## Generate PostgreSQL server certificate

PostgreSQL server generate **certificate private key** and a **certificate request** for the `CN = PostgreSQL server`

```bash
openssl genrsa -out postgres_server.key 4096
openssl req -new -key postgres_server.key -out postgres_server.csr
```

The CA root put the **certificate request** in a temporary location (ex. `generated` directory), generate a **signed certificate** and optionally create a **certificate full chain**.

Configuration file used by root CA:

```
[ req ]
default_bits       = 4096
prompt             = no
default_md         = sha256
distinguished_name = dn
req_extensions     = req_ext

[ dn ]
CN = PostgreSQL server

[ req_ext ]
subjectAltName = @alt_names

[ alt_names ]
DNS.1 = raxus.swgalaxy
DNS.2 = mobus.swgalaxy
DNS.3 = pgsql.swgalaxy
```

> **_NOTE:_**  `CN` in CA configuration file can be diffrent than the `CN` of CSR. The CA does not replace it unless explicitly configured to do so.

```bash
openssl x509 -req \
  -in generated/postgres_server.csr \
  -CA rootCA.pem -CAkey rootCA.key -CAserial rootCA.srl \
  -out generated/postgres_server.crt \
  -days 3650 \
  -sha256 \
  -extensions req_ext -extfile generated/postgres_server.cnf

cat generated/postgres_server.crt rootCA.pem > generated/postgres_server.fullchain.crt
```

To inspect a certificate in Linux command line:

```bash
openssl x509 -in rootCA.pem -text -noout
openssl x509 -in generated/postgres_server.crt -text -noout
```

## Generate PostgreSQL client certificate(s)

We will generate 2 certificates for:
- `CN=PostgreSQL client1`
- `CN=PostgreSQL client2`

```bash
openssl genrsa -out postgres_client1.key 4096
openssl req -new -key postgres_client1.key -out postgres_client1.csr

openssl genrsa -out postgres_client2.key 4096
openssl req -new -key postgres_client2.key -out postgres_client2.csr
```

Configuration file used by root CA will be the same for both certificates:

```
[ req ]
default_bits       = 4096
prompt             = no
default_md         = sha256
distinguished_name = dn
req_extensions     = req_ext

[ dn ]
CN = Generic Client PostgreSQL

[ req_ext ]
subjectAltName = @alt_names

[ alt_names ]
DNS.1 = anyhost.anydomain
```

```bash
openssl x509 -req \
  -in generated/postgres_client1.csr \
  -CA rootCA.pem -CAkey rootCA.key -CAserial rootCA.srl \
  -out generated/postgres_client1.crt \
  -days 3650 \
  -sha256 \
  -extensions req_ext -extfile generated/postgres_client.cnf

cat generated/postgres_client1.crt rootCA.pem > generated/postgres_client1.fullchain.crt

openssl x509 -req \
  -in generated/postgres_client2.csr \
  -CA rootCA.pem -CAkey rootCA.key -CAserial rootCA.srl \
  -out generated/postgres_client2.crt \
  -days 3650 \
  -sha256 \
  -extensions req_ext -extfile generated/postgres_client.cnf

cat generated/postgres_client2.crt rootCA.pem > generated/postgres_client2.fullchain.crt
```


## On PostgreSQL server, add the root CA certificate as trusted certificate. 

Put root CA certificate (`.crt` ou `.pem`)under `/etc/pki/ca-trust/source/anchors`, then update the system trust store.

```bash
cd /etc/pki/ca-trust/source/anchors
chmod 644 rootCA.pem
update-ca-trust extract
```

```bash
openssl verify -CAfile /etc/pki/tls/certs/ca-bundle.crt <path_to>/postgres_server.crt 
# Inspect the root CA certificate to get the exact CN
grep -R "<CN_name>" /etc/pki/ca-trust/extracted/
```

## Set up SSL on PostgreSQL server

Place server certificate, server private key and root CA certificate (optional but recommended) in the PostgreSQL data directory. 
Default paths:
- `$PGDATA/server.key`
- `$PGDATA/server.crt`

Or override with:

```
ssl_cert_file = '/path/to/server.crt'
ssl_key_file  = '/path/to/server.key'
ssl_ca_file   = '/path/to/root.crt'
```

 Set file pemisions: 

```bash
chmod 640 $PGDATA/postgres_server.crt
chmod 600 $PGDATA/postgres_server.key
chmod 640 $PGDATA/rootCA.pem
```

> root CA certificate is required **only for mTLS** mode when the server will validate the client certificate authenticity  
> Add (concatebate) all CA intermediate (if any) in `ssl_cert_file`

Enable TLS in `$PGDATA/postgresql.conf`.

```
ssl_cert_file = 'postgres_server.crt'
ssl_key_file  = 'postgres_server.key'
ssl_ca_file   = 'rootCA.pem'

ssl = on
ssl_ciphers = 'HIGH:!aNULL:!MD5'
ssl_prefer_server_ciphers = on
ssl_min_protocol_version = 'TLSv1.2'
ssl_max_protocol_version = 'TLSv1.3'
```
PostgreSQL will now listen for both encrypted and unencrypted connections on the **same port**.

## Server side SSL modes

### Request TLS mode

Config in `$PGDATA/pg_hba.conf`:

```
host    all             all             all                     md5
```

### Require TLS mode

Config in `$PGDATA/pg_hba.conf`:
```
hostssl all             all             0.0.0.0/0               md5
```

### Require TLS mode + client certificate

Config in `$PGDATA/pg_hba.conf`:
```
hostssl all             all             0.0.0.0/0               cert
```

## Client side SSL modes

| Mode | Encrypts | Validates CA | Validates Hostname | Typical Use | 
| --- | --- | --- | --- | --- | 
| `require` | Yes | No | No | Basic encryption | 
| `verify-ca` | Yes | Yes | No | Internal/IP-based | 
| `verify-full` | Yes | Yes | Yes | Production | 

Examples:

```bash
# mode: require
psql "host=raxus.swgalaxy port=5501 user=vplesnila dbname=postgres sslmode=require"

# mode: verify-ca
psql "host=raxus.swgalaxy port=5501 user=vplesnila dbname=postgres sslmode=verify-ca sslrootcert=rootCA.pem"
```

For `verify-full` mode the client need client certificate, client private key and root CA certificate on client side.  
In our example we will use previously generated certificats for:
- `CN=PostgreSQL client1`
- `CN=PostgreSQL client2`

Set file pemisions:

```bash
chmod 640 postgres_client1.crt
chmod 600 postgres_client1.key

chmod 640 postgres_client2.crt
chmod 600 postgres_client2.key

chmod 640 rootCA.pem
```

In `verify-full` mode we can get ride of client password by mapping the `CN` in the certificate to a **local PostgreSQL role** (aka **local user**).  
Create local roles (with no password) in PostgreSQL instance:
```sql
CREATE ROLE app1 LOGIN;
CREATE ROLE app2 LOGIN;
```

Add in `$PGDATA/pg_ident.conf`:

```
# MAPNAME       SYSTEM-IDENTITY          PG-USERNAME
certmap_app1    "PostgreSQL client1"     app1
certmap_app2    "PostgreSQL client2"     app2
```

Add in `$PGDATA/pg_hba.conf`:

```
hostssl  all  app1  0.0.0.0/0  cert  map=certmap_app1
hostssl  all  app2  0.0.0.0/0  cert  map=certmap_app2
```

> Restart PostgreSQL instance after modifying `$PGDATA/pg_ident.conf` and `$PGDATA/pg_hba.conf`.  

Connect in `verify-full` mode using certificate for authentification:

```bash
# mode: verify-full
psql "host=raxus.swgalaxy port=5501 user=app1 dbname=postgres sslmode=verify-full sslrootcert=rootCA.pem sslcert=postgres_client1.crt sslkey=postgres_client1.key"

psql "host=raxus.swgalaxy port=5501 user=app2 dbname=postgres sslmode=verify-full sslrootcert=rootCA.pem sslcert=postgres_client2.crt sslkey=postgres_client2.key"
```

As `SAN` **(Subject Alternative Name)** of the **server** certificate match to:
- `raxus.swgalaxy`
- `mobus.swgalaxy`
- `pgsql.swgalaxy`

it is possible to connect to all theses DNS enteries using the same **server** certificate.  
Example:

```bash
psql "host=pgsql.swgalaxy port=5501 user=app1 dbname=postgres sslmode=verify-full sslrootcert=rootCA.pem sslcert=postgres_client1.crt sslkey=postgres_client1.key"
```

> ⭐ Server certificates → client checks SAN/CN for hostname  
> ⭐ Client certificates → server checks only CN for user identity

> **IMPORTANT**: we can mix TLS authentification by certificate and password.  
PostgreSQL processes pg_hba.conf top‑to‑bottom, and the first matching rule wins.
Once a connection matches a line (based on type, database, user, address, etc.), PostgreSQL stops and applies that rule’s authentication method.

The folowing `$PGDATA/pg_hba.conf` allows authentification using certificates for PostgreSQL users app1 and app2 and authentification using password for all other users.

```
hostssl  all  app1  0.0.0.0/0  cert  map=certmap_app1
hostssl  all  app2  0.0.0.0/0  cert  map=certmap_app2
hostssl  all  all   0.0.0.0/0  md5
```

Check if connections are using TLS:

```sql
SELECT * FROM pg_stat_ssl;
```

Check for connection username:
```sql
select pid as process_id, 
       usename as username, 
       datname as database_name, 
       client_addr as client_address, 
       application_name,
       backend_start,
       state,
       state_change
from pg_stat_activity;
```