dns name compression write failure

Due to a logic flaw dns name compression writing was not properly implemented
which would result in the name prefix not being written for a partial match.

Fixes Bug: #757
Fix By: Brad House (@bradh352)

src/lib/ares_dns_name.c

@@ -362,6 +362,7 @@ ares_status_t ares__dns_name_write(ares__buf_t *buf, ares__llist_t **list,
 {
   const ares_nameoffset_t *off = NULL;
   size_t                   name_len;
+  size_t                   orig_name_len;
   size_t                   pos = ares__buf_len(buf);
   ares_dns_labels_t        labels;
   char                     name_copy[512];
@@ -375,7 +376,8 @@ ares_status_t ares__dns_name_write(ares__buf_t *buf, ares__llist_t **list,
 
   /* NOTE: due to possible escaping, name_copy buffer is > 256 to allow for
    *       this */
-  name_len = ares_strcpy(name_copy, name, sizeof(name_copy));
+  name_len      = ares_strcpy(name_copy, name, sizeof(name_copy));
+  orig_name_len = name_len;
 
   /* Find longest match */
   if (list != NULL) {
@@ -388,7 +390,7 @@ ares_status_t ares__dns_name_write(ares__buf_t *buf, ares__llist_t **list,
   }
 
   /* Output labels */
-  if (off == NULL || off->name_len != name_len) {
+  if (off == NULL || off->name_len != orig_name_len) {
     size_t i;
 
     status = ares_split_dns_name(&labels, validate_hostname, name_copy);
@@ -432,7 +434,7 @@ ares_status_t ares__dns_name_write(ares__buf_t *buf, ares__llist_t **list,
 
   /* Store pointer for future jumps as long as its not an exact match for
    * a prior entry */
-  if (list != NULL && (off == NULL || off->name_len != name_len) &&
+  if (list != NULL && (off == NULL || off->name_len != orig_name_len) &&
       name_len > 0) {
     status = ares__nameoffset_create(list, name /* not truncated copy! */, pos);
     if (status != ARES_SUCCESS) {

src/lib/ares_event_configchg.c

@@ -0,0 +1,152 @@
+/* MIT License
+ *
+ * Copyright (c) 2024 Brad House
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ * SPDX-License-Identifier: MIT
+ */
+#include "ares_setup.h"
+#include "ares.h"
+#include "ares_private.h"
+#include "ares_event.h"
+
+static void ares_event_configchg_reload(ares_channel_t *channel)
+{
+  /* XXX: dispatch via thread so we don't block events */
+  ares_reinit(channel);
+}
+
+#ifdef __LINUX__
+#  include <sys/inotify.h>
+struct ares_event_configchg {
+  int           inotify_fd;
+  ares_event_t *event;
+};
+
+void ares_event_configchg_destroy(ares_event_configchg_t *configchg, , ares_event_thread *e)
+{
+  if (configchg == NULL || configchg->inotify_fd == -1) {
+    return;
+  }
+  ares_event_update(NULL, e, ARES_EVENT_FLAG_NONE, NULL, configchg->inotify_fd, NULL, NULL);
+}
+
+static void ares_event_configchg_free(void *data)
+{
+  ares_event_configchg_t *configchg = data;
+  if (configchg == NULL)
+    return;
+
+  if (configchg->inotify_fd >= 0) {
+    close(configchg->inotify_fd);
+    configchg->inotify_fd = -1;
+  }
+
+  ares_free(configchg);
+}
+
+void ares_event_configchg_cb(ares_event_thread_t *e, ares_socket_t fd,
+                             void *data, ares_event_flags_t flags)
+{
+  ares_event_configchg_t *configchg = data;
+  /* Some systems cannot read integer variables if they are not
+     properly aligned. On other systems, incorrect alignment may
+     decrease performance. Hence, the buffer used for reading from
+     the inotify file descriptor should have the same alignment as
+     struct inotify_event. */
+  char buf[4096] __attribute__ ((aligned(__alignof__(struct inotify_event))));
+  const struct inotify_event *event;
+  ssize_t len;
+  ares_bool_t triggered = ARES_FALSE;
+
+  while (1) {
+    const char *ptr;
+
+    len = read(fd, buf, sizeof(buf));
+    if (len <= 0) {
+      break;
+    }
+
+    /* Loop over all events in the buffer. */
+    for (ptr = buf; ptr < buf + len;
+         ptr += sizeof(struct inotify_event) + event->len) {
+
+      event = (const struct inotify_event *)ptr;
+
+      if (event->name == NULL)
+        continue;
+
+      if (strcasecmp(name, "resolv.conf") == 0 ||
+          strcasecmp(name, "nsswitch.conf") == 0) {
+        triggered = ARES_TRUE;
+      }
+    }
+  }
+  if (triggered) {
+    ares_event_configchg_reload(e->channel);
+  }
+}
+
+
+ares_status_t ares_event_configchg_init(ares_event_configchg_t **configchg, ares_event_thread *e)
+{
+  ares_status_t status = ARES_SUCCESS;
+
+  *configchg = ares_malloc_zero(sizeof(**configchg));
+  if (*configchg == NULL) {
+    return ARES_ENOMEM;
+  }
+
+  *configchg->inotify_fd = inotify_init1(IN_NONBLOCK|IN_CLOEXEC);
+  if (*configchg->inotify_fd == -1) {
+    status = ARES_ESERVFAIL;
+    goto done;
+  }
+
+
+  /* We need to monitor /etc/resolv.conf, /etc/nsswitch.conf */
+  if (inotify_add_watch(*configchg->inotify_fd, "/etc", IN_CREATE|IN_MODIFY|IN_MOVED_TO|IN_ONLYDIR) == -1) {
+    status = ARES_ESERVFAIL;
+    goto done;
+  }
+
+  status = ares_event_update(&(*configchg)->event, e, ARES_EVENT_FLAG_READ,
+                             cb, *configchg->inotify_fd,
+                             ares_event_configchg_free, NULL);
+
+done:
+  if (status != ARES_SUCCESS) {
+    ares_event_configchg_destroy(*configchg);
+    *configchg = NULL;
+  }
+  return status;
+}
+
+
+
+
+#elif defined(_WIN32)
+
+#elif defined(__APPLE__)
+
+#else
+
+#endif