[erlang-questions] epmd regression bug in erlang-solutions.com esl-erlang 18.3?

[Michael Santos]

On Sat, Mar 19, 2016 at 11:47:49PM +0800, Khitai Pang wrote:
> On 2016/3/19 19:45, Michael Santos wrote:
> >
> >On Mar 18, 2016 10:46 PM, "Khitai Pang" <khitai.pang@REDACTED
> ><mailto:khitai.pang@REDACTED>> wrote:
> >>
> >> Hi,
> >>
> >> I have been using the esl-erlang pacakge from
> >https://packages.erlang-solutions.com/erlang/ Ubuntu PPA for
> >Trusty.
> >>
> >> I have these in my bashrc:
> >>
> >> export ERL_EPMD_ADDRESS="XXX.XXX.XXX.XXX"   (LAN IP of my VM in
> >the cloud)
> >> export ERL_EPMD_PORT="5678"
> >
> >Just tested and it worked for me.
> >
> >Does the IP address in ERL_EPMD_ADDRESS exist in the VM?
> >
> 
> Yes it does, it's the IP of the LAN in the clound, and it all works
> fine with 18.2.
> 
> I believe it's possible that the issue is caused by the
> configuration in the operating system (Ubuntu 14.04.4 x86_64) but
> currently I have no idea how to further debug this issue.
> 
> With esl-erlang 18.2 and the following configs in bashrc:
> 
> export ERL_EPMD_ADDRESS="XXX.XXX.XXX.XXX"
> export ERL_EPMD_PORT="5678"
> 
> epmd works fine:
> 
> tcp        0      0 127.0.0.1:5678          0.0.0.0:* LISTEN      6754/epmd
> tcp        0      0 10.131.70.80:5678       0.0.0.0:* LISTEN      6754/epmd
> 
> 
> Thanks
> Khitai
> 
> >>
> >> Everything worked fine with the 18.2 package.  After upgrading
> >to esl-erlang 18.3, epmd stopped to work:
> >>
> >> > epmd -debug
> >> epmd: Sat Mar 19 16:16:51 2016: epmd running - daemon = 0
> >> epmd: Sat Mar 19 16:16:51 2016: failed to bind socket: Cannot
> >assign requested address
> >>
> >> There is a workaround: if I remove the ERL_EPMD_ADDRESS
> >environment variable, epms works:
> >>
> >> tcp        0      0 0.0.0.0:5678 <http://0.0.0.0:5678> 0.0.0.0:*
> >LISTEN      3328/epmd
> >> tcp6       0      0 :::5678 :::*                    LISTEN     3328/epmd
> >>
> >> After downgrading esl-erlang from 18.3 to 18.2, the issue is gone.

Have you disabled IPv6?

    # disable IPv6 support on the loopback device
    $ sudo sysctl net.ipv6.conf.lo.disable_ipv6=1

    $ ERL_EPMD_ADDRESS="127.0.0.1" ERL_EPMD_PORT=5678 epmd
    epmd: Sat Mar 19 22:40:09 2016: failed to bind socket: Cannot assign requested address

    # enable IPv6 support on the loopback device
    $ sudo sysctl net.ipv6.conf.lo.disable_ipv6=0

    $ ERL_EPMD_ADDRESS="127.0.0.1" ERL_EPMD_PORT=5678 epmd
    <works>

If we look at what is happening:

    $ ERL_EPMD_ADDRESS="127.0.0.1" ERL_EPMD_PORT=5678 strace epmd
    ...
    bind(4, {sa_family=AF_INET6, sin6_port=htons(5678), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = -1 EADDRNOTAVAIL (Cannot assign requested address)
    ...

When you specify a specific address to bind, epmd will also bind to the
IPv4 and IPv6 loopbacks. In this case, your OS supports IPv6 but has
disabled it on the loopback device so the bind fails:

    $ ifconfig lo
    lo        Link encap:Local Loopback
              inet addr:127.0.0.1  Mask:255.0.0.0
              UP LOOPBACK RUNNING  MTU:65536  Metric:1
              RX packets:0 errors:0 dropped:0 overruns:0 frame:0
              TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
              collisions:0 txqueuelen:0
              RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

So either enable IPv6 for the loopback or disable IPv6 completely by
passing "ipv6.disable=1" at boot.