kernel-netlink: Install virtual IPv6 addresses as deprecated

This should prevent the kernel's IPv6 source address selection algorithm
from using this address unless it is forced to by our source route.
This is helpful if split tunneling is used.

Fixes #598.

src/libhydra/plugins/kernel_netlink/kernel_netlink_net.c

@@ -1868,6 +1868,17 @@ static status_t manage_ipaddr(private_kernel_netlink_net_t *this, int nlmsg_type
 
 	netlink_add_attribute(hdr, IFA_LOCAL, chunk, sizeof(request));
 
+	if (ip->get_family(ip) == AF_INET6 && this->rta_prefsrc_for_ipv6)
+	{	/* if source routes are possible we let the virtual IP get deprecated
+		 * immediately (but mark it as valid forever) so it gets only used if
+		 * forced by our route, and not by the default IPv6 address selection */
+		struct ifa_cacheinfo cache = {
+			.ifa_valid = 0xFFFFFFFF,
+			.ifa_prefered = 0,
+		};
+		netlink_add_attribute(hdr, IFA_CACHEINFO, chunk_from_thing(cache),
+							  sizeof(request));
+	}
 	return this->socket->send_ack(this->socket, hdr);
 }