sharkey

Sharkey is a service for managing certificates for use by OpenSSH.

Sharkey has a client component and a server component. The server is responsible for issuing signed host certificates, the client is responsible for installing host certificates on machines. Sharkey builds on the trust relationships of your existing X.509 PKI to manage trusted SSH certificates. Existing X.509 certificates can be minted into SSH certificates, so you don't have to maintain two separate PKI hierarchies.

Build

Check out the repository, and build client/server:

go build -o sharkey-client ./client
go build -o sharkey-server ./server

Server

The server component accepts requests and issues short lived host certificates.

Clients send their public key to the server (via TLS with mutual authentication) periodically. The server authenticates the client by checking that its certificate is valid for the requested hostname. If everything looks good, the server will take the public key in the request and issue an OpenSSH host certificate for the requested hostname.

A log of all issued certificates is stored in a database. The server can generate a known_hosts file from the issuance log if required.

Usage:

usage: sharkey-server --config=CONFIG [<flags>] <command> [<args> ...]

Certificate issuer of the ssh-ca system.

Flags:
  --help           Show context-sensitive help (also try --help-long and --help-man).
  --config=CONFIG  Path to config file for server.
  --version        Show application version.

Commands:
  help [<command>...]
    Show help.

  start
    Run the sharkey server.

  migrate [<flags>]
    Set up database/run migrations.

Configuration (example):

# SQLite database
# ---
db:
  address: /path/to/sharkey.db
  type: sqlite

# MySQL database
# ---
# db:
#   username: root
#   password: password
#   address: hostname:port
#   schema: ssh_ca
#   type: mysql
#   tls:                                       # MySQL TLS config (optional)
#     ca: /path/to/mysql-ca-bundle.pem
#     cert: /path/to/mysql-client-cert.pem     # MySQL client cert
#     key: /path/to/mysql-client-cert-key.pem  # MySQL client cert key

# Server listening address
listen_addr: "0.0.0.0:8080"

# TLS config for serving requests
# ---
tls:
  ca: /path/to/ca-bundle.pem
  cert: /path/to/server-certificate.pem
  key: /path/to/server-certificate-key.pem

# Signing key (from ssh-keygen)
signing_key: /path/to/ca-signing-key

# Lifetime/validity duration for generated host certificates
host_cert_duration: 168h

# Lifetime/validity duration for generated user certificates
user_cert_duration: 24h

# Optional suffix to strip from client hostnames when generating certificates.
# This is useful if all your machines have a common TLD/domain, and you want to
# include an alias in the generated certificate that doesn't include that suffix.
# Leave empty to disable
strip_suffix: ".example.com"

# Optional set of aliases for hosts. If a hostname matches an alias entry, the
# listed principals will be added to its certificate. This is useful if you have
# special hosts that are accessed via CNAME records.
aliases:
  "host.example.com":
    - "alias1.example.com"
    - "alias2.example.com"

# Optional set of extra entries to provide to clients when they fetch a known_hosts
# file. This is useful if you have externally-managed servers in your infrastructure
# that you want to tell clients about, of if you want to add CA entries to the
# known_hosts file.
extra_known_hosts:
  - "@cert-authority *.example.com ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDBwhA8rKPESjDy4iqTlkBqUlBU2xjwtmFUHY6cutA9TYbB5H/mjxzUpnSNw/HyFWNpysjTSQtHWWBdJdJGU/0aDgFUwbduHeDFxviGVSkOxm2AYn7XJopzITZRqmAmsYXHUBa75RQb+UgIG7EpCoi8hF4ItJV+TT777j1irkXwlMmeDiJEaA+7bPNdUdGw8zRbk0CyeotYVD0griRtkXdfgnQAu+DvBwOuW/uiZaPz/rAVjt4b9fmp6pcFKI3RsBqqn5tQVhKCPVuSwqvIQ7CTVkMClYovlH1/zGe8PG1DHbM9irP98S5j3mVD9W5v3QILpsg24RIS14M8pLarlD6t [email protected]"

# User certs are issued to users who connect through an authenticating proxy
# That user should connect with a user certificate and set the username
# in a header.
auth_proxy:
  # Hostname is validated against the incoming user certificate
  hostname: proxy.example.com
  # The HTTP header containing the username
  username_header: X-Forwarded-User

# Optional settings related to SSH
ssh:
  # List of extensions that should be set on the user certificate (default is no extensions)
  user_cert_extensions:
    - "permit-X11-forwarding"
    - "permit-agent-forwarding"
    - "permit-port-forwarding"
    - "permit-pty"
    - "permit-user-rc"

A signing key for generating host certificates can be generated with ssh-keygen.

Database

Sharkey supports both SQLite and MySQL. There is a built-in command in the server binary to manage migrations (based on goose).

To run migrations on a configured database:

# SQLite
./sharkey-server --config=[CONFIG] migrate --migrations=db/sqlite

# MySQL
./sharkey-server --config=[CONFIG] migrate --migrations=db/mysql

You can also manage migrations using the goose command-line utility. See the goose documentation for more info.

Client

The client component periodically requests a new host certificate from the server and installs it on the machine.

The client will use a TLS client certificate to make a connection to the server and authenticate itself. This assumes that there is a long-lived certificate and key installed on each machine that uses the client. We then periodically read the host key for the locally running OpenSSH (host_key), send it to the server, and retrieve a signed host certificate based on that key. The signed host certificate is then installed on the machine (signed_cert).

Usage:

usage: sharkey-client --config=CONFIG [<flags>]

Flags:
  --help           Show context-sensitive help (also try --help-long and --help-man).
  --config=CONFIG  Path to yaml config file for setup
  --version        Show application version.

Configuration (example):

# Server address
request_addr: "https://sharkey-server.example:8080"

# TLS config for making requests
# ---
tls:
  ca: /path/to/ca-bundle.pem
  cert: /path/to/client-certificate.pem
  key: /path/to/client-certificate-key.pem

# List of host keys for OpenSSH server
host_keys:
  # Here, 'key' is the public key, and 'cert' is where to install the signed cert
  - plain: "/etc/ssh/ssh_host_rsa_key.pub"
    signed: "/etc/ssh/ssh_host_rsa_key-cert.pub"
  # You can specify multiple host keys (e.g. if you have both RSA, ED25519 keys)
  - plain: "/etc/ssh/ssh_host_ed25519_key.pub"
    signed: "/etc/ssh/ssh_host_ed25519_key-cert.pub"

# Where to install the known_hosts file
known_hosts: /etc/ssh/known_hosts

# If set to true, only install authorities in known_hosts file (ignore other machine's host keys).
known_hosts_authorities_only: false

# How often to refresh/request new certificate
sleep: "24h"

# Path to sudo binary on client host
# Uses sudo to write known_hosts and signed_cert.pub if this field specified
sudo: "/usr/bin/sudo"

# Command to restart ssh daemon for the host
# If sudo is set as well, this command will be prefixed with 'sudo'
ssh_reload: ["service", "ssh", "restart"]

OpenSSH will have to be configured to read the signed host certificate (this is with the HostCertificate config option in sshd_config). If the signed host certificate is missing from disk, OpenSSH will fall back to TOFU with the default host key. Therefore, it should always be safe to configure a host certificate; even if the Sharkey client fails you can still SSH into your machine.

User Certificates

For a user to SSH into an openssh server, they can present a certificate, which should have a principal matching their username. Sharkey outsources identifying users to an SSO proxy. That proxy needs to connect to sharkey over mTLS. You can configure the DNS SAN that should appear on the server's client cert (eg, proxy.example.com) and the HTTP header it sets the username to (eg, X-Forwarded-User). See example configs.

No client helper is included with Sharkey at this time, so you have to set up a script yourself at this time to enroll the user.

Testing looks something like this: curl --cert proxy.crt --key proxy.key https://localhost:8080/enroll_user -H "X-Forwarded-User: bob" -d @~/.ssh/bob.pub

But in production use you'd expect it more like curl <auth to your proxy> https://ssoproxy.example.com/enroll_user -d @~/.ssh/bob.pub

GitHub SSH CA Support

Sharkey supports issuing user certificates that are compatible with GitHub SSH CA format by:

• Mapping a GitHub username to a SAML identity
• Including appropriate GitHub username in each certificate

GitHub supports authentication using SSH certificates for Enterprise Cloud accounts. The only requirement is that certificates include GitHub usernames, so that they can be matched to a particular user.

Sharkey already requires SSO proxy for the user certificate feature. Additionally, the GitHub integration requires that the GitHub organization is configured with SSO (i.e. non-GitHub) access.

An example config with GitHub SSH CA Support enabled can be found in test/git_server_config.yaml. A GitHub App with read/write access to Organization:members is required.

Sharkey will periodically query GitHub for a mapping of SAML identities to GitHub usernames and store it in Sharkey's DB. When issuing a certificate, Sharkey will check the DB and if a mapping exists, attaches it to the certificate as an extension.

An example cert is shown below:

        Type: [email protected] user certificate
        Public key: RSA-CERT SHA256:Eabuov2aAPLhN1FscJ6P3Lle85N6Txhj4sy4ALTkG6M
        Signing CA: ED25519 SHA256:HYgRf1dHbVtWY/e3jjfnAlwvAPPBKYxdXz8SDfhlAws (using ssh-ed25519)
        Key ID: "alice"
        Serial: 1
        Valid: from 2020-07-31T16:10:25 to 2020-08-01T16:10:25
        Principals:
                alice
        Critical Options: (none)
        Extensions:
                [email protected] UNKNOWN OPTION (len 5)
                permit-X11-forwarding
                permit-agent-forwarding
                permit-port-forwarding
                permit-pty
                permit-user-rc

Telemetry

Sharkey supports sending DogStatsD metrics. Currently only metrics regarding GitHub SSH CA are being emitted. Adding the following block to the server configuration will enable metrics:

telemetry:
  address: "127.0.0.1:8200"

Unix sockets are also supported.