[plt-scheme] IPv6?

From: Johan Liseborn (johan at liseborn.se)
Date: Fri Nov 4 04:29:36 EST 2005

On Nov 3, 2005, at 22:41, Matthew Flatt wrote:

> At Sat, 23 Jul 2005 10:35:20 +0200, Johan Liseborn wrote:
>> Are there any plans on adding IPv6 support to PLT Scheme?
> MzScheme in the SVN respository trunk supports IPv6.

Great news!

I haven't checked the code out and tried it yet, but I'll try to give  
some initial comments anyway. Note though, that I am by no means an  
expert on IPv6 (one of the reasons I wanted IPv6 support in MzScheme  
was so that I could start playing with it).

Maybe it would be a good idea to have some discussion on this thread  
before you take my word for it and start changing things... :-)

Are you using 'getaddrinfo' to resolve addresses?

>  * By default, `tcp-listen' and `udp-open-socket' create sockets that
>    work with IPv4, only. For `tcp-connect', etc., the hostname
>    determines the protocol family.
>  * If you system supports IPv6, beware that "localhost" is probably
>    mapped to an IPv6 address by default. In that case, if you're  
> trying
>    to contact an IPv4 server, it probably won't work; use ""
>    instead.

OK, so does that mean that the following will be true:

(tcp-listen 5060)

Bind a socket to on port 5060 (as before).

(tcp-listen 5060 5 "localhost")

Bind a socket to whatever address "localhost" is mapped to  
(presumably in /etc/hosts), which may be either an IPv4 address (e.g. or and IPv6 address (e.g. ::1).

(tcp-listen 5060 5 "foobar.example.org")

Bind a socket to whatever address "foobar.example.org" is mapped to,  
wither IPv4 or IPv6.

(tcp-listen 5060 5 "")

Bind to the actual IPv4 address given.

(tcp-listen 5060 5 "1080:0:0:0:8:800:200C:417A")

Bind to the actual IPv6 address given.

>  * Depending on your system's capabilities, you might supply "::" as
>    the local hostname for `tcp-listen' to get a listener that accepts
>    both IPv4 and IPv6 connections. This works under Mac OS X and
>    Linux, for example, but under Windows it creates an IPv6-only
>    listener.

Hmm, I am not 100% sure, but isn't '::' (or '0:0:0:0:0:0:0:0')  
comparable to '' for IPv4 (i.e. the unspecified address, which  
for binding usually means "bind to all addresses")? I believe ':: 
1' (or '0:0:0:0:0:0:0:1') is comparable to '' (i.e. the  
"loopback" address).

So, how does the current implementation work:

(tcp-listen 5060 5 "::")

Binds only to the loopback address


Binds to *all* addresses

I believe the Right Thing would be for (tcp-listen 5060 5 "::1") to  
bind to the loopback address, and for (tcp-listen 5060 5 "::") to  
bind to all addresses.

If I understand things correctly, whether something like '::' will  
give you a socket that accepts both v4 and v6, or v6 only depends on  
the type of stack, i.e. whether it is a "dual-mode" stack or not, and  
thus is not possible to manipulate through the socket APIs.

>  * The `udp-open-socket' function now accepts optional hostname and
>    port arguments. If they're supplied, then the new socket's protocol
>    family is the one implied by the address and port. For example,
>    `(udp-open-socket "::")' creates an IPv6 socket if the system
>    supports it. On some systems (e.g., Linux), the resulting IPv6
>    socket can also send and receive IPv4; on some systems (e.g.,  
> Mac OS
>    X), it can only receive IPv4; on some systems (e.g., Windows), it
>    doesn't support IPv4 at all.
>    Arguments to `udp-open-socket' merely determine the socket's
>    protocol family; `udp-open-socket' doesn't bind the socket. If the
>    socket is going to be used to communicate with a specific host,  
> then
>    supplying the host to `udp-open-socket' is a good way to ensure  
> that
>    the socket's protocol family is consistent with the target host.
>    Similarly, if the socket is going to be bound to a particular host
>    and port, supplying those values to `udp-open-socket' ensures that
>    the binding will work.

OK, so again, are you using 'getaddrinfo' to resolve, and the type of  
socket I get is whatever 'getaddrinfo' gives you?


Johan Liseborn

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