[Dnsmasq-discuss] DHCP simultaneous with static addresses and autoconf

[Dan Williams]

On Fri, 2015-03-27 at 15:51 -0400, Laine Stump wrote:
> (the following is on RHEL, Fedora, and CentOS clients; I'm not sure what
> behavior is on other platforms.)
> 
> First, a question about client behavior (i.e. not specific to dnsmasq):
> 
> I noticed a long time ago that IPv4 clients configured for both static
> IP addresses and dhcp would discard the static IP addresses in favor of

NetworkManager (which is default on Fedora, RHEL, and CentOS for
many/most installs) will quite happily allow both IPv4 static and DHCP
addresses to co-exist and will manage each appropriately.  You're right
that there's no reason for that not to work.  It's not typically exposed
from some GUI clients because it's an unusual configuration; but it's
explicitly supported by NM itself when then configuration specifies it.

> the one from dhcp. Likewise, I found the same behavior for IPv6 when
> checking it for this bug report:
> 
>   https://bugzilla.redhat.com/show_bug.cgi?id=1113983
> 
> This is different from IPv6 autoconf behavior - you can configure a
> static IP and set autoconf, and you will end up with both IP addresses.

Right.

> I don't see any reason why a client shouldn't behave the same with dhcp
> - just keep the static address and add the address from dhcp, and am
> thinking this is just a deficiency in the dhcp client implementation (in
> this case ISC dhclient). Is there anything in the DHCP standard that I'm

If you're using plain dhclient (and not NM), then I'd think it would be
a deficiency in the RHEL/Fedora dhclient-script then, not dhclient
itself.  dhclient itself doesn't care about the interface configuration,
it's up to /usr/sbin/dhclient-script to take the DHCP server's reply and
apply it to the interface.

> overlooking that would preclude static + dhcp ( + autoconf) on the same
> interface from working?

I don't think the DHCP standards care at all, but I'm not an expert.

> And now to a question about dnsmasq - libvirt uses dnsmasq for DNS, DHCP
> (v4 and v6), and RA. I've just noticed that when I enable dhcpv6,
> clients are no longer able to get an IPv6 via autoconf (i.e. when
> they're configured to only use autoconf). Is this expected, or a
> bug/configuration problem?

This could be a misconfiguration or a bug in dnsmasq, I'll let Simon
answer that.

If you set the A bit in the RA's Prefix Information option, that says
clients can use the given prefix to autoconfigure an address.  If you
set either of the M or O bits in the RA header, then DHCPv6 will be
invoked.  These are independent of each other.

It sounds like when dnsmasq is configured for DHCPv6, it turns off the A
bit which prevents clients from using the prefix for an autoconf
address.  While that would typically be done when using the M flag
(since M implies you are using full DHCP leases and thus probably don't
want autoconf) it's not prohibited in anyway that I know of, so perhaps
dnsmasq just doesn't have a knob to keep A=1 when M=1?  Just a guess.

Dan

> This is my dnsmasq version: dnsmasq-2.72-3.fc21.x86_64
> 
> The dnsmasq commandline:
> 
> /sbin/dnsmasq --conf-file=/var/lib/libvirt/dnsmasq/ipv6.conf
> --leasefile-ro --dhcp-script=/usr/libexec/libvirt_leaseshelper
> 
> and ipv6.conf:
> 
> strict-order
> domain=test.lainetest.org
> expand-hosts
> pid-file=/var/run/libvirt/network/ipv6.pid
> except-interface=lo
> bind-dynamic
> interface=virbr1
> dhcp-range=192.168.123.2,192.168.123.254
> dhcp-no-override
> dhcp-range=x:x:x:x::2,x:x:x:x::200
> dhcp-lease-max=764
> dhcp-hostsfile=/var/lib/libvirt/dnsmasq/ipv6.hostsfile
> addn-hosts=/var/lib/libvirt/dnsmasq/ipv6.addnhosts
> enable-ra
> 
> This is the config file when there is no dhcp-range defined for the network:
> 
> strict-order
> domain=test.lainetest.org
> expand-hosts
> pid-file=/var/run/libvirt/network/ipv6.pid
> except-interface=lo
> bind-dynamic
> interface=virbr1
> dhcp-range=192.168.123.2,192.168.123.254
> dhcp-no-override
> dhcp-lease-max=253
> dhcp-hostsfile=/var/lib/libvirt/dnsmasq/ipv6.hostsfile
> addn-hosts=/var/lib/libvirt/dnsmasq/ipv6.addnhosts
> dhcp-range=x:x:x:x::1,ra-only
> 
> (in this case, autoconf works correctly).
> 
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