rerwrite problem

[lnxa]

if($host!='www.abc.com'){
    rewrite ^/(.*)$ http://www.abc.com/$1 permanent;
    }
why has this error
: unknown directive "if($host!='www.abc.com')" in /etc/nginx/nginx.conf:110
configuration file /etc/nginx/nginx.conf test failed

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,40554,40559#msg-40559