AS overview
Test target: 2a0f:9400:690d::4 · pns.mlgt.info · address space: unknown · prefixes announced: 18
prefix(es): 2a05:4140:305::/48, 2a0f:9400:611d::/48, 2a0f:9400:6111::/48, 2a05:4140:333::/48, 2a0f:9400:690d::/48, 2a13:aac4:1000::/36, 2a05:4140:380::/42, 2a05:4140:314::/48 (+10 more)
Targets & per-vantage-point probe results (5 target(s) across 3 vantage point(s))
Source codes (hover any badge for full description): CUR=curated · NS/SOA/MX=DNS records · SPF=SPF TXT · DRV=holder-derived (www, ns1, mail, gw, …) · ATL=RIPE Atlas · PDB=PeeringDB · WHOIS=RIPE whois · RDAP=RIPE RDAP · HIT=IPv6 Hitlist · RTR=in-prefix path hop (router) · PRB=static prefix probe · SYN=synthetic prefix::1 fallback
| Target IP / hostname | Source | NL host (go6lab) | ITA host (Karsolink) | SLO host (6connect) | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ping | 1500B | :80 | :443 | reached | ping | 1500B | :80 | :443 | reached | ping | 1500B | :80 | :443 | reached | ||
2a05:4140:315:315::315www.mlgt.info | whois | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - |
2a05:4140:317::5mail.mlgt.it | MX | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - |
2a05:4140:317::c0femail.mlgt.info | MX | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - |
2a0f:9400:6111::11 | SPF | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - |
2a0f:9400:690d::4pns.mlgt.info | NS | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - | ✓ | ✗ | open | open | - |
RPKI & ASPA
18 of 18 prefix(es) covered by a valid ROA.
| Prefix | RPKI state | Reason | Covering VRP(s) |
|---|---|---|---|
2a05:4140:305::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a0f:9400:611d::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS204508 /48 maxLen 48 (ripe) |
2a0f:9400:6111::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS204508 /48 maxLen 48 (ripe) |
2a05:4140:333::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a0f:9400:690d::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS204508 /48 maxLen 48 (ripe) |
2a13:aac4:1000::/36 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS44908 /36 maxLen 48 (ripe); AS204508 /36 maxLen 48 (ripe) |
2a05:4140:380::/42 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:314::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:309::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:321::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:315::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a13:aac4:1000::/37 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS44908 /36 maxLen 48 (ripe); AS204508 /36 maxLen 48 (ripe) |
2a05:4140:306::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:307::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:317::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:304::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:316::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
2a05:4140:319::/48 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS212271 /40 maxLen 40 (ripe); AS204508 /40 maxLen 48 (ripe); AS212271 /29 maxLen 48 (ripe) |
Random-IP probe (yarrp) into the AS' prefixes
At least one router inside AS204508's announced prefix replied during the random-target probe. Hop(s): 2a0f:9400:611d::2, 2a0f:9400:690d::4. These are candidate targets for direct testing.
ICMPv6 Type 2 (Packet Too Big) acceptance - active test
Type 2 not honored. Forged ICMPv6 PTB (MTU=1280) had no effect: the next 1500-byte Echo Reply still arrived whole (1360 B, no Fragment Header). PMTUD does not work toward this address from us.
method: ICMPv6 Echo + forged PTB, tested 2026-05-30T12:48:02Z
Failure detail — what to grep in your logs
| vantage | our source | your target | method | result | tested (UTC) |
|---|---|---|---|---|---|
| go6lab | 2a00:8642:42::75 | 2a0f:9400:690d::4 | icmp6-echo | not_honored | 2026-05-30T12:48:02Z |
| karsolink | 2a12:d8c0:105a:9001::a154 | 2a0f:9400:690d::4 | tls | not_honored | 2026-05-30T12:57:26Z |
| odin | 2607:fae0:a000::42 | 2a0f:9400:690d::4 | icmp6-echo | not_honored | 2026-05-30T12:47:41Z |
Attempt log (go6lab):
icmp6-echo→ not_honored (size_before=1360, size_after=1360)dns-tcp→ inconclusive (size_before=31, size_after=None): max DNS-over-TCP segment 31B; not big enough to test PMTU shrinktls→ not_honored (size_before=1386, size_after=2772)http→ inconclusive (size_before=432, size_after=None): natural max segment 432B already <= 1220B; nothing to shrink
Attempt log (karsolink):
icmp6-echo→ no_echo: no Echo Reply to 1300-byte probedns-tcp→ inconclusive (size_before=31, size_after=None): max DNS-over-TCP segment 31B; not big enough to test PMTU shrinktls→ not_honored (size_before=1428, size_after=1428)http→ inconclusive (size_before=432, size_after=None): natural max segment 432B already <= 1220B; nothing to shrink
Attempt log (odin):
icmp6-echo→ not_honored (size_before=1360, size_after=1360)dns-tcp→ inconclusive (size_before=31, size_after=None): max DNS-over-TCP segment 31B; not big enough to test PMTU shrinktls→ not_honored (size_before=1386, size_after=1386)http→ inconclusive (size_before=432, size_after=None): natural max segment 432B already <= 1220B; nothing to shrink
To match the corresponding ICMPv6 packet on your side (host firewall, AS edge, or transit tap), look for our PTBs around the timestamps above:
sudo tcpdump -i any -n -e 'icmp6 and ip6[40] = 2 and (src host 2607:fae0:a000::42 or src host 2a00:8642:42::75 or src host 2a12:d8c0:105a:9001::a154)'
If you see our PTBs arriving but the destination's TCP/Echo flow does not shrink, the drop is in the destination kernel (cause 3 below). If you don't see them at all, drop is upstream of you (cause 2). If you only see them from one of our two source IPs, the drop is path-asymmetric — one transit on the asymmetric route is filtering, the other is not.
What does "Type 2 not honored" actually mean? — click to expand
What this test does
Using the icmp6-echo method, we send a 1500-byte ICMPv6 Echo Request, then a forged ICMPv6 Type 2 (Packet Too Big) declaring path MTU=1280, and observe whether the next Echo Reply arrives split into IPv6 fragments. RFC 4890 requires hosts and intermediate networks not to filter ICMPv6 Type 2; the destination's TCP/UDP stack must act on a received PTB by lowering its Path MTU cache for that destination, which makes subsequent segments smaller.
What we measured
The TCP segment size your server emitted before our forged Type 2 was 1360 B; after, it was 1360 B. No change. RFC 4890 ("Type 2 messages MUST NOT be filtered") expects subsequent segments to shrink to fit a Path MTU of 1280 B.
Three plausible causes
- Your host firewall is dropping ICMPv6 Type 2 inbound. Many default firewall rule sets only allow Echo Request/Reply and Neighbor Discovery, silently dropping all other ICMPv6 types - including Packet Too Big.
- An upstream / transit network is dropping ICMPv6 Type 2 before it reaches you. Some transit ASes filter ICMPv6 messages other than Echo at the edge. The forged PTB never arrives, so your stack never has a chance to act on it.
- Your kernel is ignoring the PTB. Linux / BSD stacks normally accept ICMPv6 PTB and update the route cache, but a few sysctls (or a hardened kernel) can be configured to ignore PMTU updates - typically as part of an over-aggressive anti-spoofing or uRPF policy.
How to check & fix (Linux examples)
1. Confirm Type 2 is not blocked at the host firewall:
sudo ip6tables -L INPUT -nv | grep -iE 'icmpv6|packet-too-big' sudo nft list ruleset 2>/dev/null | grep -A1 'icmpv6'
If you see rules dropping ICMPv6 unconditionally, change them to permit at least icmpv6 type packet-too-big (and destination-unreachable, time-exceeded, parameter-problem per RFC 4890).
2. Confirm the kernel accepts incoming PTB:
sudo sysctl net.ipv6.conf.all.accept_redirects net.ipv4.ip_no_pmtu_disc net.ipv6.route.mtu_expires
The defaults (accept_redirects=1, ip_no_pmtu_disc=0) are the right values for honoring PTB.
3. Live trace: while we have an open TCP flow with a small MSS (we run our test from 2607:fae0:a000::42 on odin, 2a00:8642:42::75 on go6lab and 2a12:d8c0:105a:9001::a154 on karsolink), watch for our forged Type 2 arriving on your interface:
sudo tcpdump -i any -n -e 'icmp6 and ip6[40] = 2 and (src host 2607:fae0:a000::42 or src host 2a00:8642:42::75 or src host 2a12:d8c0:105a:9001::a154)'
If you see our PTBs arriving but TCP segments still stay big, the drop is in your kernel or NIC offload (cause 3). If you don't see them at all, the drop is upstream of you (cause 2) - ask your upstream(s) to permit ICMPv6 Type 2.
4. If your test target above (2026-05-30T12:48:02Z) is a host that you don't own (e.g. a third-party DNS / TLS server you happen to operate prefixes for), the verdict reflects that specific host's behaviour - try the test against a server you do own and we'll happily re-run.
RFC 4890 references: §4.3.1 (Packet Too Big - MUST NOT be dropped), and RFC 8201 for the broader PMTUD requirement.
From NL host (go6lab) echo onlyRFC 4890 ✗
filter likely at: past AS47152 (Fibersix) · min PMTU on path: 1458
Path (traceroute + mtr + PTR)
| # | IP / PTR | RTT | mtr loss | AS | AS holder |
|---|---|---|---|---|---|
| 1 | 2a00:8642:42::3 | 1.1ms | 0% | AS203993 | STEFFANN-DC-AS - S.J.M. Steffann, NL |
| 2 | gw.friends.steffann.nl 2a00:8642:1000:f000::1 | 2.0ms | 0%* | AS203993 | STEFFANN-DC-AS - S.J.M. Steffann, NL |
| 3 | 2a00:1ca8:1::194 | 2.3ms | 0% | AS50673 | Serverius-as - Serverius Holding B.V., NL |
| 4 | 2a03:3f40::10:33 | 9.9ms | 0% | AS50673 | Serverius-as - Serverius Holding B.V., NL |
| 5 | 2a02:2d8:1:a001:232a | 4.3ms | 0% | - | |
| 6 | RT.MIX.MIL.IT.retn.net 2a02:2d8::57f5:e080 | 18.7ms | 0% | AS9002 | RETN-AS - RETN Limited, GB |
| 7 | GW-AS47152.retn.net 2a02:2d8:1:b832:232a::1 | 22.6ms | 0% | AS9002 | RETN-AS - RETN Limited, GB |
| 8 | itcap0-r0.fibersix.it 2a10:ba87:fef0::1 | 40.2ms | 0% | AS47152 | Fibersix - Francesco Ascione trading as FIBERSIX, IT |
| 9 | * | - | 0% | - | |
| 10 | * | - | - | - | |
| 11 | * | - | - | - | |
| 12 | * | - | - | - | |
| 13 | * | - | - | - | |
| 14 | * | - | - | - | |
| 15 | * | - | - | - | |
| 16 | * | - | - | - | |
| 17 | * | - | - | - | |
| 18 | * | - | - | - | |
| 19 | * | - | - | - | |
| 20 | * | - | - | - | |
| 21 | * | - | - | - | |
| 22 | * | - | - | - | |
| 23 | * | - | - | - | |
| 24 | * | - | - | - |
tracepath -6
1?: [LOCALHOST] 0.049ms pmtu 1458
1: 2a00:8642:42::3 1.550ms
1: 2a00:8642:42::3 1.645ms
2: gw.friends.steffann.nl 2.428ms
3: 2a00:1ca8:1::194 3.145ms
4: 2a03:3f40::10:33 6.584ms
5: ae1-499.RT.SRV.DRO.NL.retn.net 5.040ms
6: RT.MIX.MIL.IT.retn.net 19.112ms
7: GW-AS47152.retn.net 22.445ms asymm 11
8: itcap0-r0.fibersix.it 41.565ms asymm 12
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Too many hops: pmtu 1458
Resume: pmtu 1458 From ITA host (Karsolink) echo onlyRFC 4890 ✗
filter likely at: past AS47152 (Fibersix) · min PMTU on path: 1500
Path (traceroute + mtr + PTR)
| # | IP / PTR | RTT | mtr loss | AS | AS holder |
|---|---|---|---|---|---|
| 1 | karsolink-01.net.karsolink.com 2a12:d8c0:105a:9001::1 | 0.4ms | 0%* | AS204471 | KARSOLINK - 2S Computers SRL, IT |
| 2 | 2a12:d8c0:109f:121::a1 | 0.5ms | 0%* | AS204471 | KARSOLINK - 2S Computers SRL, IT |
| 3 | 2a12:d8c0:101f:103::1 | 8.4ms | 10% | AS204471 | KARSOLINK - 2S Computers SRL, IT |
| 4 | * | - | - | - | |
| 5 | * | - | - | - | |
| 6 | itcap0-r0.fibersix.it 2a10:ba87:fef0::1 | 28.0ms | 0% | AS47152 | Fibersix - Francesco Ascione trading as FIBERSIX, IT |
| 7 | * | - | 0% | - | |
| 8 | * | - | - | - | |
| 9 | * | - | - | - | |
| 10 | * | - | - | - | |
| 11 | * | - | - | - | |
| 12 | * | - | - | - | |
| 13 | * | - | - | - | |
| 14 | * | - | - | - | |
| 15 | * | - | - | - | |
| 16 | * | - | - | - | |
| 17 | * | - | - | - | |
| 18 | * | - | - | - | |
| 19 | * | - | - | - | |
| 20 | * | - | - | - | |
| 21 | * | - | - | - | |
| 22 | * | - | - | - | |
| 23 | * | - | - | - | |
| 24 | * | - | - | - |
Rate-limited ICMPv6: hop 3 (loss between 5% and 95% across mtr cycles - the router replies but only sometimes).
tracepath -6
1?: [LOCALHOST] 0.030ms pmtu 1500
1: karsolink-01.net.karsolink.com 0.691ms
1: karsolink-01.net.karsolink.com 0.675ms
2: 2a12:d8c0:109f:121::a1 1.020ms
3: 2a12:d8c0:101f:103::1 9.073ms
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Too many hops: pmtu 1500
Resume: pmtu 1500 From SLO host (6connect) echo onlyRFC 4890 ✗
filter likely at: past AS47152 (Fibersix) · min PMTU on path: 1458
Path (traceroute + mtr + PTR)
| # | IP / PTR | RTT | mtr loss | AS | AS holder |
|---|---|---|---|---|---|
| 1 | fw1-lju.6connect.com 2607:fae0:a000::2 | 4.4ms | 0%* | AS8038 | 6CONNECT - 6connect, Inc., US |
| 2 | * | - | 0%* | - | |
| 3 | 2a00:ee1:800:9::1 | 1.7ms | 0% | AS5603 | SIOL-NET - Telekom Slovenije, d.d., SI |
| 4 | 2a00:ee0:1:10::2 | 0.9ms | 0% | AS5603 | SIOL-NET - Telekom Slovenije, d.d., SI |
| 5 | de-cix1.rt.act.fkt.de.retn.net 2001:7f8::232a:0:1 | 14.2ms | 0% | - | NA |
| 6 | RT.MIX.MIL.IT.retn.net 2a02:2d8::57f5:e080 | 16.0ms | 0% | AS9002 | RETN-AS - RETN Limited, GB |
| 7 | GW-AS47152.retn.net 2a02:2d8:1:b832:232a::1 | 16.4ms | 0% | AS9002 | RETN-AS - RETN Limited, GB |
| 8 | itcap0-r0.fibersix.it 2a10:ba87:fef0::1 | 33.5ms | 0% | AS47152 | Fibersix - Francesco Ascione trading as FIBERSIX, IT |
| 9 | * | - | 0% | - | |
| 10 | * | - | - | - | |
| 11 | * | - | - | - | |
| 12 | * | - | - | - | |
| 13 | * | - | - | - | |
| 14 | * | - | - | - | |
| 15 | * | - | - | - | |
| 16 | * | - | - | - | |
| 17 | * | - | - | - | |
| 18 | * | - | - | - | |
| 19 | * | - | - | - | |
| 20 | * | - | - | - | |
| 21 | * | - | - | - | |
| 22 | * | - | - | - | |
| 23 | * | - | - | - | |
| 24 | * | - | - | - |
tracepath -6
1?: [LOCALHOST] 0.031ms pmtu 1500
1: fw1-lju.6connect.com 0.572ms
1: fw1-lju.6connect.com 0.498ms
2: no reply
3: 2a00:ee1:800:9::1 1.905ms
4: 2a00:ee0:1:15::2 1.294ms asymm 3
5: de-cix1.rt.act.fkt.de.retn.net 52.439ms asymm 9
6: RT.MIX.MIL.IT.retn.net 16.080ms asymm 9
7: GW-AS47152.retn.net 15.989ms asymm 8
8: GW-AS47152.retn.net 15.777ms pmtu 1458
8: itcap0-r0.fibersix.it 34.538ms asymm 9
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Too many hops: pmtu 1458