[avahi] how to determine if an AvahiAddress is the local machine's?

[Lennart Poettering]

On Fri, 09.09.05 03:16, Charles Schmidt (cschmidt2 at emich.edu) wrote:

> -
> 
> As far as I can tell, there is no API for determining if a given
> AvahiAddress refers to an IP that the local machine has.  Something like
> avahi_address_is_localhost() or avahi_address_is_this_host() would be
> quite nice for filtering through resolve responses to get only those
> that aren't services from the local machine
> 
> Right now, I'm using:
> 
> static gboolean
> is_local_address (const gchar *address)
> {
> /* address here was got with avahi_address_snprint() */
> 	static struct hostent *he = NULL;
> 	struct in_addr addr;
> 	gint i;
> 	gint ret;
> 	gchar hostname[255 + 6];
> 	
> 	ret = gethostname (hostname, 255);
> 	if (ret) {
> 		return FALSE;
> 	}
> 
> 	strncat (hostname, ".local", 6);
> 	he = gethostbyname (hostname);
> 
> 	if (he == NULL) {
> 		return FALSE;
> 	}
> 
> 	for (i = 0; he->h_addr_list[i]; i++) {
> 		memcpy (&addr, he->h_addr_list[0], sizeof (struct in_addr));
> 
> 		if (strcmp (inet_ntoa (addr), address) == 0) {
> 			return TRUE;
> 		}
> 	}
> 
> 	return FALSE;
> }
> 
> But I'm not sure that this is a) the best way or b) something that will
> work in all situations (multiple interfaces).  Can anyone suggest a
> better way to do this?

Hmmh, nowadays the local hostname returned gethostname() needs not to
be available via gethostbyname() at all. Some people consider
depending on that behaviour a bug. Especially since not everybody has
nss-mdns running, i.e. on a host foo the name foo.local may not be
resolvable. (another issue: some people set the kernel hostname to
the FQDN, which breaks you code. You should truncate the kernel hostname
at the first dot.)

I acknowledge that some way to determine whether a service is on the
local machine is useful. Unfortunately these things are not that
easy. I had a short discussion with Marc Krochmal on a related issue:
I'd like to see a way to detect if two services with the same name on
different interfaces are actually identical. The idea is to add a
random cookie to each service TXT RR implicitly. If the cookie
matches services are identical. This could be used for your stuff,
since the cookie is the same for all services on the local machine.

The downside of this cookie thing is that it requires the consumer to
resolve a service before one can detect if it is identical to
another. Which is contrary to what mDNS/DNS-SD was designed for. 

Same issue with your "is address local" suggestion: you need to
resolve a service before you can make this check.

However, due to some reasons we can optimize for this stuff a little
and so I decided to add support for this cookie thingy soon. 

The client api of avahi will gain a new function
avahi_client_get_local_service_cookie() (wrapping the DBUS method
"GetLocalServiceCookie()" which returns the cookie used by the local
daemon for you. You have to parse incoming TXT records yourself the
special field "avahi.cookie" for the cookie.

Second I will add a function avahi_client_is_service_local() (wrapping
DBUS "IsServiceLocal()") which can be used to check if a specific tuple
(interface,protocol,name,type,domain) refers to a locally defined
service. This is probably what you want to use, since it doesn't
require resolving of a service to check if it is local.

Third I will add a new DBUS API function "IsAddressLocal()" for
you. However, we will not add a wrapper in avahi-client for that. If
you use avahi-client you can use netlink or SIOCGIFCONF directly. Or
use the cookie stuff/avahi_client_is_service_local(). The reason why I
add it as DBUS method is that a similar function is already available
in avahi's core and people who make direct use of DBUS from something
that is not C/C++ (without going through avahi-client) might need it.

I hope this would satisfy your needs?

Lennart

-- 
Lennart Poettering; lennart [at] poettering [dot] de
ICQ# 11060553; GPG 0x1A015CC4; http://0pointer.de/lennart/