hostip: cache negative name resolves

Hold them for half the normal lifetime. Helps when told to transfer N
URLs in quick succession that all use the same non-resolving hostname.

Done by storing a DNS entry with a NULL pointer for 'addr'.

Previously an attempt was made in #12406 by Björn Stenberg that was
ultimately never merged.

Closes #18157

docs/TODO

@@ -24,7 +24,6 @@
  1.5 get rid of PATH_MAX
  1.6 thread-safe sharing
  1.8 CURLOPT_RESOLVE for any port number
- 1.9 Cache negative name resolves
  1.10 auto-detect proxy
  1.11 minimize dependencies with dynamically loaded modules
  1.12 updated DNS server while running
@@ -263,11 +262,6 @@
 
  See https://github.com/curl/curl/issues/1264
 
-1.9 Cache negative name resolves
-
- A name resolve that has failed is likely to fail when made again within a
- short period of time. Currently we only cache positive responses.
-
 1.10 auto-detect proxy
 
  libcurl could be made to detect the system proxy setup automatically and use

docs/libcurl/opts/CURLOPT_DNS_CACHE_TIMEOUT.md

@@ -10,6 +10,7 @@ See-also:
   - CURLOPT_DNS_USE_GLOBAL_CACHE (3)
   - CURLOPT_MAXAGE_CONN (3)
   - CURLOPT_RESOLVE (3)
+  - CURLMOPT_NETWORK_CHANGED (3)
 Protocol:
   - All
 Added-in: 7.9.3
@@ -48,8 +49,11 @@ DNS entries have a "TTL" property but libcurl does not use that. This DNS
 cache timeout is entirely speculative that a name resolves to the same address
 for a small amount of time into the future.
 
-Since version 8.1.0, libcurl prunes entries from the DNS cache if it exceeds
-30,000 entries no matter which timeout value is used.
+libcurl prunes entries from the DNS cache if it exceeds 30,000 entries no
+matter which timeout value is used. (Added in version 8.1.0)
+
+Since curl 8.16.0, failed name resolves are stored in the DNS cache for half
+the set timeout period.
 
 # DEFAULT
 

docs/tests/FILEFORMAT.md

@@ -747,3 +747,13 @@ should be cut off from the upload data before comparing it.
 
 ### `<valgrind>`
 disable - disables the valgrind log check for this test
+
+### `<dns [host="name"]>`
+
+This specify the input the DNS server is expected to get from curl. Because of
+differences in implementations, this section is sorted automatically before
+compared.
+
+Because of local configurations in machines running tests, there may be
+additional requests sent to `[host].[custom suffix]`. To prevent such requests
+to mess up comparisons, we can set the hostname to check in the `<dns>` tag.