Ocamlnet 3 Reference Manual : Rpc_auth_dh

# Module Rpc_auth_dh

    module Rpc_auth_dh: sig .. end

Diffie-Hellman authentication (AUTH_DH alias AUTH_DES)

This module implements DH authentication, the simplest form of Secure RPC. Despite its name, this form of authentication provides only a medium level of security, see below.

To use AUTH_DH you need the public-key infrastructure for Secure RPC. This requires that a special daemon, the so-called `keyserv`, runs on both the client's system and the server's system. The task of `keyserv` is to store public and private keys. We do not have a `keyserv` in Ocamlnet, so you must use the `keyserv` your system provides. (Note that `keyserv` is often distributed together with NIS+. However, you can run `keyserv` without needing to set up NIS+.)

In order to make a remote call, the keyserv `daemon` of the client must know the private key of the client user, and the public key of the server user. The `keyserv` daemon of the server must know the public key of the client user and the private key of the server user. Note that you can load a key pair into `keyserv` with the command `keylogin`. (This is not necessary for the root user, root's key pair is loaded at daemon startup time.)

See the manual pages of your OS for `keyserv`, `keylogin`, `keylogout`, and `/etc/publickey`.

Furthermore, it is strictly necessary that time synchronization is enabled between the client and the server. The recommended solution is to synchronize both clocks independently using a time normal (with NTP). Alternatively, the server can provide a time service on port 37 ("netdate").

To identify users, AUTH_DH uses so-called netnames. These have the form "<osflavor>.<user>@<domain>", where <osflavor> determines the kind of operating system (usually "unix"), <user> is an identifier for the user, and <domain> determines where the user identifiers are valid. In UNIX environments, the netnames are formed like:

- For root users: "unix.<hostname>@<nisdomain>"
- For non-privileged users: "unix.<uid>@<nisdomain>". Note that <uid> is the numeric user ID.

The `keyserv` daemon provides a service `net_get` that returns the netname of the calling user. AUTH_DH uses this service to determine the netname of the current process, but this does not hide netnames from the user of AUTH_DH:

- The client must know the netname of the server
- The server must interpret the netname of the client and decide whether the client user is authorized or not

How secure is AUTH_DH? As pointed out, the security is not the best.

- The chosen prime for DH is too short
- It uses DES (56 bits) to encrypt the verifiers

The DES weakness can be reduced by changing the conversation key frequently. This AUTH_DH implementation allows it to specify the maximum lifetime of a DES key.

Note that it is hard to attack AUTH_DH without knowing the public key. So it is best not to make it accessible for third parties.

    val domainname : unit -> string

Returns the NIS domain name. The name is determined by calling the external command `domainname`. Note: This function refuses to work for setuid or setgid programs.

    val client_auth_method : ?ttl:int ->
           ?getdeviation:(Unix.inet_addr -> float) ->
           ?key_lifetime:int ->
           ?keyserv:Rpc_key_service.connector -> string -> Rpc_client.auth_method

Creates a new authentication method using AUTH_DH. The passed string is the netname of the called server.

Pass the resulting auth_method to `Rpc_client.set_auth_methods` to configure AUTH_DH for an RPC client.

`ttl` : The "time to live" for the network packets. Effectively, this number is the maximum time deviation the server will tolerate. It defaults to 60 seconds meaning that it is acceptable if the server gets the network packet 60/2 seconds before or after the time the packet is sent by the client.
`getdeviation` : This function is called when the time has to be resynchronized. The argument is the internet address of the server, and the expected result is the number of seconds the server is ahead to the client. By default, a function is used that connects to the netdate time service of the server, and compares the time of the client and the server. If the clocks can be assumed to always be synchronous, it is safe to pass `fun _ -> 0.0` as deviation function.
`key_lifetime` : After this number of seconds the DES key (conversation key) expires. Default: 3600

    val server_auth_method : ?max_sessions:int ->
           ?max_ttl:int ->
           ?key_lifetime:int ->
           ?attack_detector:bool ->
           ?keyserv:Rpc_key_service.connector -> unit -> Rpc_server.auth_method

Pass the result of this function to `Rpc_server.set_auth_methods` to configure AUTH_DH for an RPC server.

Note that the current implementation of AUTH_DH blocks until the `keyserv` responds. For most applications, this is not a big problem, as `keyserv` lookups are seldom. Perhaps I will rewrite the code some day such that `keyserv` lookups are done in an asynchronous way. (The `Rpc_server.auth_method` interface allows it already.)

`max_sessions` : The maximum number of authenticated connections the server can manage. If more clients connect, the lifetime of the conversation keys will decrease, but the server will still be functional.
`max_ttl` : The maximum number for the ttl value. The ttl value is passed by the client, but if it is bigger than `max_ttl`, the maximum is used instead.
`key_lifetime` : After this number of seconds the conversation key expires and must be renewed.
`attack_detector` : Whether an attack detector is to be installed. It detects if there are many failed connection attempts for a certain user (more than 10 failures in 10 seconds). If this criterion matches no more logins are allowed for this user in the current 10 seconds period. The detector contains a heuristics that makes it unlikely that a TCP connection breaks when just a key must be renewed and the server is currently being attacked.
`keyserv` : The `keyserv` daemon to use. Defaults to the same default as `Rpc_key_service.create`.