How can I lookup host name from snap core using avahi (mDNS)?

7675t · February 2, 2018, 6:54am

Hi,

I want to lookup host names (I mean, to get the IP address from a host name) from a snap core machine (say, named raspi). I don’t want to setup local DNS server, so I want to use avahi-daemon.

I installed the avahi snap package into raspi and it works. Now I can access raspi from remote machine by its hostname as:

remote: $ ping raspi.local
PING raspi.local (192.168.2.2) 56(84) bytes of data.
64 bytes from 192.168.2.2: icmp_seq=1 ttl=64 time=17.3 ms

However, I cannot access the remote machine FROM raspi:

raspi: $ ping remote.local
ping: unknown host remote.local

I’m sure avahi-daemon is running in the remote machine.
How can I resolve host name from the snap core machine using avahi?

I know the file ‘/etc/nsswitch.conf’ in raspi controls the order to resolve host name.
The file is:

# /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# If you have the `glibc-doc-reference' and `info' packages installed, try:
# `info libc "Name Service Switch"' for information about this file.

passwd:         compat extrausers
group:          compat extrausers
shadow:         compat extrausers
gshadow:        files

hosts:          files dns myhostname
networks:       files

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis

I think it needs some setting to use mDNS, like:

hosts:          files mdns4_minimal dns myhostname

but I cannot edit the file on the core package.
Is there any way to edit this configuration file in the core package?

Thank you,

ogra · February 2, 2018, 5:46pm

not currently, no, but you could create a “ping” snap that consumes the avahi interfaces … IIRC that is how @ondra and @till.kamppeter are currently implementing the cups snap …

(after all there should be nothing running on your core system that is not coming from a snap, so your snap should be able to consume the avahi interface here)

7675t · February 3, 2018, 4:39pm

I’m not really sure of the interfaces of snap… maybe I should read the doc through. I will try to make a simple ping snap package. Thanks for advice.

7675t · February 5, 2018, 3:44am

Now I create a snap ‘my-ping’ to learn how can I lookup the host name from the snap package using avahi. It contains ping, avahi-resolve and avahi-browse to check the functionality.

My snapcfaft.yaml is here:

name: my-ping
version: '0.1'
summary: My ping snap
description: |
  ping snap package.

grade: devel
confinement: devmode

parts:
  ping:
    plugin: nil
    stage-packages:
      - coreutils
      - avahi-daemon
      - avahi-utils
    stage:
      - bin
      - lib
      - usr
      - etc
    prime:
      - bin
      - lib
      - usr
      - etc
      
apps:
  ping:
    command: ping
    plugs: [network, network-bind, network-observe, avahi-observe]
  avahi-resolve:
    command: avahi-resolve
    plugs: [network, network-bind, network-observe, avahi-observe]
  avahi-browse:
    command: avahi-browse
    plugs: [network, network-bind, network-observe, avahi-observe]

After install it, I tried

$ my-ping.avahi-resolve -n carbon.local
Failed to create client object: Daemon not running

It’s reasonable, we have to connect my-ping:avahi-observe to avahi:avahi-observe.

$ snap connect my-ping:avahi-observe avahi
$ snap interfaces my-ping
Slot                 Plug
avahi:avahi-observe  my-ping
:network             avahi,my-ping,snapweb,tork-ros-roomblock
:network-bind        avahi,my-ping,snapweb,tork-ros-roomblock
-                    my-ping:network-observe

However, I still have a problem, the command never returns.

$ my-ping.avahi-resolve -n carbon.local
(Never come back until Ctrl-C)

The /var/log/syslog saids:

Feb  5 12:38:42 lidarkick avahi.avahi-daemon[7692]: dbus-protocol.c: interface=org.freedesktop.Avahi.Server, path=/, member=GetAPIVersion
Feb  5 12:38:42 lidarkick avahi.avahi-daemon[7692]: dbus-protocol.c: interface=org.freedesktop.Avahi.Server, path=/, member=GetState
Feb  5 12:38:42 lidarkick avahi.avahi-daemon[7692]: dbus-protocol.c: interface=org.freedesktop.Avahi.Server, path=/, member=HostNameResolverNew
Feb  5 12:38:42 lidarkick kernel: [ 4511.723096] audit: type=1107 audit(1517801922.269:746): pid=1113 uid=100 auid=4294967295 ses=4294967295 msg='apparmor="DENIED" operation="dbus_signal"  bus="system" path="/Client5/HostNameResolver1" interface="org.freedesktop.Avahi.HostNameResolver" member="Found" mask="send" name=":1.28" pid=7692 label="snap.avahi.avahi-daemon" peer_pid=12085 peer_label="snap.my-ping.avahi-resolve"
Feb  5 12:38:42 lidarkick kernel: [ 4511.723096]  exe="/usr/bin/dbus-daemon" sauid=100 hostname=? addr=? terminal=?'

It may complains about Dbus, but I have no idea to solve this. How can I setup my package to resolve the host name using avahi package?

Thanks,

ogra · February 5, 2018, 10:14am

perhaps @ondra has an idea here ?

ondra · February 6, 2018, 4:12pm

@7675t I just tested with simple avahi client snap and you are right, there is problem with avahi interface. I will see if I can find time to prepare PR for it.
In the mean time file bug against snapd here https://bugs.launchpad.net/snappy/+filebug

jdstrand · February 6, 2018, 4:55pm

@ondra - if it would help, if you can give me all the denials, I can roll this in into the next batch of policy updates for 2.32.

7675t · February 7, 2018, 1:04am

Thanks @ondra and @ogra

I filed this in your link. https://bugs.launchpad.net/snappy/+bug/1747794
Thank you for maintaining your quite useful apps.

ondra · February 7, 2018, 2:20am

@jdstrand I just run tests with following fix: https://github.com/kubiko/snapd/tree/fix-avahi-observe
This seems to fix the problem.
All denials are about slot provider (avahi daemon) not being able to send given dbus messages. Looks like peer_name is not provided but required by the profile, so it blocks sending
I created test ‘avahi-client’ snap to test this case (once lp builds it it should be added to the store)

ondra · February 7, 2018, 3:45pm

@7675t this has been now merged in and it should be in next milestone
thanks for reporting this!

7675t · February 7, 2018, 11:57pm

Thank you for your quick support, impressive!

ondra · February 8, 2018, 12:14pm

Fix is now available in edge channel
$ snap refresh core --edge
You can use my test client:
$ snap install avahi-client
connect interfaces
$ avahi-client.browse -a

7675t · February 8, 2018, 3:18pm

Thanks, it works perfectly. (I mean, avahi part of my problem)

Now I wonder how I can run ping command from the snap package using avahi,
but it may be a separated question.

ondra · February 8, 2018, 3:52pm

you can run ping directly, all you need is to add network-observe plug for your application, then you will have permissions to call ping

7675t · February 8, 2018, 4:12pm

I can run ping by IP address, but how can I run ping by host name distributed by avahi? like:

$ my-ping.ping 192.168.0.18
PING 192.168.0.18 (192.168.0.18) 56(84) bytes of data.
64 bytes from 192.168.0.18: icmp_seq=1 ttl=64 time=11.9 ms

$ my-ping.avahi-resolve -n remote.local
remote.local	192.168.0.18

$ my-ping.ping remote.local
ping: unknown host remote.local

If I could edit /etc/nsswitch.conf, it seems to be easy to add mdns entry, but I can’t edit that file.
So glibc gethostbyname() would only look into /etc/hosts and DNS then it fails.

I may be misunderstanding something… I appreciate any kind of advice. Thanks!

ondra · February 21, 2018, 2:49pm

Hi @7675t

Sorry for silence, I wan enjoying holidays without internet Anyway I have checked and made some changes to my example snaps. And we are very close to make this use case also work. So install latest avahi and avahi-client from --edge channel and connect all needed interfaces. $ snap connect avahi-client:avahi-control avahi; snap connect avahi-client:avahi-observe avahi; snap connect avahi-client:network-observe core For your testing you will need to, for the moment, bind mount one file $ sudo mount -o bind /snap/avahi-client/current/nsswitch.conf /etc/nsswitch.conf

Now you should be able to run: $ sudo avahi-client.ping <something.local>

To get this working properly you need to follow landing of this feature: layouts-re-mapping-snap-directories

Once this lands you will be able to add to your snapcraft.yaml

layouts:
    /etc/nsswitch.conf:
        bind-file: $SNAP/nsswitch.conf

this will create impression for file overlay only inside the snap context without affecting rest of the system, without need to run that manual bind mount

jdstrand · February 21, 2018, 3:47pm

Please note that this should be singular, ie layout:.

ondra · February 21, 2018, 4:26pm

@jdstrand aha this could explain why I had trouble testing it. Thanks bunch for pointing this out!

Also thanks for quick manual review! All is now working

ondra · February 21, 2018, 4:41pm

@7675t so all is ready now to test. Thanks to @jdstrand pointing out my mistake in layout definition I was able to get it working on edge channel without any additional hack.
Ignore my previous post with bind mount, simply install my example avahi-client snap and you should be able to test ping on local domain

$ snap install avahi --edge
$ snap install avahi-snap --edge
$ snap set avahi host-name=core
$ snap connect avahi-client:avahi-observe avahi; snap connect avahi-client:avahi-control avahi; snap connect avahi-client:network-observe core
$ sudo avahi-client.ping core.local

Mind layout is fresh feature so snapcraft build does not support it yet and you will need to modify manually prime/meta/snap.yaml once you run build and add there:

layout:
  /etc/nsswitch.conf:`
    bind-file: $SNAP/nsswitch.conf`

jdstrand · February 21, 2018, 7:22pm

I think you need the core snap from edge too.