1
Diagnostic step 1
From the client, run 'tracepath -6 <hostname>' or 'mtr -6' to detect where IPv6 packets stop; capture tcpdump at the client and (if possible) at router WAN to confirm where SYNs disappear.
FULL AI ANALYSIS
IPv6 connections open but certain websites time out after enabling SLAAC on home router
Generated Oct 3, 2026, 12:35:03 AM
Analysis summary
Symptoms indicate IPv6 reachability is partial: name resolution and local IPv6 addressing are fine but end-to-end TCP connectivity to some prefixes fails. Likely causes include: ISP-side IPv6 routing blackholes or filtering for specific provider prefixes, PMTU/ICMPv6 filtering causing handshake stalls for paths requiring path MTU discovery, router misconfiguration of RA/RDNSS or incorrect next-hop delegation, or transit middlebox (CDN/edge) blocking from the assigned IPv6 prefix. Local client problems are less likely since tethered/cellular networks succeed and multiple clients show same behavior.
Possible causes
ISP or transit provider has incorrect routing for specific IPv6 prefix ranges (packets lost after border).
90%ICMPv6 messages blocked on router or by ISP (broken Path MTU Discovery causes TCP stalls for some sites).
55%MTU mismatch on PPPoE or tunnel causing packets to be dropped when DF set and ICMPv6 unreachable replies blocked by middlebox/ISP.
45%Router RA / prefix delegation misconfiguration producing addresses that are routable locally but not advertised correctly to upstream.
10%Suggested fixes
2
Diagnostic step 2
Check router IPv6 prefix delegation and routing table; ensure upstream gateway using the delegated prefix is present and that the router advertises the correct prefix via RA.
3
Diagnostic step 3
Temporarily set MTU lower on client (e.g., 1280) and retry to see if PMTU issues are the cause; if lowering MTU fixes connections, investigate ICMPv6 filtering.
4
Diagnostic step 4
Enable detailed IPv6 logging on router and inspect for dropped packets or RA/ND errors; if router firmware is custom, test with vendor firmware or factory reset to rule out firmware bugs.
5
Diagnostic step 5
If diagnostics show packets reaching ISP border but no replies, contact ISP with traceroute/tcpdump evidence and request they check IPv6 routing and ICMPv6 handling; escalate to ISP support if they confirm upstream routing issues (provide captured traces).