[Dnsmasq-discuss] [PATCH] Issues with TCP queries on recreated interfaces.

[Vladislav Grishenko]

Hi Petr,

Regarding 0002-Compare-address-and-interface-index-for-allowed-inte.patch, does it support case with different valid interfaces with the same address?
For example:
	eth0 192.168.1.1/24
	tun0 192.168.1.1./16 (created/destroyed dynamically)

Regarding appearance, seems newly added code doesn’t fully follow dnsmasq code style in several places:
* indentation (should be ident ==2 spaces, 8 spaces == \t)
* brackets on the same code lines
* function args on the next line are not aligned with the first argument
* prettyprint_addr() result is forcibly ignored with (void) unlike other places

Best Regards, Vladislav Grishenko

-----Original Message-----
From: Dnsmasq-discuss <dnsmasq-discuss-bounces at lists.thekelleys.org.uk> On Behalf Of Petr Mensik
Sent: Tuesday, July 9, 2019 5:31 PM
To: dnsmasq-discuss at lists.thekelleys.org.uk
Subject: [Dnsmasq-discuss] [PATCH] Issues with TCP queries on recreated interfaces.

Hello Simon and others,

we have discovered issues with TCP DNS query on dnsmasq, when running in bind-dynamic or bind-interfaces mode. dnsmasq scans automatically new interfaces or do that on new query in second case. However, because used speedup comparing only IP adresses in iface_allowed function, it never gets updated index of an interface.

In case where named interface is destroyed and created again, that drops TCP queries on that interface. They are checked for incoming interface number. If such number is not found in interfaces list, query is denied.

Luckily, there was a bug in checking, hiding this problem from usual configuration. If IPv6 address is enabled on the new device, new iface entry would be created, because scope_id of sockaddr_in6 does not match previous. That makes even IPv4 queries succeed.

Bug on bugzilla [1] is partly private.

I propose three changes. First is just helper to log what happens with listeners on bind-dynamic configuration.

Second is the most important. Create new interface every time index changes. Also test address family of incoming TCP query when checking allowed clients.

Third is cleanup of unused interfaces. On some virtual machines hosts, interfaces may often be created and destroyed. It might have negative effect on walking trough interfaces list. I think listeners should be garbage collected also on bind-interfaces configuration. But for now, release memory for unused interfaces at least for bind-dynamic.

1. https://bugzilla.redhat.com/show_bug.cgi?id=1721668
--
Petr Menšík
Software Engineer
Red Hat, http://www.redhat.com/
email: pemensik at redhat.com  PGP: 65C6C973