Find a free port for management interface

Bind a socket and then close to identify
a free port and use it when starting openvpn.exe.

Try port = offset + config-index is first, matching
the current usage, and fallback to a dynamic port if
the former fails.

Trac: #1051
Signed-off-by: Selva Nair <selva.nair@gmail.com>

misc.c

@@ -773,3 +773,54 @@ set_openssl_env_vars()
         }
     }
 }
+
+/*
+ * Find a free port to bind and return it in addr.sin_port
+ */
+BOOL
+find_free_tcp_port(SOCKADDR_IN *addr)
+{
+    BOOL ret = false;
+    SOCKADDR_IN addr_bound;
+    WSADATA wsaData;
+    int len = sizeof(*addr);
+
+    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0)
+    {
+        return ret;
+    }
+
+    u_short old_port = addr->sin_port;
+
+    SOCKET sk = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
+    if (sk == INVALID_SOCKET)
+    {
+        MsgToEventLog(EVENTLOG_ERROR_TYPE, L"%hs: socket open failed", __func__);
+        goto out;
+    }
+    while (bind(sk, (SOCKADDR *) addr, len))
+    {
+        if (addr->sin_port == 0)
+        {
+            MsgToEventLog(EVENTLOG_ERROR_TYPE, L"%hs: bind to dynamic port failed", __func__);
+            goto out;
+        }
+        addr->sin_port = 0;
+    }
+    if (getsockname(sk, (SOCKADDR *) &addr_bound, &len))
+    {
+        MsgToEventLog(EVENTLOG_ERROR_TYPE, L"%hs: getsockname failed", __func__);
+        goto out;
+    }
+
+    ret = true;
+
+out:
+    if (sk != INVALID_SOCKET)
+    {
+        closesocket(sk);
+    }
+    addr->sin_port = ret ? addr_bound.sin_port : old_port;
+    WSACleanup();
+    return ret;
+}

misc.h

@@ -89,4 +89,13 @@ void set_openssl_env_vars(void);
 /* Return escaped copy of a string */
 char *escape_string(const char *str);
 
+/**
+ * Find a free port to bind to
+ * @param addr : Address to bind to -- if port >0 it's tried first.
+ *               On return the port is set to the one found.
+ * @returns true on success, false on error. In case of error
+ * addr is unchanged.
+ */
+BOOL find_free_tcp_port(SOCKADDR_IN *addr);
+
 #endif

openvpn.c

@@ -2193,6 +2193,8 @@ StartOpenVPN(connection_t *c)
     /* Create a management interface password */
     GetRandomPassword(c->manage.password, sizeof(c->manage.password) - 1);
 
+    find_free_tcp_port(&c->manage.skaddr);
+
     /* Construct command line -- put log first */
     _sntprintf_0(cmdline, _T("openvpn --log%ls \"%ls\" --config \"%ls\" "
         "--setenv IV_GUI_VER \"%hs\" --setenv IV_SSO openurl,crtext --service %ls 0 --auth-retry interact "