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Allow ping from specific subnets to AWS EC2 instances using Terraform

If you’re using Terraform to set up EC2 instances on AWS you may be a little confused about how to allow ping through the AWS VPC firewall, especially if you want to limit ping so that it only works from specific IPs or subnets.

To do this just add a Terraform ingress security group rule to the aws_security_group:

ingress {
  cidr_blocks = ["1.2.3.4/32"]
  from_port   = 8
  to_port     = 0
  protocol    = "icmp"
  description = "Allow ping from 1.2.3.4"
}

The above rule will only allow ping from the single IPv4 address “1.2.3.4”. You can use the cidr_blocks setting to allow ping from any set of IPv4 IP address and subnets that you wish. If you want to allow IPv6 addresses use the ipv6_cidr_blocks setting:

ingress {
  cidr_blocks       = ["1.2.3.4/32"]
  ipv6_cidr_blocks  = [aws_vpc.example.ipv6_cidr_block]
  from_port         = 8
  to_port           = 0
  protocol          = "icmp"
  description       = "Allow ping from 1.2.3.4 and the example.ipv6_cidr_block"
}

Right about now you should be scratching your head and asking why a port range is specified from port 8 to port 0? Isn’t that backwards? Also, this is ICMP, so why are we specifying port ranges at all?

Well, for ICMP security group rules Terraform uses the from_port field to define the ICMP message type, and “ping” is an ICMP “echo request” type 8 message.

So why is to_port = 0? Since ICMP is a network-layer protocol there is no TCP or UDP port number associated with ICMP packets as these numbers are associated with the transport layer, which is above the network layer. So you might think it’s set to 0 because it’s a “don’t care” setting, but that is not the case.

It’s actually set to 0 because Terraform (and AWS) use the to_port field to define the ICMP code of the ICMP packet being allowed through the firewall, and “ping” is defined as a type 8, code 0 ICMP message.

I have no idea why Terraform chose to obscure the usage this way, but I suspect it’s because the AWS API reuses the from_port field for storing the ICMP message type, and reuses the to_port for storing the ICMP code, and Terraform just copied their bad design. A more user-friendly implementation of Terraform would have created an icmp_message_type and icmp_message_code fields (or aliases) that are mapped to the AWS from_port and to_port fields to make it obvious what you’re setting and why it works.

Hope you find this useful.

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Generate a crypted password for Ansible

The Ansible user: command allows you to add a user to a Linux system with a password. The password must be passed to Ansible in a hashed password format using one of the hash formats supported by /etc/shadow.

Some Ansible docs suggest storing your passwords in plain text and using the Ansible SHA512 filter to hash the plaintext passwords before passing them to the user module. This is a bad practice for a number of reasons.

Storing your passwords in plain text is a bad idea

  • Storing your passwords in plain text is a bad idea, since anyone who can read your Ansible playbook now knows your password.
  • The play is not idempotent, since the SHA512 filter will re-hash the password every time you run the play, resetting the password each time the play is run.
  • Attempting to make the play idempotent, by using update_password: on_create, means that you can no longer update the password using Ansible. This might be OK if you’re just updating one machine. It’s a giant pain in the ass if you need to update the password on many machines.

A better way is to hash the password once using openssl and store the hashed version of the password in your Ansible playbooks:

- name: Set the user's password
  user:
    name: earl
    password: "$6$wLZ77bHhLVJsHaMz$WqJhNW2VefjhnupK0FBj5LDPaONaAMRoiaWle4rU5DkXz7hxhl3Gxcwshuy.KQWRFt6YPWXNbdKq9B/Rk9q7A."

To generate the hashed password use the openssl passwd command on any Linux host:

openssl passwd -6 -stdin

This opens an interactive shell to openssl. Just enter the password that you want to use, hit enter, and openssl will respond with the hashed version. Copy and paste the hashed version of the password into Ansible, and the next time you run Ansible on a host the user’s password will be updated.

Type Ctrl-D to exit the interactive openssl shell.

Since you used the interactive shell the plaintext password that you entered is not saved into the Linux host’s history file and is not visible to anyone running ps.

The crypted password is encrypted with a SHA512 one-way hash and a random 16 character salt, so you can check the playbook into a Git repository without revealing your password.

Hope you find this useful.

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Automatically decrypt multiple LUKS-encrypted volumes

I’ve written in the past on Adding an external encrypted drive with LVM to Ubuntu Linux and Adding a LUKS-encrypted iSCSI volume to Synology DS414 NAS but I neglected to mention how to automatically decrypt additional volumes.

When installing a fresh copy of Ubuntu one of the options is to install with a LUKS-encrypted Logical Volume Manager Volume Group (LVM VG). This puts your root volume on the encrypted LVM VG. When you power up your machine Ubuntu prompts you to enter the decryption passphrase in order to decrypt the VG and start your computer. Without the passphrase the contents of your hard drive are unreadable.

If you add encrypted external drives and/or additional VGs you will end up with multiple encrypted volumes. Ubuntu will prompt you for the passphrase of each additional encrypted volume when you boot up the machine.

If you don’t want to enter multiple, different passphrases each time you boot, you can store the passphrases for additional volumes on the encrypted root filesystem of your first drive using the /etc/crypttab file. You’ll just be prompted for one passphrase, of the first VG, and that decrypts the passphrases needed to decrypt the additional volumes.

Here’s how it works.

The /etc/crypttab file contains 4 fields per line: the name of the encrypted volume, a UUID identifying the storage device, the name of a file with the decryption passphrase, and encryption options.

nvme0n1p5   UUID=405d8c73-1cf9-4b2c-9b8e-c76b90d27c67 none                        luks,discard
datastorage UUID=f2d73ac8-1ef1-4735-9dd4-9e778fc9e781 /root/.luks-datastorage     luks,discard
external1   UUID=0140476b-dd0b-4aab-b7d4-2f5fa14d1a0c /root/.luks-backupexternal1 luks
external2   UUID=610a67d4-c4f6-4b73-a824-a437971e8d24 /root/.luks-backupexternal2 luks
iscsi       UUID=b106b749-f4ab-44be-8962-6ff867dc074e /root/.luks-backupiscsi     luks

The first volume, nvme0n1p5, is the encrypted boot volume. It contains the root filesystem and the /root home directory. The third field is “none” which means that Ubuntu will prompt you for a decryption passphrase in order to unlock and decrypt the drive.

The remaining volumes have files defined that contain the decryption passphrase for each volume. Those files are hidden files in the /root home directory. Once the nvme0n1p5 volume is decrypted and mounted, the remaining volumes are automatically decrypted using the passphrases stored in the hidden files.

The end result is that all of your drives are encrypted, but you only have to enter one passphrase to unlock all of your drives.

Hope you find this useful.

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Too many authentication failures

I was working with a new Linux distro and after creating a brand-new VM with a single login I attempted to ssh into the VM only to be greeted with:

Received disconnect from 10.0.0.180 port 22:2: Too many authentication failures
Disconnected from 10.0.0.180 port 22

It was a new VM, and I hadn’t loaded an ssh key (there was no option to do so in the install). I’d set up a user and password, so I expected to get a password prompt. I didn’t get to a password prompt, just an immediate disconnect.

I used ssh -vvv to connect and found that my ssh client was attempting to use my ssh keys, as ssh is supposed to, and on the third key the VM spat back the error:

Received disconnect from 10.0.0.180 port 22:2: Too many authentication failures
Disconnected from 10.0.0.180 port 22

Well, I wanted to connect with a password anyhow, so I tried:

ssh -o PubkeyAuthentication=no username@10.0.0.180

I was greeted with a password: prompt.

I checked the /etc/ssh/sshd_config and found that someone who’d built the install image had changed the default setting for MaxAuthTries from 6 to 2.

The MaxAuthTries setting tells the ssh daemon how many different authentication attempts a user can try before it disconnects. Each ssh key loaded into ssh-agent counts as one authentication attempt. The default is 6 because many users (like me) have multiple ssh keys loaded into ssh-agent so that we can automatically log into different hosts that use different ssh keys. Trying more than one ssh key isn’t the same as thumb-fingering a password — ssh is designed to allow for multiple key attempts. After the ssh connection attempts all of your ssh keys and you haven’t run out of attempts and passwords are enabled you’ll eventually get a password prompt.

Setting MaxAuthTries back to the more reasonable default of 6 and reloading the sshd daemon fixed the issue. Apparently whoever tested the setup only has one ssh key and wasn’t aware of what changing the MaxAuthTries setting does when people with more than one key attempt to log in.

If you’re concerned about ssh security sshd_config allows you to control what versions of the ssh protocol are supported, which ciphers you trust (or don’t trust), and to tune other settings that lock down what you will or won’t allow ssh to do in your environment. It may be that for some applications in some environments setting MaxAuthTries 2 makes sense, but using it for an out of the box installation just breaks ssh for no good reason.

Hope you find this useful.

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How to get the IP address of a KVM/virsh VM

Since virsh domifaddr doesn’t work to get the IP addresses of VMs on a bridged network, I wrote a get-vm-ip script (which you can download from Github) which uses this to get the IP of a running VM:

HOSTNAME=[your vm name]
MAC=$(virsh domiflist $HOSTNAME | awk '{ print $5 }' | tail -2 | head -1)
arp -a | grep $MAC | awk '{ print $2 }' | sed 's/[()]//g'

The virsh command gets the MAC address, the last line finds the IP address using arp.

Hope you find this useful.

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