How to allow all ICMP types in EC2 Security Group using Terraform

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With the aws_security_group module, security groups (firewall rules) can be created using Terraform.

Each security group resource can define multiple ingress rules. Either by setting a single port or a port range:

resource "aws_security_group" "my_ec2_sg" {
  name = "my-ec2-sg"
  vpc_id = myvpcid

  # Allow tcp 80 from ALB
  ingress {
    from_port = 80
    to_port = 80
    protocol = "tcp"
    security_groups = [aws_security_group.my_alb_sg.id]
  }

  # Allow tcp 6081+6082 from Mgmt
  ingress {
    description = "Allow Access to Varnish from Mgmt"
    from_port = 6081
    to_port = 6082
    protocol = "tcp"
    cidr_blocks = ["172.31.100.100/32"]
  }

  egress {
    from_port = 0
    to_port = 0
    protocol = "-1"
    cidr_blocks = ["0.0.0.0/0"]
  }

}

But when I needed to enable ICMP for monitoring, I had do dig a bit deeper.  

ICMP doesn't use ports

ICMP is a different protocol than TCP or UDP, which are mostly used in data transmissions between applications. Besides being a different protocol, there's another major difference: ICMP doesn't use ports - instead different "code fields" are used to identify the ICMP type.

How can the ingress rule with port definition be "translated" to ICMP? What if I need all ICMP codes?

AI's wrong suggestion

Let's ask AI. I prompted for an ingress rule allowing ICMP pings from a specific source. And AI (Grok) spit out the following ingress rule:

  ingress {
    description = "Allow ICMP ping from 172.31.100.100/32"
    from_port   = -1
    to_port     = -1
    protocol    = "icmp"
    cidr_blocks = ["172.31.100.100/32"]
    icmp_type   = 8  # Echo Request (ping)
    icmp_code   = 0
  }

To my untrained Terraform eyes this seems to look OK. Let's do a terraform plan and see how the security group would be altered...

Terraform plan ran into an error after suggested icmp options

The plan failed!

 An argument named "icmp_type" is not expected here. 
 An argument named "icmp_code" is not expected here. 

The AI suggestion didn't work. 

Allowing all ICMP types

After digging deeper, with both AI and classical Search Engine research, the trick is to use the ingress ports as ICMP types/codes.This is also described in the aws_security_group module documentation:

from_port - (Required) Start port (or ICMP type number if protocol is icmp or icmpv6).
to_port - (Required) End range port (or ICMP code if protocol is icmp).

To allow all ICMP types from a specific source, you can use the following example:

  ingress {
    description = "Allow ICMP from Mgmt"
    from_port   = -1
    to_port     = -1
    protocol    = "icmp"
    cidr_blocks = ["172.31.100.100/32"]
  }

In the AWS Console, the ingress rule now shows "Port range All":

Terraform plan ran into an error after suggested icmp options

Pings and other ICMP codes, such as traceroute, are now working from the Mgmt machine. 


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