Add Server Name Identification to ALTCP TLS

This is a known missing feature;

* [lwip-tcpip#47][gh-lwip-pr]
* [lwip-tcpip/lwip@c53c9d020][gh-lwip-commit]

Added here again for compatibility with [pico-sdk][gh-pico] v1.5.x.
See discussion in [marceloalcocer/picohttps#1][gh-issue] for more
details.

[gh-lwip-pr]: lwip-tcpip#47
[gh-lwip-commit] lwip-tcpip@c53c9d0
[gh-pico]: https://github.com/raspberrypi/pico-sdk
[gh-issue]: marceloalcocer/picohttps#1 (comment)

Author: marceloalcocer

src/apps/altcp_tls/altcp_tls_mbedtls.c

@@ -107,6 +107,7 @@ struct altcp_tls_config {
   u8_t pkey_count;
   u8_t pkey_max;
   mbedtls_x509_crt *ca;
+  char host[256];
 #if defined(MBEDTLS_SSL_CACHE_C) && ALTCP_MBEDTLS_USE_SESSION_CACHE
   /** Inter-connection cache for fast connection startup */
   struct mbedtls_ssl_cache_context cache;
@@ -633,6 +634,7 @@ altcp_mbedtls_setup(void *conf, struct altcp_pcb *conn, struct altcp_pcb *inner_
   /* tell mbedtls about our I/O functions */
   mbedtls_ssl_set_bio(&state->ssl_context, conn, altcp_mbedtls_bio_send, altcp_mbedtls_bio_recv, NULL);
 
+  mbedtls_ssl_set_hostname(&state->ssl_context, config->host);
   altcp_mbedtls_setup_callbacks(conn, inner_conn);
   conn->inner_conn = inner_conn;
   conn->fns = &altcp_mbedtls_functions;
@@ -942,7 +944,7 @@ altcp_tls_create_config_server_privkey_cert(const u8_t *privkey, size_t privkey_
 }
 
 static struct altcp_tls_config *
-altcp_tls_create_config_client_common(const u8_t *ca, size_t ca_len, int is_2wayauth)
+altcp_tls_create_config_client_common(const u8_t *ca, size_t ca_len, int is_2wayauth, char* host)
 {
   int ret;
   struct altcp_tls_config *conf = altcp_tls_create_config(0, (is_2wayauth) ? 1 : 0, (is_2wayauth) ? 1 : 0, ca != NULL);
@@ -964,13 +966,14 @@ altcp_tls_create_config_client_common(const u8_t *ca, size_t ca_len, int is_2way
 
     mbedtls_ssl_conf_ca_chain(&conf->conf, conf->ca, NULL);
   }
+  memcpy(conf->host, host, sizeof(conf->host));
   return conf;
 }
 
 struct altcp_tls_config *
-altcp_tls_create_config_client(const u8_t *ca, size_t ca_len)
+altcp_tls_create_config_client(const u8_t *ca, size_t ca_len, char* host)
 {
-  return altcp_tls_create_config_client_common(ca, ca_len, 0);
+  return altcp_tls_create_config_client_common(ca, ca_len, 0, host);
 }
 
 struct altcp_tls_config *
@@ -986,7 +989,7 @@ altcp_tls_create_config_client_2wayauth(const u8_t *ca, size_t ca_len, const u8_
     return NULL;
   }
 
-  conf = altcp_tls_create_config_client_common(ca, ca_len, 1);
+  conf = altcp_tls_create_config_client_common(ca, ca_len, 1, NULL);
   if (conf == NULL) {
     return NULL;
   }

src/include/lwip/altcp_tls.h

@@ -92,7 +92,7 @@ struct altcp_tls_config *altcp_tls_create_config_server_privkey_cert(const u8_t
 /** @ingroup altcp_tls
  * Create an ALTCP_TLS client configuration handle
  */
-struct altcp_tls_config *altcp_tls_create_config_client(const u8_t *cert, size_t cert_len);
+struct altcp_tls_config *altcp_tls_create_config_client(const u8_t *cert, size_t cert_len, char* host);
 
 /** @ingroup altcp_tls
  * Create an ALTCP_TLS client configuration handle with two-way server/client authentication