# Kerberos Server

MIT Kerberos V KDC + admin server container running on Ubuntu 26.04.

## Prerequisites

Kerberos is sensitive to hostname resolution. Before running this container:

- `KRB5_KDC_HOST` must resolve to the container from every client machine (forward DNS or `/etc/hosts`).
- The container's own hostname should match `KRB5_KDC_HOST` so that service tickets are issued for the correct principal. Set it with `--hostname` / the container runtime's hostname option.
- Reverse DNS (PTR records) for the KDC host is strongly recommended — some Kerberos operations fail without it.

DNS lookup of KDC addresses is disabled in the generated `krb5.conf` (`dns_lookup_kdc = false`); the value of `KRB5_KDC_HOST` is used directly instead.

## Environment variables

| Variable | Default | Description |
|---|---|---|
| `KRB5_REALM` | `EXAMPLE.ORG` | Kerberos realm (uppercase) |
| `KRB5_DOMAIN` | `example.org` | DNS domain mapped to the realm |
| `KRB5_KDC_HOST` | *(required)* | FQDN of this KDC, used by clients and in service ticket names |
| `KRB5_MASTER_PASSWORD` | *(required on first start)* | Database master key — set once, cannot be changed without destroying the database |
| `KRB5_ADMIN_PRINCIPAL` | `admin` | Name of the bootstrap admin principal |
| `KRB5_ADMIN_PASSWORD` | *(required on first start)* | Password for `<admin>/admin@<REALM>` |

Copy `env.example` to `~/app-data/kerberos/kerberos.env` and fill in real values before first run.

> **Important:** `KRB5_MASTER_PASSWORD` and `KRB5_ADMIN_PASSWORD` are only required on first start. Once the realm is initialised (the database file exists in the volume), these variables are not read and can be removed from the env file for enhanced security. The master password cannot be changed without wiping the `kerberos_data` volume and reinitialising the realm, which invalidates all issued tickets and keytabs.

## Build

```bash
./scripts/build.sh
```

## Run

```bash
./scripts/run-container.sh
```

The `kerberos_data` volume (`/var/lib/krb5kdc`) holds the realm database, configuration, and keytab. All files are written once on first start. On subsequent starts the container requires no environment variables — the persisted configuration is used as-is. Sensitive variables (`KRB5_MASTER_PASSWORD`, `KRB5_ADMIN_PASSWORD`) can be removed from the env file after the realm is initialised.

## Ports

| Port | Protocol | Service |
|---|---|---|
| 88 | TCP/UDP | KDC (ticket granting) |
| 464 | TCP/UDP | kpasswd (password changes) |
| 749 | TCP | kadmin (remote administration) |

## Client configuration

Pre-authentication is required (`+preauth` is set by default), so clients must supply a password or keytab — anonymous TGT requests are rejected.

The KDC hostname (`KRB5_KDC_HOST`) must be resolvable from every client machine via DNS or a local hosts file entry.

### Linux

Install the MIT Kerberos client tools:

```bash
sudo apt install krb5-user        # Debian / Ubuntu
sudo dnf install krb5-workstation # Fedora / RHEL
```

Create `/etc/krb5.conf`:

```ini
[libdefaults]
    default_realm = EXAMPLE.ORG

[realms]
    EXAMPLE.ORG = {
        kdc = kerberos.example.org
        admin_server = kerberos.example.org
    }

[domain_realm]
    .example.org = EXAMPLE.ORG
    example.org = EXAMPLE.ORG
```

```bash
kinit user@EXAMPLE.ORG   # obtain a ticket
klist                     # verify
kdestroy                  # release
```

### macOS

Kerberos (Heimdal) is built into macOS — no installation required. It is interoperable with MIT KDCs.

Create `/etc/krb5.conf` with the same content as the Linux example above.

```bash
kinit user@EXAMPLE.ORG
klist
kdestroy
```

Acquired tickets are also visible in **Ticket Viewer** (`/System/Library/CoreServices/Ticket Viewer.app`).

For SSH with GSSAPI, add to `~/.ssh/config`:

```
Host *.example.org
    GSSAPIAuthentication yes
    GSSAPIDelegateCredentials yes
```

### Windows (standalone, not domain-joined)

Install **MIT Kerberos for Windows** (KfW) from [web.mit.edu/kerberos/dist](https://web.mit.edu/kerberos/dist/). The built-in Windows Kerberos provider (SSPI) is designed for Active Directory domain membership and is not suitable for standalone use against a custom KDC.

Create the configuration file at `%ProgramData%\MIT\Kerberos5\krb5.ini` (system-wide) or set the `KRB5_CONFIG` environment variable to point to a file of your choice:

```ini
[libdefaults]
    default_realm = EXAMPLE.ORG

[realms]
    EXAMPLE.ORG = {
        kdc = kerberos.example.org
        admin_server = kerberos.example.org
    }

[domain_realm]
    .example.org = EXAMPLE.ORG
    example.org = EXAMPLE.ORG
```

Use the KfW **Network Identity Manager** GUI or the bundled command-line tools from a Command Prompt:

```cmd
kinit user@EXAMPLE.ORG
klist
kdestroy
```

## Managing principals

Exec into the running container, then use `kadmin.local` (no password required):

```bash
container exec -it kerberos bash
```

```bash
# List all principals
kadmin.local -q "listprincs"

# Add a user principal
kadmin.local -q "addprinc username@REALM"

# Add a service principal and extract a keytab
kadmin.local -q "addprinc -randkey ldap/ldap.example.org@REALM"
kadmin.local -q "ktadd -k /tmp/ldap.keytab ldap/ldap.example.org@REALM"
```

## OpenLDAP SASL/GSSAPI integration

1. Create the LDAP service principal and extract a keytab:
   ```bash
   kadmin.local -q "addprinc -randkey ldap/ldap.example.org@REALM"
   kadmin.local -q "ktadd -k /tmp/ldap.keytab ldap/ldap.example.org@REALM"
   ```
2. Copy the keytab into the OpenLDAP container at `/etc/ldap/ldap.keytab`.
3. Set `KRB5_KTNAME=/etc/ldap/ldap.keytab` in the OpenLDAP container environment.
4. Install `libsasl2-modules-gssapi-mit` in the OpenLDAP image and enable the `GSSAPI` SASL mechanism.