From Fedora Project Wiki

Testing IPv6 support and dualstack networking can be very tricky. The purpose of this document is to provide a live resource on testing situations arising from existance and usage of two network layer protocols.

Tracker bug dependency tree.


IPv4 and IPv6 connectivity

Applications may behave differently based on connectivity checks. Those can be performed using the following per protocol characteristics.

  • Existence of any IP address (including or ::1
  • Existence of non-loopback IP address (including IPv6 link-local addresses)
  • Existence of a global IP address
  • Existence of a default route (or route for the target we want to reach)

Example: To test application behavior on a host connected via IPv6-only network you should make sure you only have the loopback IPv4 address and that you have a global IPv6 address and a default IPv6 gateway as that what you would normally call an IPv6-only setup. Similarly for IPv4-only setup you would make sure that there are only IPv6 loopback and link-local addresses and that there is a global IPv4 address and a default IPv4 gateway. But there are still many corner cases that may trigger unexpected issues.

IPv4 and IPv6 DNS records

You need to carefully choose a target hostname for dual-stack networking tests.

Examples of scenarios:

  • Hostname with a single A record or a single AAAA record
  • Hostname with one A record and one AAAA record
  • Hostname with multiple records per family, some of which are obsolete and inacceessible

IPv4 and IPv6 recursive DNS servers

This is entirely orthogonal to IPv4 and IPv6 DNS records and doesn't cause many issues.

IPv4 and IPv6 authoritative DNS servers

This is a rather special case important for domain operators. A bad configuration can make otherwise working IPv6 hosts inaccessible from IPv6-only networks with their own recursive servers.

Name resolution (using libc functions)

Name resolution features are provided by the GNU C Library (glibc) which is not yet ready for proper IPv6 and dual-stack operation as you can see when performing your tests. The C library comes with its own testing tool getent that has a special database called ahosts that runs getaddrinfo(), the library function that translates names to objects with addressing information. For your testing it is best used together with tools like strace, ltrace or even gdb so that you know exactly what is happening behind the scenes.

As the getent tool is very primitive, we created a tool called getaddrinfo just as the library function that handles a larger subset of the function's API.

Name resolution input

When application requests addressing information for a hostname with an optional service name, the library returns a list of addressing information objects. The order of objects in the list is significant and depends on operating system configuration and connectivity.

From the application

  • nodename
  • servname
  • protocol
  • socktype
  • flags
    • ...

From local configuration and connectivity checks

  • Files in /etc/ including nsswitch, hosts, services and more
  • To what extend is IPv4 and IPv6 available

From the outside world

  • DNS information
  • Multicast DNS information
  • LDAP information

Name resolution processing

What is requested

Not all information is requested at all times. Some information like canonical name must be explicitly requested by the application via AI_CANONNAME flag. It may be desirable to suppress other requests by local configuration or connectivity checks, a notable example being suppression of DNS AAAA queries on hosts without global connectivity.

What is passed to the application

Not all information that is learnt via requests is presented to the application. It is typically filtered according to input from the application. It is sometimes also filtered according to connectivity checks but that has caused more problems than improvements.

How it is sorted

There are rules for sorting addressing information returned by getaddrinfo(). One of the basic features is to return global IPv6 destinations before global IPv4 destinations. But when the library detects that IPv6 connectivity is not available, the reverse applies.

Example tests

Dual-stack host, destination with global IPv4 and IPv6

On a host with IPv4 and IPv6 connectivity we request addressing information of another host that is announced as dual-stack in DNS.

$ getent ahosts
2a02:38::1001   STREAM
2a02:38::1001   DGRAM  
2a02:38::1001   RAW    STREAM    DGRAM    RAW

You can see that getaddrinfo() returned six items for two unique IP addresses with the IPv6 address sorted first. If you only expected only two items, one for each IP address, see upstream bug 14990.

IPv4-only host, destination with global IPv4 and IPv6

We do the same on a host without IPv6 connectivity.

$ getent ahosts    STREAM    DGRAM    RAW    
2a02:38::1001   STREAM 
2a02:38::1001   DGRAM  
2a02:38::1001   RAW    

You can see that the result is the same as before except that IPv4 is sorted first.

Known issues

There are so many problems in glibc's name resolution API implementation that it's not practical to list them here.

Name resolution (using netresolve)

There is an experimental package called netresolve (TODO: not yet in Fedora) that consists of a library somewhat similar to the glibc name resolution API implementation and a set of debugging tools. Any application using libc API and a couple of other APIs can be run using wrapresolve to use the netresolve implementation instead and benefit from some advanced features and more extensive debugging. When using the libc backend, netresolve can be also used to test the glibc implementation. With the nss backend it can be used to test glibc nsswitch backends directly.

$ ./netresolve --node
response netresolve 0.0.1
ip 2a02:38::1001 any any 0 0 0 21599
ip any any 0 0 0 12589

You can see that netresolve behaves slightly different than getent ahosts and it by default returns one item per IP address. But you can easily tweak it to behave the same way.

$ ./netresolve --node --service ''
response netresolve 0.0.1
ip 2a02:38::1001 stream tcp 0 0 0 21021
ip 2a02:38::1001 dgram udp 0 0 0 21021
ip 2a02:38::1001 raw any 0 0 0 21021
ip stream tcp 0 0 0 12592
ip dgram udp 0 0 0 12592
ip raw any 0 0 0 12592

Note that netresolve is both the name of the command and the name of the library. The above tests are done using the command but the same results would be given to an application using the library. Unless you explicitly request, netresolve uses its internal name resolution modules and not the libc functions.


Tools with issues:

Testing client applications

Client application checklist

A client application should:

  • avoid using obsolete functions:
    • gethostbyname(), gethostbyname2()
    • gethostbyaddr()
    • ...
  • use system wide name resolution configuration
    • avoid implementing name resolution code in the application
    • avoid calling DNS libraries directly (there are other name resolution sources than DNS)
  • reduce timeouts when a fallback is available
    • reduce excessive DNS timeouts, possibly using a decent system wide name resolution library
    • reduce excessive TCP timeouts, possibly using a decent socket library
  • attempt all provided addresses until one succeeds or all fail
    • possibly using a decent socket library

Testing server applications

Server applications should be capable of accepting IPv4 and IPv6 clients and logging their addresses. Some server applications make use of addresses extensively, some just need to translate them to a meaningful name. Many server applications initiate connections to other hosts and thus the section about client applications applies to them as well.


Packages with decent IPv6 support

Following packages have decent IPv6 support but there may still be a lot of space for improvement. Virtually all packages in this category have known IPv6 issues or suboptimal fallback handling and could use further testing.

  • bind – The Berkeley Internet Name Domain (BIND) DNS (Domain Name System) server
  • curl – A utility for getting files from remote servers (FTP, HTTP, and others)
  • cyrus-imapd – A high-performance mail server with IMAP, POP3, NNTP and SIEVE support
  • c-ares – A library that performs asynchronous DNS operations
  • dnsmasq – A lightweight DHCP/caching DNS server
  • dnssec-trigger – NetworkManager plugin to update/reconfigure DNSSEC resolving
  • httpd – Apache HTTP Server
  • NetworkManager – Network connection manager and user applications
  • openssh – An open source implementation of SSH protocol versions 1 and 2
  • firefox – Mozilla Firefox Web browser
  • iproute – Advanced IP routing and network device configuration tools
  • iptables – Tools for managing Linux kernel packet filtering capabilities

Packages with broken or incomplete IPv6 support

Core system:

  • avahi – Local network service discovery
    • IPv6 turned off by default
    • nss-mdns doesn't do IPv6LL due to glibc limitations
  • glibc – The GNU libc libraries
    • limited and buggy getaddrinfo()


Packages that need porting to IPv6

Following packages contain tools or libraries that are IPv4-only but should be extended towards dual-stack networking.

Packages that are intended for IPv4-only operation

Following packages do not need any IPv6 testing as they contain IPv4 specific tools.

  • arptables
  • arpwatch

Notable uncategorized packages

Following packages weren't categorized yet but are considered important enough to be checked for IPv6 support. Please add more important system packages.

  • abrt – Automatic bug detection and reporting tool
  • apr – Apache Portable Runtime library
  • apr-util – Apache Portable Runtime Utility library
  • busybox – Statically linked binary providing simplified versions of system commands
    • various networking tools including ping
  • bzr – Friendly distributed version control system
  • cups – Common Unix Printing System
  • cvs – Concurrent Versions System
  • docker – Automates deployment of containerized applications
    • IPv6 in /etc/resolv.conf
    • IPv6 on bridged networks
  • dovecot – Secure imap and pop3 server
  • dracut – Initramfs generator using udev
  • firewalld – A firewall daemon with D-BUS interface providing a dynamic firewall
  • gdb – A GNU source-level debugger for C, C++, Fortran and other languages
  • glib2/glib-networking – Networking support for GLib
  • grub2 – Bootloader with support for Linux, Multiboot and more
  • gvfs – Backends for the gio framework in GLib
  • ipxe – A network boot loader
  • iptstate – A top-like display of IP Tables state table entries
  • ldns – Low–level DNS(SEC) library with API
  • lftp – A sophisticated file transfer program
  • libvirt – Library providing a simple virtualization API
  • mercurial – Mercurial – a distributed SCM
  • net-tools – Basic networking tools
  • nmap – Network exploration tool and security scanner
  • nss-myhostname – glibc plugin for local system host name resolution
  • nss – Network Security Services
  • openswan/libreswan/strongswan – IPSEC implementation with IKEv1 and IKEv2 keying protocols
  • postfix – Postfix Mail Transport Agent
  • postgresql – PostgreSQL client programs
  • quagga/bird – Routing daemon
  • rsync – A program for synchronizing files over a network
  • sendmail – A widely used Mail Transport Agent (MTA)
  • squid – The Squid proxy caching serverb
  • sssd – System Security Services Daemon
  • syslinux – Simple kernel loader which boots from a FAT filesystem
  • systemd – A System and Service Manager
  • tcpdump – A network traffic monitoring tool
  • telnet – The client program for the Telnet remote login protocol
  • traceroute – Traces the route taken by packets over an IPv4/IPv6 network
  • vinagre – VNC client for GNOME
  • vino – A remote desktop system for GNOME
  • virt-manager – Virtual Machine Manager
  • virt-viewer – Virtual Machine Viewer
  • wireshark – Network traffic analyzer
  • yum – RPM package installer/updater/manager