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Level 0: resolver Environment

Azure

Set the VNet DNS servers to the assigned Securd resolvers and deny port 53 to other destinations with an NSG rule.

Configuration and reference code for Azure

Terraform

main.tf
resource "azurerm_virtual_network" "agents" {
  name                = "agents-vnet"
  address_space       = ["10.40.0.0/16"]
  location            = var.location
  resource_group_name = var.rg
  dns_servers         = [var.securd_primary, var.securd_secondary]
}

resource "azurerm_network_security_rule" "allow_securd_dns" {
  name                        = "allow-securd-dns"
  priority                    = 100
  direction                   = "Outbound"
  access                      = "Allow"
  protocol                    = "*"
  source_port_range           = "*"
  destination_port_range      = "53"
  source_address_prefix       = "*"
  destination_address_prefixes = [var.securd_primary, var.securd_secondary]
  resource_group_name         = var.rg
  network_security_group_name = azurerm_network_security_group.agents.name
}

resource "azurerm_network_security_rule" "deny_other_dns" {
  name                        = "deny-other-dns"
  priority                    = 110
  direction                   = "Outbound"
  access                      = "Deny"
  protocol                    = "*"
  source_port_range           = "*"
  destination_port_range      = "53"
  source_address_prefix       = "*"
  destination_address_prefix  = "*"
  resource_group_name         = var.rg
  network_security_group_name = azurerm_network_security_group.agents.name
}

Evaluate Agent DNS with your own agent traffic

Deploy on a single policy in learning mode and review the recorded destinations with your team.