nginx: trust X-Forwarded-For only from TRUSTED_PROXIES (closes #64)

nginx trusted X-Forwarded-For from every RFC1918 address, so a client
reaching it from one could write a new address on each request and
get a fresh rate-limit allowance. The container's TRUSTED_PROXIES now
names the reverse proxies nginx trusts, none by default.
bin/entrypoint.sh makes each entry a CIDR, checks it with the new
"netwatch-server check-cidr", which runs the server's own
TRUSTED_PROXIES parsing, and writes one set_real_ip_from line per
entry into /etc/nginx/trusted-proxies.conf, which nginx.conf includes.
The backend is started with TRUSTED_PROXIES=127.0.0.1/32, since nginx
is its only client. The viewport test mounts an empty file there.

Model: opus-5-5

backend/internal/middleware/export_test.go

@@ -29,7 +29,3 @@ func ClientIP(
 ) string {
 	return clientIP(remoteAddr, header, trusted)
 }
-
-func ParseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
-	return parseTrustedProxies(cidrs)
-}

backend/internal/middleware/middleware.go

@@ -64,7 +64,7 @@ func New(
 	_ fx.Lifecycle,
 	params Params,
 ) (*Middleware, error) {
-	trusted, err := parseTrustedProxies(params.Config.TrustedProxies)
+	trusted, err := ParseTrustedProxies(params.Config.TrustedProxies)
 	if err != nil {
 		return nil, err
 	}
@@ -77,9 +77,11 @@ func New(
 	return s, nil
 }
 
-// parseTrustedProxies converts the TRUSTED_PROXIES entries into
-// prefixes, failing fast on any malformed entry.
-func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
+// ParseTrustedProxies converts the TRUSTED_PROXIES entries into
+// prefixes, failing fast on any malformed entry. Each entry must be
+// a CIDR; a lone address is refused. "netwatch-server check-cidr"
+// runs it too.
+func ParseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
 	prefixes := make([]netip.Prefix, 0, len(cidrs))
 
 	for _, cidr := range cidrs {

backend/internal/middleware/middleware_test.go

@@ -36,15 +36,32 @@ func mustPrefixes(t *testing.T, cidrs ...string) []netip.Prefix {
 	return prefixes
 }
 
+// TestParseTrustedProxiesRejectsMalformed includes entries nginx would
+// read as another address or look up as a hostname, in the CIDR form
+// bin/entrypoint.sh gives "netwatch-server check-cidr".
 func TestParseTrustedProxiesRejectsMalformed(t *testing.T) {
 	t.Parallel()
 
-	_, err := middleware.ParseTrustedProxies([]string{"not-a-cidr"})
-	if err == nil || !strings.Contains(err.Error(), "TRUSTED_PROXIES") {
-		t.Fatalf("error = %v, want one naming TRUSTED_PROXIES", err)
+	for _, cidr := range []string{
+		"not-a-cidr", "10.0.0.1", "1.2.3/32", "172.30/32", "10/32",
+		"cafe/32", "999.1.1.1/32", "10.0.0.0/33", "::1/129",
+		"fe80::1%eth0/128",
+	} {
+		_, err := middleware.ParseTrustedProxies([]string{cidr})
+		if err == nil || !strings.Contains(err.Error(), "TRUSTED_PROXIES") {
+			t.Errorf("%q: error = %v, want one naming TRUSTED_PROXIES",
+				cidr, err)
+		}
 	}
 }
 
+func TestParseTrustedProxiesAcceptsCIDRs(t *testing.T) {
+	t.Parallel()
+
+	mustPrefixes(t, "172.17.0.1/32", "10.0.0.0/8", "2001:db8::1/128",
+		"2001:db8::/32", "::ffff:192.0.2.1/128")
+}
+
 type clientIPCase struct {
 	name       string
 	remoteAddr string