[Proposal] Add new erl_epmd:names/2 function

[Gsantomaggio]

With erl_epmd:names/2 it is possible to specify the Family address
as: net_adm:names(localhost,inet6)..
It would help to handle the nodes name resolution when the EPMD is bound
to IPV6 port:
See #602 (comment).

for example:

bin/erl -sname hello -proto_dist inet6_tcp

(hello@linux-s1)5> net_adm:names(localhost).
{error,address}

with the new function, it is possible to specify the Family address, it would be:

bin/erl -sname hello -proto_dist inet6_tcp

(hello@linux-s1)4> net_adm:names(localhost,inet6).
{ok,[{"hello",33353}]}

Note:
This is just a proposal, please let me know what you think about if it makes sense I will add the documentation.

Thank you for your attention.

[CLAassistant]

All committers have signed the CLA.

[Gsantomaggio]

I created and Vagrant image to help to test the issue.

1. setup

vagrant up && vagrant ssh

2. try erl

~> erl -sname hello
Protocol 'inet_tcp': register/listen error: econnrefused

2. Try erl with inet6_tcp support

:~> sudo ss -tulpen | grep epmd
tcp    LISTEN     0      128       [::1]:4369               [::]:*                   users:(("epmd",pid=1072,fd=3),("systemd",pid=1,fd=46)) ino:15225 sk:4 v6only:1 <->

:~> erl -sname hello -proto_dist inet6_tcp
Erlang/OTP 21 [erts-10.3.3] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:1] [hipe]
Eshell V10.3.3  (abort with ^G)
(hello@node0)1>  net_adm:names(localhost).
{error,address}

[garazdawi]

I need to read up some more before deciding, but your approach seems to be good.

What I'm wondering is if kernel could possibly figure out this by itself, so that the user does not have to provide an argument every time it calls this function.

[Gsantomaggio]

Thank you Lukas,
I was wondering the same, I did not understand why inet_db:res_option(inet6) return false in this trace: #602 (comment) and also if there is a way to set it externally. I could not find a way.

[garazdawi]

I did not understand why inet_db:res_option(inet6) return false in this trace: #602 (comment) and also if there is a way to set it externally. I could not find a way.

If you configure an erl_inetrc with {inet6,true}., then you will get inet_db:res_option(inet6) to return true and everything works. This will change all name resolutions to ipv6 though, not only the distribution.

vagrant@localhost:~> cat erl_inetrc
{inet6,true}.
vagrant@localhost:~> erl -kernel inetrc '"./erl_inetrc"' -sname hello -proto_dist inet6_tcp
Erlang/OTP 21 [erts-10.3.5.1] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:1] [hipe]

Eshell V10.3.5.1  (abort with ^G)
(hello@localhost)1> net_adm:names().
{ok,[{"hello",36515}]}

Does this solve your problem? It will be complicated to fix to work without setting that option and I'm not sure it actually makes sense to not set it when running ipv6 distro. Can you think of any reason why you would want to resolve ipv4 addresses per default and use ipv6 distro?

[Gsantomaggio]

It seems to work, thank you @garazdawi
If you think that this API does not need, we can close the PR.

[garazdawi]

If you think that this API does not need, we can close the PR.

I don't know if it needed or not :) What I think would be great is to somewhere document what you need to configure in order to run erlang distribution in an ipv6 environment.

[garazdawi]

I'll close this PR for now. If anyone thinks that it is still a good idea feel free to re-open or create a new PR.