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IPv6 And MTU Woes

I've just recently deployed IPv6. Everything went well except for one painful issue that is still not really resolved my satisfaction. To recount the story requires covering quite a bit of ground and digging through a pile of acronyms. Hold on tight!

My ISP provides a native /48 network per customer. That means that I get a mere 1208925819614629174706176 public IP addresses spread over 65536 /64 networks to use as I please.

I want to use my existing FreeBSD router to do the routing for the individual networks. I want to do this for several reasons:

  1. The ISP-provided box is a standard consumer router and is fairly limited in what it can do. It's not actively harmful; it's a respectable brand and fairly powerful hardware, but it's still only a consumer-grade box with a web interface.

  2. I'd rather have the intricate configuration details of my network be stored in text configuration files on commodity hardware and on an operating system that I mostly trust. The ISP-provided box runs Linux on proprietary hardware and only provides shell access via an undocumented (authenticated) backdoor (side-door?).

  3. I trust myself to write safe pf rules.

  4. Exposing the FreeBSD machine directly to the WAN eliminates one routing hop.

However, in order to allow my FreeBSD machine to do the routing of the individual networks (as opposed to letting the entirely optional ISP-provided box do it), I had to get it to handle the PPP connection. The machine doesn't have a modem, so instead I have to run the ISP-provided modem/router in bridging mode and get the FreeBSD machine to send PPP commands using the PPPoE protocol. Encouragingly, my ISP suggested that yes, I should be using FreeBSD for this. It's a testament to the quality of IDNet: They are a serious technical ISP, they don't treat their customers like idiots, and they respect the freedom of choice of their customers to use whatever hardware and software they want.

For those that don't know, limitations in PPPoE mean that the MTU of the link is limited to at most 1492. For reference, most networks on the internet are using an MTU of 1500. In IPv4, if you send a packet that's larger than your router's MTU, the packet will be fragmented into separate pieces and then reassembled at the destination. This has, historically, turned out to be a rather nasty way to deal with oversized packets and therefore, in IPv6, packets that are larger than the MTU will be rejected by routers and will result in Packet Too Large ICMPv6 messages being returned to the sender.

In effect, this means that IPv6 networks are somewhat less tolerant of misconfigured MTU values than IPv4 networks. Various companies have written extensively about fragmentation issues.

So why am I mentioning this? Well, shortly after I'd enabled IPv6 for the network and all services, I suddenly ran into a problem where I couldn't send mail. The symptom was that my mail client would connect to the SMTP server, authenticate successfully, send an initial DATA command, and then sit there doing nothing. Eventually, the server would kick the client due to lack of activity. After asking on the mailing list for my mail client, Andrej Kacian pointed me at a thread that documented someone dealing with MTU issues. After some examination with Wireshark, I realized that my workstation was sending packets that were larger than the PPPoE link's MTU of 1492. My FreeBSD machine was dilligently responding with Packet Too Large errors, but for whatever reason, my Linux workstation was essentially ignoring them. Some conversations on the #ipv6 Freenode IRC channel have suggested that Linux handles this very badly. Worse, it seems that the MTU related issues are sporadic: Sometimes it works without issue, other times not.

The "solution" seems to be this: Set the MTUs of all interfaces on all machines in my network to 1492. If I, for example, set the MTU of my workstation's network interface to 1500 and set the FreeBSD router's interfaces to 1492, I can no longer SSH reliably into remote sites, and the majority of TLS handshakes fail. No Packet Too Large errors are generated, which seems counter to my understanding of how this stuff is supposed to work. I very much dislike having to use a non-default MTU on my network: It seems like I will inevitably forget to set it on one or more machines and will run into bizarre and intermittent network issues on that machine.

Some further conversation on the #ipv6 IRC channel suggests that I should not have to do this at all. However, I've so far spent roughly ten hours trying to debug the problem and am exhausted. Using a non-standard MTU in my LAN(s) works around the issue for now, and I'll re-examine the problem after my capacity for suffering has been replenished.

2018-02-23: Update: IPv6 And Linux