1.31 backport: Relax recent SNI restrictions (#36950)

changelogs/current.yaml

@@ -8,6 +8,10 @@ minor_behavior_changes:
 
 bug_fixes:
 # *Changes expected to improve the state of the world and are unlikely to have negative effects*
+- area: access_log
+  change: |
+    Relaxed the restriction on SNI logging to allow the ``_`` character, even if
+    ``envoy.reloadable_features.sanitize_sni_in_access_log`` is enabled.
 
 removed_config_or_runtime:
 # *Normally occurs at the end of the* :ref:`deprecation period <deprecated>`

source/common/common/utility.cc

@@ -505,7 +505,8 @@ std::string StringUtil::sanitizeInvalidHostname(const absl::string_view source)
   std::string ret_str = std::string(source);
   bool sanitized = false;
   for (size_t i = 0; i < ret_str.size(); ++i) {
-    if (absl::ascii_isalnum(ret_str[i]) || ret_str[i] == '.' || ret_str[i] == '-') {
+    if (absl::ascii_isalnum(ret_str[i]) || ret_str[i] == '.' || ret_str[i] == '-' ||
+        ret_str[i] == '_') {
       continue;
     }
     sanitized = true;

source/common/common/utility.h

@@ -431,8 +431,8 @@ class StringUtil {
 
   /**
    * Sanitize host name strings for logging purposes. Replace invalid hostname characters (anything
-   * that's not alphanumeric, hyphen, or period) with underscore. The sanitized string is not a
-   * valid host name.
+   * that's not alphanumeric, hyphen, or period) with underscore. The sanitized string
+   * is not a valid host name.
    * @param source supplies the string to sanitize.
    * @return sanitized string.
    */

test/common/formatter/substitution_formatter_test.cc

@@ -959,6 +959,15 @@ TEST(SubstitutionFormatterTest, streamInfoFormatter) {
                 ProtoEq(ValueUtil::nullValue()));
   }
 
+  {
+    StreamInfoFormatter upstream_format("REQUESTED_SERVER_NAME");
+    std::string requested_server_name = "outbound_.8080_._.example.com";
+    stream_info.downstream_connection_info_provider_->setRequestedServerName(requested_server_name);
+    EXPECT_EQ("outbound_.8080_._.example.com", upstream_format.formatWithContext({}, stream_info));
+    EXPECT_THAT(upstream_format.formatValueWithContext({}, stream_info),
+                ProtoEq(ValueUtil::stringValue("outbound_.8080_._.example.com")));
+  }
+
   {
     StreamInfoFormatter upstream_format("REQUESTED_SERVER_NAME");
     std::string requested_server_name = "stub-server";