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Postfix IPv6 Support

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Postfix IPv6 Support


Postfix 2.2 introduces support for the IPv6 (IP version 6) protocol. IPv6 support older Postfix versions was available as an add-on patch. The section "Compatibility with Postfix <2.2 IPv6 support" below discusses the differences between these implementations.

The main feature of interest is that IPv6 uses 128-bit IP addresses instead of the 32-bit addresses used by IPv4. It can therefore accommodate a much larger number of hosts and networks without ugly kluges such as NAT. A side benefit of the much larger address space is that it makes network scanning unpractical.

Postfix uses the same SMTP protocol over IPv6 as it already uses over the older IPv4 network, and does AAAA record lookups in the DNS in addition to the older A records. Information about IPv6 can be found at http://www.ipv6.org/.

This document provides information on the following topics:

Supported Platforms

Postfix version 2.2 supports IPv4 and IPv6 on the following platforms:

  • AIX 5.1+
  • Darwin 7.3+
  • FreeBSD 4+
  • Linux 2.4+
  • NetBSD 1.5+
  • OpenBSD 2+
  • Solaris 8+
  • Tru64Unix V5.1+

On other platforms Postfix will simply use IPv4 as it has always done.

See below for tips how to port Postfix IPv6 support to other environments.


Postfix IPv6 support introduces two new main.cf configuration parameters, and introduces an important change in address syntax notation in match lists such as mynetworks or debug_peer_list.

Postfix IPv6 address syntax is a little tricky, because there are a few places where you must enclose an IPv6 address inside "[]" characters, and a few places where you must not. It is a good idea to use "[]" only in the few places where you have to. Check out the postconf(5) manual whenever you do IPv6 related configuration work with Postfix.

  • Instead of hard-coding and ::1 loopback addresses in master.cf, specify "inet_interfaces = loopback-only" in main.cf. This way you can use the same master.cf file regardless of whether or not Postfix will run on an IPv6-enabled system.

  • The first new parameter is called inet_protocols. This specifies what protocols Postfix will use when it makes or accepts network connections, and also controls what DNS lookups Postfix will use when it makes network connections.

        # You must stop/start Postfix after changing this parameter.
        inet_protocols = ipv4       (DEFAULT: enable IPv4 only)
        inet_protocols = all        (enable IPv4, and IPv6 if supported)
        inet_protocols = ipv4, ipv6 (enable both IPv4 and IPv6)
        inet_protocols = ipv6       (enable IPv6 only)

    By default, Postfix uses IPv4 only, because most systems aren't attached to an IPv6 network.

    • On systems with combined IPv4/IPv6 stacks, attempts to deliver mail via IPv6 would always fail with "network unreachable", and those attempts would only slow down Postfix.

    • Linux kernels don't even load IPv6 protocol support by default. Any attempt to use it would fail immediately.

    Note 1: you must stop and start Postfix after changing the inet_protocols configuration parameter.

    Note 2: if you see error messages like the following, then you're running Linux and need to turn on IPv6 in the kernel: see http://www.ipv6.org/ for hints and tips. Unlike other systems, Linux does not have a combined stack for IPv4 and IPv6, and IPv6 protocol support is not loaded by default.

    postconf: warning: inet_protocols: IPv6 support is disabled: Address family not supported by protocol
    postconf: warning: inet_protocols: configuring for IPv4 support only

    Note 3: on older Linux and Solaris systems, the setting "inet_protocols = ipv6" will not prevent Postfix from accepting IPv4 connections. Postfix will present the client IP addresses in IPv6 format, though. In all other cases, Postfix always presents IPv4 client IP addresses in the traditional dotted quad IPv4 format.

  • The other new parameter is smtp_bind_address6. This sets the local interface address for outgoing IPv6 SMTP connections, just like the smtp_bind_address parameter does for IPv4:

        smtp_bind_address6 = 2001:240:5c7:0:250:56ff:fe89:1
  • If you left the value of the mynetworks parameter at its default (i.e. no mynetworks setting in main.cf) Postfix will figure out by itself what its network addresses are. This is what a typical setting looks like:

    % postconf mynetworks
    mynetworks = [::1]/128 [fe80::]/10 [2001:240:5c7::]/64 

    If you did specify the mynetworks parameter value in main.cf, you need update the mynetworks value to include the IPv6 networks the system is in. Be sure to specify IPv6 address information inside "[]", like this:

        mynetworks = ...IPv4 networks... [::1]/128 [2001:240:5c7::]/64 ...

NOTE: when configuring Postfix match lists such as mynetworks or debug_peer_list, you must specify IPv6 address information inside "[]" in the main.cf parameter value and in files specified with a "/file/name" pattern. IPv6 addresses contain the ":" character, and would otherwise be confused with a "type:table" pattern.

Known Limitations

  • The order of IPv6/IPv4 outgoing connection attempts is not yet configurable. Currently, IPv6 is tried before IPv4.

  • Postfix currently does not support DNSBL (real-time blackhole list) lookups for IPv6 client IP addresses; currently there are no blacklists that cover the IPv6 address space.

  • IPv6 does not have class A, B, C, etc. networks. With IPv6 networks, the setting "mynetworks_style = class" has the same effect as the setting "mynetworks_style = subnet".

  • On Tru64Unix and AIX, Postfix can't figure out the local subnet mask and always assumes a /128 network. This is a problem only with "mynetworks_style = subnet" and no explicit mynetworks setting in main.cf.

Compatibility with Postfix <2.2 IPv6 support

Postfix version 2.2 IPv6 support is based on the Postfix/IPv6 patch by Dean Strik and others, but differs in a few minor ways.

  • main.cf: The inet_interfaces parameter does not support the notation "ipv6:all" or "ipv4:all". Use the inet_protocols parameter instead.

  • main.cf: Specify "inet_protocols = all" or "inet_protocols = ipv4, ipv6" in order to enable both IPv4 and IPv6 support.

  • main.cf: The inet_protocols parameter also controls what DNS lookups Postfix will attempt to make when delivering or receiving mail.

  • main.cf: Specify "inet_interfaces = loopback-only" to listen on loopback network interfaces only.

  • The lmtp_bind_address and lmtp_bind_address6 features were omitted. The Postfix LMTP client will be absorbed into the SMTP client, so there is no reason to keep adding features to the LMTP client.

  • The SMTP server now requires that IPv6 addresses in SMTP commands are specified as [ipv6:ipv6address], as described in RFC 2821.

  • The IPv6 network address matching code was rewritten from the ground up, and is expected to be closer to the specification. The result may be incompatible with the Postfix/IPv6 patch.

IPv6 Support for unsupported platforms

Getting Postfix IPv6 working on other platforms involves the following steps:

  • Specify how Postfix should find the local network interfaces. Postfix needs this information to avoid mailer loops and to find out if mail for [email protected][ipaddress] is a local or remote destination.

    If your system has the getifaddrs() routine then add the following to your platform-specific section in src/util/sys_defs.h:

    #ifndef NO_IPV6
    # define HAS_IPV6
    # define HAVE_GETIFADDRS

    Otherwise, if your system has the SIOCGLIF ioctl() command in /usr/include/*/*.h, add the following to your platform-specific section in src/util/sys_defs.h:

    #ifndef NO_IPV6
    # define HAS_IPV6
    # define HAS_SIOCGLIF

    Otherwise, Postfix will have to use the old SIOCGIF commands and get along with reduced IPv6 functionality (it won't be able to figure out your IPv6 netmasks, which are needed for "mynetworks_style = subnet". Add this to your platform-specific section in src/util/sys_defs.h:

    #ifndef NO_IPV6
    # define HAS_IPV6
  • Test if Postfix can figure out its interface information.

    After compiling Postfix in the usual manner, step into the src/util directory and type "make inet_addr_local". Running this file by hand should produce all the interface addresses and network masks, for example:

    % make
    % cd src/util
    % make inet_addr_local
    [... some messages ...]
    % ./inet_addr_local
    [... some messages ...]
    ./inet_addr_local: inet_addr_local: configured 2 IPv4 addresses
    ./inet_addr_local: inet_addr_local: configured 4 IPv6 addresses

    The above is for an old FreeBSD machine. Other systems produce slightly different results, but you get the idea.

If none of all this produces a usable result, send email to the [email protected] mailing list and we'll try to help you through this.


The following information is in part based on information that was compiled by Dean Strik.

  • Mark Huizer wrote the original Postfix IPv6 patch.

  • Jun-ichiro 'itojun' Hagino of the KAME project made substantial improvements. Since then, we speak of the KAME patch.

  • The PLD Linux Distribution ported the code to other stacks (notably USAGI). We speak of the PLD patch. A very important feature of the PLD patch was that it can work with Lutz Jaenicke's TLS patch for Postfix.

  • Dean Strik extended IPv6 support to platforms other than KAME and USAGI, updated the patch to keep up with Postfix development, and provided a combined IPv6 + TLS patch. Information about his effort can be found on Dean Strik's Postfix website at http://www.ipnet6.org/postfix/.

  • Wietse Venema took Dean Strik's IPv6 patch, merged it into Postfix 2.2, and took the opportunity to eliminate all IPv4-specific code from Postfix that could be removed. For systems without IPv6 support in the kernel and system libraries, Postfix has a simple compatibility layer, so that it will use IPv4 as before.

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