# Kerberos Server

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

## Prerequisites

### DNS setup

Kerberos is sensitive to hostname resolution. The following DNS records must be in place before starting the container.

**Forward record** — resolves the KDC hostname to its IP address:

```dns
krb5.example.org.                IN  A               192.168.1.10
_kerberos._tcp.example.org.      IN  SRV  1  0  88   krb5.example.org.
_kerberos._udp.example.org.      IN  SRV  1  0  88   krb5.example.org.
_kerberos-adm._tcp.example.org.  IN  SRV  1  0  749  krb5.example.org.
_kpasswd._udp.example.org.       IN  SRV  1  0  464  krb5.example.org.
```

**Reverse record** — resolves the IP back to the hostname (required by some Kerberos operations):

```dns
10.1.168.192.in-addr.arpa.  IN  PTR  krb5.example.org.
```

The container's hostname must match `KRB5_KDC_HOST` exactly — it is set via the `hostname:` field in the compose file.

**Note**: DNS-based KDC discovery (SRV records) is disabled for the server container in the generated configuration (`dns_lookup_kdc = false`), however it is required for clients to locate the KDC. The KDC hostname must be resolvable by clients via DNS or a local hosts file entry.

## 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>@<REALM>` |

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

### Admin principal naming

By default the bootstrap principal is created as `admin@REALM` — a flat, UPN-style name. This is the value of `KRB5_ADMIN_PRINCIPAL` used verbatim.

MIT Kerberos also supports the traditional `primary/instance` convention, where the `/admin` instance signals administrative intent. If you prefer that style, set the full name in the variable:

```
KRB5_ADMIN_PRINCIPAL=slawek/admin
```

This creates `slawek/admin@REALM` and grants it full kadmin rights via the ACL. Either form works — it is purely a naming preference.

> **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
```

Runs as user `krb5`; `CAP_NET_BIND_SERVICE` is granted to the daemon binaries at build time via `setcap`.


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.