1 files changed, 68 insertions, 14 deletions

diff --git a/ssl/ssl_lib.c b/ssl/ssl_lib.c
index 5ea310d5c4..b5239d6eb2 100644
--- a/ssl/ssl_lib.c
+++ b/ssl/ssl_lib.c
@@ -2753,31 +2753,85 @@ char *SSL_get_shared_ciphers(const SSL *s, char *buf, int size)
     return buf;
 }
 
-/** return a servername extension value if provided in Client Hello, or NULL.
- * So far, only host_name types are defined (RFC 3546).
+/**
+ * Return the requested servername (SNI) value. Note that the behaviour varies
+ * depending on:
+ * - whether this is called by the client or the server,
+ * - if we are before or during/after the handshake,
+ * - if a resumption or normal handshake is being attempted/has occurred
+ * - whether we have negotiated TLSv1.2 (or below) or TLSv1.3
+ * 
+ * Note that only the host_name type is defined (RFC 3546).
  */
-
 const char *SSL_get_servername(const SSL *s, const int type)
 {
+    /*
+     * If we don't know if we are the client or the server yet then we assume
+     * client.
+     */
+    int server = s->handshake_func == NULL ? 0 : s->server;
     if (type != TLSEXT_NAMETYPE_host_name)
         return NULL;
 
-    /*
-     * SNI is not negotiated in pre-TLS-1.3 resumption flows, so fake up an
-     * SNI value to return if we are resuming/resumed.  N.B. that we still
-     * call the relevant callbacks for such resumption flows, and callbacks
-     * might error out if there is not a SNI value available.
-     */
-    if (s->hit)
-        return s->session->ext.hostname;
+    if (server) {
+        /**
+         * Server side
+         * In TLSv1.3 on the server SNI is not associated with the session
+         * but in TLSv1.2 or below it is.
+         *
+         * Before the handshake:
+         *  - return NULL
+         *
+         * During/after the handshake (TLSv1.2 or below resumption occurred):
+         * - If a servername was accepted by the server in the original
+         *   handshake then it will return that servername, or NULL otherwise.
+         *
+         * During/after the handshake (TLSv1.2 or below resumption did not occur):
+         * - The function will return the servername requested by the client in
+         *   this handshake or NULL if none was requested.
+         */
+         if (s->hit && !SSL_IS_TLS13(s))
+            return s->session->ext.hostname;
+    } else {
+        /**
+         * Client side
+         *
+         * Before the handshake:
+         *  - If a servername has been set via a call to
+         *    SSL_set_tlsext_host_name() then it will return that servername
+         *  - If one has not been set, but a TLSv1.2 resumption is being
+         *    attempted and the session from the original handshake had a
+         *    servername accepted by the server then it will return that
+         *    servername
+         *  - Otherwise it returns NULL
+         *
+         * During/after the handshake (TLSv1.2 or below resumption occurred):
+         * - If the session from the orignal handshake had a servername accepted
+         *   by the server then it will return that servername.
+         * - Otherwise it returns the servername set via
+         *   SSL_set_tlsext_host_name() (or NULL if it was not called).
+         *
+         * During/after the handshake (TLSv1.2 or below resumption did not occur):
+         * - It will return the servername set via SSL_set_tlsext_host_name()
+         *   (or NULL if it was not called).
+         */
+        if (SSL_in_before(s)) {
+            if (s->ext.hostname == NULL
+                    && s->session != NULL
+                    && s->session->ssl_version != TLS1_3_VERSION)
+                return s->session->ext.hostname;
+        } else {
+            if (!SSL_IS_TLS13(s) && s->hit && s->session->ext.hostname != NULL)
+                return s->session->ext.hostname;
+        }
+    }
+
     return s->ext.hostname;
 }
 
 int SSL_get_servername_type(const SSL *s)
 {
-    if (s->session
-        && (!s->ext.hostname ? s->session->
-            ext.hostname : s->ext.hostname))
+    if (SSL_get_servername(s, TLSEXT_NAMETYPE_host_name) != NULL)
         return TLSEXT_NAMETYPE_host_name;
     return -1;
 }