tool: print client's SNI value, if any.

Change-Id: I4fbce046090f0b0e78c9de923643268cfe6f142f
Reviewed-on: https://boringssl-review.googlesource.com/12241
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

tool/transport_common.cc

@@ -267,6 +267,11 @@ void PrintConnectionInfo(const SSL *ssl) {
   SSL_get0_alpn_selected(ssl, &alpn, &alpn_len);
   fprintf(stderr, "  ALPN protocol: %.*s\n", alpn_len, alpn);
 
+  const char *host_name = SSL_get_servername(ssl, TLSEXT_NAMETYPE_host_name);
+  if (host_name != nullptr && SSL_is_server(ssl)) {
+    fprintf(stderr, "  Client sent SNI: %s\n", host_name);
+  }
+
   // Print the server cert subject and issuer names.
   bssl::UniquePtr<X509> peer(SSL_get_peer_certificate(ssl));
   if (peer != nullptr) {