AS213115 - MMITECHPIRFC 4890 ✓RPKI -ASPA -

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AS overview

Test target: 2001:67c:2474::3586:3761 · (SPF ip6: literal) · address space: PI · prefixes announced: 1

prefix(es): 2001:67c:2474::/48

Targets & per-vantage-point probe results (8 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 / hostnameSourceNL host (go6lab)ITA host (Karsolink)SLO host (6connect)
ping1500B:80:443reachedping1500B:80:443reachedping1500B:80:443reached
2001:67c:2474::1hit??-??-??-
2001:67c:2474::3586:3761SPFopenopen-openopen-openopen-
2001:67c:2474::5db9:24c2SPFopenopen-openopen-openopen-
2001:67c:2474::6627:478b
ns1.mmitech.si
drv??-refusedrefused-refusedrefused-
2001:67c:2474::69fe:b543hitopenopen-openopen-openopen-
2001:67c:2474::7a41:9792
ns2.mmitech.si
drv??refusedrefusedrefusedrefused
2001:67c:2474::a536:1a31hitopen?-openrefused-openrefused-
2001:67c:2474::b474:1c43SPF??-refusedrefused-refusedrefused-

SIX peering

Not an SIX member, but our traceroute path crosses the SIX LAN from SLO host (6connect) - reached via an SIX-resident transit operator.

RPKI & ASPA

0 of 1 prefix(es) covered by a valid ROA, 1 not-found.

PrefixRPKI stateReasonCovering VRP(s)
2001:67c:2474::/48- not-foundno covering VRP

Random-IP probe (yarrp) into the AS' prefixes

We probed 15 arbitrary IPv6 address(es) inside AS213115's announced prefix(es). No router inside the prefix responded. The deepest visible hop was 2001:15c7:a000:b::2 at hop 18 - that router answers ICMPv6 Echo. This is not in AS213115's announced prefix; operationally it's the AS-edge / peering interface (often on a SIX peering address or an upstream's /127 link). The traceroute boundary is the AS edge: ICMPv6 Time-Exceeded responses from anything inside this AS are filtered.

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:

Why they disagree - different probe methods: These vantages picked different probe methods, so their verdicts are not directly comparable. The active test tries methods in order (icmp6-echodns-tcptlshttp) and stops at the first that produces a definitive verdict; if the chosen method differs, the underlying evidence differs too. Most often this is because the path PMTU on one vantage is below 1500 B, so the natural Echo Reply is already fragmented and the icmp6-echo method falls through to a TCP-based one. This is largely a measurement artifact, not a destination-behaviour difference.

Failure detail — what to grep in your logs
vantageour sourceyour targetmethodresulttested (UTC)
go6lab2a00:8642:42::752001:67c:2474::3586:3761icmp6-echohonored2026-05-30T10:32:00Z
karsolink2a12:d8c0:105a:9001::a1542001:67c:2474::3586:3761tlsnot_honored2026-05-30T10:35:29Z
odin2607:fae0:a000::422001:67c:2474::3586:3761icmp6-echohonored2026-05-30T10:31:56Z

Attempt log (go6lab):

  1. icmp6-echohonored (size_before=1360, size_after=None): natural Echo Reply at 1400B was already split into IPv6 fragments without our PTB intervention -- the destination's stack has a cached path MTU below 1400B from a previously-honored ICMPv6 Type 2, which is itself evidence that this destination honors PTB on this path.
  2. dns-tcpinconclusive (size_before=156, size_after=None): max DNS-over-TCP segment 156B; not big enough to test PMTU shrink
  3. tlsnot_honored (size_before=2856, size_after=2856)
  4. httpinconclusive (size_before=295, size_after=None): natural max segment 295B already <= 1220B; nothing to shrink

Attempt log (karsolink):

  1. icmp6-echono_echo: no Echo Reply to 1300-byte probe
  2. dns-tcpinconclusive (size_before=156, size_after=None): max DNS-over-TCP segment 156B; not big enough to test PMTU shrink
  3. tlsnot_honored (size_before=2856, size_after=2856)
  4. httpinconclusive (size_before=295, size_after=None): natural max segment 295B already <= 1220B; nothing to shrink

Attempt log (odin):

  1. icmp6-echohonored (size_before=1360, size_after=None): natural Echo Reply at 1400B was already split into IPv6 fragments without our PTB intervention -- the destination's stack has a cached path MTU below 1400B from a previously-honored ICMPv6 Type 2, which is itself evidence that this destination honors PTB on this path.
  2. dns-tcpinconclusive (size_before=156, size_after=None): max DNS-over-TCP segment 156B; not big enough to test PMTU shrink
  3. tlsnot_honored (size_before=3166, size_after=3166)
  4. httpinconclusive (size_before=295, size_after=None): natural max segment 295B 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 tls method, we open a TLS handshake to your TCP server, observe its TCP segment size, forge an ICMPv6 PTB declaring path MTU=1280, and observe whether subsequent segments shrink. 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 2856 B; after, it was 2856 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

  1. 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.
  2. 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.
  3. 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 (2026-05-30T10:35:29Z) 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.

tracepath6 per-hop PMTU drilldown

For each target where Type 2 verdicts disagreed across vantages, tracepath -6 ran from every vantage to capture per-hop PMTU evolution along that vantage's actual forward path. A pmtu change entry on a hop means that hop's router generated a PTB and we observed the shrink; absence of any change combined with a not_honored verdict suggests a router somewhere downstream is silently dropping >MTU packets (an RFC 4890 violation) rather than sending a PTB.

Target 2001:67c:2474::3586:3761

go6lab — verdict honored

verdict = honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22a00:8642:1000:f000::1
32a00:1ca8:1::194
42a03:3f40::10:33
52001:7f8:13::a503:4779:1
62a01:260:1:1::16
72a01:260:1:1::9f
82a01:260:1021::a
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karsolink — verdict not_honored

verdict = not_honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22a12:d8c0:109f:121::a1
32a12:d8c0:101f:6::1
4<span class='muted'>no reply</span>
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102001:850:501:bc::2
112001:15c7:a000:b::1
122001:15c7:a000:b::2
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odin — verdict honored

verdict = honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22607:fae0:a000:2::2
32a03:a100:0:201:1::1
42001:7f8:46::2:1283
52001:15c7:a000:b::1
62001:15c7:a000:b::2
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Target 2001:67c:2474::5db9:24c2

go6lab — verdict honored

verdict = honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22a00:8642:1000:f000::1
32a00:1ca8:1::194
42a03:3f40::10:33
52001:7f8:13::a503:4779:1
62a01:260:1:1::16
72a01:260:1:1::9f
82a01:260:1021::a
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karsolink — verdict not_honored

verdict = not_honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22a12:d8c0:109f:121::a1
32a12:d8c0:101f:6::1
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102001:850:501:bc::2
112001:15c7:a000:b::1
122001:15c7:a000:b::2
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14<span class='muted'>no reply</span>
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odin — verdict honored

verdict = honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22607:fae0:a000:2::2
32a03:a100:0:201:1::1
42001:7f8:46::2:1283
52001:15c7:a000:b::1
62001:15c7:a000:b::2
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Target 2001:67c:2474::69fe:b543

go6lab — verdict honored

verdict = honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22a00:8642:1000:f000::1
32a00:1ca8:1::194
42a03:3f40::10:33
52001:7f8:13::a503:4779:1
62a01:260:1:1::16
72a01:260:1:1::9f
82a01:260:1021::a
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karsolink — verdict not_honored

verdict = not_honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22a12:d8c0:109f:121::a1
32a12:d8c0:101f:6::1
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102001:850:501:bc::2
112001:15c7:a000:b::1
122001:15c7:a000:b::2
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odin — verdict not_honored

verdict = not_honored; final pmtu = 1500

#hop IPpmtu change
1<span class='muted'>no reply</span>
22607:fae0:a000:2::2
32a03:a100:0:201:1::1
42001:7f8:46::2:1283
52001:15c7:a000:b::1
62001:15c7:a000:b::2
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From NL host (go6lab)   openRFC 4890 ✓

filter likely at: (none) · min PMTU on path: 1500

4/4
Echo small (56B) 33.2ms
4/4
Echo 1500B (DF) 33.3ms
no
Type 1 dest-unreach
443,80
TCP responds on

Path (traceroute + mtr + PTR)

#IP / PTRRTTmtr lossASAS holder
12a00:8642:42::31.3ms0%AS203993STEFFANN-DC-AS - S.J.M. Steffann, NL
2gw.friends.steffann.nl
2a00:8642:1000:f000::1
2.3ms0%*AS203993STEFFANN-DC-AS - S.J.M. Steffann, NL
32a00:1ca8:1::1942.6ms0%AS50673Serverius-as - Serverius Holding B.V., NL
42a03:3f40::10:335.4ms0%AS50673Serverius-as - Serverius Holding B.V., NL
5t-2.interxionfra11.nlsix.net
2001:7f8:13::a503:4779:1
33.9ms0%-NA
62a01-260-1-1--16.core6.t-2.net
2a01:260:1:1::16
42.5ms0%AS34779T-2-AS - T-2, d.o.o., SI
72a01-260-1-1--9f.core6.t-2.net
2a01:260:1:1::9f
35.6ms0%AS34779T-2-AS - T-2, d.o.o., SI
82a01-260-1021--a.link6.t-2.net
2a01:260:1021::a
33.9ms0%AS34779T-2-AS - T-2, d.o.o., SI
9*-0%-
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tracepath -6

 1?: [LOCALHOST]                        0.032ms pmtu 1500
 1:  2a00:8642:42::3                                       1.191ms 
 1:  2a00:8642:42::3                                       1.190ms 
 2:  gw.friends.steffann.nl                                2.923ms 
 3:  2a00:1ca8:1::194                                      2.450ms 
 4:  2a03:3f40::10:33                                      4.506ms 
 5:  t-2.interxionfra11.nlsix.net                         34.651ms asymm  6 
 6:  2a01-260-1-1--16.core6.t-2.net                       42.257ms asymm  7 
 7:  2a01-260-1-1--9f.core6.t-2.net                       38.415ms asymm  8 
 8:  2a01-260-1021--a.link6.t-2.net                       34.781ms asymm 13 
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19:  no reply
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24:  no reply
     Too many hops: pmtu 1500
     Resume: pmtu 1500 

From ITA host (Karsolink)   openRFC 4890 ✗

filter likely at: (none) · min PMTU on path: 1500

4/4
Echo small (56B) 28.2ms
4/4
Echo 1500B (DF) 28.1ms
no
Type 1 dest-unreach
443,80
TCP responds on

Path (traceroute + mtr + PTR)

#IP / PTRRTTmtr lossASAS holder
1*---
22a12:d8c0:109f:121::a10.4ms-AS204471KARSOLINK - 2S Computers SRL, IT
32a12:d8c0:101f:6::19.9ms90%AS204471KARSOLINK - 2S Computers SRL, IT
4*---
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8*-0%-
9*-0%-
102001:850:501:bc::224.8ms0%AS8447A1TELEKOM-AT - A1 Telekom Austria AG, AT
112001:15c7:a000:b::127.1ms0%AS21283A1SI-AS - A1 Slovenija telekomunikacijske storitve,d.d., SI
122001:15c7:a000:b::227.7ms0%AS21283A1SI-AS - A1 Slovenija telekomunikacijske storitve,d.d., SI
13*-0%-
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Rate-limited ICMPv6: hop 3 (loss between 5% and 95% across mtr cycles - the router replies but only sometimes).

tracepath -6

 1?: [LOCALHOST]                        0.062ms pmtu 1500
 1:  karsolink-01.net.karsolink.com                        0.797ms 
 2:  2a12:d8c0:109f:121::a1                                7.763ms 
 3:  no reply
 4:  no reply
 5:  no reply
 6:  no reply
 7:  no reply
 8:  no reply
 9:  no reply
10:  2001:850:501:bc::2                                   25.045ms asymm  9 
11:  2001:15c7:a000:b::1                                  27.693ms 
12:  2001:15c7:a000:b::2                                  28.157ms 
13:  no reply
14:  no reply
15:  no reply
16:  no reply
17:  no reply
18:  no reply
19:  no reply
20:  no reply
21:  no reply
22:  no reply
23:  no reply
24:  no reply
     Too many hops: pmtu 1500
     Resume: pmtu 1500 

From SLO host (6connect)   openRFC 4890 ✓

filter likely at: (none) · min PMTU on path: 1500

4/4
Echo small (56B) 77.6ms
4/4
Echo 1500B (DF) 2.3ms
no
Type 1 dest-unreach
443,80
TCP responds on

Path (traceroute + mtr + PTR)

#IP / PTRRTTmtr lossASAS holder
1fw1-lju.6connect.com
2607:fae0:a000::2
0.3ms0%*AS80386CONNECT - 6connect, Inc., US
2*---
32a03:a100:0:201:1::10.7ms20%AS56635XENYA - XENYA inzeniring, proizvodnja in trgovina, d.o.o. Ljubljana, SI
4six.a1.si @SIX
2001:7f8:46::2:1283
1.0ms0%AS21283A1 Slovenija
52001:15c7:a000:b::11.5ms0%AS21283A1SI-AS - A1 Slovenija telekomunikacijske storitve,d.d., SI
62001:15c7:a000:b::22.1ms0%AS21283A1SI-AS - A1 Slovenija telekomunikacijske storitve,d.d., SI
7*-0%-
8*---
9*---
10*---
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24*---

Rate-limited ICMPv6: hop 3 (loss between 5% and 95% across mtr cycles - the router replies but only sometimes).

tracepath -6

 1?: [LOCALHOST]                        0.025ms pmtu 1500
 1:  fw1-lju.6connect.com                                  0.828ms 
 1:  fw1-lju.6connect.com                                  0.608ms 
 2:  no reply
 3:  no reply
 4:  six.a1.si                                             1.408ms 
 5:  2001:15c7:a000:b::1                                   2.731ms 
 6:  2001:15c7:a000:b::2                                   2.306ms asymm  7 
 7:  no reply
 8:  no reply
 9:  no reply
10:  no reply
11:  no reply
12:  no reply
13:  no reply
14:  no reply
15:  no reply
16:  no reply
17:  no reply
18:  no reply
19:  no reply
20:  no reply
21:  no reply
22:  no reply
23:  no reply
24:  no reply
     Too many hops: pmtu 1500
     Resume: pmtu 1500