docs: Add more HTTPS proxy documentation

- Document HTTPS proxy type.

- Document --write-out %{proxy_ssl_verify_result}.

- Document SOCKS proxy + HTTP/HTTPS proxy combination.

HTTPS proxy support was added in 7.52.0 for OpenSSL, GnuTLS and NSS.

Ref: https://github.com/curl/curl/commit/cb4e2be

docs/cmdline-opts/proxy.d

@@ -5,11 +5,16 @@ Help: Use this proxy
 ---
 Use the specified proxy.
 
-The proxy string can be specified with a protocol:// prefix to specify
+The proxy string can be specified with a protocol:// prefix. No protocol
-alternative proxy protocols. Use socks4://, socks4a://, socks5:// or
+specified or http:// will be treated as HTTP proxy. Use socks4://, socks4a://,
-socks5h:// to request the specific SOCKS version to be used. No protocol
+socks5:// or socks5h:// to request a specific SOCKS version to be used.
-specified, http:// and all others will be treated as HTTP proxies. (The
+(The protocol support was added in curl 7.21.7)
-protocol support was added in curl 7.21.7)
+
+HTTPS proxy support via https:// protocol prefix was added in 7.52.0 for
+OpenSSL, GnuTLS and NSS.
+
+Unrecognized and unsupported proxy protocols cause an error since 7.52.0.
+Prior versions may ignore the protocol and use http:// instead.
 
 If the port number is not specified in the proxy string, it is assumed to be
 1080.

docs/cmdline-opts/socks4.d

@@ -6,10 +6,12 @@ Added: 7.15.2
 Use the specified SOCKS4 proxy. If the port number is not specified, it is
 assumed at port 1080.
 
-This option overrides any previous use of --proxy, as they are mutually
-exclusive.
-
 Since 7.21.7, this option is superfluous since you can specify a socks4 proxy
 with --proxy using a socks4:// protocol prefix.
 
+This option typically overrides any previous use of --proxy; however since
+7.52.0 if --proxy is used to specify an HTTP or HTTPS proxy then curl will
+attempt to use it with the SOCKS proxy. In such a case curl first connects to
+the SOCKS proxy and then connects (through SOCKS) to the HTTP or HTTPS proxy.
+
 If this option is used several times, the last one will be used.

docs/cmdline-opts/socks4a.d

@@ -6,10 +6,12 @@ Added: 7.18.0
 Use the specified SOCKS4a proxy. If the port number is not specified, it is
 assumed at port 1080.
 
-This option overrides any previous use of --proxy, as they are mutually
-exclusive.
-
 Since 7.21.7, this option is superfluous since you can specify a socks4a proxy
 with --proxy using a socks4a:// protocol prefix.
 
+This option typically overrides any previous use of --proxy; however since
+7.52.0 if --proxy is used to specify an HTTP or HTTPS proxy then curl will
+attempt to use it with the SOCKS proxy. In such a case curl first connects to
+the SOCKS proxy and then connects (through SOCKS) to the HTTP or HTTPS proxy.
+
 If this option is used several times, the last one will be used.

docs/cmdline-opts/socks5-hostname.d

@@ -6,10 +6,12 @@ Added: 7.18.0
 Use the specified SOCKS5 proxy (and let the proxy resolve the host name). If
 the port number is not specified, it is assumed at port 1080.
 
-This option overrides any previous use of --proxy, as they are mutually
-exclusive.
-
 Since 7.21.7, this option is superfluous since you can specify a socks5
 hostname proxy with --proxy using a socks5h:// protocol prefix.
 
+This option typically overrides any previous use of --proxy; however since
+7.52.0 if --proxy is used to specify an HTTP or HTTPS proxy then curl will
+attempt to use it with the SOCKS proxy. In such a case curl first connects to
+the SOCKS proxy and then connects (through SOCKS) to the HTTP or HTTPS proxy.
+
 If this option is used several times, the last one will be used.

docs/cmdline-opts/socks5.d

@@ -6,12 +6,14 @@ Added: 7.18.0
 Use the specified SOCKS5 proxy - but resolve the host name locally. If the
 port number is not specified, it is assumed at port 1080.
 
-This option overrides any previous use of --proxy, as they are mutually
-exclusive.
-
 Since 7.21.7, this option is superfluous since you can specify a socks5 proxy
 with --proxy using a socks5:// protocol prefix.
 
+This option typically overrides any previous use of --proxy; however since
+7.52.0 if --proxy is used to specify an HTTP or HTTPS proxy then curl will
+attempt to use it with the SOCKS proxy. In such a case curl first connects to
+the SOCKS proxy and then connects (through SOCKS) to the HTTP or HTTPS proxy.
+
 If this option is used several times, the last one will be used.
 
 This option (as well as --socks4) does not work with IPV6, FTPS or LDAP.

docs/cmdline-opts/write-out.d

@@ -60,6 +60,10 @@ Number of new connects made in the recent transfer. (Added in 7.12.3)
 .B num_redirects
 Number of redirects that were followed in the request. (Added in 7.12.3)
 .TP
+.B proxy_ssl_verify_result
+The result of the HTTPS proxy's SSL peer certificate verification that was
+requested. 0 means the verification was successful. (Added in 7.52.0)
+.TP
 .B redirect_url
 When an HTTP request was made without -L to follow redirects, this variable
 will show the actual URL a redirect \fIwould\fP take you to. (Added in 7.18.2)

docs/libcurl/opts/CURLOPT_PROXY.3

@@ -38,10 +38,22 @@ option \fICURLOPT_PROXYPORT(3)\fP. If not specified, libcurl will default to
 using port 1080 for proxies.
 
 The proxy string may be prefixed with [scheme]:// to specify which kind of
-proxy is used. Use socks4://, socks4a://, socks5:// or socks5h:// (the last
+proxy is used.
-one to enable socks5 and asking the proxy to do the resolving, also known as
+
-\fICURLPROXY_SOCKS5_HOSTNAME\fP type) to request the specific SOCKS version to
+.RS
-be used. No scheme specified or http://, will be treated as HTTP proxies.
+.IP http://
+HTTP Proxy. Default when no scheme or proxy type is specified.
+.IP https://
+HTTPS Proxy. (Added in 7.52.0 for OpenSSL, GnuTLS and NSS)
+.IP socks4://
+SOCKS4 Proxy.
+.IP socks4a://
+SOCKS4a Proxy. Proxy resolves URL hostname.
+.IP socks5://
+SOCKS5 Proxy.
+.IP socks5h://
+SOCKS5 Proxy. Proxy resolves URL hostname.
+.RE
 
 Without a scheme prefix, \fICURLOPT_PROXYTYPE(3)\fP can be used to specify
 which kind of proxy the string identifies.

docs/libcurl/opts/CURLOPT_PROXYTYPE.3

@@ -28,15 +28,26 @@ CURLOPT_PROXYTYPE \- proxy protocol type
 
 CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PROXYTYPE, long type);
 .SH DESCRIPTION
-Pass a long with this option to set type of the proxy. Available options for
+Pass one of the values below to set the type of the proxy.
-this are \fICURLPROXY_HTTP\fP, \fICURLPROXY_HTTP_1_0\fP
-\fICURLPROXY_SOCKS4\fP, \fICURLPROXY_SOCKS5\fP, \fICURLPROXY_SOCKS4A\fP and
-\fICURLPROXY_SOCKS5_HOSTNAME\fP. The HTTP type is default.
 
-If you set \fICURLOPT_PROXYTYPE(3)\fP to \fICURLPROXY_HTTP_1_0\fP, it will
+.RS
-only affect how libcurl speaks to a proxy when CONNECT is used. The HTTP
+.IP CURLPROXY_HTTP
-version used for "regular" HTTP requests is instead controlled with
+HTTP Proxy. Default.
-\fICURLOPT_HTTP_VERSION(3)\fP.
+.IP CURLPROXY_HTTPS
+HTTPS Proxy. (Added in 7.52.0 for OpenSSL, GnuTLS and NSS)
+.IP CURLPROXY_HTTP_1_0
+HTTP 1.0 Proxy. This is very similar to CURLPROXY_HTTP except it uses HTTP/1.0
+for any CONNECT tunnelling. It does not change the HTTP version of the actual
+HTTP requests, controlled by \fICURLOPT_HTTP_VERSION(3)\fP.
+.IP CURLPROXY_SOCKS4
+SOCKS4 Proxy.
+.IP CURLPROXY_SOCKS4A
+SOCKS4a Proxy. Proxy resolves URL hostname.
+.IP CURLPROXY_SOCKS5
+SOCKS5 Proxy.
+.IP CURLPROXY_SOCKS5_HOSTNAME
+SOCKS5 Proxy. Proxy resolves URL hostname.
+.RE
 
 Often it is more convenient to specify the proxy type with the scheme part of
 the \fICURLOPT_PROXY(3)\fP string.