foo.js

var http = require('http');

function handle(req, rep) {
    req.pipe(process.stdout);
    req.on('end', function () {
        rep.end();
        console.log('');
    });
}

var server = http.createServer(handle);

server.on('checkContinue', function (req, rep) {
    if (!req.headers['x-foo']) {
        console.log('did not have foo');
        rep.writeHead(400);
        rep.end();
        return;
    }

    rep.writeContinue();
    handle(req, rep);
});

server.listen(8080);

with-header

~$ curl localhost:8080 -d{0..1023} -H 'X-Foo: value' -v
* About to connect() to localhost port 8080 (#0)
*   Trying ::1...
* Connection refused
*   Trying 127.0.0.1...
* connected
* Connected to localhost (127.0.0.1) port 8080 (#0)
> POST / HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: localhost:8080
> Accept: */*
> X-Foo: value
> Content-Length: 4009
> Content-Type: application/x-www-form-urlencoded
> Expect: 100-continue
> 
< HTTP/1.1 100 Continue
< HTTP/1.1 200 OK
< Date: Fri, 03 May 2013 02:06:47 GMT
< Connection: keep-alive
< Transfer-Encoding: chunked
< 
* Connection #0 to host localhost left intact
* Closing connection #0

without-header

~$ curl localhost:8080 -d{0..1023} -v
* About to connect() to localhost port 8080 (#0)
*   Trying ::1...
* Connection refused
*   Trying 127.0.0.1...
* connected
* Connected to localhost (127.0.0.1) port 8080 (#0)
> POST / HTTP/1.1
> User-Agent: curl/7.24.0 (x86_64-apple-darwin12.0) libcurl/7.24.0 OpenSSL/0.9.8r zlib/1.2.5
> Host: localhost:8080
> Accept: */*
> Content-Length: 4009
> Content-Type: application/x-www-form-urlencoded
> Expect: 100-continue
> 
< HTTP/1.1 400 Bad Request
< Date: Fri, 03 May 2013 02:07:41 GMT
< Connection: close
< Transfer-Encoding: chunked
< 
* Closing connection #0