AS overview
Test target: 2a01:2d8::ad1 · adns1.intercom.it · address space: PA · prefixes announced: 1
prefix(es): 2a01:2d8::/29
Targets & per-vantage-point probe results (3 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 | ||
2a01:2d8::ad1adns1.intercom.it | NS | ✓ | ✓ | ? | ? | - | ✓ | ✓ | ? | ? | - | ✓ | ✓ | ? | ? | - |
2a01:2d8::ad2adns2.intercom.it | NS | ✓ | ✓ | ? | ? | - | ✓ | ✓ | ? | ? | - | ✓ | ✓ | ? | ? | - |
2a01:2d8::ad3adns3.intercom.it | NS | ✓ | ✓ | ? | ? | - | ✓ | ✓ | ? | ? | - | ✓ | ✓ | ? | ? | - |
MIX-IT / NAMEX / MINAP / TOP-IX / VSIX / EQUINIX-MILAN / BGPX-ROME / NAMEX-BARI / PCIX / DECIX-PALERMO peering
MIX-IT / NAMEX / MINAP / TOP-IX / VSIX / EQUINIX-MILAN / BGPX-ROME / NAMEX-BARI / PCIX / DECIX-PALERMO member - Open peering, peering member, since 2010-07-29. IPv6 LAN: 2001:7f8:b:100:1d1:a5d0:8224:54. Locations: -.
RPKI & ASPA
1 of 1 prefix(es) covered by a valid ROA.
| Prefix | RPKI state | Reason | Covering VRP(s) |
|---|---|---|---|
2a01:2d8::/29 | ✓ valid | matched VRP (correct origin, length within maxLength) | AS8224 /29 maxLen 29 (ripe) |
Random-IP probe (yarrp) into the AS' prefixes
At least one router inside AS8224's announced prefix replied during the random-target probe. Hop(s): 2a01:2d8::1:100, 2a01:2d8::1:101, 2a01:2d8::1:103, 2a01:2d8::1:104, 2a01:2d8::1:8. These are candidate targets for direct testing.
ICMPv6 Type 2 (Packet Too Big) acceptance - active test
Vantage points disagree about Type 2 acceptance - transit ASes on one path may be filtering Type 2 even though the destination's stack accepts it on another path:
- NL host (go6lab): Type 2 honored. Forged PTB accepted; next response shrunk. (ICMPv6 Echo + forged PTB)
- ITA host (Karsolink): inconclusive: no Echo Reply to 1300-byte probe. (ICMPv6 Echo + forged PTB)
- SLO host (6connect): Type 2 honored. Forged PTB accepted; next response shrunk. (ICMPv6 Echo + forged PTB)
Why they disagree - reachability mismatch: One vantage couldn't drive a flow at all to this destination (no Echo Reply or no responding TCP port), so the active test had no behavioural change to observe. The other vantage's verdict is the only meaningful one here.
Failure detail — what to grep in your logs
| vantage | our source | your target | method | result | tested (UTC) |
|---|---|---|---|---|---|
| go6lab | 2a00:8642:42::75 | 2a01:2d8::ad1 | icmp6-echo | honored | 2026-05-30T12:43:00Z |
| karsolink | 2a12:d8c0:105a:9001::a154 | 2a01:2d8::ad1 | icmp6-echo | no_echo | 2026-05-30T12:46:48Z |
| odin | 2607:fae0:a000::42 | 2a01:2d8::ad1 | icmp6-echo | honored | 2026-05-30T12:42:48Z |
Attempt log (go6lab):
icmp6-echo→ honored (size_before=1460, size_after=None)dns-tcp→ inconclusive (size_before=156, size_after=None): max DNS-over-TCP segment 156B; not big enough to test PMTU shrinktls→ no_tcp: TLS connect failedhttp→ no_tcp: tcp/80 not open
Attempt log (karsolink):
icmp6-echo→ no_echo: no Echo Reply to 1300-byte probedns-tcp→ inconclusive (size_before=156, size_after=None): max DNS-over-TCP segment 156B; not big enough to test PMTU shrinktls→ no_tcp: TLS connect failedhttp→ no_tcp: tcp/80 not open
Attempt log (odin):
icmp6-echo→ honored (size_before=1460, size_after=None)dns-tcp→ inconclusive (size_before=156, size_after=None): max DNS-over-TCP segment 156B; not big enough to test PMTU shrinktls→ no_tcp: TLS connect failedhttp→ no_tcp: tcp/80 not open
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.
Where the path divergence is
Per-vantage probe + verdict (headline):
- go6lab: probe
icmp6-echo→ verdicthonored - karsolink: probe
icmp6-echo→ verdictno_echo - odin: probe
icmp6-echo→ verdicthonored
Full per-method matrix (all four methods run at each vantage):
| vantage | icmp6-echo | dns-tcp | tls | http |
|---|---|---|---|---|
| go6lab | honored | inconclusive | no_tcp | no_tcp |
| karsolink | no_echo | inconclusive | no_tcp | no_tcp |
| odin | honored | inconclusive | no_tcp | no_tcp |
✓ All vantages agree on method(s): icmp6-echo — the headline-method spread above is dispatcher noise, not a real Type 2 disagreement.
These vantage-level disagreements are rooted somewhere in the forward paths. Joining each per-vantage traceroute against the IP→AS lookup from our global yarrp mesh, the first hop where the paths land in different ASes is the most likely site of the offending filter / unreachable AS / Type 2 drop.
Suspect transit ASes (ranked by how often they appear at the divergence point on the path of the worse-classifying vantage): AS8224, AS174.
- From go6lab (open/Type-2=honored) the path enters AS174 at hop 7; from karsolink (open/Type-2=no_echo) the same hop is in AS8224. Last common AS: AS8224. The disagreement is most likely rooted in one of those two transit ASes.
- From go6lab (open/Type-2=honored) the path enters AS174 at hop 9; from odin (open/Type-2=honored) the same hop is in AS8224. Last common AS: AS8224. The disagreement is most likely rooted in one of those two transit ASes.
- From karsolink (open/Type-2=no_echo) the path enters an opaque hop at hop 9; from odin (open/Type-2=honored) the same hop is in AS8224. Last common AS: AS8224. The disagreement is most likely rooted in one of those two transit ASes.
Diagnosis is path-level, not packet-level: it tells you which transit AS is the prime suspect, not exactly which firewall rule is to blame. Use the tracepath6 output below (when available) for per-hop PMTU evidence on the same path.
✨ Diagnostic interpretation
Per-hop PTB acceptance walk
From the local vantage, we walk the forward path hop-by-hop, sending each hop a 1500-byte ICMPv6 Echo, then a forged PTB (MTU=1280) sourced from us, then another 1500-byte Echo. If the second reply arrives fragmented or smaller, that hop honoured the PTB. If unchanged, it didn't. The first ✗ in an otherwise-✓ path is the most likely filter location. Cross-country aggregation: see the global PTB filter atlas for transit ASes ranked by filter rate across all measurements.
| # | hop IP | AS | Holder | PTB acceptance |
|---|---|---|---|---|
| 1 | 2607:fae0:a000::2 | AS8038 | 6CONNECT - 6connect, Inc., US | - skipped (CoPP) |
| 2 | * | - | - (no IP) | |
| 3 | * | - | - (no IP) | |
| 4 | * | - | - (no IP) | |
| 5 | * | - | - (no IP) | |
| 6 | * | - | - (no IP) | |
| 7 | * | - | - (no IP) | |
| 8 | 2001:7f8:b:100:1d1:a5d0:8224:54 | - | NA | ? no_response |
| 9 | 2a01:2d8::1:100 | AS8224 | INTERCOM-AS - Intercom s.r.l., IT | ? no_response |
| 10 | 2a01:2d8::1:100 | AS8224 | INTERCOM-AS - Intercom s.r.l., IT | ✓ honored |
| 11 | 2a01:2d8::1:a1 | AS8224 | INTERCOM-AS - Intercom s.r.l., IT | ? no_response |
Tests host-mode PTB acceptance (PTBs aimed at the hop itself). A router that honours PTBs to itself can still be filtering PTBs transiting through it; this is one indicator, not proof of full PTB transparency. Hops in CoPP-rate-limit ranges are skipped to avoid false signals.
From NL host (go6lab) openRFC 4890 ✓
filter likely at: (none) · min PMTU on path: 1500
Path (traceroute + mtr + PTR)
| # | IP / PTR | RTT | mtr loss | AS | AS holder |
|---|---|---|---|---|---|
| 1 | 2a00:8642:42::3 | 0.7ms | 0% | AS203993 | STEFFANN-DC-AS - S.J.M. Steffann, NL |
| 2 | * | - | 0%* | - | |
| 3 | 2a00:1ca8:1::194 | 2.3ms | 20% | AS50673 | Serverius-as - Serverius Holding B.V., NL |
| 4 | 2a03:3f40::10:41 | 2.1ms | 10% | AS50673 | Serverius-as - Serverius Holding B.V., NL |
| 5 | * | - | 90% | - | |
| 6 | * | - | - | - | |
| 7 | be3343.ccr41.fra05.atlas.cogentco.com 2001:550:0:1000::9a36:3e8d | 11.1ms | 0% | AS174 | COGENT-174 - Cogent Communications, LLC, US |
| 8 | * | - | - | - | |
| 9 | port-channel5892.ccr92.mil02.atlas.cogentco.com 2001:550:0:1000::9a36:3ed9 | 25.0ms | 0% | AS174 | COGENT-174 - Cogent Communications, LLC, US |
| 10 | be5012.rcr51.b019138-1.mil02.atlas.cogentco.com 2001:550:0:1000::9a36:4bda | 26.9ms | 0% | AS174 | COGENT-174 - Cogent Communications, LLC, US |
| 11 | 2001:978:2:2a::1:2 | 24.9ms | 0% | AS174 | COGENT-174 - Cogent Communications, LLC, US |
| 12 | mix-cr1.intercom.it 2a01:2d8::1:8 | 26.5ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
| 13 | mlc-bkb-1.intercom.it 2a01:2d8::1:100 | 24.3ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
| 14 | 2a01:2d8::1:a1 | 26.2ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
| 15 | 2a01:2d8::1:a1 | 25.1ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
Rate-limited ICMPv6: hop 3, hop 4, hop 5 (loss between 5% and 95% across mtr cycles - the router replies but only sometimes).
tracepath -6
1?: [LOCALHOST] 0.093ms pmtu 1500
1: 2a00:8642:42::3 1.416ms
1: 2a00:8642:42::3 1.254ms
2: gw.friends.steffann.nl 2.338ms
3: 2a00:1ca8:1::194 2.730ms
4: 2a03:3f40::10:41 3.343ms
5: no reply
6: no reply
7: be2950.ccr42.fra05.atlas.cogentco.com 11.517ms
8: no reply
9: port-channel5891.ccr91.mil02.atlas.cogentco.com 25.580ms
10: be4466.rcr51.b019138-1.mil02.atlas.cogentco.com 26.328ms
11: 2001:978:2:2a::1:2 24.451ms
12: mix-cr1.intercom.it 25.863ms asymm 14
13: mlc-bkb-1.intercom.it 24.489ms
14: 2a01:2d8::1:a1 24.465ms
15: 2a01:2d8::1:a1 24.380ms !A
Resume: pmtu 1500 From ITA host (Karsolink) open
filter likely at: (none) · min PMTU on path: 1500
Path (traceroute + mtr + PTR)
| # | IP / PTR | RTT | mtr loss | AS | AS holder |
|---|---|---|---|---|---|
| 1 | * | - | 0%* | - | |
| 2 | * | - | 0%* | - | |
| 3 | 2a12:d8c0:101f:103::1 | 8.3ms | 80% | AS204471 | KARSOLINK - 2S Computers SRL, IT |
| 4 | intercom-v6.mix-it.net @MIX-IT / NAMEX / MINAP / TOP-IX / VSIX / EQUINIX-MILAN / BGPX-ROME / NAMEX-BARI / PCIX / DECIX-PALERMO 2001:7f8:b:100:1d1:a5d0:8224:54 | 9.5ms | 0% | - | NA |
| 5 | mix-cr1.intercom.it 2a01:2d8::1:8 | 9.4ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
| 6 | mlc-bkb-1.intercom.it 2a01:2d8::1:100 | 9.3ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
| 7 | 2a01:2d8::1:a1 | 9.2ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
| 8 | 2a01:2d8::1:a1 | 9.2ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
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: no reply
2: 2a12:d8c0:109f:121::a1 0.681ms
3: 2a12:d8c0:101f:103::1 8.919ms
4: intercom-v6.mix-it.net 9.897ms asymm 5
5: mix-cr1.intercom.it 9.985ms asymm 6
6: mlc-bkb-1.intercom.it 10.102ms asymm 5
7: 2a01:2d8::1:a1 10.108ms asymm 6
8: 2a01:2d8::1:a1 10.186ms !A
Resume: pmtu 1500 From SLO host (6connect) openRFC 4890 ✓
filter likely at: (none) · min PMTU on path: 1500
Path (traceroute + mtr + PTR)
| # | IP / PTR | RTT | mtr loss | AS | AS holder |
|---|---|---|---|---|---|
| 1 | fw1-lju.6connect.com 2607:fae0:a000::2 | 0.4ms | 0%* | AS8038 | 6CONNECT - 6connect, Inc., US |
| 2 | * | - | - | - | |
| 3 | * | - | - | - | |
| 4 | * | - | 60% | - | |
| 5 | * | - | - | - | |
| 6 | * | - | - | - | |
| 7 | * | - | - | - | |
| 8 | intercom-v6.mix-it.net @MIX-IT / NAMEX / MINAP / TOP-IX / VSIX / EQUINIX-MILAN / BGPX-ROME / NAMEX-BARI / PCIX / DECIX-PALERMO 2001:7f8:b:100:1d1:a5d0:8224:54 | 12.2ms | 0% | - | NA |
| 9 | mlc-bkb-1.intercom.it 2a01:2d8::1:100 | 12.2ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
| 10 | mlc-bkb-1.intercom.it 2a01:2d8::1:100 | 12.4ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
| 11 | 2a01:2d8::1:a1 | 12.5ms | 0% | AS8224 | INTERCOM-AS - Intercom s.r.l., IT |
Rate-limited ICMPv6: hop 4 (loss between 5% and 95% across mtr cycles - the router replies but only sometimes).
tracepath -6
1?: [LOCALHOST] 0.028ms pmtu 1500
1: fw1-lju.6connect.com 0.631ms
1: fw1-lju.6connect.com 0.426ms
2: no reply
3: no reply
4: e0-1.core1.lju1.he.net 2.247ms
5: no reply
6: port-channel1.core1.zag2.he.net 3.844ms
7: no reply
8: intercom.minap.it 12.444ms
9: 2a01:2d8::1:8 12.503ms asymm 10
10: 2a01:2d8::1:100 12.353ms asymm 9
11: 2a01:2d8::1:a1 12.425ms asymm 10
12: 2a01:2d8::1:a1 12.718ms !A
Resume: pmtu 1500