forms authentication, not able to save web.config programmatically - asp.net

Question

im using visual studio 2008. in my project im using forms authentication, my project structure is as follows
root
|
login.aspx
home.aspx
web.config
admin (folder)
|
admin.aspx
web.config
here in root web.config is as follows
<authentication mode="Forms">
<forms name="Authen" protection="All" timeout="60" loginUrl="Login.aspx" enableCrossAppRedirects="true" cookieless="AutoDetect"/>
</authentication>
<authorization>
<deny users="?"/>
</authorization>
and in inner web.config
and my users are
Username Role
admin1 Admin
admin2 Admin
user1 User
user2 User
as you can see that in second web.config, i gave access to users with "Admin" role and "user1" user.
for giving access to "admin" folder, i wrote the following:
Configuration config = WebConfigurationManager.OpenWebConfiguration("~/HR");
SystemWebSectionGroup systemWeb = (SystemWebSectionGroup)config.GetSectionGroup("system.web");
AuthorizationSection section = (AuthorizationSection)systemWeb.Sections["authorization"];
AuthorizationRule newRule = new AuthorizationRule(AuthorizationRuleAction.Allow);
newRule.Users.Add("user1");
section.Rules.Add(newRule);
and saved the web.config as
config.Save();
this is working fine when i run it from visual studio development server, when i host it in IIS, im getting the following error
Configuration Error
Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.
Parser Error Message: An error occurred loading a configuration file: Access to the path 'F:\dotnet\samples\myproj\UI\hr\f-q_g-yn.tmp' is denied.
Source Error:
[No relevant source lines]
Source File: F:\dotnet\samples\myproj\UI\hr\web.config Line: 0
here the problem is, it is not able to save the web.config
i got one solution, that is, i added
<identity impersonate="true" userName="myusername" password="mypassword"/>
to root web.config and it is working fine. but my boss says that is not the correct way. what is the correct way to solve this problem ??
please help

Answer 1

The account your website is running under does not have permissions to write to the directory your web.config is in. There are two options how to solve that:
Run your website under another account which does have the required permissions. You set the account in the settings of the respective Application Pool in IIS Manager.
Modify the security settinsg (ACLs) of the directory/directories, and add write permissions for the user your application pool is running under.
You should probably combine both methods – use a separate user account and add the write permissions only to this account, not to e.g. Network Service.
(But note that I am not sure whether a website modifying its own web.config files is a good idea at all.)