A Deutsche Telekom customer experiences a Path MTU Discovery (PMTUD) failure when connecting to Azure-hosted services over IPv6, a problem which may be hidden for many customers because common...

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  "detected_at": "2026-09-18T09:10:28+00:00",
  "company": {
    "name": "Deutsche Telekom",
    "domain": "telekom.com"
  },
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    "topics": [
      "ipv6",
      "networking",
      "isp",
      "reliability",
      "pmtud"
    ],
    "post_id": "1wjl4xu",
    "summary": "A Deutsche Telekom customer experiences a Path MTU Discovery (PMTUD) failure when connecting to Azure-hosted services over IPv6, a problem which may be hidden for many customers because common consumer routers like the FRITZ!Box automatically apply a workaround.",
    "category": "reliability",
    "comments": [
      {
        "url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/comment/pajnn16/",
        "depth": 0,
        "score": 32,
        "author": "zekica",
        "excerpt": "This is not a problem with any of the IPv6 RFCs but instead a problem with Azure's implementation and until they fix it, good luck.",
        "posted_at": "2026-09-18T10:36:46.000Z",
        "author_url": "https://www.reddit.com/user/zekica/"
      },
      {
        "url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/comment/pajoc3o/",
        "depth": 0,
        "score": 26,
        "author": "philsbln",
        "excerpt": "Report to the Website operator. That’s the only thing you can do. I am not surprised that Azure Front Door has issues with IPv6 as many Azure things have.",
        "posted_at": "2026-09-18T10:41:41.000Z",
        "author_url": "https://www.reddit.com/user/philsbln/"
      },
      {
        "url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/comment/pajdm9k/",
        "depth": 0,
        "score": 15,
        "author": "UNF0RM4TT3D",
        "excerpt": "I've set my Mikrotik's RA to have the MTU of my PPPoE tunnel to my ISP. It works very well. And the firewall rule doesn't get hit as often. So it seems to work way better than just the mss clamping rule.",
        "posted_at": "2026-09-18T09:17:43.000Z",
        "author_url": "https://www.reddit.com/user/UNF0RM4TT3D/"
      },
      {
        "url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/comment/pajdy9w/",
        "depth": 0,
        "score": 11,
        "author": "snapilica2003",
        "excerpt": "Are you sure Deutsche Telekom doesn’t support baby jumbo frame (RFC 4638)?\n\n Most ISPs who still rely on PPPoE should have implemented this. I’m on Digi Romania which also has PPPoE and because they support it, I’m using MTU 1500 on my WAN connection (which means MTU 1508 on the actual physical interface).",
        "posted_at": "2026-09-18T09:20:32.000Z",
        "author_url": "https://www.reddit.com/user/snapilica2003/"
      },
      {
        "url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/comment/pajx6aw/",
        "depth": 0,
        "score": 10,
        "author": "netsx",
        "excerpt": "The internet IS broken, and it has been for decades. Adjusting MSS is still practically necessary, and essentially mandatory if your egress MTU < 1500.",
        "posted_at": "2026-09-18T11:39:08.000Z",
        "author_url": "https://www.reddit.com/user/netsx/"
      },
      {
        "url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/comment/pak5p3t/",
        "depth": 0,
        "score": 9,
        "author": "gtuminauskas",
        "excerpt": "I noticed this issue last year, it mainly happens on Azure/Microsoft network, edge routers and even CDNs, where they have IPv6 ebabled but ICMPv6 is disabled!!! <-- IPv6 relies on ICMP.\n\n If Microsoft keeps ICMP disabled, then no wonder why on their end, they dont receive ICMP packets to lower PMTUD... Because they are blocking that negotiation ...",
        "posted_at": "2026-09-18T12:27:34.000Z",
        "author_url": "https://www.reddit.com/user/gtuminauskas/"
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      {
        "url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/comment/pak3vd5/",
        "depth": 0,
        "score": 6,
        "author": "dgx-g",
        "excerpt": "Thank you so much for this post. I just assumed azure had blacklisted my static v6 prefix for some reason and never looked into it. I can use my online banking on my home wifi again.",
        "posted_at": "2026-09-18T12:17:42.000Z",
        "author_url": "https://www.reddit.com/user/dgx-g/"
      },
      {
        "url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/comment/pajzur7/",
        "depth": 0,
        "score": 6,
        "author": "yrro",
        "excerpt": "Azure famously drops all ICMPv6, which breaks PMTUD.\n\n Azure's IPv6 support is a joke, and is borderline mis-selling.",
        "posted_at": "2026-09-18T11:54:57.000Z",
        "author_url": "https://www.reddit.com/user/yrro/"
      }
    ],
    "evidence": [
      "[post] My setup is a Deutsche Telekom PPPoE connection. The actual PPPoE MTU is 1492, while clients on the LAN use the normal Ethernet MTU of 1500.",
      "[post] What makes this even more interesting is that this may explain why relatively few Telekom users notice it: consumer routers such as FRITZ!Box do MSS clamping automatically.",
      "[comment u/sbujdoso] Anecdata: I haver encontered the same problem for years: azure is broken mostly towards deutse Telekom."
    ],
    "virality": "high",
    "post_date": "2026-09-18T09:10:28.000Z",
    "post_kind": "text",
    "post_text": "My setup is a Deutsche Telekom PPPoE connection. The actual PPPoE MTU is 1492, while clients on the LAN use the normal Ethernet MTU of 1500.\n\nOne particular site, login.schwaebisch-hall.de (behind Azure Front Door), reliably fails over IPv6.\n\nIPv4 works.\n\nIPv6 through another ISP works.\n\nDNS works.\n\nThe TCP connection works.\n\nThe TLS handshake starts - and then stalls after the initial ClientHello message.\n\nAfter packet captures and testing, the problem is very clearly MTU-related.\n\nWith the client at MTU 1500:\n\nIPv6 HTTPS fails\n\nclient advertises TCP MSS 1440\n\nthe first 99 bytes from the server arrive\n\nthen server TCP bytes 100–2955 are missing\n\nlater packets starting at SEQ 2956 arrive\n\nthe client repeatedly ACKs 100 and SACKs the later data\n\nThat missing range is 2856 bytes.\n\nInterestingly:\n\n2856 = 2 * 1428\n\nAnd with IPv6 + TCP timestamps:\n\n40 IPv6 + 32 TCP + 1428 payload = 1500\n\nSo it fits exactly two full-size 1500-byte packets disappearing at a path that only supports 1492. I can't prove the size of the missing packets because, obviously, they never reach my capture point, but the numbers are rather suggestive.\n\nNow the fun part:\n\nIf I change the client MTU to 1492, everything works.\n\nIf I leave the client MTU at 1500 and configure my MikroTik to clamp the outgoing IPv6 TCP MSS to 1432, everything works.\n\nWith MSS 1432, the Azure endpoint sends lots of packets that are...",
    "sentiment": "negative",
    "subreddit": "ipv6",
    "post_flair": [
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    "post_title": "IPv6 is “broken” when PMTUD fails - and Happy Eyeballs doesn't save you",
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    "source_url": "https://www.reddit.com/r/ipv6/comments/1wjl4xu/ipv6_is_broken_when_pmtud_fails_and_happy/",
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  -d '{"domain":"telekom.com","limit":20}'