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If IPv6 is unavailable while IPv4 works, Linux is not automatically offline. IPv4-only services and many dual-stack services can still work, but IPv6-only destinations are unreachable. Dual-stack connections may succeed over IPv4, pause while an IPv6 attempt times out, or fail outright, depending on DNS results, address selection, routing, and how the application retries.
“No working IPv6” describes several different conditions
The visible symptom depends first on what has failed. A Linux host can have IPv6 disabled at the kernel or interface level, or it can have IPv6 enabled locally but no usable route to the wider network. Those states are not interchangeable.
IPv6 disabled in the kernel
The Linux IPv6 module has a disable parameter. With disable=1, the kernel does not add IPv6 addresses to interfaces and IPv6 sockets cannot be opened. The kernel documentation states: “No IPv6 addresses will be added to interfaces, and it will not be possible to open an IPv6 socket.” Enabling IPv6 after this module-level disable requires a reboot; changing an ordinary interface setting is not enough.
A distribution may pass the parameter through its boot configuration, commonly in a kernel command-line option such as an IPv6 disable setting. Remove that boot-time setting and reboot when you need to restore module-level IPv6.
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IPv6 disabled on one or more interfaces
The disable_ipv6 per-interface sysctl prevents IPv6 addresses and routes from being added on that interface. Other interfaces may still have IPv6 enabled. This is a narrower failure than disabling the IPv6 module, but a host using the affected interface will have no normal IPv6 path through it.
IPv6 enabled but unroutable
A host can display IPv6 addresses and still be unable to reach an IPv6 destination. The default route may be missing, the router advertisement may be absent, the upstream provider may not be forwarding traffic, or a firewall may be dropping packets. In this state an application can usually create an IPv6 socket; packet delivery fails later.
Autoconfiguration disabled is not the same as IPv6 disabled
With autoconf=0, Linux does not perform normal IPv6 address autoconfiguration. The kernel documentation says that only the IPv6 loopback address (::1) and link-local addresses are added to interfaces. Those addresses support local functions, but they do not by themselves provide general Internet reachability. Seeing a link-local address therefore does not prove that global IPv6 works.
What users can reach in each case
| Destination | Expected result when IPv6 is unavailable but IPv4 works | Why |
|---|---|---|
| IPv4-only service | Normally works over IPv4 | The service has an A record and an IPv4 endpoint; IPv6 is not required. |
| IPv6-only service | Fails | There is no IPv4 endpoint that can substitute for a destination available only through IPv6. |
| Dual-stack service | May work quickly, work after a pause, or fail | The result depends on returned addresses, source and destination selection, routing, and the application’s retry strategy. |
| Local loopback or link-local service | May still work locally | ::1 and link-local addresses can remain present even when Internet IPv6 is unavailable. |
IPv4 continuing to work is not evidence that IPv6 is healthy, and an IPv6 failure does not by itself mean that IPv4 has been disabled.
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Why dual-stack applications behave differently
DNS can return both address families
A dual-stack hostname can publish an A record for IPv4 and an AAAA record for IPv6. Linux’s getaddrinfo(3) interface can return a list containing both AF_INET and AF_INET6 results. The resolver supplies candidates; the application decides how to use them.
Address ordering is only a starting point
RFC 6724 defines default destination and source address selection. The preferred family depends on which source addresses are actually available, so a host with a usable global IPv6 source may prefer IPv6 while another host may prefer IPv4. The returned order is not a guarantee that a connection will succeed over the first address.
The RFC also makes the application boundary explicit: “The algorithms specified here do not override choices made by applications or upper-layer protocols.” A client can iterate through the list, race multiple attempts, impose its own policy, or stop after the first failure.
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Fallback is not universal
A well-behaved client may attempt IPv6 and then promptly try IPv4, or start both attempts close together and use the first successful connection. Another program may try one IPv6 address, wait for its connect timeout, and never attempt the IPv4 result. Thus two programs on the same Linux host can show different behavior against the same hostname.
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There is no single Linux-wide timeout for this situation. The pause is determined by the application’s connect timeout, retry count, resolver behavior, and network stack interactions. A browser, package manager, command-line tool, and custom program can all make different choices.
DNS delay can look like an IPv6 routing failure
glibc resolver documentation says IPv4 and IPv6 lookups are performed in parallel by default. Some DNS appliances mishandle those parallel A and AAAA requests. The single-request resolver option makes the queries sequential as a workaround, but sequential queries can make name resolution slower. That is a DNS query-handling problem, not proof that IPv6 packets are failing in the network.
How to diagnose the failure without guessing
Test the layers in order. Changing a global IPv6 switch before identifying the failed layer can hide the real cause and break services that do require IPv6.
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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →- Check kernel and interface disable controls. Inspect the running settings, for example:
cat /sys/module/ipv6/parameters/disable sysctl net.ipv6.conf.all.disable_ipv6 sysctl net.ipv6.conf.default.disable_ipv6 sysctl net.ipv6.conf.<interface>.disable_ipv6The module parameter indicates a kernel-level disable. The sysctl values show whether IPv6 is disabled globally, for newly created interfaces, or on a particular interface. Replace
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- Inspect addresses and routes.
ip -6 addr show dev <interface> ip -6 routeLook for more than
::1and a link-local address beginning withfe80::. A global or delegated address is useful evidence of configuration, but it still does not prove end-to-end reachability. Check for a usable IPv6 route, normally including a default route, and remember that a route can exist while an upstream device drops traffic. - Separate DNS from transport. Query the name and note whether it returns A and AAAA records:
getent ahosts example.comIf your distribution provides it,
resolvectl query example.comshows resolver results and timing. A slow or incomplete query points toward resolver or DNS infrastructure; a fast query followed by a stalled connection points toward transport, routing, firewall, or application behavior. - Test IPv4 and IPv6 independently.
curl -4 -I https://example.com curl -6 -I --connect-timeout 5 https://example.comThe first command forces IPv4 and the second forces IPv6. Use a hostname you are authorized to test. A failed
curl -6demonstrates that this particular HTTPS attempt could not complete; it does not by itself identify whether the cause is address configuration, routing, filtering, or the remote service. ICMP tests can also be filtered, so an unsuccessful ping is not conclusive on its own.Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy. - Compare more than one application. If one dual-stack client quickly succeeds while another waits or fails, examine how each consumes the
getaddrinfolist and how it handles connection errors. The difference is often in application retry policy rather than in the kernel’s ability to resolve the name.
How to read common symptoms
| Symptom | Most useful interpretation | Next check |
|---|---|---|
| IPv4-only sites work; one IPv6-only site fails immediately | The host has no usable IPv6 path to a destination that has no IPv4 alternative. | Check global IPv6 address, default route, upstream service, and firewall policy. |
| Some websites open quickly, others hang first | Those names are likely presenting different address sets, or the clients use different retry strategies. | Compare A/AAAA answers and test each family separately. |
| All names take a long time to resolve | Resolver communication may be delayed; this is not automatically an IPv6 routing failure. | Inspect DNS server behavior and whether parallel A/AAAA queries are mishandled. |
| A command fails instantly with “network unreachable” | The local stack may lack an IPv6 route or the interface may be disabled. | Inspect ip -6 addr, ip -6 route, and the relevant sysctls. |
| One application fails while another reaches the same host | The applications are likely ordering, racing, or retrying addresses differently. | Check application logs and connection timeout settings before changing system IPv6. |
| An interface has an IPv6 address but external tests fail | An address proves local configuration, not end-to-end connectivity. | Verify the default route, router advertisements, upstream delegation, and filtering. |
Fix the cause instead of disabling IPv6 blindly
Restore a missing or broken path
If IPv6 is supposed to be available, verify that the router advertises a prefix and default route, the provider has delegated usable space, and firewalls allow the traffic required by your policy. Correct the network or host route rather than changing address-family preference merely to hide the outage.
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Re-enable an interface that was intentionally disabled
For a per-interface setting, an administrator can clear the disable flag, using the actual interface name:
sudo sysctl -w net.ipv6.conf.<interface>.disable_ipv6=0Changing the value back can cause Linux to add a link-local address and start Duplicate Address Detection. Wait for that process and for router advertisements before judging Internet reachability. A persistent configuration must also be corrected in the distribution’s sysctl files; the command above changes the running system only.
Remove a module-level boot disable
If the IPv6 module was loaded with disable=1, remove the corresponding boot configuration and reboot. Because the kernel cannot open IPv6 sockets while that module-level disable is active, per-interface sysctls cannot restore full functionality in the current boot.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsAddress autoconfiguration deliberately
If autoconf=0 was set intentionally, configure a valid static address and route or restore the required autoconfiguration mechanism. Do not assume that the presence of fe80:: means the interface has an Internet-routable address.
Change resolver behavior only for a resolver problem
If testing shows that a DNS appliance mishandles parallel A and AAAA requests, single-request may be an appropriate glibc workaround. It serializes the lookups and can increase lookup time, so use it for the DNS failure you observed, not as a general response to broken IPv6 forwarding.
Account for application limitations
When the network is correctly configured but a particular client still stalls, inspect its address-family settings, retry count, and connect timeout. Updating or configuring that client may be safer than disabling IPv6 for the entire host. A client that stops after one failed address cannot be made reliable by DNS records alone; it must handle the complete list returned by getaddrinfo.
A practical decision path
- Only IPv6-only destinations fail: treat the problem as an IPv6 reachability outage.
- Dual-stack destinations fail after a pause but IPv4 tests pass: investigate IPv6 route failure and the client’s fallback timeout.
- Dual-stack destinations fail immediately with no IPv6 socket possible: inspect module and interface disable settings.
- Name lookups themselves are slow: investigate resolver and DNS appliance behavior before altering IPv6 routing.
- Different programs produce different results: compare their address ordering and retry logic.
- A link-local address is present but global tests fail: check for a global address, default route, upstream service, and filtering; link-local presence is insufficient.
What to remember
IPv4 and IPv6 are separate protocol paths. Losing one does not automatically remove the other. IPv6-only services need IPv6, while dual-stack services depend on how DNS, Linux address selection, routes, and the application interact. Fully disabling IPv6, disabling autoconfiguration, and leaving IPv6 enabled with a broken upstream route produce different symptoms and require different fixes. Diagnose those layers separately before applying a system-wide change.
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