Lightweight SSH key management on AWS EC2
Keymaker is the missing link between SSH and IAM accounts on Amazon AWS. It’s a stateless synchronization engine that securely manages the process of SSH public key sharing and verification, user and group synchronization, and home directory sharing (via optional EFS integration). You, the AWS account administrator, define or import user and group identities in IAM, and instances in your account dynamically retrieve and use those identities to authenticate your users. Keymaker is the modern, minimalistic alternative to LDAP or Active Directory authentication.
Run pip install keymaker.
On instances that accept SSH logins:
- Run keymaker install.
- Ensure processes launched by sshd have the IAM permissions iam:GetSSHPublicKey, iam:ListSSHPublicKeys, iam:GetUser, iam:ListGroups, iam:GetGroup, iam:ListGroupsForUser, iam:GetRole, and sts:GetCallerIdentity. The easiest way to do this is by running keymaker configure --instance-iam-role INSTANCE_ROLE as a privileged IAM user, which will create and attach a Keymaker IAM policy to the role INSTANCE_ROLE (which you should then assign, via an IAM Instance Profile, to any instances you launch). You can also manually configure these permissions, or attach the IAMReadOnlyAccess managed policy.
Keymaker requires OpenSSH v6.2+, provided by Ubuntu 14.04+ and RHEL7+.
Run keymaker with no arguments to get usage information. In client mode (running on a computer that you will connect from), you can run keymaker <subcommand>, where subcommand is:
upload_key Upload public SSH key for a user. Run this command for each user who will be accessing EC2 hosts. list_keys Get public SSH keys for a given or current IAM/SSH user. disable_key Disable a given public SSH key for a given or current IAM/SSH user. enable_key Enable a given public SSH key for a given or current IAM/SSH user. delete_key Delete a given public SSH key for a given or current IAM/SSH user. configure Perform administrative configuration tasks on the current AWS account.
Use keymaker <subcommand> --help to get a full description and list of options for each command.
Principle of operation
Amazon Web Services IAM user accounts provide the ability to add SSH public keys to their metadata (up to 5 keys can be added; individual keys can be disabled). Keymaker uses this metadata to authenticate SSH logins. Keymaker provides an integrated way for a user to upload their public SSH key to their IAM account with keymaker upload_key.
Run keymaker install on instances that you want your users to connect to. This installs three components:
- An AuthorizedKeysCommand sshd configuration directive, which acts as a login event hook and dynamically retrieves public SSH keys from IAM for the user logging in, using the default boto3 credentials (which default to the instance’s IAM role credentials).
- A pam_exec PAM configuration directive, which causes sshd to call keymaker-create-account-for-iam-user early in the login process. This script detects if a Linux user account does not exist for the authenticating principal but an authorized IAM account exists with the same name, and creates the account on demand. The UID of the account is computed from a hash of the user’s SSH key, making it stable across instances that run Keymaker.
- A cron job that runs on your instance once an hour and synchronizes IAM group membership information. Only IAM groups whose names start with a configurable prefix (by default, keymaker_*) are synchronized as Linux groups.
As a result, users who connect to your instances over SSH are given access based on information centralized in your AWS account. Users must have an active IAM account with active matching SSH public keys in order for authentication to succeed. Users’ UIDs and group memberships are also synchronized across your instances, so any UID-based checks or group-based privileges remain current as well.
Some AWS security models put IAM users in one AWS account, and resources (EC2 instances, S3 buckets, etc.) in a family of other federated AWS accounts (for example, a dev account and a prod account). Users then assume roles in those federated accounts, subject to their permissions, with sts:AssumeRole. When users connect via SSH to instances running in federated accounts, Keymaker can be instructed to look up the user identity and SSH public key in the other AWS account (called the “ID resolver” account).
Keymaker expects to find this configuration information by introspecting the instance’s own IAM role description. The description is expected to contain a list of space-separated config tokens, for example, keymaker_id_resolver_account=123456789012 keymaker_id_resolver_iam_role=id_resolver. For sts:AssumeRole to work, the role id_resolver in account 123456789012 is expected to have a trust policy allowing the instance’s IAM role to perform sts:AssumeRole on id_resolver.
Run the following command in the ID resolver account (that contains the IAM users) to apply this configuration automatically: keymaker configure --instance-iam-role arn:aws:iam::987654321098:role/INSTANCE_ROLE --cross-account-profile AWS_CLI_PROFILE_NAME. Here, 987654321098 is the account ID of the federated account where EC2 instances will run, and AWS_CLI_PROFILE_NAME is the name of the AWS CLI role profile that you have set up to access the federated account.
Requiring IAM group membership
Group membership is asserted if the instance’s IAM role description contains the config token keymaker_require_iam_group=prod_ssh_users. The user logging in is then required to be a member of the prod_ssh_users IAM group. Apply this configuration automatically by running keymaker configure --require-iam-group IAM_GROUP_NAME.
Integrating IAM user identities with Unix user identities has implications for your security threat model. With Keymaker, a principal with the ability to set SSH public keys on an IAM user account can impersonate that user when logging in to an EC2 instance. As an example, this can expand the scope of a compromised AWS secret key. You can mitigate this threat with an IAM policy restricting access to the UploadSSHPublicKey method.
Does running Keymaker require root access?
The answer depends on what you mean by “running Keymaker”.
- To install keymaker into the sshd config and crontab (keymaker install), you need access to those daemons’ config files, which normally means you need root access.
- The keymaker authorization hook (runs when sshd logs you in) does not need root access.
- None of the client-side commands (listed under “Usage” above) need root access.
Email email@example.com for details on the EFS integration.
Licensed under the terms of the Apache License, Version 2.0.
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