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If IPv6 is unavailable while IPv4 still works, a Linux computer is not automatically offline. IPv4-only services usually remain reachable, but an IPv6-only service cannot be reached through ordinary IPv4. Dual-stack services may work immediately, take longer, or fail, depending on DNS results, address-selection rules, and how each application retries connections.
The first diagnostic question is therefore not simply “Is IPv6 broken?” It is whether IPv6 is disabled, enabled without a usable route, delayed by DNS, or being handled poorly by a particular application. Those conditions produce different symptoms and require different fixes.
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“No working IPv6” describes more than one failure
Two Linux systems can show similar browser or application errors while having fundamentally different IPv6 states. Separate the local configuration from actual end-to-end reachability before changing anything.
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IPv6 disabled in the kernel or on an interface
The Linux IPv6 module has a disable=1 parameter. The kernel documentation says that, with this module-level disablement, no IPv6 addresses are added to interfaces and IPv6 sockets cannot be opened. Re-enabling IPv6 after this particular module setting requires a reboot.
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A separate per-interface control, disable_ipv6, disables IPv6 on interfaces and prevents IPv6 addresses from being added there. This is different from merely losing an upstream route: an application may be unable to create an IPv6 socket at all when IPv6 is disabled.
IPv6 enabled locally, but no usable path
IPv6 can remain enabled, with addresses visible in Linux, while packets cannot reach the wider network. A missing default route, a broken router advertisement, an upstream outage, or a firewall can leave the host with no functioning end-to-end path. In this state, creating an IPv6 socket may succeed, but the connection can fail or wait for a response.
Autoconfiguration disabled is not the same as disabling IPv6
With the kernel’s autoconf=0 setting, interfaces receive only the IPv6 loopback address (::1) and link-local addresses unless another configuration supplies additional addresses. Those addresses support local or link-local communication; they do not, by themselves, prove Internet IPv6 reachability.
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| Destination or condition | What DNS may return | Likely result without working IPv6 | Why |
|---|---|---|---|
| IPv6-only service | An AAAA record, with no usable A record | Connection fails | IPv4 cannot substitute for a destination that has no IPv4 endpoint. |
| Dual-stack service and an application that retries alternatives | Both AAAA and A records | Often succeeds over IPv4 after an IPv6 attempt, or succeeds quickly if addresses are raced in parallel | The application uses more than one address returned by the resolver. |
| Dual-stack service and an application that stops after the first failure | Both AAAA and A records | Can fail even though IPv4 works | The application does not continue to the IPv4 address. |
| Resolver or DNS appliance mishandles parallel queries | A and AAAA lookups may be delayed or malformed | Name resolution is slow or unreliable before any connection is attempted | The problem is DNS query handling, not proof that IPv6 packet routing is broken. |
These outcomes explain why one browser, package manager, or command-line client may work while another appears to hang. “The network works over IPv4” does not guarantee that every dual-stack program will fall back in the same way.
How address selection and fallback actually work
For a hostname, Linux applications commonly call getaddrinfo(3). The result can contain multiple addresses, including both AF_INET6 (IPv6) and AF_INET (IPv4) entries. The resolver supplies a list; the application decides how to consume it.
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RFC 6724 defines default destination-address selection. The preferred order can favor IPv6 or IPv4 according to the available source addresses and policy. A host that has no suitable IPv6 source address may therefore prefer IPv4, while a host that appears IPv6-capable may receive an IPv6 destination first.
The RFC also makes an important boundary clear: its algorithms do not override decisions made by applications or upper-layer protocols. A well-behaved client can iterate through returned addresses, use a “Happy Eyeballs”-style race, or apply its own timeout and retry policy. Another client can try one address and stop. There is no Linux-wide promise that every program will automatically switch to IPv4.
Why a website can appear to hang
If an application tries an unreachable IPv6 endpoint first, it may wait until its connect attempt times out before trying IPv4. The delay is not universal: it depends on the application’s connection strategy, socket timeouts, resolver behavior, and whether attempts are serial or parallel. Do not infer a standard number of seconds from one program’s behavior.
Why DNS can be the real source of the delay
glibc resolver documentation says IPv4 and IPv6 lookups are performed in parallel by default. Some DNS appliances mishandle those simultaneous requests. The resolver’s single-request option makes the queries sequential as a workaround, but sequential lookups can be slower. A slow lookup in this situation points to DNS query handling, not necessarily to a broken IPv6 route.
Diagnostic workflow for a Linux host
Run the checks in order. Keep name resolution, local configuration, routing, and application behavior as separate questions.
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1. Check whether IPv6 is disabled
Inspect the active sysctl values:
sysctl net.ipv6.conf.all.disable_ipv6
sysctl net.ipv6.conf.default.disable_ipv6
sysctl net.ipv6.conf.all.autoconfA value of 1 for disable_ipv6 indicates that IPv6 is disabled for the corresponding scope. Check individual interfaces too, because a global value and an interface value can differ:
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sysctl net.ipv6.conf.<interface>.disable_ipv6Replace <interface> with a real name such as eth0 or wlan0. If the machine was started with the IPv6 module parameter disable=1, inspect the system’s kernel/module configuration and plan for a reboot when reversing that module-level setting. Do not treat autoconf=0 as equivalent to a complete IPv6 disable.
2. Inspect addresses, remembering what they do not prove
ip -6 addr show
ip -6 addr show dev <interface>::1 is loopback. Addresses beginning with fe80:: are link-local. Seeing either kind confirms only local IPv6 functionality; neither demonstrates Internet reachability. A global-looking address is useful evidence that configuration exists, but an address alone still does not prove that packets can complete an end-to-end trip.
If you change disable_ipv6 from enabled to disabled on an interface, Linux can create a link-local address and begin Duplicate Address Detection. That transition is expected and does not by itself demonstrate that an upstream route is available.
3. Inspect IPv6 routes
ip -6 route show
ip -6 route get 2001:db8::1The first command displays configured IPv6 routes. The second asks the kernel which route it would use for a destination; use a real IPv6 destination in an operational test rather than the documentation prefix shown here. A host can have addresses but no usable default route. Conversely, a displayed route can still lead to an unreachable gateway, so route presence is not the same as successful delivery.
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4. Separate DNS from transport
First see what address families name resolution returns:
getent ahosts example.comLook for IPv4 and IPv6 results. The presence of an AAAA result says that DNS published an IPv6 address; it does not say that your host has a working IPv6 path. If your system uses a resolver service, its equivalent query tool can show the same distinction.
Measure lookup time independently from connection time:
time getent ahosts example.comIf this command is slow while direct IPv4 and IPv6 tests are quick, investigate resolver configuration or DNS appliance behavior. Changing host IPv6 settings will not repair a DNS server that mishandles parallel A and AAAA queries.
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For a hostname that is expected to support both protocols, compare explicit requests:
curl -4 -I https://example.com
curl -6 -I https://example.comThe IPv4 request tests the IPv4 path; the IPv6 request tests name resolution for IPv6 plus the IPv6 transport path. An IPv6 failure with a successful IPv4 request establishes that IPv4 remains usable, but it does not identify whether the IPv6 fault is local, routed, filtered, or upstream.
ICMP tools such as ping -4 and ping -6 can provide additional clues, but a failed ping is not conclusive when a network blocks ICMP. Prefer a protocol-specific connection test for the service you actually need.
6. Compare applications
Repeat the same destination test with the affected programs. If one dual-stack client quickly succeeds over IPv4 while another stalls, the difference is likely in address ordering, parallel connection attempts, retry limits, or timeout policy. getaddrinfo returns a list; each application determines whether it tries all entries, races them, or stops after an initial error.
Choosing the right remedy
When the host was intentionally disabled
If policy or a deliberate configuration disabled IPv6, restore it only when the network is meant to provide IPv6. Remove the relevant module or sysctl disablement according to your distribution’s configuration method, then reboot when the module-level disable=1 parameter was used. Recheck addresses, routes, and an actual service connection afterward.
When IPv6 is enabled but unroutable
Investigate the default route, router advertisements, gateway reachability, firewall rules, and upstream service. Disabling IPv6 may hide symptoms for some applications, but it does not repair the missing path and can make IPv6-only destinations permanently inaccessible.
When only one application fails
Look for application-specific address-selection or timeout settings, and update or reconfigure that client if appropriate. Do not assume that a system-wide IPv6 change is justified when other dual-stack applications already recover successfully over IPv4.
When name resolution is slow
Focus on the resolver and DNS appliance. The glibc single-request behavior can work around appliances that mishandle parallel A and AAAA requests, but sequential queries may add latency. Treat this as a DNS configuration decision, not as evidence that IPv6 routing must be disabled.
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Common misconceptions
- “IPv6 failed, so the whole Linux system is offline.” IPv4-capable destinations can continue to work.
- “Every dual-stack program automatically falls back.” Fallback and retry behavior belong partly to the application.
- “An IPv6 address proves Internet connectivity.” Loopback, link-local, or otherwise unroutable addresses do not establish end-to-end reachability.
- “An AAAA record proves the route works.” DNS publication and packet delivery are separate checks.
- “Disabling IPv6 fixes any slow website.” The delay may be DNS lookup handling or an application’s connection strategy.
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