[Dnsmasq-discuss] [PATCH] Wrong DHCPv6 packet originated interface

[Lung-Pin Chang]

Hi folks,

I'm currently using dnsmasq 2.70 (compiled from source) in my Linux
environment for IPv6 RA/DHCPv6.

The following section is my dnsmasq config:
interface=lbr0
dhcp-range=set:lbr0v60,2002:01ab:68c7:beef::5566,2002:01ab:68c7:beef::5600,64,86400
dhcp-option=tag:lbr0v60,option6:dns-server,[2001:4860:4860::8888],[2001:4860:4860::8844]
enable-ra
dhcp-leasefile=/etc/dhcpd/dhcpd.conf.leases
dhcp-script=/usr/syno/etc/dhcpd/dhcpd-script.sh

and my interface address configuration:
8: lbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue
    link/ether 00:00:15:69:67:15 brd ff:ff:ff:ff:ff:ff
    inet6 2002:1ab:68c7:beef::1/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::200:15ff:fe69:6715/64 scope link
       valid_lft forever preferred_lft forever
66: 6in4: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1472 qdisc noqueue
    link/sit 1.171.104.199 peer 192.88.99.1
    inet6 2002:1ab:68c7::1/16 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::1ab:68c7/128 scope link
       valid_lft forever preferred_lft forever

With logging facilities in my dhcp-script, I noticed that the script
is invoked by the "old" event when I kill -HUP <dnsmasq pid>.

The problem is: when my script got invoked, the DNSMASQ_INTERFACE
environment is set to a wrong interface (i.e., 6in4, instead of the
incoming interface lbr0).

After digging a while in the source code, I found that the
find_interface_v6 callback (src/lease.c) might be the cause:

Given several interfaces share common prefix with different prefix length,

current interface searching mechanism (lease_find_interfaces ->
iface_enumerate) would

enumerate all the addresses on the host by netlink RTM_GETADDR
request, trying to figure out

which interface is the packet originated from by checking if they're
in the same subnet.

But with the netlink parsing flow, when multiple interfaces match, the
originated interface would

always be the one that comes later in the netlink message. So in my
case, the originated

interface would always be 6in4, even if the prefix length on it is
apparently shorter than lbr0.

I came  up with a quick fix[1] to save the interface prefix length in
the lease context,

so longer prefixes are always preferred and the script got invoked
with correct interface.

But I'm not sure if this also applies to IPv4, as I'm testing in a
IPv6-only environment in Linux

(so I only traced the netlink part).

Would like to hear your thoughts via list, mail or GitHub, thanks.

[1]: https://github.com/iamben/dnsmasq/commit/4ee9751e13de3f7941e2ef1e086dc0768d819d69

-- 
Lung-Pin Chang 張瓏繽

Wireless Internet Laboratory & Computer Center
Department of Computer Science
National Chiao Tung University