additional_connection_limits

Since: Dev Builds Only

The functionality described in this section requires a dev build of KumoMTA. You can obtain a dev build by following the instructions in the Installation section.

Specifies additional connection limit constraints that cut across the per-site-per-source scoping of the connection_limit option.

The value is a map from the limit name to the desired limit.

For example, you could implement a global outbound connection limit of 100 connections like this:

kumo.on('get_egress_path_config', function(domain, source_name, site_name)
  return kumo.make_egress_path {
    additional_connection_limits = {
      ['global-connection-limit'] = 100,
    },
  }
end)

or you could set up a source-specific connection limit that is shared by all domains with a particular suffix match something like this, including both the source and your candidate domain name in the limit name:

local utils = require 'policy-extras.policy_utils'

local LIMITS = {
  ['.outlook.com'] = 100,
  ['.example.com'] = 32,
}

kumo.on('get_egress_path_config', function(domain, source_name, site_name)
  local limits = {}
  for suffix, value in pairs(LIMITS) do
    if utils.ends_with(domain, suffix) then
      limits[string.format('site-limit-for-%s-%s', suffix, source)] = value
    end
  end
  return kumo.make_egress_path {
    additional_connection_limits = limits,
  }
end)

You can mix all of the above with the built-in connection_limit:

local utils = require 'policy-extras.policy_utils'

local LIMITS = {
  ['.outlook.com'] = 100,
  ['.example.com'] = 32,
}

kumo.on('get_egress_path_config', function(domain, source_name, site_name)
  local limits = {
    -- No more than 100 connections globally
    ['global-connection-limit'] = 100,
  }
  -- Apply domain+source specific limits
  for suffix, value in pairs(LIMITS) do
    if utils.ends_with(domain, suffix) then
      limits[string.format('site-limit-for-%s-%s', suffix, source)] = value
    end
  end
  return kumo.make_egress_path {
    -- no more than 10 connections from a given source to this specific site
    connection_limit = 10,
    additional_connection_limits = limits,
  }
end)

When a connection is eligible to be established, the system will sort the overall set of connection limits, including the connection_limit limit option, from smallest to highest, then acquire a lease to connect in that order. This minimizes the chances that we'll redundantly consume an available slot from the larger allocation only to trip over one of the smaller ones.

Note

When choosing a name for your limit, you can select any name you like, but you should avoid using the prefix kumomta. as that is used by kumomta and you do not want to collide with its own limit names.