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IPv6 is _still_ not at the feature parity with IPv4! I'm not kidding. For example, Android doesn't support stateful DHCPv6. And DHCPv6 doesn't have the _basic_

IPv6 is _still_ not at the feature parity with IPv4! I'm not kidding. For example, Android doesn't support stateful DHCPv6. And DHCPv6 doesn't have the _basic_ feature of DHCPv4: hostnames. You can't easily use it to do a quick survey of your network. Then you have that @#&(^(&!@^ that is ULA. With IPv4 we have a very useful pattern: you create an "internal" network that is stable and predictable. It's routed to the outside world through NAT. If the external connection goes down, the internal network is unaffected. With IPv6 you're supposed to have ULA and the global routed addresses in parallel. So now the external connection goes down, and the router withdraws the prefix from the router advertisement. Half of the hosts lose their external addresses, but keep the ULAs. Half of the hosts don't implement prefix withdrawal, and keep both their ULAs and the normal addresses. Congrats, now these hosts can't talk to each other due to the ULA addresses being less preferred. And of course, IPv6 hasn't improved on the PMTU. So if you're running an Internet service, you need to use something like 1400 MTU to make sure some of the misconfigured tunneled clients don't get shafted. There's now an RFC that makes it useful: https://datatracker.ietf.org/doc/html/rfc9268 https://datatracker.ietf.org/doc/html/rfc9268 , but it's Experimental and it'll need ~20 years to be deployed anyways. IPv6, a story of recursive utter failure at all levels...

matrix2003 2y ago
> Congrats, now these hosts can't talk to each other due to the ULA addresses being less preferred. What do you mean by this? Are you taking about mDNS still referencing the withdrawn prefix?
- cyberax 2y ago
  If two hosts have both mutually accessible ULA and GUA addresses, they will prefer _GUAs_ to talk to each other. So the connection will be susceptible to the prefix withdrawal if the upstream goes down (BTW, IPv6 did nothing sane for multihoming either). > Are you taking about mDNS still referencing the withdrawn prefix? That too.
  - matrix2003 2y ago
    Oh, interesting. My machines have not had good mDNS support, so I have been hardcoding ULAs for LAN-only traffic.
elcritch 2y ago
> dhcpv6 Just skip DHCPV6, just use SLAAC. Plus I've never seen DHCP hostnames work. Now I just ping ff02::1 multicast to see what devices are on my network. Unfortunately much software makes it a pain to use link-local addresses but they're really convenient as they normally don't change across networks. > Half of the hosts don't implement prefix withdrawal, and keep both their ULAs and the normal addresses. Congrats, now these hosts can't talk to each other due to the ULA addresses being less preferred. I've had similar issues with crappy devices not relinquishing DHCPv4 IPs properly. Always fun trying to figure out why your laptop is dropped off your network after 20 minutes because it honors DHCP. The lack of proper prefix widthdrawl sucks. Though it's something software should be able to handle by preferring ULA addresses when communicating locally.
- blueflow 2y ago
  > ping ff02::1 multicast Wasn't this disabled for IPv4 for being a security concern? See the Linux net.ipv4.icmp_echo_ignore_broadcasts sysctl. I wonder if this will stay enabled for IPv6.
- aidenn0 2y ago
  > Plus I've never seen DHCP hostnames work Over half the workstations at my office use DHCP hostnames and they work just fine. In fact I'll say exactly the opposite: I've never seen DHCP hostnames not work.
  - elcritch 2y ago
    To be fair work is the wrong term, but more "used" as in be implemented and utilized.
- cyberax 2y ago
  > Now I just ping ff02::1 multicast to see what devices are on my network. Yeah, you see a bunch of MACs and... what next? How do you get the names?
- yjftsjthsd-h 2y ago
  > Now I just ping ff02::1 multicast to see what devices are on my network. Unfortunately much software makes it a pain to use link-local addresses but they're really convenient as they normally don't change across networks. How does that help? I don't want a list of IPs, I want to reach my devices by name (which DHCP makes easy).
tucnak 2y ago
You know that dhcpv6 servers like odhcpd (Openwrt) implement hairpin NAT along with IPv6-PD right?
- cyberax 2y ago
  How would it help? The LAN traffic between hosts in that LAN does not go through the router.

matrix2003 2y ago

> Congrats, now these hosts can't talk to each other due to the ULA addresses being less preferred. What do you mean by this? Are you taking about mDNS still referencing the withdrawn prefix?

cyberax 2y ago
If two hosts have both mutually accessible ULA and GUA addresses, they will prefer _GUAs_ to talk to each other. So the connection will be susceptible to the prefix withdrawal if the upstream goes down (BTW, IPv6 did nothing sane for multihoming either). > Are you taking about mDNS still referencing the withdrawn prefix? That too.
- matrix2003 2y ago
  Oh, interesting. My machines have not had good mDNS support, so I have been hardcoding ULAs for LAN-only traffic.

elcritch 2y ago

> dhcpv6 Just skip DHCPV6, just use SLAAC. Plus I've never seen DHCP hostnames work. Now I just ping ff02::1 multicast to see what devices are on my network. Unfortunately much software makes it a pain to use link-local addresses but they're really convenient as they normally don't change across networks. > Half of the hosts don't implement prefix withdrawal, and keep both their ULAs and the normal addresses. Congrats, now these hosts can't talk to each other due to the ULA addresses being less preferred. I've had similar issues with crappy devices not relinquishing DHCPv4 IPs properly. Always fun trying to figure out why your laptop is dropped off your network after 20 minutes because it honors DHCP. The lack of proper prefix widthdrawl sucks. Though it's something software should be able to handle by preferring ULA addresses when communicating locally.

blueflow 2y ago
> ping ff02::1 multicast Wasn't this disabled for IPv4 for being a security concern? See the Linux net.ipv4.icmp_echo_ignore_broadcasts sysctl. I wonder if this will stay enabled for IPv6.
aidenn0 2y ago
> Plus I've never seen DHCP hostnames work Over half the workstations at my office use DHCP hostnames and they work just fine. In fact I'll say exactly the opposite: I've never seen DHCP hostnames not work.
- elcritch 2y ago
  To be fair work is the wrong term, but more "used" as in be implemented and utilized.
cyberax 2y ago
> Now I just ping ff02::1 multicast to see what devices are on my network. Yeah, you see a bunch of MACs and... what next? How do you get the names?
yjftsjthsd-h 2y ago
> Now I just ping ff02::1 multicast to see what devices are on my network. Unfortunately much software makes it a pain to use link-local addresses but they're really convenient as they normally don't change across networks. How does that help? I don't want a list of IPs, I want to reach my devices by name (which DHCP makes easy).

tucnak 2y ago

You know that dhcpv6 servers like odhcpd (Openwrt) implement hairpin NAT along with IPv6-PD right?

cyberax 2y ago
How would it help? The LAN traffic between hosts in that LAN does not go through the router.