# SSL Preread
Enables extracting information from the
[ClientHello](https://datatracker.ietf.org/doc/html/rfc5246#section-7.4.1.2)
message without terminating TLS,
such as the server name requested via
[SNI](https://datatracker.ietf.org/doc/html/rfc6066#section-3)
or protocols advertised in
[ALPN](https://datatracker.ietf.org/doc/html/rfc7301).
When [building from the source code](https://en.angie.software//angie/docs/installation/sourcebuild.md#sourcebuild),
this module isn't built by default;
it should be enabled with the
`‑‑with‑stream_ssl_preread_module`
[build option](https://en.angie.software//angie/docs/installation/sourcebuild.md#configure).
In packages and images from [our repos](https://en.angie.software//angie/docs/installation/index.md#install-packages),
the module is included in the build.
## Configuration Example
### Selecting an upstream by server name
```nginx
map $ssl_preread_server_name $name {
backend.example.com backend;
default backend2;
}
upstream backend {
server 192.168.0.1:12345;
server 192.168.0.2:12345;
}
upstream backend2 {
server 192.168.0.3:12345;
server 192.168.0.4:12345;
}
server {
listen 12346;
proxy_pass $name;
ssl_preread on;
}
```
### Selecting a server by protocol
```nginx
map $ssl_preread_alpn_protocols $proxy {
~\bh2\b 127.0.0.1:8001;
~\bhttp/1.1\b 127.0.0.1:8002;
~\bxmpp-client\b 127.0.0.1:8003;
}
server {
listen 9000;
proxy_pass $proxy;
ssl_preread on;
}
```
### Selecting a server by SSL protocol version
```nginx
map $ssl_preread_protocol $upstream {
"" ssh.example.com:22;
"TLSv1.2" new.example.com:443;
default tls.example.com:443;
}
# ssh and https at the same port
server {
listen 192.168.0.1:443;
proxy_pass $upstream;
ssl_preread on;
}
```
## Directives
### ssl_preread
| [Syntax](https://en.angie.software//angie/docs/configuration/configfile.md#configfile) | `ssl_preread` `on` | `off`; |
|------------------------------------------------------------------------------------------|-------------------------------|
| Default | `ssl_preread off;` |
| [Context](https://en.angie.software//angie/docs/configuration/configfile.md#configfile) | stream, server |
Enables extracting information from the ClientHello message at the
[preread](https://en.angie.software//angie/docs/configuration/processing.md#stream-sessions) phase.
## Built-in Variables
### `$ssl_preread_protocol`
Highest SSL protocol version supported by the client.
### `$ssl_preread_server_name`
Server name requested via SNI.
### `$ssl_preread_alpn_protocols`
List of protocols advertised by the client through ALPN.
The values are comma separated.