4 Using TLS for Erlang Distribution

This section describes how the Erlang distribution can use TLS to get extra verification and security.

The Erlang distribution can in theory use almost any connection-based protocol as bearer. However, a module that implements the protocol-specific parts of the connection setup is needed. The default distribution module is inet_tcp_dist in the Kernel application. When starting an Erlang node distributed, net_kernel uses this module to set up listen ports and connections.

In the SSL application, an extra distribution module, inet_tls_dist, can be used as an alternative. All distribution connections will use TLS and all participating Erlang nodes in a distributed system must use this distribution module.

The security level depends on the parameters provided to the TLS connection setup. Erlang node cookies are however always used, as they can be used to differentiate between two different Erlang networks.

To set up Erlang distribution over TLS:

• Step 1: Build boot scripts including the SSL application.
• Step 2: Specify the distribution module for net_kernel.
• Step 3: Specify the security options and other SSL options.
• Step 4: Set up the environment to always use TLS.

The following sections describe these steps.

4.1  Building Boot Scripts Including the SSL Application

Boot scripts are built using the systools utility in the SASL application. For more information on systools, see the SASL documentation. This is only an example of what can be done.

The simplest boot script possible includes only the Kernel and STDLIB applications. Such a script is located in the bin directory of the Erlang distribution. The source for the script is found under the Erlang installation top directory under releases/<OTP version>/start_clean.rel.

Do the following:

• Copy that script to another location (and preferably another name).

• Add the applications Crypto, Public Key, and SSL with their current version numbers after the STDLIB application.

The following shows an example .rel file with TLS added:

      {release, {"OTP  APN 181 01","R15A"}, {erts, "5.9"},
      [{kernel,"2.15"},
      {stdlib,"1.18"},
      {crypto, "2.0.3"},
      {public_key, "0.12"},
      {asn1, "4.0"},
      {ssl, "5.0"}
      ]}.

The version numbers differ in your system. Whenever one of the applications included in the script is upgraded, change the script.

Do the following:

• Build the boot script.

  Assuming the .rel file is stored in a file start_ssl.rel in the current directory, a boot script can be built as follows:

   1> systools:make_script("start_ssl",[]).    

There is now a start_ssl.boot file in the current directory.

Do the following:

• Test the boot script. To do this, start Erlang with the -boot command-line parameter specifying this boot script (with its full path, but without the .boot suffix). In UNIX it can look as follows:

$ erl -boot /home/me/ssl/start_ssl
Erlang (BEAM) emulator version 5.0
 
Eshell V5.0  (abort with ^G)
1> whereis(ssl_manager).
<0.41.0>    

The whereis function-call verifies that the SSL application is started.

As an alternative to building a bootscript, you can explicitly add the path to the SSL ebin directory on the command line. This is done with command-line option -pa. This works as the SSL application does not need to be started for the distribution to come up, as a clone of the SSL application is hooked into the Kernel application. So, as long as the SSL application code can be reached, the distribution starts. The -pa method is only recommended for testing purposes.

4.2  Specifying Distribution Module for net_kernel

The distribution module for TLS is named inet_tls_dist and is specified on the command line with option -proto_dist. The argument to -proto_dist is to be the module name without suffix _dist. So, this distribution module is specified with -proto_dist inet_tls on the command line.

Extending the command line gives the following:

$ erl -boot /home/me/ssl/start_ssl -proto_dist inet_tls    

For the distribution to be started, give the emulator a name as well:

$ erl -boot /home/me/ssl/start_ssl -proto_dist inet_tls -sname ssl_test
Erlang (BEAM) emulator version 5.0 [source]
 
Eshell V5.0  (abort with ^G)
(ssl_test@myhost)1>     

However, a node started in this way refuses to talk to other nodes, as no TLS parameters are supplied (see the next section).

4.3  Specifying TLS Options

The TLS distribution options can be written into a file that is consulted when the node is started. This file name is then specified with the command line argument -ssl_dist_optfile.

Any available TLS option can be specified in an options file, but note that options that take a fun() has to use the syntax fun Mod:Func/Arity since a function body cannot be compiled when consulting a file.

Do not tamper with the socket options list, binary, active, packet, nodelay and deliver since they are used by the distribution protocol handler itself. Other raw socket options such as packet_size may interfere severely, so beware!

For TLS to work, at least a public key and a certificate must be specified for the server side. In the following example, the PEM file "/home/me/ssl/erlserver.pem" contains both the server certificate and its private key.

Create a file named for example "/home/me/ssl/ssl_test@myhost.conf":

[{server,
  [{certfile, "/home/me/ssl/erlserver.pem"},
   {secure_renegotiate, true}]},
 {client,
  [{secure_renegotiate, true}]}].

And then start the node like this (line breaks in the command are for readability, and shall not be there when typed):

$ erl -boot /home/me/ssl/start_ssl -proto_dist inet_tls
  -ssl_dist_optfile "/home/me/ssl/ssl_test@myhost.conf"
  -sname ssl_test

The options in the {server, Opts} tuple are used when calling ssl:ssl_accept/3, and the options in the {client, Opts} tuple are used when calling ssl:connect/4.

For the client, the option {server_name_indication, atom_to_list(TargetNode)} is added when connecting. This makes it possible to use the client option {verify, verify_peer}, and the client will verify that the certificate matches the node name you are connecting to. This only works if the the server certificate is issued to the name atom_to_list(TargetNode).

For the server it is also possible to use the option {verify, verify_peer} and the server will only accept client connections with certificates that are trusted by a root certificate that the server knows. A client that presents an untrusted certificate will be rejected. This option is preferably combined with {fail_if_no_peer_cert, true} or a client will still be accepted if it does not present any certificate.

A node started in this way is fully functional, using TLS as the distribution protocol.

4.4  Specifying TLS Options (Legacy)

As in the previous section the PEM file "/home/me/ssl/erlserver.pem" contains both the server certificate and its private key.

On the erl command line you can specify options that the TLS distribution adds when creating a socket.

The simplest TLS options in the following list can be specified by adding the prefix server_ or client_ to the option name:

• certfile
• keyfile
• password
• cacertfile
• verify
• verify_fun (write as {Module, Function, InitialUserState})
• crl_check
• crl_cache (write as Erlang term)
• reuse_sessions
• secure_renegotiate
• depth
• hibernate_after
• ciphers (use old string format)

Note that verify_fun needs to be written in a different form than the corresponding TLS option, since funs are not accepted on the command line.

The server can also take the options dhfile and fail_if_no_peer_cert (also prefixed).

client_-prefixed options are used when the distribution initiates a connection to another node. server_-prefixed options are used when accepting a connection from a remote node.

Raw socket options, such as packet and size must not be specified on the command line.

The command-line argument for specifying the TLS options is named -ssl_dist_opt and is to be followed by pairs of SSL options and their values. Argument -ssl_dist_opt can be repeated any number of times.

An example command line doing the same as the example in the previous section can now look as follows (line breaks in the command are for readability, and shall not be there when typed):

$ erl -boot /home/me/ssl/start_ssl -proto_dist inet_tls
  -ssl_dist_opt server_certfile "/home/me/ssl/erlserver.pem"
  -ssl_dist_opt server_secure_renegotiate true client_secure_renegotiate true
  -sname ssl_test
Erlang (BEAM) emulator version 5.0 [source]

Eshell V5.0  (abort with ^G)
(ssl_test@myhost)1>

4.5  Setting up Environment to Always Use TLS (Legacy)

A convenient way to specify arguments to Erlang is to use environment variable ERL_FLAGS. All the flags needed to use the TLS distribution can be specified in that variable and are then interpreted as command-line arguments for all subsequent invocations of Erlang.

In a Unix (Bourne) shell, it can look as follows (line breaks are for readability, they are not to be there when typed):

$ ERL_FLAGS="-boot /home/me/ssl/start_ssl -proto_dist inet_tls
  -ssl_dist_opt server_certfile /home/me/ssl/erlserver.pem
  -ssl_dist_opt server_secure_renegotiate true client_secure_renegotiate true"
$ export ERL_FLAGS
$ erl -sname ssl_test
Erlang (BEAM) emulator version 5.0 [source]
 
Eshell V5.0  (abort with ^G)
(ssl_test@myhost)1> init:get_arguments().
[{root,["/usr/local/erlang"]},
 {progname,["erl "]},
 {sname,["ssl_test"]},
 {boot,["/home/me/ssl/start_ssl"]},
 {proto_dist,["inet_tls"]},
 {ssl_dist_opt,["server_certfile","/home/me/ssl/erlserver.pem"]},
 {ssl_dist_opt,["server_secure_renegotiate","true",
                "client_secure_renegotiate","true"]
 {home,["/home/me"]}]    

The init:get_arguments() call verifies that the correct arguments are supplied to the emulator.

4.6  Using TLS distribution over IPv6

It is possible to use TLS distribution over IPv6 instead of IPv4. To do this, pass the option -proto_dist inet6_tls instead of -proto_dist inet_tls when starting Erlang, either on the command line or in the ERL_FLAGS environment variable.

An example command line with this option would look like this:

$ erl -boot /home/me/ssl/start_ssl -proto_dist inet6_tls
  -ssl_dist_optfile "/home/me/ssl/ssl_test@myhost.conf"
  -sname ssl_test

A node started in this way will only be able to communicate with other nodes using TLS distribution over IPv6.