Re: [Bug-wget] Anyone want to add libcares support to wget?

[Tim Ruehsen]

Tim

On Wednesday 16 March 2016 07:35:44 Ben Greear wrote:
> On 03/15/2016 06:40 PM, Ángel González wrote:
> > On 15/03/16 17:03, Ben Greear wrote:
> >> I want to use wget on multi-homed machines, and I use routing rules to
> >> route pkts out specific interfaces based on the source IP.
> >> 
> >> So, I need to be able to bind to a local IP address, including for the
> >> resolver logic, which is why I want to use libcares.
> >> 
> >> To test, I think it would be sufficient to put two IPs on a port
> >> (1.1.1.1, 1.1.1.2), and then use cmd-line args to choose one or the
> >> other.  Sniff and watch for proper source address.
> >> 
> >> For DNS servers, have 1.1.1.3 and 1.1.1.4 be specified.  Sniff and watch
> >> to see that the resolver destination IP is as specified.  This should be
> >> completely independent of whatever is in /etc/resolv.conf, etc.
> >> 
> >> I (and others) have patched libcares and curl in the past to handle all
> >> of this, so I know libcares can do it.  I think the main work is to
> >> handle new cmd-line arguments, pass appropriate info to libcares, and
> >> make wget use libcares for it's resolve logic.
> >> 
> >> Thanks,
> >> Ben
> > 
> > I think it's ok to add a --bind parameter to wget to choose the interface
> > for download, but I don't think it should use a different resolver just
> > for this.
> > 
> > You _could_ change the libc nameservers from the application by changing
> > _res.nsaddr_list after res_init(). It may be a bit fragile, but should
> > work fine. And even better than changing each application, I would
> > recommend you to create a simple LD_PRELOAD library that overrides
> > res_init() & friends, and allows you to specify the target dns server
> > with an environment variable. It's a pity there's no RESOLV_CONF var
> > similar to RESOLV_HOST_CONF, for using a separate /etc/resolv.conf.
> wget can already bind to a local IP.  It might be nice to add support for
> SO_BINDTODEVICE, but not sure it is required for what I need.

@Ángel The LD_PRELOAD idea is good. I really like it !

> 
> The rest of your suggestions are total hacks!
> 
> Anyway, I'm quite sure of what I want, and it should not be horribly
> difficult to accomplish.  I'll work on it if no one else is interested.

Thanks for your offer.
I expect a configure option to switch such a feature on (default=off) to avoid 
a new dependency (c-ares) for a feature that just one out of a million users 
need. How would you design an automated test ?

Did you consider using a container (e.g. docker) for such a task ? Easy to set 
up and you'll have your feature not only for wget. IMO, that is much more 
flexible. (It was Giuseppe's idea during a private talk).

Regards, Tim

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