linux-kernel - Re: Run a script with cap_net_bind_service

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Date:	Wed, 17 Sep 2014 01:41:30 +0200
From:	Junien Fridrick <linux.kernel@...ien.fridrick.net>
To:	Aleksei Besogonov <alex.besogonov@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Run a script with cap_net_bind_service - mission impossible.

On Mon, Sep 15, 2014 at 01:07:03PM +0000, Aleksei Besogonov wrote:
> Hi!

Hi,

> 
> It seems that it's totally impossible to start a script with
> cap_net_bind_service capability and as a non-root user without modifying
> system-wide settings.
> 
> I've trawled the Net for a solution that should be exceedingly simple. I
> want to run a daemon under a non-privileged account AND allow it to bind to
> 'secure' ports (443, 589 and 53). So far I found the following non-solutions:
> 
> - Use iptables to redirect ports. Doesn't work with local traffic.

Unless I'm missing something, you can use iptables to redirect ports even for
local traffic : you need to use the OUTPUT chain. For example, if I run :
# iptables -t nat -I OUTPUT -p tcp -d 192.168.0.1 --dport 1245 -j REDIRECT --to-port 1246

and then from the same machine, run :
$ nc -v 192.168.0.1 1245

The connection will be redirected to port 1246. Doesn't that answer your use
case ?

> - Use an HTTP proxy server (yeah, and also a DNS proxy server).
> - Set cap_net_bind_service capability bit on the script interpreter (so
> it'll break during upgrades). 
> - Fuck you, run it under the root user. With several permutations like:
>   * Dropping caps after opening sockets (can't do this)
>   * Dropping all caps before starting the interpreter (fucks up the file
> ownership)
> 
> I've tried without any luck various permutations of capsh like: capsh
> --keep=1 --secbits=5 --user=cyberax --caps=cap_net_bind_service+eip -- -c
> 'nc -l 443'
> 
> So is it possible at all?
> 
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