[Dnsmasq-discuss] RFC 5908 - DHCPv6 NTP option 56

Simon Kelley simon at thekelleys.org.uk
Thu Sep 10 21:48:50 BST 2015

On 10/09/15 13:55, Kevin Darbyshire-Bryant wrote:
> Hi All,
> I've been looking at providing NTP server addresses to my DHCPv6 clients
> using dnsmasq.  2 RFCs seem applicable, Simple NTP provision RFC4075
> defines option 31 and known to dnsmasq as 'sntp-server'.  RFC5908
> defines a more flexible/complicated provision mechanism using option 56,
> known to dnsmasq as 'ntp-server'.
> RFC5908 uses sub-options to option 56 defining a choice of sub-option 1 
> unicast NTP server address, sub-option 2 Multicast server address and
> sub-option 3 provision of a FQDN of the NTP server.
> Looking at the dnsmasq logs (I've not yet got to packet capture) and
> limited understanding of the code, I'm not convinced that dnsmasq's
> option 56 implementation understands the usage of sub-options.#

You're right. It doesn't support sub-options. It's expecting one or more
IPv6 addresses, which is all wrong.

DHCPv4 allows you to construct arbitrary encapsulated options, which
would be useful in this case. Sadly, that's not supported for DHCPv6.

A short term fix would be to remove that expectation of an address-list.
That would at least allow you to construct the option as colon-separated
hex. Better would be special processing which handled addresses and
FQDNs and did the right thing. This form of option encoding seems quite
general, the ability to specify some server as either an address or a
FQDN is quite sensible. I wonder if any other DHCPv6 options are
specified to use it.
> As ever, assuming I've mis-understood something here!

No, you're right!

I'll commit the minimum change so it doesn't expect and IPv6 address
list right now.



> Best regards,
> Kevin
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