Allow TLS Email sends as a compile-time option

[alexbartlow]

To avoid setting up and configuring a sendmail server as a proxy (https://www.digitalocean.com/community/tutorials/how-to-send-email-through-an-external-smtp-service-with-sendmail-on-freebsd-10-1) We'd like the ability to simply point ossec-hids at an existing SMTP server.

Instead of re-implementing the wheel, I've added a compile-time flag to use curl to send with credentials over TLS to an external SMTP server. When compiling ossec-hids, you can use the SENDMAIL_CURL=1 env var to trigger a build that uses curl to send emails to the server you specify, instead of passing the information to a local sendmail install.

We've been using this patch ourselves, and it's working great.

I thought I'd at least offer it up to the community, though I can see the virtue of not wanting to take on this added complexity concern within the product itself. It does, however, greatly simplify the process of setting it up. Here's a snip from our relevant chef cookbook:

  #config template
  <global>
    <email_notification>yes</email_notification>
    <email_to><%= @smtp_to %></email_to>
    <smtp_server><%= @smtp_server %></smtp_server>
    <email_from><%= @smtp_from %></email_from>
    <auth_smtp>yes</auth_smtp>
    <smtp_user><%= @smtp_user %></smtp_user>
    <smtp_password><%= @smtp_password %></smtp_password>
    <secure_smtp>yes</secure_smtp>
  </global>

  #install recipe
execute "run installer" do
  command "bash /opt/ossec/ossec-hids-master/install.sh"
  environment "SENDMAIL_CURL" => "yes"
  user "root"
  group "root"
end

[nbuuck]

Should these really be hard-coded rather than relying on the system-configured name servers?

[alexbartlow]

@nbuuck thanks for pointing that out - I've removed the hard-coded DNS servers.

[sempervictus]

#1381 looks to be adding libsodium, if that PR is to be implemented, would it make sense to convert the mail functionality to a thinner internal implementation? Primarily asking because curl does not come without concerns, nor does any library, but the depth of opaque function calls relative to this code is probably a bit deeper than if the send/recv/fmt was implemented here, and crypto was the sole component being farmed out.