Enforce NULL context for hardcoded RNG
diff --git a/programs/ssl/dtls_client.c b/programs/ssl/dtls_client.c
index 5bd5cbe..39a9f76 100644
--- a/programs/ssl/dtls_client.c
+++ b/programs/ssl/dtls_client.c
@@ -95,6 +95,20 @@
 }
 #endif /* MBEDTLS_DEBUG_C */
 
+#if defined(MBEDTLS_SSL_CONF_RNG)
+int rng_wrap( void *ctx, unsigned char *dst, size_t len );
+
+mbedtls_ctr_drbg_context *rng_ctx_global = NULL;
+int rng_wrap( void *ctx, unsigned char *dst, size_t len )
+{
+    /* We expect the NULL parameter here. */
+    if( ctx != NULL )
+        return( -1 );
+
+    return( mbedtls_ctr_drbg_random( rng_ctx_global, dst, len ) );
+}
+#endif /* MBEDTLS_SSL_CONF_RNG */
+
 int main( int argc, char *argv[] )
 {
     int ret, len;
@@ -192,7 +206,13 @@
      * Production code should set a proper ca chain and use REQUIRED. */
     mbedtls_ssl_conf_authmode( &conf, MBEDTLS_SSL_VERIFY_OPTIONAL );
     mbedtls_ssl_conf_ca_chain( &conf, &cacert, NULL );
+
+#if !defined(MBEDTLS_SSL_CONF_RNG)
     mbedtls_ssl_conf_rng( &conf, mbedtls_ctr_drbg_random, &ctr_drbg );
+#else
+    rng_ctx_global = &ctr_drbg;
+#endif
+
 #if defined(MBEDTLS_DEBUG_C)
     mbedtls_ssl_conf_dbg( &conf, my_debug, stdout );
 #endif