基本想法和代码来自论坛这个帖子， http://cnodejs.org/topic/4f16442ccae1f4aa27001101

做了一些修改，放到git上去了。支持HTTPS访问外部网站。 https://github.com/xuduo35/simpleproxy.git

var net = require('net');
var serverip = "127.0.0.1";
var local_port = 8894;
var localflag = 0;
if (process.argv.length >= 3) {
serverip = process.argv[2];
local_port = 8893;
localflag = 1;
}
//在本地创建一个server监听本地local_port端口
net.createServer(function (client) {
//首先监听浏览器的数据发送事件，直到收到的数据包含完整的http请求头
var buffer = new Buffer(0);
client.on('data', function (data) {
    buffer = buffer_add(buffer, data);
    
    if (buffer_find_body(buffer) == -1) return;
    
    var req = parse_request(buffer);
    
    if (req === false) return;
    
    client.removeAllListeners('data');
    relay_connection(req);
});

//从http请求头部取得请求信息后，继续监听浏览器发送数据，同时连接目标服务器，并把目标服务器的数据传给浏览器
function relay_connection(req) {
    console.log(req.method + ' ' + req.host + ':' + req.port);
    
    //如果请求不是CONNECT方法（GET, POST），那么替换掉头部的一些东西
    if (req.method != 'CONNECT') {
        //先从buffer中取出头部
        var _body_pos = buffer_find_body(buffer);
        
        if (_body_pos < 0) _body_pos = buffer.length;
        
        var header = buffer.slice(0, _body_pos).toString('utf8');
        
        //替换connection头
        header = header.replace(/(proxy-)?connection\:.+\r\n/ig, '')
            .replace(/Keep-Alive\:.+\r\n/i, '')
            .replace("\r\n", '\r\nConnection: close\r\n');
        
        //替换网址格式(去掉域名部分)
        if (req.httpVersion == '1.1') {
            var url = req.path.replace(/http\:\/\/[^\/]+/, '');
            if (url.path != url) header = header.replace(req.path, url);
        }
        
        buffer = buffer_add(new Buffer(header, 'utf8'), buffer.slice(_body_pos));
    }

    if (localflag) {
        // encrypt in local, decrypt for proxy in buffer_add
        for (var i = 0; i < buffer.length; i++) {
            buffer[i] += 1;
        }
    }
    
    client.pause();
    
    //交换服务器与浏览器的数据
    client.on("data", function (data) {
        if (!server.closeflag) {
            server.write(data);
        }
    });
    
    //建立到目标服务器的连接
    var server = localflag ? net.createConnection(8894, serverip) : net.createConnection(req.port, req.host);

    server.pause();
    
    server.on("data", function (data) {
        if (!client.closeflag) {
            // encrypt for local, decrypt for proxy
            for (var i = 0; i < data.length; i++) {
                data[i] += localflag ? 1 : -1;
            }
                
            client.write(data);
        }
    });
   
    client.on("end", function () {
        client.closeflag = 1
    });
    
    server.on("end", function () {
        server.closeflag = 1
    });
    
    server.on("connect", function (socket) {
        client.resume();
        server.resume();
        
        if (req.method == 'CONNECT') {
            if (localflag) {
                server.write(buffer);
                client.write(new Buffer("HTTP/1.1 200 Connection established\r\nConnection: close\r\n\r\n"));
            }
        } else {
            server.write(buffer);
        }
    });
}
}).listen(local_port);
console.log('Proxy server running at localhost:’+local_port);
//处理各种错误
process.on('uncaughtException’, function (err) {
console.log(“\nError!!!”);
console.log(err);
});
/*
从请求头部取得请求详细信息
如果是 CONNECT 方法，那么会返回 { method,host,port,httpVersion}
如果是 GET/POST 方法，那么返回 { metod,host,port,path,httpVersion}
*/
function parse_request(buffer) {
var s = buffer.toString(‘utf8’);
var method = s.split('\n')[0].match(/^([A-Z]+)\s/)[1];

if (method == 'CONNECT') {
    var arr = s.match(/^([A-Z]+)\s([^\:\s]+)\:(\d+)\sHTTP\/(\d.\d)/);
    
    if (arr && arr[1] && arr[2] && arr[3] && arr[4])
        return {
            method: arr[1],
            host: arr[2],
            port: arr[3],
            httpVersion: arr[4]
        };
} else {
    var arr = s.match(/^([A-Z]+)\s([^\s]+)\sHTTP\/(\d.\d)/);
    
    if (arr && arr[1] && arr[2] && arr[3]) {
        var host = s.match(/Host\:\s+([^\n\s\r]+)/)[1];
        
        if (host) {
            var _p = host.split(':', 2);
            return {
                method: arr[1],
                host: _p[0],
                port: _p[1] ? _p[1] : 80,
                path: arr[2],
                httpVersion: arr[3]
            };
        }
    }
}

return false;
}
/*
两个buffer对象加起来
*/
function buffer_add(buf1, buf2) {
if (!localflag) {
// decrypt
for (var i = 0; i < buf2.length; i++) {
buf2[i] -= 1;
}
}
var re = new Buffer(buf1.length + buf2.length);

buf1.copy(re);
buf2.copy(re, buf1.length);

return re;
}
/*
从缓存中找到头部结束标记(“\r\n\r\n”)的位置
*/
function buffer_find_body(b) {
for (var i = 0, len = b.length - 3; i < len; i++) {
if (b[i] == 0x0d && b[i + 1] == 0x0a && b[i + 2] == 0x0d && b[i + 3] == 0x0a) {
return i + 4;
}
}
return -1;
}