ProxyPass a WebSocket connection to a UNIX socket

Any redirect to localhost doesn't make sense from a remote system (e.g. client's Web browser). So the rewrite flags permanent (301) or redirect (302) are not usable in your case.

Please try following setup using a transparent rewrite rule:

location  /foo {
  rewrite /foo/(.*) /$1  break;
  proxy_pass         http://localhost:3200;
  proxy_redirect     off;
  proxy_set_header   Host $host;
}

Use curl -i to test your rewrites. A very subtle change to the rule can cause nginx to perform a redirect.

Use Rewrite conditions to match for this special case:

RewriteEngine On
RewriteCond %{REQUEST_URI}  ^/socket.io            [NC]
RewriteCond %{QUERY_STRING} transport=websocket    [NC]
RewriteRule /(.*)           ws://localhost:8082/$1 [P,L]

ProxyPass        /socket.io http://localhost:8082/socket.io
ProxyPassReverse /socket.io http://localhost:8082/socket.io

NOTE As Mark W noted below. These must be entered at vhost level and not at server or .htaccess level.

You can also reference a balancer:

<Proxy balancer://http-localhost/>
    BalancerMember http://localhost:8082 keepalive=On smax=1 connectiontimeout=10 retry=600 timeout=900 ttl=900
    BalancerMember http://localhost:8083 keepalive=On smax=1 connectiontimeout=10 retry=600 timeout=900 ttl=900
    ProxySet lbmethod=bytraffic
</Proxy>

<Proxy balancer://ws-localhost/>
    BalancerMember ws://localhost:8082 keepalive=On smax=1 connectiontimeout=10 retry=600 timeout=900 ttl=900
    BalancerMember ws://localhost:8083 keepalive=On smax=1 connectiontimeout=10 retry=600 timeout=900 ttl=900
    ProxySet lbmethod=bytraffic
</Proxy>

RewriteEngine On
RewriteCond %{REQUEST_URI}  ^/socket.io                [NC]
RewriteCond %{QUERY_STRING} transport=websocket        [NC]
RewriteRule /(.*)           balancer://ws-localhost/$1 [P,L]

ProxyPass        /socket.io balancer://http-localhost/socket.io
ProxyPassReverse /socket.io balancer://http-localhost/socket.io