[erlang-questions] Supporting a port number in spawn/4

Amit K klg.amit@REDACTED
Tue Oct 22 14:10:50 CEST 2019

Hi Lukas,

It's a great question!
And actually, choosing between disabling epmd completely and protecting
communication via TLS, will have somewhat different security benefits.

1. *Protecting epmd communication via TLS* - This only protects us from an
attacker that wants to perform epmd impersonation, because TLS here is
authenticating only the server node to the client node.
Then perhaps if we have two Erlang nodes on the same machine, let's say
Node1 and Node2, an attacker can make the client node connect to Node2
instead of Node1 by returning the wrong port by a simple manipulation of
the plain epmd traffic, and in that way expose sensitive data to the client
node, which exists in Node2 but not in Node1. Maybe it's a bit far fetched
but from my limited knowledge it looks like a possible attack scenario.
Another thing that the encryption is good for here is probably in
preventing a covert channel.

2. *Disabling epmd completely* - This approach eliminates a lot more attack
vectors actually.
Just as one simple example, assume for a moment that we have a buffer
overflow bug in epmd, then using this approach you will not be affected by
it. By completely disabling an unnecessary service (if it's really
unnecessary of course, in some situations I assume epmd is very much
required!) the attack surface reduces much more so it's difficult to list
in detail everything we'll be protecting against.
Nevertheless, we all know that it's a good security practice to disable
unrequired services whenever you can :) and especially ones that listen to
requests from the outside world.


On Tue, Oct 22, 2019 at 2:49 PM Lukas Larsson <lukas@REDACTED> wrote:

> On Tue, Oct 22, 2019 at 1:02 PM Amit K <klg.amit@REDACTED> wrote:
>> Hi Greg,
>> Thank you for your feedback!
>> In general, I think it makes sense to have a solution for this that's
>> built in to OTP and not have to use an external proxy method.
>> Going more along the route of what you suggest, a sensible alternative
>> would be for epmd.exe to be able to accept TLS settings like the Erlang
>> node (erl.exe) does, and then communication with it will also be protected
>> by TLS. a quick look at the code makes me  guess that the reason that was
>> not done originally is that epmd is a small & separate utility written in C
>> and therefore it doesn't have the "crypto" framework like the other OTP
>> parts do.
> I'm kind of curious about what type of attacks you are trying to protect
> against? You mention "man in the middle", however, I cannot see how someone
> could exploit that to do anything but disrupt the distributed erlang
> connections. Though that could only be a lack of imagination on my part,
> and when it comes to security we must be extra careful.
> Lukas
>> Regards,
>> Amit
>> On Tue, Oct 22, 2019 at 10:54 AM Grzegorz Junka <list1@REDACTED> wrote:
>>> On 21/10/2019 21:25, Amit K wrote:
>>> Hi all,
>>> I am very new to Erlang, am considering to use it in a project and I
>>> have some security concerns.
>>> I can see it's quite easy to configure TLS for the node-to-node
>>> communication, but making the name-to-port resolution service (epmd) secure
>>> seem a bit too complex to me, such as the one suggested here:
>>> https://www.erlang-solutions.com/blog/erlang-and-elixir-distribution-without-epmd.html
>>> So I was thinking, seeing that there are already options to:
>>> 1. Start a distributed node without epmd (-start_epmd false)
>>> 2. Limit a node's port numbers to a specific range (via inet_dist_listen_min
>>> & inet_dist_listen_max).
>>> Wouldn't it be nice if we could also specify a predefined port to
>>> spawn/4, to complete that picture? That is allow spawn to look like:
>>> spawn("Name@REDACTED:Port", Mod, Func, ArgList).
>>> Then when spawn sees that a port was provided, it can completely skip
>>> the "epmd resolution" part and proceed with connecting to the target node
>>> via the provided port.
>>> Note: I realize that the "Name" becomes slightly redundant when the Port
>>> is explicit. However this can still be useful - it would be good if the
>>> implementation will also verify that the port belongs to the provided name
>>> at the receiving side, so that a node will not accidentally process a
>>> message that wasn't meant for it.
>>> Again, I'm a complete newbie to Erlang in general, so I may be missing
>>> something essential here :) But I would love to know what that is, if
>>> that's the case, or hear your thoughts in general otherwise :)
>>> Hi Amit,
>>> There is also another option, run any communication between nodes via IP
>>> tunnels <https://en.wikipedia.org/wiki/IP_tunnel>. There are some tools
>>> to automate that
>>> <https://www.virtualthoughts.co.uk/2019/07/15/application-security-with-mutual-tls-mtls-via-istio/>.
>>> They are mostly used between docker containers or pods but it's just a
>>> detail, equally well they can support a microarchitecture build on Erlang
>>> nodes.
>>> Regards
>>> Greg
>>> _______________________________________________
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