#16 RFC-3484: Traceroute should defaults to IPv6 instead of IPv4 if possible

[Jan Macku]

Traceroute defaults to IPv4 addresses even when IPv6 addresses are available. While there is the traceroute6 option, The RFC below appears to indicate that this should not be the case.

RFC-3484 - Default Address Selection for Internet Protocol version 6 (IPv6) https://www.rfc-editor.org/info/rfc3484

[Dmitry Butskoy]

Jan Macku wrote:

Traceroute defaults to IPv4 addresses even when IPv6 addresses are
available. While there is the traceroute6 option, The RFC below
appears to indicate that this should not be the case.

RFC-3484 - Default Address Selection for Internet Protocol version 6
(IPv6) https://www.rfc-editor.org/info/rfc3484

Well, sounds reasonable, since ping(8) has long used the results of
getaddrinfo(3) to determine the default address family.

Can you imagine some negative use cases in practice due to the removal
of the current mandatory IPv4 default? (I mean the risk of significant
breakage of someone's habits, scripts, etc.)

Regards,
Dmitry Butskoy

[Jan Macku]

Hello Dmitry,
Thank you for your prompt response.

Can you imagine some negative use cases in practice due to the removal
of the current mandatory IPv4 default? (I mean the risk of significant
breakage of someone's habits, scripts, etc.)

I think this change should be safe. But even if it affects some scripts, mitigation should be straightforward by using the -4 option. As you mentioned, ping already has such a behavior.

[root@R9 ~]# ping  kernel.org
PING kernel.org(dfw.source.kernel.org (2604:1380:4641:c500::1)) 56 data bytes

[root@Kombat9 ~]# traceroute kernel.org
traceroute to kernel.org (139.178.84.217), 30 hops max, 60 byte packets
[root@R9 ~]# traceroute6 kernel.org
traceroute to kernel.org (2604:1380:4641:c500::1), 30 hops max, 80 byte packets

I forgot to mention that my request is coming from the RHEL issue tracker - https://issues.redhat.com/browse/RHEL-58449

Thank you
Regards,
Jan

[Dmitry Butskoy]

Fixed in 2.1.6 .

[Jan Macku]

Thank you for quick fix.