How to use "forward socks5" with my DNS server?

1. Example Sets

- Client: Chrome.
- System DNS: 127.0.0.1 (Adblock-enabled DNS).
- Privoxy.
- Some SOCKS5 proxy server.

2. What I want

I want privoxy to forward Socks5 to SocksProxy server _but_ first lookup
DNS to see whether the domain is blocked or not.

Chrome -[HTTP Proxy]-> Privoxy <--> DNS/SOCKS5

3. Actual result

The privoxy forward all requests to So5 proxy _without_ looking up
system-set DNS,
bypassing adblocked bad domains.

4. Current horrible alternative

For now I just use "forward socks4" to force Privoxy look up DNS before
connecting
but this is horrible because

- Remote server can distinguish between direct-Socks5-traffic and
privoxied-traffic 
by looking for "Connection: closed" header.
-> Could you please modify the request header to "Connection:
keep-alive" just to spoof this?