hsts: add support for Strict-Transport-Security

- enable in the build (configure)
- header parsing
- host name lookup
- unit tests for the above
- CI build
- CURL_VERSION_HSTS bit
- curl_version_info support
- curl -V output
- curl-config --features
- CURLOPT_HSTS_CTRL
- man page for CURLOPT_HSTS_CTRL
- curl --hsts (sets CURLOPT_HSTS_CTRL and works with --libcurl)
- man page for --hsts
- save cache to disk
- load cache from disk
- CURLOPT_HSTS
- man page for CURLOPT_HSTS
- added docs/HSTS.md
- fixed --version docs
- adjusted curl_easy_duphandle

38 files changed, 1122 insertions, 21 deletions

diff --git a/configure.ac b/configure.ac
index a81d96a39..d60ccc14e 100755
--- a/configure.ac
+++ b/configure.ac
@@ -4882,6 +4882,31 @@ AC_HELP_STRING([--disable-alt-svc],[Disable alt-svc support]),
        AC_MSG_RESULT(no)
 )
 
+dnl ************************************************************
+dnl switch on/off hsts
+dnl
+curl_hsts_msg="no      (--enable-hsts)";
+AC_MSG_CHECKING([whether to support HSTS])
+AC_ARG_ENABLE(hsts,
+AC_HELP_STRING([--enable-hsts],[Enable HSTS support])
+AC_HELP_STRING([--disable-hsts],[Disable HSTS support]),
+[ case "$enableval" in
+  no)
+       AC_MSG_RESULT(no)
+       ;;
+  *) AC_MSG_RESULT(yes)
+       curl_hsts_msg="enabled";
+       enable_hsts="yes"
+       ;;
+  esac ],
+       AC_MSG_RESULT(no)
+)
+
+if test "$enable_hsts" = "yes"; then
+  AC_DEFINE(USE_HSTS, 1, [to enable HSTS])
+  experimental="$experimental HSTS"
+fi
+
 dnl *************************************************************
 dnl check whether ECH support, if desired, is actually available
 dnl
@@ -4998,6 +5023,9 @@ fi
 if test "x$enable_altsvc" = "xyes"; then
   SUPPORT_FEATURES="$SUPPORT_FEATURES alt-svc"
 fi
+if test "x$enable_hsts" = "xyes"; then
+  SUPPORT_FEATURES="$SUPPORT_FEATURES HSTS"
+fi
 
 if test "x$CURL_DISABLE_CRYPTO_AUTH" != "x1" -a \
     \( "x$HAVE_GSSAPI" = "x1" -o "x$USE_WINDOWS_SSPI" = "x1" \); then
diff --git a/docs/EXPERIMENTAL.md b/docs/EXPERIMENTAL.md
index ee5898944..5b2d36c14 100644
--- a/docs/EXPERIMENTAL.md
+++ b/docs/EXPERIMENTAL.md
@@ -20,3 +20,5 @@ Experimental support in curl means:
 
  - HTTP/3 support and options
  - CURLSSLOPT_NATIVE_CA (No configure option, feature built in when supported)
+ - HSTS support and options
+ 
diff --git a/docs/HSTS.md b/docs/HSTS.md
new file mode 100644
index 000000000..c3f08393c
--- /dev/null
+++ b/docs/HSTS.md
@@ -0,0 +1,44 @@
+# HSTS support
+
+curl features **EXPERIMENTAL** support for the Strict-Transport-Security: HTTP
+header. Added in curl 7.74.0
+
+## Standard
+
+[HTTP Strict Transport Security](https://tools.ietf.org/html/rfc6797)
+
+## Behavior
+
+libcurl features an in-memory cache for HSTS hosts, so that subsequent
+HTTP-only requests to a host name present in the cache will get internally
+"redirected" to the HTTPS version.
+
+## `curl_easy_setopt()` options:
+
+ - `CURLOPT_HSTS_CTRL` - enable HSTS for this easy handle
+ - `CURLOPT_HSTS` - specify file name where to store the HSTS cache on close
+  (and possibly read from at startup)
+
+## curl cmdline options
+
+ - `--hsts [filename]` - enable HSTS, use the file as HSTS cache. If filename
+   is `""` (no length) then no file will be used, only in-memory cache.
+
+## HSTS cache file format
+
+Lines starting with `#` are ignored.
+
+For each hsts entry:
+
+    [host name] "YYYYMMDD HH:MM:SS"
+
+The `[host name]` is dot-prefixed if it is a includeSubDomain.
+
+The time stamp is when the entry expires.
+
+I considered using wget's file format for the HSTS cache. However, they store the time stamp as the epoch (number of seconds since 1970) and I strongly disagree with using that format. Instead I opted to use a format similar to the curl alt-svc cache file format.
+
+## Possible future additions
+
+ - `CURLOPT_HSTS_PRELOAD` - provide a set of preloaded HSTS host names
+ - ability to save to something else than a file
diff --git a/docs/Makefile.am b/docs/Makefile.am
index 9d67084d9..4a1984010 100644
--- a/docs/Makefile.am
+++ b/docs/Makefile.am
@@ -63,6 +63,7 @@ EXTRA_DIST =                                    \
  GOVERNANCE.md                                  \
  HELP-US.md                                     \
  HISTORY.md                                     \
+ HSTS.md                                        \
  HTTP-COOKIES.md                                \
  HTTP2.md                                       \
  HTTP3.md                                       \
diff --git a/docs/cmdline-opts/Makefile.inc b/docs/cmdline-opts/Makefile.inc
index 792cadb3c..7e8529c1a 100644
--- a/docs/cmdline-opts/Makefile.inc
+++ b/docs/cmdline-opts/Makefile.inc
@@ -85,6 +85,7 @@ DPAGES =					\
   head.d header.d				\
   help.d					\
   hostpubmd5.d					\
+  hsts.d                                        \
   http0.9.d					\
   http1.0.d					\
   http1.1.d http2.d				\
diff --git a/docs/cmdline-opts/hsts.d b/docs/cmdline-opts/hsts.d
new file mode 100644
index 000000000..2399084bf
--- /dev/null
+++ b/docs/cmdline-opts/hsts.d
@@ -0,0 +1,18 @@
+Long: hsts
+Arg: <file name>
+Protocols: HTTPS
+Help: Enable HSTS with this cache file
+Added: 7.74.0
+Category: http
+---
+WARNING: this option is experimental. Do not use in production.
+
+This option enables HSTS for the transfer. If the file name points to an
+existing HSTS cache file, that will be used. After a completed transfer, the
+cache will be saved to the file name again if it has been modified.
+
+Specify a "" file name (zero length) to avoid loading/saving and make curl
+just handle HSTS in memory.
+
+If this option is used several times, curl will load contents from all the
+files but the last one will be used for saving.
diff --git a/docs/cmdline-opts/version.d b/docs/cmdline-opts/version.d
index 52c29f177..f6c091707 100644
--- a/docs/cmdline-opts/version.d
+++ b/docs/cmdline-opts/version.d
@@ -28,6 +28,8 @@ This curl uses a libcurl built with Debug. This enables more error-tracking
 and memory debugging etc. For curl-developers only!
 .IP "GSS-API"
 GSS-API is supported.
+.IP "HSTS"
+HSTS support is present.
 .IP "HTTP2"
 HTTP/2 support has been built-in.
 .IP "HTTP3"
diff --git a/docs/libcurl/curl_easy_setopt.3 b/docs/libcurl/curl_easy_setopt.3
index 362cfef0a..3434b158c 100644
--- a/docs/libcurl/curl_easy_setopt.3
+++ b/docs/libcurl/curl_easy_setopt.3
@@ -319,6 +319,10 @@ Add or control cookies. See \fICURLOPT_COOKIELIST(3)\fP
 Specify the Alt-Svc: cache file name. See \fICURLOPT_ALTSVC(3)\fP
 .IP CURLOPT_ALTSVC_CTRL
 Enable and configure Alt-Svc: treatment. See \fICURLOPT_ALTSVC_CTRL(3)\fP
+.IP CURLOPT_HSTS
+Set HSTS cache file. See \fICURLOPT_HSTS(3)\fP
+.IP CURLOPT_HSTS_CTRL
+Enable HSTS. See \fICURLOPT_HSTS_CTRL(3)\fP
 .IP CURLOPT_HTTPGET
 Do an HTTP GET request. See \fICURLOPT_HTTPGET(3)\fP
 .IP CURLOPT_REQUEST_TARGET
diff --git a/docs/libcurl/curl_version_info.3 b/docs/libcurl/curl_version_info.3
index 5c5f16a3d..a6fa5e3b9 100644
--- a/docs/libcurl/curl_version_info.3
+++ b/docs/libcurl/curl_version_info.3
@@ -143,6 +143,9 @@ to use the current user credentials without the app having to pass them on.
 (Added in 7.38.0)
 .IP CURL_VERSION_GSSNEGOTIATE
 supports HTTP GSS-Negotiate (added in 7.10.6)
+.IP CURL_VERSION_HSTS
+libcurl was built with support for HSTS (HTTP Strict Transport Security)
+(Added in 7.74.0)
 .IP CURL_VERSION_HTTPS_PROXY
 libcurl was built with support for HTTPS-proxy.
 (Added in 7.52.0)
diff --git a/docs/libcurl/opts/CURLOPT_HSTS.3 b/docs/libcurl/opts/CURLOPT_HSTS.3
new file mode 100644
index 000000000..b90dbff8b
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_HSTS.3
@@ -0,0 +1,66 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.\"
+.TH CURLOPT_HSTS 3 "5 Feb 2019" "libcurl 7.74.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_HSTS \- set HSTS cache file name
+.SH SYNOPSIS
+.nf
+#include <curl/curl.h>
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTS, char *filename);
+.fi
+.SH EXPERIMENTAL
+Warning: this feature is early code and is marked as experimental. It can only
+be enabled by explicitly telling configure with \fB--enable-hsts\fP. You are
+advised to not ship this in production before the experimental label is
+removed.
+.SH DESCRIPTION
+Make the \fIfilename\fP point to a file name to load an existing HSTS cache
+from, and to store the cache in when the easy handle is closed. Setting a file
+name with this option will also enable HSTS for this handle (the equivalent of
+setting \fICURLHSTS_ENABLE\fP with \fICURLOPT_HSTS_CTRL(3)\fP).
+
+If the given file does not exist or contains no HSTS entries at startup, the
+HSTS cache will simply start empty. Setting the file name to NULL or "" will
+only enable HSTS without reading from or writing to any file.
+
+If this option is set multiple times, libcurl will load cache entries from
+each given file but will only store the last used name for later writing.
+.SH DEFAULT
+NULL, no file name
+.SH PROTOCOLS
+HTTPS and HTTP
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_HSTS, "/home/user/.hsts-cache");
+  curl_easy_perform(curl);
+}
+.fi
+.SH AVAILABILITY
+Added in 7.74.0
+.SH RETURN VALUE
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+.SH "SEE ALSO"
+.BR CURLOPT_HSTS_CTRL "(3), " CURLOPT_ALTSVC "(3), " CURLOPT_RESOLVE "(3), "
diff --git a/docs/libcurl/opts/CURLOPT_HSTS_CTRL.3 b/docs/libcurl/opts/CURLOPT_HSTS_CTRL.3
new file mode 100644
index 000000000..ffdfee93f
--- /dev/null
+++ b/docs/libcurl/opts/CURLOPT_HSTS_CTRL.3
@@ -0,0 +1,73 @@
+.\" **************************************************************************
+.\" *                                  _   _ ____  _
+.\" *  Project                     ___| | | |  _ \| |
+.\" *                             / __| | | | |_) | |
+.\" *                            | (__| |_| |  _ <| |___
+.\" *                             \___|\___/|_| \_\_____|
+.\" *
+.\" * Copyright (C) 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" *
+.\" * This software is licensed as described in the file COPYING, which
+.\" * you should have received as part of this distribution. The terms
+.\" * are also available at https://curl.haxx.se/docs/copyright.html.
+.\" *
+.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+.\" * copies of the Software, and permit persons to whom the Software is
+.\" * furnished to do so, under the terms of the COPYING file.
+.\" *
+.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+.\" * KIND, either express or implied.
+.\" *
+.\" **************************************************************************
+.\"
+.TH CURLOPT_HSTS_CTRL 3 "4 Sep 2020" "libcurl 7.74.0" "curl_easy_setopt options"
+.SH NAME
+CURLOPT_HSTS_CTRL \- control HSTS behavior
+.SH SYNOPSIS
+.nf
+#include <curl/curl.h>
+
+#define CURLHSTS_ENABLE       (1<<0)
+#define CURLHSTS_READONLYFILE (1<<1)
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTS_CTRL, long bitmask);
+.fi
+.SH EXPERIMENTAL
+Warning: this feature is early code and is marked as experimental. It can only
+be enabled by explicitly telling configure with \fB--enable-hsts\fP. You are
+advised to not ship this in production before the experimental label is
+removed.
+.SH DESCRIPTION
+HSTS (HTTP Strict Transport Security) means that an HTTPS server can instruct
+the client to not contact it again over clear-text HTTP for a certain period
+into the future. libcurl will then automatically redirect HTTP attempts to
+such hosts to instead use HTTPS. This is done by libcurl retaining this
+knowledge in an in-memory cache.
+
+Populate the long \fIbitmask\fP with the correct set of features to instruct
+libcurl how to handle HSTS for the transfers using this handle.
+.SH BITS
+.IP "CURLHSTS_ENABLE"
+Enable the in-memory HSTS cache for this handle.
+.IP "CURLHSTS_READONLYFILE"
+Make the HSTS file (if specified) read-only - makes libcurl not save the cache
+to the file when closing the handle.
+.SH DEFAULT
+0. HSTS is disabled by default.
+.SH PROTOCOLS
+HTTPS and HTTP
+.SH EXAMPLE
+.nf
+CURL *curl = curl_easy_init();
+if(curl) {
+  curl_easy_setopt(curl, CURLOPT_HSTS_CTRL, CURLHSTS_ENABLE);
+  curl_easy_perform(curl);
+}
+.fi
+.SH AVAILABILITY
+Added in 7.74.0
+.SH RETURN VALUE
+Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
+.SH "SEE ALSO"
+.BR CURLOPT_HSTS "(3), " CURLOPT_CONNECT_TO "(3), " CURLOPT_RESOLVE "(3), "
+.BR CURLOPT_ALTSVC "(3), "
diff --git a/docs/libcurl/opts/Makefile.inc b/docs/libcurl/opts/Makefile.inc
index fe4177579..9d1eb2bba 100644
--- a/docs/libcurl/opts/Makefile.inc
+++ b/docs/libcurl/opts/Makefile.inc
@@ -180,6 +180,8 @@ man_MANS =                                      \
   CURLOPT_HEADERDATA.3                          \
   CURLOPT_HEADERFUNCTION.3                      \
   CURLOPT_HEADEROPT.3                           \
+  CURLOPT_HSTS.3                                \
+  CURLOPT_HSTS_CTRL.3                           \
   CURLOPT_HTTP09_ALLOWED.3                      \
   CURLOPT_HTTP200ALIASES.3                      \
   CURLOPT_HTTPAUTH.3                            \
diff --git a/docs/libcurl/symbols-in-versions b/docs/libcurl/symbols-in-versions
index 1b37d13d1..cc35fc57b 100644
--- a/docs/libcurl/symbols-in-versions
+++ b/docs/libcurl/symbols-in-versions
@@ -213,6 +213,8 @@ CURLGSSAPI_DELEGATION_NONE      7.22.0
 CURLGSSAPI_DELEGATION_POLICY_FLAG 7.22.0
 CURLHEADER_SEPARATE             7.37.0
 CURLHEADER_UNIFIED              7.37.0
+CURLHSTS_ENABLE                 7.74.0
+CURLHSTS_READONLYFILE           7.74.0
 CURLINFO_ACTIVESOCKET           7.45.0
 CURLINFO_APPCONNECT_TIME        7.19.0
 CURLINFO_APPCONNECT_TIME_T      7.61.0
@@ -443,6 +445,8 @@ CURLOPT_HEADER                  7.1
 CURLOPT_HEADERDATA              7.10
 CURLOPT_HEADERFUNCTION          7.7.2
 CURLOPT_HEADEROPT               7.37.0
+CURLOPT_HSTS                    7.74.0
+CURLOPT_HSTS_CTRL               7.74.0
 CURLOPT_HTTP09_ALLOWED          7.64.0
 CURLOPT_HTTP200ALIASES          7.10.3
 CURLOPT_HTTPAUTH                7.10.6
@@ -1001,6 +1005,7 @@ CURL_VERSION_CURLDEBUG          7.19.6
 CURL_VERSION_DEBUG              7.10.6
 CURL_VERSION_GSSAPI             7.38.0
 CURL_VERSION_GSSNEGOTIATE       7.10.6        7.38.0
+CURL_VERSION_HSTS               7.74.0
 CURL_VERSION_HTTP2              7.33.0
 CURL_VERSION_HTTP3              7.66.0
 CURL_VERSION_HTTPS_PROXY        7.52.0
diff --git a/docs/options-in-versions b/docs/options-in-versions
index 683363239..97c416d83 100644
--- a/docs/options-in-versions
+++ b/docs/options-in-versions
@@ -79,6 +79,7 @@
 --header (-H)                        5.0
 --help (-h)                          4.0
 --hostpubmd5                         7.17.1
+--hsts                               7.74.0
 --http0.9                            7.64.0
 --http1.0 (-0)                       7.9.1
 --http1.1                            7.33.0
diff --git a/include/curl/curl.h b/include/curl/curl.h
index ad1a2097b..568408942 100644
--- a/include/curl/curl.h
+++ b/include/curl/curl.h
@@ -954,6 +954,10 @@ typedef enum {
 #define CURLALTSVC_H2           (1<<4)
 #define CURLALTSVC_H3           (1<<5)
 
+/* CURLHSTS_* are bits for the CURLOPT_HSTS option */
+#define CURLHSTS_ENABLE       (long)(1<<0)
+#define CURLHSTS_READONLYFILE (long)(1<<1)
+
 /* CURLPROTO_ defines are for the CURLOPT_*PROTOCOLS options */
 #define CURLPROTO_HTTP   (1<<0)
 #define CURLPROTO_HTTPS  (1<<1)
@@ -2029,6 +2033,11 @@ typedef enum {
    */
   CURLOPT(CURLOPT_SSL_EC_CURVES, CURLOPTTYPE_STRINGPOINT, 298),
 
+  /* HSTS bitmask */
+  CURLOPT(CURLOPT_HSTS_CTRL, CURLOPTTYPE_LONG, 299),
+  /* HSTS file name */
+  CURLOPT(CURLOPT_HSTS, CURLOPTTYPE_STRINGPOINT, 300),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;
 
@@ -2900,6 +2909,7 @@ typedef struct curl_version_info_data curl_version_info_data;
 #define CURL_VERSION_HTTP3        (1<<25) /* HTTP3 support built-in */
 #define CURL_VERSION_ZSTD         (1<<26) /* zstd features are present */
 #define CURL_VERSION_UNICODE      (1<<27) /* Unicode support on Windows */
+#define CURL_VERSION_HSTS         (1<<28) /* HSTS is supported */
 
  /*
  * NAME curl_version_info()
diff --git a/lib/Makefile.inc b/lib/Makefile.inc
index a2fd57a83..ea7a37496 100644
--- a/lib/Makefile.inc
+++ b/lib/Makefile.inc
@@ -61,7 +61,7 @@ LIB_CFILES = altsvc.c amigaos.c asyn-ares.c asyn-thread.c base64.c            \
   socks_gssapi.c socks_sspi.c speedcheck.c splay.c strcase.c strdup.c         \
   strerror.c strtok.c strtoofft.c system_win32.c telnet.c tftp.c timeval.c    \
   transfer.c urlapi.c version.c warnless.c wildcard.c x509asn1.c dynbuf.c     \
-  version_win32.c easyoptions.c easygetopt.c
+  version_win32.c easyoptions.c easygetopt.c hsts.c
 
 LIB_HFILES = altsvc.h amigaos.h arpa_telnet.h asyn.h conncache.h connect.h    \
   content_encoding.h cookie.h curl_addrinfo.h curl_base64.h curl_ctype.h      \
@@ -80,7 +80,7 @@ LIB_HFILES = altsvc.h amigaos.h arpa_telnet.h asyn.h conncache.h connect.h    \
   smb.h smtp.h sockaddr.h socketpair.h socks.h speedcheck.h splay.h strcase.h \
   strdup.h strerror.h strtok.h strtoofft.h system_win32.h telnet.h tftp.h     \
   timeval.h transfer.h urlapi-int.h urldata.h warnless.h wildcard.h           \
-  x509asn1.h dynbuf.h version_win32.h easyoptions.h
+  x509asn1.h dynbuf.h version_win32.h easyoptions.h hsts.h
 
 LIB_RCFILES = libcurl.rc
 
diff --git a/lib/curl_get_line.c b/lib/curl_get_line.c
index ffe4256ba..aa524d8fe 100644
--- a/lib/curl_get_line.c
+++ b/lib/curl_get_line.c
@@ -22,7 +22,8 @@
 
 #include "curl_setup.h"
 
-#if !defined(CURL_DISABLE_COOKIES) || !defined(CURL_DISABLE_ALTSVC)
+#if !defined(CURL_DISABLE_COOKIES) || !defined(CURL_DISABLE_ALTSVC)) || \
+  defined(USE_HSTS)
 
 #include "curl_get_line.h"
 #include "curl_memory.h"
diff --git a/lib/easy.c b/lib/easy.c
index 60e2befd7..ca1117a46 100644
--- a/lib/easy.c
+++ b/lib/easy.c
@@ -79,6 +79,7 @@
 #include "http2.h"
 #include "dynbuf.h"
 #include "altsvc.h"
+#include "hsts.h"
 
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -881,6 +882,15 @@ struct Curl_easy *curl_easy_duphandle(struct Curl_easy *data)
       (void)Curl_altsvc_load(outcurl->asi, outcurl->set.str[STRING_ALTSVC]);
   }
 #endif
+#ifdef USE_HSTS
+  if(data->hsts) {
+    outcurl->hsts = Curl_hsts_init();
+    if(!outcurl->hsts)
+      goto fail;
+    if(outcurl->set.str[STRING_HSTS])
+      (void)Curl_hsts_load(outcurl->hsts, outcurl->set.str[STRING_HSTS]);
+  }
+#endif
   /* Clone the resolver handle, if present, for the new handle */
   if(Curl_resolver_duphandle(outcurl,
                              &outcurl->state.resolver,
@@ -929,6 +939,7 @@ struct Curl_easy *curl_easy_duphandle(struct Curl_easy *data)
     Curl_safefree(outcurl->change.url);
     Curl_safefree(outcurl->change.referer);
     Curl_altsvc_cleanup(&outcurl->asi);
+    Curl_hsts_cleanup(&outcurl->hsts);
     Curl_freeset(outcurl);
     free(outcurl);
   }
diff --git a/lib/easyoptions.c b/lib/easyoptions.c
index 0ab6a3fc6..e5b9ffb70 100644
--- a/lib/easyoptions.c
+++ b/lib/easyoptions.c
@@ -115,6 +115,8 @@ struct curl_easyoption Curl_easyopts[] = {
   {"HEADERDATA", CURLOPT_HEADERDATA, CURLOT_CBPTR, 0},
   {"HEADERFUNCTION", CURLOPT_HEADERFUNCTION, CURLOT_FUNCTION, 0},
   {"HEADEROPT", CURLOPT_HEADEROPT, CURLOT_VALUES, 0},
+  {"HSTS", CURLOPT_HSTS, CURLOT_STRING, 0},
+  {"HSTS_CTRL", CURLOPT_HSTS_CTRL, CURLOT_LONG, 0},
   {"HTTP09_ALLOWED", CURLOPT_HTTP09_ALLOWED, CURLOT_LONG, 0},
   {"HTTP200ALIASES", CURLOPT_HTTP200ALIASES, CURLOT_SLIST, 0},
   {"HTTPAUTH", CURLOPT_HTTPAUTH, CURLOT_VALUES, 0},
@@ -342,6 +344,6 @@ struct curl_easyoption Curl_easyopts[] = {
  */
 int Curl_easyopts_check(void)
 {
-  return (CURLOPT_LASTENTRY != (298 + 1));
+  return (CURLOPT_LASTENTRY != (300 + 1));
 }
 #endif
diff --git a/lib/hsts.c b/lib/hsts.c
new file mode 100644
index 000000000..7eb3cda03
--- /dev/null
+++ b/lib/hsts.c
@@ -0,0 +1,430 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.haxx.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+/*
+ * The Strict-Transport-Security header is defined in RFC 6797:
+ * https://tools.ietf.org/html/rfc6797
+ */
+#include "curl_setup.h"
+
+#if !defined(CURL_DISABLE_HTTP) && defined(USE_HSTS)
+#include <curl/curl.h>
+#include "urldata.h"
+#include "llist.h"
+#include "hsts.h"
+#include "curl_get_line.h"
+#include "strcase.h"
+#include "sendf.h"
+#include "strtoofft.h"
+#include "parsedate.h"
+#include "rand.h"
+#include "rename.h"
+
+/* The last 3 #include files should be in this order */
+#include "curl_printf.h"
+#include "curl_memory.h"
+#include "memdebug.h"
+
+#define MAX_HSTS_LINE 4095
+#define MAX_HSTS_HOSTLEN 256
+#define MAX_HSTS_HOSTLENSTR "256"
+#define MAX_HSTS_SUBLEN 4
+#define MAX_HSTS_SUBLENSTR "4"
+#define MAX_HSTS_DATELEN 64
+#define MAX_HSTS_DATELENSTR "64"
+
+#ifdef DEBUGBUILD
+/* to play well with debug builds, we can *set* a fixed time this will
+   return */
+time_t deltatime; /* allow for "adjustments" for unit test purposes */
+static time_t debugtime(void *unused)
+{
+  char *timestr = getenv("CURL_TIME");
+  (void)unused;
+  if(timestr) {
+    unsigned long val = strtol(timestr, NULL, 10) + deltatime;
+    return (time_t)val;
+  }
+  return time(NULL);
+}
+#define time(x) debugtime(x)
+#endif
+
+struct hsts *Curl_hsts_init(void)
+{
+  struct hsts *h = calloc(sizeof(struct hsts), 1);
+  if(h) {
+    Curl_llist_init(&h->list, NULL);
+  }
+  return h;
+}
+
+static void hsts_free(struct stsentry *e)
+{
+  free((char *)e->host);
+  free(e);
+}
+
+void Curl_hsts_cleanup(struct hsts **hp)
+{
+  struct hsts *h = *hp;
+  if(h) {
+    struct Curl_llist_element *e;
+    struct Curl_llist_element *n;
+    for(e = h->list.head; e; e = n) {
+      struct stsentry *sts = e->ptr;
+      n = e->next;
+      hsts_free(sts);
+    }
+    free(h->filename);
+    free(h);
+    *hp = NULL;
+  }
+}
+
+static struct stsentry *hsts_entry(void)
+{
+  return calloc(sizeof(struct stsentry), 1);
+}
+
+static CURLcode hsts_create(struct hsts *h,
+                            const char *hostname,
+                            bool subdomains,
+                            curl_off_t expires)
+{
+  struct stsentry *sts = hsts_entry();
+  if(!sts)
+    return CURLE_OUT_OF_MEMORY;
+
+  sts->expires = expires;
+  sts->includeSubDomains = subdomains;
+  sts->host = strdup(hostname);
+  if(!sts->host) {
+    free(sts);
+    return CURLE_OUT_OF_MEMORY;
+  }
+  fprintf(stderr, "*** Add %s %s %d\n", hostname,
+          subdomains?"SUB":"-", (int)expires);
+  Curl_llist_insert_next(&h->list, h->list.tail, sts, &sts->node);
+  return CURLE_OK;
+}
+
+CURLcode Curl_hsts_parse(struct hsts *h, const char *hostname,
+                         const char *header)
+{
+  const char *p = header;
+  curl_off_t expires = 0;
+  bool gotma = FALSE;
+  bool gotinc = FALSE;
+  bool subdomains = FALSE;
+  struct stsentry *sts;
+  time_t now = time(NULL);
+
+  do {
+    while(*p && ISSPACE(*p))
+      p++;
+    if(Curl_strncasecompare("max-age=", p, 8)) {
+      bool quoted = FALSE;
+      CURLofft offt;
+      char *endp;
+
+      if(gotma)
+        return CURLE_BAD_FUNCTION_ARGUMENT;
+
+      p += 8;
+      while(*p && ISSPACE(*p))
+        p++;
+      if(*p == '\"') {
+        p++;
+        quoted = TRUE;
+      }
+      offt = curlx_strtoofft(p, &endp, 10, &expires);
+      if(offt == CURL_OFFT_FLOW)
+        expires = CURL_OFF_T_MAX;
+      else if(offt)
+        /* invalid max-age */
+        return CURLE_BAD_FUNCTION_ARGUMENT;
+      p = endp;
+      if(quoted) {
+        if(*p != '\"')
+          return CURLE_BAD_FUNCTION_ARGUMENT;
+        p++;
+      }
+      gotma = TRUE;
+    }
+    else if(Curl_strncasecompare("includesubdomains", p, 17)) {
+      if(gotinc)
+        return CURLE_BAD_FUNCTION_ARGUMENT;
+      subdomains = TRUE;
+      p += 17;
+      gotinc = TRUE;
+    }
+    else {
+      /* unknown directive, do a lame attempt to skip */
+      while(*p && (*p != ';'))
+        p++;
+    }
+
+    while(*p && ISSPACE(*p))
+      p++;
+    if(*p == ';')
+      p++;
+  } while (*p);
+
+  if(!gotma)
+    /* max-age is mandatory */
+    return CURLE_BAD_FUNCTION_ARGUMENT;
+
+  if(!expires) {
+    /* remove the entry if present verbatim (without subdomain match) */
+    sts = Curl_hsts(h, hostname, FALSE);
+    if(sts) {
+      Curl_llist_remove(&h->list, &sts->node, NULL);
+      hsts_free(sts);
+    }
+    return CURLE_OK;
+  }
+
+  if(CURL_OFF_T_MAX - now < expires)
+    /* would overflow, use maximum value */
+    expires = CURL_OFF_T_MAX;
+  else
+    expires += now;
+
+  /* check if it already exists */
+  sts = Curl_hsts(h, hostname, FALSE);
+  if(sts) {
+    /* just update these fields */
+    sts->expires = expires;
+    sts->includeSubDomains = subdomains;
+  }
+  else
+    return hsts_create(h, hostname, subdomains, expires);
+
+  return CURLE_OK;
+}
+
+/*
+ * Return TRUE if the given host name is currently an HSTS one.
+ *
+ * The 'subdomain' argument tells the function if subdomain matching should be
+ * attempted.
+ */
+struct stsentry *Curl_hsts(struct hsts *h, const char *hostname,
+                           bool subdomain)
+{
+  if(h) {
+    time_t now = time(NULL);
+    size_t hlen = strlen(hostname);
+    struct Curl_llist_element *e;
+    struct Curl_llist_element *n;
+    for(e = h->list.head; e; e = n) {
+      struct stsentry *sts = e->ptr;
+      n = e->next;
+      if(sts->expires <= now) {
+        /* remove expired entries */
+        Curl_llist_remove(&h->list, &sts->node, NULL);
+        hsts_free(sts);
+        continue;
+      }
+      if(subdomain && sts->includeSubDomains) {
+        size_t ntail = strlen(sts->host);
+        if(ntail < hlen) {
+          size_t offs = hlen - ntail;
+          if((hostname[offs-1] == '.') &&
+             Curl_strncasecompare(&hostname[offs], sts->host, ntail))
+            return sts;
+        }
+      }
+      if(Curl_strcasecompare(hostname, sts->host))
+        return sts;
+    }
+  }
+  return NULL; /* no match */
+}
+
+/*
+ * Write this single hsts entry to a single output line
+ */
+static CURLcode hsts_out(struct stsentry *sts, FILE *fp)
+{
+  struct tm stamp;
+  CURLcode result = Curl_gmtime(sts->expires, &stamp);
+  if(result)
+    return result;
+
+  fprintf(fp, "%s%s \"%d%02d%02d %02d:%02d:%02d\"\n",
+          sts->includeSubDomains ? ".": "", sts->host,
+          stamp.tm_year + 1900, stamp.tm_mon + 1, stamp.tm_mday,
+          stamp.tm_hour, stamp.tm_min, stamp.tm_sec);
+  return CURLE_OK;
+}
+
+
+/*
+ * Curl_https_save() writes the HSTS cache to a file.
+ */
+CURLcode Curl_hsts_save(struct Curl_easy *data, struct hsts *h,
+                        const char *file)
+{
+  struct Curl_llist_element *e;
+  struct Curl_llist_element *n;
+  CURLcode result = CURLE_OK;
+  FILE *out;
+  char *tempstore;
+  unsigned char randsuffix[9];
+
+  if(!h)
+    /* no cache activated */
+    return CURLE_OK;
+
+  /* if not new name is given, use the one we stored from the load */
+  if(!file && h->filename)
+    file = h->filename;
+
+  if((h->flags & CURLHSTS_READONLYFILE) || !file || !file[0])
+    /* marked as read-only, no file or zero length file name */
+    return CURLE_OK;
+
+  if(Curl_rand_hex(data, randsuffix, sizeof(randsuffix)))
+    return CURLE_FAILED_INIT;
+
+  tempstore = aprintf("%s.%s.tmp", file, randsuffix);
+  if(!tempstore)
+    return CURLE_OUT_OF_MEMORY;
+
+  out = fopen(tempstore, FOPEN_WRITETEXT);
+  if(!out)
+    result = CURLE_WRITE_ERROR;
+  else {
+    fputs("# Your HSTS cache. https://curl.haxx.se/docs/hsts.html\n"
+          "# This file was generated by libcurl! Edit at your own risk.\n",
+          out);
+    for(e = h->list.head; e; e = n) {
+      struct stsentry *sts = e->ptr;
+      n = e->next;
+      result = hsts_out(sts, out);
+      if(result)
+        break;
+    }
+    fclose(out);
+    if(!result && Curl_rename(tempstore, file))
+      result = CURLE_WRITE_ERROR;
+
+    if(result)
+      unlink(tempstore);
+  }
+  free(tempstore);
+  return result;
+}
+
+/* only returns SERIOUS errors */
+static CURLcode hsts_add(struct hsts *h, char *line)
+{
+  /* Example lines:
+     example.com "20191231 10:00:00"
+     .example.net "20191231 10:00:00"
+   */
+  char host[MAX_HSTS_HOSTLEN + 1];
+  char date[MAX_HSTS_DATELEN + 1];
+  int rc;
+
+  rc = sscanf(line,
+              "%" MAX_HSTS_HOSTLENSTR "s \"%" MAX_HSTS_DATELENSTR "[^\"]\"",
+              host, date);
+  if(2 == rc) {
+    time_t expires = Curl_getdate_capped(date);
+    CURLcode result;
+    char *p = host;
+    bool subdomain = FALSE;
+    if(p[0] == '.') {
+      p++;
+      subdomain = TRUE;
+    }
+    result = hsts_create(h, p, subdomain, expires);
+    if(result)
+      return result;
+  }
+
+  return CURLE_OK;
+}
+
+/*
+ * Load the HSTS cache from the given file. The text based line-oriented file
+ * format is documented here:
+ * https://github.com/curl/curl/wiki/HSTS
+ *
+ * This function only returns error on major problems that prevents hsts
+ * handling to work completely. It will ignore individual syntactical errors
+ * etc.
+ */
+static CURLcode hsts_load(struct hsts *h, const char *file)
+{
+  CURLcode result = CURLE_OK;
+  char *line = NULL;
+  FILE *fp;
+
+  /* we need a private copy of the file name so that the hsts cache file
+     name survives an easy handle reset */
+  free(h->filename);
+  h->filename = strdup(file);
+  if(!h->filename)
+    return CURLE_OUT_OF_MEMORY;
+
+  fp = fopen(file, FOPEN_READTEXT);
+  if(fp) {
+    line = malloc(MAX_HSTS_LINE);
+    if(!line)
+      goto fail;
+    while(Curl_get_line(line, MAX_HSTS_LINE, fp)) {
+      char *lineptr = line;
+      while(*lineptr && ISBLANK(*lineptr))
+        lineptr++;
+      if(*lineptr == '#')
+        /* skip commented lines */
+        continue;
+
+      hsts_add(h, lineptr);
+    }
+    free(line); /* free the line buffer */
+    fclose(fp);
+  }
+  return result;
+
+  fail:
+  Curl_safefree(h->filename);
+  free(line);
+  fclose(fp);
+  return CURLE_OUT_OF_MEMORY;
+}
+
+/*
+ * Curl_hsts_load() loads HSTS from file.
+ */
+CURLcode Curl_hsts_load(struct hsts *h, const char *file)
+{
+  CURLcode result;
+  DEBUGASSERT(h);
+  result = hsts_load(h, file);
+  return result;
+}
+
+#endif /* CURL_DISABLE_HTTP || USE_HSTS */
diff --git a/lib/hsts.h b/lib/hsts.h
new file mode 100644
index 000000000..60b3c2df7
--- /dev/null
+++ b/lib/hsts.h
@@ -0,0 +1,60 @@
+#ifndef HEADER_CURL_HSTS_H
+#define HEADER_CURL_HSTS_H
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.haxx.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+#include "curl_setup.h"
+
+#if !defined(CURL_DISABLE_HTTP) && defined(USE_HSTS)
+#include <curl/curl.h>
+#include "llist.h"
+
+#ifdef DEBUGBUILD
+extern time_t deltatime;
+#endif
+
+struct stsentry {
+  struct Curl_llist_element node;
+  const char *host;
+  bool includeSubDomains;
+  time_t expires; /* the timestamp of this entry's expiry */
+};
+
+/* The HSTS cache. Needs to be able to tailmatch host names. */
+struct hsts {
+  struct Curl_llist list;
+  char *filename;
+  unsigned int flags;
+};
+
+struct hsts *Curl_hsts_init(void);
+void Curl_hsts_cleanup(struct hsts **hp);
+CURLcode Curl_hsts_parse(struct hsts *h, const char *hostname,
+                         const char *sts);
+struct stsentry *Curl_hsts(struct hsts *h, const char *hostname,
+                           bool subdomain);
+CURLcode Curl_hsts_save(struct Curl_easy *data, struct hsts *h,
+                        const char *file);
+CURLcode Curl_hsts_load(struct hsts *h, const char *file);
+#else
+#define Curl_hsts_cleanup(x)
+#endif /* CURL_DISABLE_HTTP || USE_HSTS */
+#endif /* HEADER_CURL_HSTS_H */
diff --git a/lib/http.c b/lib/http.c
index 3a0a32df4..4db1c9589 100644
--- a/lib/http.c
+++ b/lib/http.c
@@ -77,6 +77,7 @@
 #include "connect.h"
 #include "strdup.h"
 #include "altsvc.h"
+#include "hsts.h"
 
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -3990,6 +3991,23 @@ CURLcode Curl_http_readwrite_headers(struct Curl_easy *data,
         }
       }
     }
+
+#ifdef USE_HSTS
+    /* If enabled, the header is incoming and this is over HTTPS */
+    else if(data->hsts && checkprefix("Strict-Transport-Security:", headp) &&
+            (conn->handler->flags & PROTOPT_SSL)) {
+      CURLcode check =
+        Curl_hsts_parse(data->hsts, data->state.up.hostname,
+                        &headp[ sizeof("Strict-Transport-Security:") -1 ]);
+      if(check)
+        infof(data, "Illegal STS header skipped\n");
+#ifdef DEBUGBUILD
+      else
+        infof(data, "Parsed STS header fine (%d entries)\n",
+              data->hsts->list.size);
+#endif
+    }
+#endif
 #ifndef CURL_DISABLE_ALTSVC
     /* If enabled, the header is incoming and this is over HTTPS */
     else if(data->asi && checkprefix("Alt-Svc:", headp) &&
diff --git a/lib/setopt.c b/lib/setopt.c
index 3b96289c2..4aa31bb39 100644
--- a/lib/setopt.c
+++ b/lib/setopt.c
@@ -45,6 +45,7 @@
 #include "setopt.h"
 #include "multiif.h"
 #include "altsvc.h"
+#include "hsts.h"
 
 /* The last 3 #include files should be in this order */
 #include "curl_printf.h"
@@ -2839,6 +2840,33 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
     data->set.trailer_data = va_arg(param, void *);
 #endif
     break;
+#ifdef USE_HSTS
+  case CURLOPT_HSTS:
+    if(!data->hsts) {
+      data->hsts = Curl_hsts_init();
+      if(!data->hsts)
+        return CURLE_OUT_OF_MEMORY;
+    }
+    argptr = va_arg(param, char *);
+    result = Curl_setstropt(&data->set.str[STRING_HSTS], argptr);
+    if(result)
+      return result;
+    if(argptr)
+      (void)Curl_hsts_load(data->hsts, argptr);
+    break;
+  case CURLOPT_HSTS_CTRL:
+    arg = va_arg(param, long);
+    if(arg & CURLHSTS_ENABLE) {
+      if(!data->hsts) {
+        data->hsts = Curl_hsts_init();
+        if(!data->hsts)
+          return CURLE_OUT_OF_MEMORY;
+      }
+    }
+    else
+      Curl_hsts_cleanup(&data->hsts);
+    break;
+#endif
 #ifndef CURL_DISABLE_ALTSVC
   case CURLOPT_ALTSVC:
     if(!data->asi) {
diff --git a/lib/url.c b/lib/url.c
index 0176517d9..1e3f025ad 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -96,6 +96,7 @@ bool curl_win32_idn_to_ascii(const char *in, char **out);
 #include "getinfo.h"
 #include "urlapi-int.h"
 #include "system_win32.h"
+#include "hsts.h"
 
 /* And now for the protocols */
 #include "ftp.h"
@@ -411,6 +412,7 @@ CURLcode Curl_close(struct Curl_easy **datap)
   Curl_flush_cookies(data, TRUE);
   Curl_altsvc_save(data, data->asi, data->set.str[STRING_ALTSVC]);
   Curl_altsvc_cleanup(&data->asi);
+  Curl_hsts_cleanup(&data->hsts);
 #if !defined(CURL_DISABLE_HTTP) && !defined(CURL_DISABLE_CRYPTO_AUTH)
   Curl_http_auth_cleanup_digest(data);
 #endif
@@ -1911,6 +1913,19 @@ static CURLcode parseurlandfillconn(struct Curl_easy *data,
   if(uc)
     return Curl_uc_to_curlcode(uc);
 
+  uc = curl_url_get(uh, CURLUPART_HOST, &data->state.up.hostname, 0);
+  if(uc) {
+    if(!strcasecompare("file", data->state.up.scheme))
+      return CURLE_OUT_OF_MEMORY;
+  }
+
+#ifdef USE_HSTS
+  if(data->hsts && strcasecompare("http", data->state.up.scheme)) {
+    if(Curl_hsts(data->hsts, data->state.up.hostname, TRUE))
+      infof(data, "Switch from HTTP to HTTPS due to HSTS!\n");
+  }
+#endif
+
   result = findprotocol(data, conn, data->state.up.scheme);
   if(result)
     return result;
@@ -1956,12 +1971,6 @@ static CURLcode parseurlandfillconn(struct Curl_easy *data,
   else if(uc != CURLUE_NO_OPTIONS)
     return Curl_uc_to_curlcode(uc);
 
-  uc = curl_url_get(uh, CURLUPART_HOST, &data->state.up.hostname, 0);
-  if(uc) {
-    if(!strcasecompare("file", data->state.up.scheme))
-      return CURLE_OUT_OF_MEMORY;
-  }
-
   uc = curl_url_get(uh, CURLUPART_PATH, &data->state.up.path, 0);
   if(uc)
     return Curl_uc_to_curlcode(uc);
diff --git a/lib/urldata.h b/lib/urldata.h
index e8b54aa30..ea7060ec5 100644
--- a/lib/urldata.h
+++ b/lib/urldata.h
@@ -1531,35 +1531,26 @@ enum dupstring {
   STRING_RTSP_SESSION_ID, /* Session ID to use */
   STRING_RTSP_STREAM_URI, /* Stream URI for this request */
   STRING_RTSP_TRANSPORT,  /* Transport for this session */
-
   STRING_SSH_PRIVATE_KEY, /* path to the private key file for auth */
   STRING_SSH_PUBLIC_KEY,  /* path to the public key file for auth */
   STRING_SSH_HOST_PUBLIC_KEY_MD5, /* md5 of host public key in ascii hex */
   STRING_SSH_KNOWNHOSTS,  /* file name of knownhosts file */
-
   STRING_PROXY_SERVICE_NAME, /* Proxy service name */
   STRING_SERVICE_NAME,    /* Service name */
   STRING_MAIL_FROM,
   STRING_MAIL_AUTH,
-
   STRING_TLSAUTH_USERNAME_ORIG,  /* TLS auth <username> */
   STRING_TLSAUTH_USERNAME_PROXY, /* TLS auth <username> */
   STRING_TLSAUTH_PASSWORD_ORIG,  /* TLS auth <password> */
   STRING_TLSAUTH_PASSWORD_PROXY, /* TLS auth <password> */
-
   STRING_BEARER,                /* <bearer>, if used */
-
   STRING_UNIX_SOCKET_PATH,      /* path to Unix socket, if used */
-
   STRING_TARGET,                /* CURLOPT_REQUEST_TARGET */
   STRING_DOH,                   /* CURLOPT_DOH_URL */
-
   STRING_ALTSVC,                /* CURLOPT_ALTSVC */
-
+  STRING_HSTS,                  /* CURLOPT_HSTS */
   STRING_SASL_AUTHZID,          /* CURLOPT_SASL_AUTHZID */
-
   STRING_TEMP_URL,              /* temp URL storage for proxy use */
-
   STRING_DNS_SERVERS,
   STRING_DNS_INTERFACE,
   STRING_DNS_LOCAL_IP4,
@@ -1899,6 +1890,9 @@ struct Curl_easy {
                                   NOTE that the 'cookie' field in the
                                   UserDefined struct defines if the "engine"
                                   is to be used or not. */
+#ifdef USE_HSTS
+  struct hsts *hsts;
+#endif
 #ifndef CURL_DISABLE_ALTSVC
   struct altsvcinfo *asi;      /* the alt-svc cache */
 #endif
diff --git a/lib/version.c b/lib/version.c
index 70e456658..5b759be38 100644
--- a/lib/version.c
+++ b/lib/version.c
@@ -418,6 +418,9 @@ static curl_version_info_data version_info = {
 #ifndef CURL_DISABLE_ALTSVC
   | CURL_VERSION_ALTSVC
 #endif
+#if defined(USE_HSTS)
+  | CURL_VERSION_HSTS
+#endif
   ,
   NULL, /* ssl_version */
   0,    /* ssl_version_num, this is kept at zero */
diff --git a/src/tool_cfgable.c b/src/tool_cfgable.c
index e99602c4f..3c0bbfa64 100644
--- a/src/tool_cfgable.c
+++ b/src/tool_cfgable.c
@@ -54,6 +54,7 @@ static void free_config_fields(struct OperationConfig *config)
   Curl_safefree(config->egd_file);
   Curl_safefree(config->useragent);
   Curl_safefree(config->altsvc);
+  Curl_safefree(config->hsts);
   Curl_safefree(config->cookie);
   Curl_safefree(config->cookiejar);
   Curl_safefree(config->cookiefile);
diff --git a/src/tool_cfgable.h b/src/tool_cfgable.h
index 489f9ca0e..ac4c7fadc 100644
--- a/src/tool_cfgable.h
+++ b/src/tool_cfgable.h
@@ -58,6 +58,7 @@ struct OperationConfig {
   char *cookiejar;          /* write to this file */
   char *cookiefile;         /* read from this file */
   char *altsvc;             /* alt-svc cache file name */
+  char *hsts;               /* HSTS cache file name */
   bool cookiesession;       /* new session? */
   bool encoding;            /* Accept-Encoding please */
   bool tr_encoding;         /* Transfer-Encoding please */
diff --git a/src/tool_getparam.c b/src/tool_getparam.c
index 910a5a2f9..d2e4eb498 100644
--- a/src/tool_getparam.c
+++ b/src/tool_getparam.c
@@ -219,6 +219,7 @@ static const struct LongShort aliases[]= {
   {"A",  "user-agent",               ARG_STRING},
   {"b",  "cookie",                   ARG_STRING},
   {"ba", "alt-svc",                  ARG_STRING},
+  {"bb", "hsts",                     ARG_STRING},
   {"B",  "use-ascii",                ARG_BOOL},
   {"c",  "cookie-jar",               ARG_STRING},
   {"C",  "continue-at",              ARG_STRING},
@@ -1291,6 +1292,12 @@ ParameterError getparameter(const char *flag, /* f or -long-flag */
         else
           return PARAM_LIBCURL_DOESNT_SUPPORT;
         break;
+      case 'b': /* --hsts */
+        if(curlinfo->features & CURL_VERSION_HSTS)
+          GetStr(&config->hsts, nextarg);
+        else
+          return PARAM_LIBCURL_DOESNT_SUPPORT;
+        break;
       default:  /* --cookie string coming up: */
         if(nextarg[0] == '@') {
           nextarg++;
diff --git a/src/tool_help.c b/src/tool_help.c
index 544dbbab0..0833a0d23 100644
--- a/src/tool_help.c
+++ b/src/tool_help.c
@@ -328,6 +328,9 @@ static const struct helptxt helptext[] = {
   {"    --hostpubmd5 <md5>",
    "Acceptable MD5 hash of the host public key",
    CURLHELP_SFTP | CURLHELP_SCP},
+  {"    --hsts <file name>",
+   "Enable HSTS with this cache file",
+   CURLHELP_HTTP},
   {"    --http0.9",
    "Allow HTTP 0.9 responses",
    CURLHELP_HTTP},
@@ -862,6 +865,7 @@ static const struct feat feats[] = {
   {"MultiSSL",       CURL_VERSION_MULTI_SSL},
   {"PSL",            CURL_VERSION_PSL},
   {"alt-svc",        CURL_VERSION_ALTSVC},
+  {"HSTS",           CURL_VERSION_HSTS},
 };
 
 static void print_category(curlhelp_t category)
diff --git a/src/tool_operate.c b/src/tool_operate.c
index 4ad5052ff..e0fde724b 100644
--- a/src/tool_operate.c
+++ b/src/tool_operate.c
@@ -2072,6 +2072,9 @@ static CURLcode single_transfer(struct GlobalConfig *global,
         if(config->altsvc)
           my_setopt_str(curl, CURLOPT_ALTSVC, config->altsvc);
 
+        if(config->hsts)
+          my_setopt_bitmask(curl, CURLOPT_HSTS_CTRL, CURLHSTS_ENABLE);
+
 #ifdef USE_METALINK
         if(!metalink && config->use_metalink) {
           outs->metalink_parser = metalink_parser_context_new();
diff --git a/src/tool_setopt.c b/src/tool_setopt.c
index 0dd7a57a2..2159db6cd 100644
--- a/src/tool_setopt.c
+++ b/src/tool_setopt.c
@@ -62,6 +62,11 @@ const struct NameValue setopt_nv_CURL_SOCKS_PROXY[] = {
   NVEND,
 };
 
+const struct NameValueUnsigned setopt_nv_CURLHSTS[] = {
+  NV(CURLHSTS_ENABLE),
+  NVEND,
+};
+
 const struct NameValueUnsigned setopt_nv_CURLAUTH[] = {
   NV(CURLAUTH_ANY),             /* combination */
   NV(CURLAUTH_ANYSAFE),         /* combination */
diff --git a/src/tool_setopt.h b/src/tool_setopt.h
index 3db88c6bf..f8d3320d3 100644
--- a/src/tool_setopt.h
+++ b/src/tool_setopt.h
@@ -64,8 +64,10 @@ extern const struct NameValueUnsigned setopt_nv_CURLSSLOPT[];
 extern const struct NameValue setopt_nv_CURL_NETRC[];
 extern const struct NameValue setopt_nv_CURLPROTO[];
 extern const struct NameValueUnsigned setopt_nv_CURLAUTH[];
+extern const struct NameValueUnsigned setopt_nv_CURLHSTS[];
 
 /* Map options to NameValue sets */
+#define setopt_nv_CURLOPT_HSTS_CTRL setopt_nv_CURLHSTS
 #define setopt_nv_CURLOPT_HTTP_VERSION setopt_nv_CURL_HTTP_VERSION
 #define setopt_nv_CURLOPT_HTTPAUTH setopt_nv_CURLAUTH
 #define setopt_nv_CURLOPT_SSLVERSION setopt_nv_CURL_SSLVERSION
diff --git a/tests/data/Makefile.inc b/tests/data/Makefile.inc
index 2e0c092ad..04e23c6fc 100644
--- a/tests/data/Makefile.inc
+++ b/tests/data/Makefile.inc
@@ -197,6 +197,7 @@ test1620 test1621 \
 test1630 test1631 test1632 test1633 \
 \
 test1650 test1651 test1652 test1653 test1654 test1655 \
+test1660 \
 \
 test1700 test1701 test1702 \
 \
diff --git a/tests/data/test1660 b/tests/data/test1660
new file mode 100644
index 000000000..f64765c3f
--- /dev/null
+++ b/tests/data/test1660
@@ -0,0 +1,81 @@
+<testcase>
+<info>
+<keywords>
+unittest
+HSTS
+</keywords>
+</info>
+
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+HSTS
+</features>
+
+<file name="log/input1660">
+# Your HSTS cache. https://curl.haxx.se/docs/hsts.html
+# This file was generated by libcurl! Edit at your own risk.
+.readfrom.example "20211001 04:47:41"
+.old.example "20161001 04:47:41"
+</file>
+
+# This date is exactly "20190124 22:34:21" UTC
+<setenv>
+CURL_TIME=1548369261
+</setenv>
+<name>
+HSTS
+</name>
+<command>
+-
+</command>
+</client>
+
+<verify>
+<stdout>
+readfrom.example [readfrom.example]: 1633063661 includeSubDomains
+'old.example' is not HSTS
+'readfrom.example' is not HSTS
+example.com [example.com]: 1579905261
+example.com [example.com]: 1569905261
+example.com [example.com]: 1569905261
+example.com [example.com]: 1569905261 includeSubDomains
+example.org [example.org]: 1579905261
+Input 8: error 43
+Input 9: error 43
+this.example [this.example]: 1548400797
+'this.example' is not HSTS
+Input 12: error 43
+Input 13: error 43
+Input 14: error 43
+3.example.com [example.com]: 1569905261 includeSubDomains
+3.example.com [example.com]: 1569905261 includeSubDomains
+foo.example.com [example.com]: 1569905261 includeSubDomains
+'foo.xample.com' is not HSTS
+'forexample.net' is not HSTS
+'forexample.net' is not HSTS
+'example.net' is not HSTS
+expire.example [expire.example]: 1548369268
+Number of entries: 3
+expire.example [expire.example]: 1548369268
+expire.example [expire.example]: 1548369268
+expire.example [expire.example]: 1548369268
+expire.example [expire.example]: 1548369268
+expire.example [expire.example]: 1548369268
+expire.example [expire.example]: 1548369268
+expire.example [expire.example]: 1548369268
+'expire.example' is not HSTS
+'expire.example' is not HSTS
+'expire.example' is not HSTS
+</stdout>
+<file name="log/hsts1660">
+# Your HSTS cache. https://curl.haxx.se/docs/hsts.html
+# This file was generated by libcurl! Edit at your own risk.
+.example.com "20191001 04:47:41"
+example.org "20200124 22:34:21"
+</file>
+</verify>
+</testcase>
diff --git a/tests/runtests.pl b/tests/runtests.pl
index 4bcf61bbf..a4e330661 100755
--- a/tests/runtests.pl
+++ b/tests/runtests.pl
@@ -256,6 +256,7 @@ my $has_cares;      # set if built with c-ares
 my $has_threadedres;# set if built with threaded resolver
 my $has_psl;        # set if libcurl is built with PSL support
 my $has_altsvc;     # set if libcurl is built with alt-svc support
+my $has_hsts;       # set if libcurl is built with HSTS support
 my $has_ldpreload;  # set if curl is built for systems supporting LD_PRELOAD
 my $has_multissl;   # set if curl is build with MultiSSL support
 my $has_manual;     # set if curl is built with built-in manual
@@ -2762,6 +2763,7 @@ sub compare {
 
 sub setupfeatures {
     $feature{"alt-svc"} = $has_altsvc;
+    $feature{"HSTS"} = $has_hsts;
     $feature{"brotli"} = $has_brotli;
     $feature{"crypto"} = $has_crypto;
     $feature{"debug"} = $debug_build;
@@ -3035,6 +3037,9 @@ sub checksystem {
                 # alt-svc enabled
                 $has_altsvc=1;
             }
+            if($feat =~ /HSTS/i) {
+                $has_hsts=1;
+            }
             if($feat =~ /AsynchDNS/i) {
                 if(!$has_cares) {
                     # this means threaded resolver
diff --git a/tests/unit/Makefile.inc b/tests/unit/Makefile.inc
index f63724f91..ee6816823 100644
--- a/tests/unit/Makefile.inc
+++ b/tests/unit/Makefile.inc
@@ -34,7 +34,8 @@ UNITPROGS = unit1300 unit1301 unit1302 unit1303 unit1304 unit1305 unit1307 \
  unit1600 unit1601 unit1602 unit1603 unit1604 unit1605 unit1606 unit1607 \
  unit1608 unit1609 unit1610 unit1611 unit1612 \
  unit1620 unit1621 \
- unit1650 unit1651 unit1652 unit1653 unit1654 unit1655
+ unit1650 unit1651 unit1652 unit1653 unit1654 unit1655 \
+ unit1660
 
 unit1300_SOURCES = unit1300.c $(UNITFILES)
 unit1300_CPPFLAGS = $(AM_CPPFLAGS)
@@ -154,3 +155,5 @@ unit1654_CPPFLAGS = $(AM_CPPFLAGS)
 unit1655_SOURCES = unit1655.c $(UNITFILES)
 unit1655_CPPFLAGS = $(AM_CPPFLAGS)
 
+unit1660_SOURCES = unit1660.c $(UNITFILES)
+unit1660_CPPFLAGS = $(AM_CPPFLAGS)
diff --git a/tests/unit/unit1660.c b/tests/unit/unit1660.c
new file mode 100644
index 000000000..1687cafa1
--- /dev/null
+++ b/tests/unit/unit1660.c
@@ -0,0 +1,172 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.haxx.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ***************************************************************************/
+#include "curlcheck.h"
+
+#include "urldata.h"
+#include "hsts.h"
+
+static CURLcode
+unit_setup(void)
+{
+  return CURLE_OK;
+}
+
+static void
+unit_stop(void)
+{
+  curl_global_cleanup();
+}
+
+#if defined(CURL_DISABLE_HTTP) || !defined(USE_HSTS)
+UNITTEST_START
+{
+  return 0; /* nothing to do when HTTP or HSTS are disabled */
+}
+UNITTEST_STOP
+#else
+
+struct testit {
+  const char *host;
+  const char *chost; /* if non-NULL, use to lookup with */
+  const char *hdr; /* if NULL, just do the lookup */
+  const CURLcode result; /* parse result */
+};
+
+static const struct testit headers[] = {
+  /* two entries read from disk cache, verify first */
+  { "-", "readfrom.example", NULL, CURLE_OK},
+  { "-", "old.example", NULL, CURLE_OK},
+  /* delete the remaining one read from disk */
+  { "readfrom.example", NULL, "max-age=\"0\"", CURLE_OK},
+
+  { "example.com", NULL, "max-age=\"31536000\"\r\n", CURLE_OK },
+  { "example.com", NULL, "max-age=\"21536000\"\r\n", CURLE_OK },
+  { "example.com", NULL, "max-age=\"21536000\"; \r\n", CURLE_OK },
+  { "example.com", NULL, "max-age=\"21536000\"; includeSubDomains\r\n",
+    CURLE_OK },
+  { "example.org", NULL, "max-age=\"31536000\"\r\n", CURLE_OK },
+  { "this.example", NULL, "max=\"31536\";", CURLE_BAD_FUNCTION_ARGUMENT },
+  { "this.example", NULL, "max-age=\"31536", CURLE_BAD_FUNCTION_ARGUMENT },
+  { "this.example", NULL, "max-age=31536\"", CURLE_OK },
+  /* max-age=0 removes the entry */
+  { "this.example", NULL, "max-age=0", CURLE_OK },
+  { "another.example", NULL, "includeSubDomains; ",
+    CURLE_BAD_FUNCTION_ARGUMENT },
+
+  /* Two max-age is illegal */
+  { "example.com", NULL,
+    "max-age=\"21536000\"; includeSubDomains; max-age=\"3\";",
+    CURLE_BAD_FUNCTION_ARGUMENT },
+  /* Two includeSubDomains is illegal */
+  { "2.example.com", NULL,
+    "max-age=\"21536000\"; includeSubDomains; includeSubDomains;",
+    CURLE_BAD_FUNCTION_ARGUMENT },
+  /* use a unknown directive "include" that should be ignored */
+  { "3.example.com", NULL, "max-age=\"21536000\"; include; includeSubDomains;",
+    CURLE_OK },
+  /* remove the "3.example.com" one, should still match the example.com */
+  { "3.example.com", NULL, "max-age=\"0\"; includeSubDomains;",
+    CURLE_OK },
+  { "-", "foo.example.com", NULL, CURLE_OK},
+  { "-", "foo.xample.com", NULL, CURLE_OK},
+
+  /* should not match */
+  { "example.net", "forexample.net", "max-age=\"31536000\"\r\n", CURLE_OK },
+
+  /* should not match either, since forexample.net is not in the example.net
+     domain */
+  { "example.net", "forexample.net",
+    "max-age=\"31536000\"; includeSubDomains\r\n", CURLE_OK },
+  /* remove example.net again */
+  { "example.net", NULL, "max-age=\"0\"; includeSubDomains\r\n", CURLE_OK },
+
+  /* make this live for 7 seconds */
+  { "expire.example", NULL, "max-age=\"7\"\r\n", CURLE_OK },
+  { NULL, NULL, NULL, 0 }
+};
+
+static void showsts(struct stsentry *e, const char *chost)
+{
+  if(!e)
+    printf("'%s' is not HSTS\n", chost);
+  else {
+    printf("%s [%s]: %" CURL_FORMAT_CURL_OFF_T "%s\n",
+           chost, e->host, e->expires,
+           e->includeSubDomains ? " includeSubDomains" : "");
+  }
+}
+
+UNITTEST_START
+{
+  CURLcode result;
+  struct stsentry *e;
+  struct hsts *h = Curl_hsts_init();
+  int i;
+  const char *chost;
+  CURL *easy;
+  if(!h)
+    return 1;
+
+  Curl_hsts_load(h, "log/input1660");
+
+  for(i = 0; headers[i].host ; i++) {
+    if(headers[i].hdr) {
+      result = Curl_hsts_parse(h, headers[i].host, headers[i].hdr);
+
+      if(result != headers[i].result) {
+        fprintf(stderr, "Curl_hsts_parse(%s) failed: %d\n",
+                headers[i].hdr, result);
+        unitfail++;
+        continue;
+      }
+      else if(result) {
+        printf("Input %u: error %d\n", i, (int) result);
+        continue;
+      }
+    }
+
+    chost = headers[i].chost ? headers[i].chost : headers[i].host;
+    e = Curl_hsts(h, chost, TRUE);
+    showsts(e, chost);
+  }
+
+  printf("Number of entries: %d\n", h->list.size);
+
+  /* verify that it is exists for 7 seconds */
+  chost = "expire.example";
+  for(i = 100; i < 110; i++) {
+    e = Curl_hsts(h, chost, TRUE);
+    showsts(e, chost);
+    deltatime++; /* another second passed */
+  }
+
+  easy = curl_easy_init();
+  if(easy) {
+    (void)Curl_hsts_save(easy, h, "log/hsts1660");
+    curl_easy_cleanup(easy);
+  }
+
+  Curl_hsts_cleanup(&h);
+  return unitfail;
+}
+UNITTEST_STOP
+#endif