148,6 +176,23 @@ func TestClientIP(t *testing.T) {
 	}
 }
 
+// TestClientIPReadsEveryForwardedForLine checks that an entry a proxy
+// adds as an X-Forwarded-For line of its own is read, and the line the
+// client sent before it is not taken.
+func TestClientIPReadsEveryForwardedForLine(t *testing.T) {
+	t.Parallel()
+
+	header := http.Header{}
+	header.Add("X-Forwarded-For", "203.0.113.9")
+	header.Add("X-Forwarded-For", "198.51.100.7")
+
+	got := middleware.ClientIP(loopbackPeer, header,
+		mustPrefixes(t, "127.0.0.1/32"))
+	if got != "198.51.100.7" {
+		t.Errorf("ClientIP() = %q, want %q", got, "198.51.100.7")
+	}
+}
+
 func TestSecurityHeaders(t *testing.T) {
 	t.Parallel()