Re: [dhcwg] recommendation on DHCP6 source port numbers

[Tomoyuki Sahara <[e-mail]>]

Hi, Bernie:

Some (at least 3) developers do not think so and their DHCP6
implementations send from/to other port numbers.

As written in the bug report I quoted in the last message:
https://bugzilla.redhat.com/show_bug.cgi?id=952126

- A DHCP6 client sends request messages from a random port number but
  listens on port 546 and expects the server to send response meessages
  to port 546.
- ISC-DHCP was "fixed" to send response messages to port 546, not to
  the source port of the request message.
- Windows 2008 DHCP6 server does same as ISC-DHCP server.

I think it is not a very good situation.  To maximize compatibility,
clients and servers should use only one port number for each (546/547).
That is why I proposed to add recommendations on the specification.


Thanks,
Tomoyuki

>No. Normal UDP communication rules apply. A client sends traffic to a well-known destination port and it is free to select whatever port number it likes as the source port. The server’s response is sent from that well known port (as source port) and sent to the client’s selected port (as destination port). This is normal communication and dhcpv6 follows it. That is why nothing is said or needs to be said about the client source port.
>
>- Bernie Volz
>
>> On Feb 26, 2024, at 1:00 AM, Tomoyuki Sahara <tsahara=[e-mail]> wrote:
>> 
>> Hi, DHC wg members:
>> 
>> Can we make recommendations on source port numbers of DHCP6 messages
>> in rfc8415bis?
>> 
>> DHCP6 specification says that DHCP6 clients and servers listen on UDP
>> port 546 and 547 respectively, in RFC8415 section 7.2.  It implies
>> that DHCP6 clients MUST send messages to UDP port 547 (server port) and
>> servers MUST send messages to UDP port 546 (client port) to work with
>> their counterpart correctly (though restrictions can be relaxed with
>> RFC8357 for relays).
>> 
>> But it says nothing about source port numbers.  Without any
>> restrictions, some implementations use ephemeral source port
>> (e.g. 12345) to send their messages.  DHCP6 conversations look like:
>> 
>>  1. client send Solicit    fe80::2#49876    -> ff02::1:2#547
>>  2. server send Advertise  fe80::1#547      -> fe80::2#546 (!)
>>  3. client send Request    fe80::2#49877(?) -> ff02::1:2#547
>>  4. server send Confirm    fe80::1#547      -> fe80::2#546
>> 
>> This behavior is not prohibited by the specification but makes
>> confusions for DHCP6 implementer and network/firewall operators (*1).
>> Most Internet protocols nowadays assume that servers send response
>> messages from the port number they received on.
>> (*1 e.g. https://bugzilla.redhat.com/show_bug.cgi?id=952126 )
>> 
>> In my humble opinion, it is too late to require that DHCP6 client and
>> server MUST send messages from the fixed port number (546/547) because
>> there are too many DHCP6 implementations in the wild.  But making a
>> recommendation is helpful for new implementations/deployments of DHCP6.
>> 
>> An idea to make such recommendation is adding a text in rfc8415bis:
>> 
>>  OLD:
>>    7.2. UDP Ports
>>      Clients listen for DHCP messages on UDP port 546.  Servers and
>>      relay agents listen for DHCP messages on UDP port 547.
>> 
>>  NEW:
>>    7.2. UDP Ports
>>      Clients listen for DHCP messages on UDP port 546.  Servers and
>>      relay agents listen for DHCP messages on UDP port 547.
>> 
>>      Clients are RECOMMENDED to send DHCP messages from UDP port 546.
>>      Servers and relay agents are RECOMMENDED to send DHCP messages
>>      from UDP port 547 (unless relay agent includes Relay Source Port
>>      Option for DHCP6 [RFC8357]).
>> 
>> I know WGLC has been concluded but I believe the recommendations above
>> encourage new implementations to use the standard DHCP6 port numbers
>> on UDP source port.
>> 
>> 
>> Best regards,
>> Tomoyuki Sahara
>> 
>> 
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