caddyhttp: Remote IP prefix placeholders

See https://github.com/mholt/caddy-ratelimit/issues/12

modules/caddyhttp/replacer.go

@@ -31,6 +31,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"net/netip"
 	"net/textproto"
 	"net/url"
 	"path"
@@ -196,6 +197,37 @@ func addHTTPVarsToReplacer(repl *caddy.Replacer, req *http.Request, w http.Respo
 				return or.URL.RawQuery, true
 			}
 
+			// remote IP range/prefix (e.g. keep top 24 bits of 1.2.3.4  => "1.2.3.0/24")
+			// syntax: "/V4,V6" where V4 = IPv4 bits, and V6 = IPv6 bits; if no comma, then same bit length used for both
+			// (EXPERIMENTAL)
+			if strings.HasPrefix(key, "http.request.remote.host/") {
+				host, _, err := net.SplitHostPort(req.RemoteAddr)
+				if err != nil {
+					host = req.RemoteAddr // assume no port, I guess?
+				}
+				addr, err := netip.ParseAddr(host)
+				if err != nil {
+					return host, true // not an IP address
+				}
+				// extract the bits from the end of the placeholder (start after "/") then split on ","
+				bitsBoth := key[strings.Index(key, "/")+1:]
+				ipv4BitsStr, ipv6BitsStr, cutOK := strings.Cut(bitsBoth, ",")
+				bitsStr := ipv4BitsStr
+				if addr.Is6() && cutOK {
+					bitsStr = ipv6BitsStr
+				}
+				// convert to integer then compute prefix
+				bits, err := strconv.Atoi(bitsStr)
+				if err != nil {
+					return "", true
+				}
+				prefix, err := addr.Prefix(bits)
+				if err != nil {
+					return "", true
+				}
+				return prefix.String(), true
+			}
+
 			// hostname labels
 			if strings.HasPrefix(key, reqHostLabelsReplPrefix) {
 				idxStr := key[len(reqHostLabelsReplPrefix):]

modules/caddyhttp/replacer_test.go

@@ -32,7 +32,7 @@ func TestHTTPVarReplacement(t *testing.T) {
 	ctx := context.WithValue(req.Context(), caddy.ReplacerCtxKey, repl)
 	req = req.WithContext(ctx)
 	req.Host = "example.com:80"
-	req.RemoteAddr = "localhost:1234"
+	req.RemoteAddr = "192.168.159.32:1234"
 
 	clientCert := []byte(`-----BEGIN CERTIFICATE-----
 MIIB9jCCAV+gAwIBAgIBAjANBgkqhkiG9w0BAQsFADAYMRYwFAYDVQQDDA1DYWRk
@@ -61,7 +61,7 @@ eqp31wM9il1n+guTNyxJd+FzVAH+hCZE5K+tCgVDdVFUlDEHHbS/wqb2PSIoouLV
 	req.TLS = &tls.ConnectionState{
 		Version:                    tls.VersionTLS13,
 		HandshakeComplete:          true,
-		ServerName:                 "foo.com",
+		ServerName:                 "example.com",
 		CipherSuite:                tls.TLS_AES_256_GCM_SHA384,
 		PeerCertificates:           []*x509.Certificate{cert},
 		NegotiatedProtocol:         "h2",
@@ -97,7 +97,19 @@ eqp31wM9il1n+guTNyxJd+FzVAH+hCZE5K+tCgVDdVFUlDEHHbS/wqb2PSIoouLV
 		},
 		{
 			get:    "http.request.remote.host",
-			expect: "localhost",
+			expect: "192.168.159.32",
+		},
+		{
+			get:    "http.request.remote.host/24",
+			expect: "192.168.159.0/24",
+		},
+		{
+			get:    "http.request.remote.host/24,32",
+			expect: "192.168.159.0/24",
+		},
+		{
+			get:    "http.request.remote.host/999",
+			expect: "",
 		},
 		{
 			get:    "http.request.remote.port",
@@ -146,7 +158,7 @@ eqp31wM9il1n+guTNyxJd+FzVAH+hCZE5K+tCgVDdVFUlDEHHbS/wqb2PSIoouLV
 		},
 		{
 			get:    "http.request.tls.server_name",
-			expect: "foo.com",
+			expect: "example.com",
 		},
 		{
 			get:    "http.request.tls.version",