allocate a file descriptor to listen on a port later

This is handy if you want to run a server on a low port but want to drop privileges as soon as possible.

Here we allocate a file descriptor for tcp port 80 and then drop permissions right away:

var http = require('http');
var alloc = require('tcp-bind');
var fd = alloc(80);

process.setgid(process.argv[3]);
process.setuid(process.argv[2]);

var server = http.createServer(function (req, res) {
    res.write('uid=' + process.getuid() + '\n');
    res.end('gid=' + process.getgid() + '\n');
});
server.listen({ fd: fd });

Now we're listening on port 80, but the rest is running under a regular user:

$ sudo node drop.js substack cdrom &
[1] 2855
$ curl localhost
uid=1000
gid=24

var alloc = require('tcp-bind')

Allocate a file descriptor to listen on port at an ipv4 or ipv6 string address addr.

You can create a tcp server from the file descriptor fd later by passing it to the server's .listen() method:

server.listen({ fd: fd })

This method throws when the port is already bound and when the current user doesn't have permission to open the port requested.

With npm do:

npm install tcp-bind

MIT