Add exec/subsystem versions of SSH proxying.

This is a simple tweak to the existing in-process SSH jump host
support, where instead of opening a direct-tcpip channel to the
destination host, we open a session channel and run a process in it to
make the connection to the destination.

So, where the existing jump host support replaced a local proxy
command along the lines of "plink %proxyhost -nc %host %port", this
one replaces "plink %proxyhost run-some-command".

Also added a corresponding option to use a subsystem to make the
connection. (Someone could configure an SSH server to support specific
subsystem names for particular destinations, or a general schema of
subsystem names that include the destination address in some standard
format.)

To avoid overflowing the already-full Proxy config panel with an extra
subtype selector, I've put these in as additional top-level proxy
types, so that instead of just PROXY_SSH we now have three
PROXY_SSH_foo.

config.c

@@ -1738,7 +1738,9 @@ void proxy_type_handler(union control *ctrl, dlgparam *dlg,
         ADD(PROXY_SOCKS4, "SOCKS 4");
         ADD(PROXY_HTTP, "HTTP CONNECT");
         if (ssh_proxy_supported) {
-            ADD(PROXY_SSH, "SSH to proxy and use port forwarding");
+            ADD(PROXY_SSH_TCPIP, "SSH to proxy and use port forwarding");
+            ADD(PROXY_SSH_EXEC, "SSH to proxy and execute a command");
+            ADD(PROXY_SSH_SUBSYSTEM, "SSH to proxy and invoke a subsystem");
         }
         if (ctrl->generic.context.i & PROXY_UI_FLAG_LOCAL) {
             ADD(PROXY_CMD, "Local (run a subprogram to connect)");

proxy/proxy.c

@@ -499,7 +499,9 @@ Socket *new_connection(SockAddr *addr, const char *hostname,
         char *proxy_canonical_name;
         Socket *sret;
 
-        if (type == PROXY_SSH &&
+        if ((type == PROXY_SSH_TCPIP ||
+             type == PROXY_SSH_EXEC ||
+             type == PROXY_SSH_SUBSYSTEM) &&
             (sret = sshproxy_new_connection(addr, hostname, port, privport,
                                             oobinline, nodelay, keepalive,
                                             plug, conf, itr)) != NULL)

proxy/sshproxy.c

@@ -10,6 +10,7 @@
 #include "ssh.h"
 #include "network.h"
 #include "storage.h"
+#include "proxy.h"
 
 const bool ssh_proxy_supported = true;
 
@@ -636,12 +637,47 @@ Socket *sshproxy_new_connection(SockAddr *addr, const char *hostname,
      */
     conf_set_bool(sp->conf, CONF_ssh_simple, true);
 
+    int proxy_type = conf_get_int(clientconf, CONF_proxy_type);
+    switch (proxy_type) {
+      case PROXY_SSH_TCPIP:
         /*
          * Configure the main channel of this SSH session to be a
          * direct-tcpip connection to the destination host/port.
          */
         conf_set_str(sp->conf, CONF_ssh_nc_host, hostname);
         conf_set_int(sp->conf, CONF_ssh_nc_port, port);
+        break;
+
+      case PROXY_SSH_SUBSYSTEM:
+      case PROXY_SSH_EXEC: {
+        Conf *cmd_conf = conf_copy(clientconf);
+
+        /*
+         * Unlike the Telnet and Local proxy types, we don't use the
+         * proxy username and password fields in the formatted
+         * command, because if we use them at all, it's for
+         * authenticating to the proxy SSH server.
+         */
+        conf_set_str(cmd_conf, CONF_proxy_username, "");
+        conf_set_str(cmd_conf, CONF_proxy_password, "");
+
+        char *cmd = format_telnet_command(sp->addr, sp->port, cmd_conf, NULL);
+        conf_free(cmd_conf);
+
+        conf_set_str(sp->conf, CONF_remote_cmd, cmd);
+        sfree(cmd);
+
+        conf_set_bool(sp->conf, CONF_nopty, true);
+
+        if (proxy_type == PROXY_SSH_SUBSYSTEM)
+            conf_set_bool(sp->conf, CONF_ssh_subsys, true);
+
+        break;
+      }
+
+      default:
+        unreachable("bad SSH proxy type");
+    }
 
     sp->logctx = log_init(&sp->logpolicy, sp->conf);
 

putty.h

@@ -475,7 +475,8 @@ enum {
      * Proxy types.
      */
     PROXY_NONE, PROXY_SOCKS4, PROXY_SOCKS5,
-    PROXY_HTTP, PROXY_TELNET, PROXY_CMD, PROXY_SSH,
+    PROXY_HTTP, PROXY_TELNET, PROXY_CMD, PROXY_SSH_TCPIP,
+    PROXY_SSH_EXEC, PROXY_SSH_SUBSYSTEM,
     PROXY_FUZZ
 };