Linux service avahi daemon

Linux OS Service ‘avahi-daemon’

The avahi-daemon Linux service runs on client machines to perform network-based Zeroconf service discovery. Avahi is an implementation of the DNS Service Discovery and Multicast DNS specifications for Zeroconf Networking. User applications receive notice of discovered network services and resources using the Linux D-Bus message passing. The daemon coordinates application efforts in caching replies, helping minimize network traffic.

Avahi provides a set of language bindings, including Python and Mono. Because of its modularized architecture, Avahi is already integrated into major desktop components like GNOME’s Virtual File System or KDE’s input/output architecture. Refer http://avahi.org/ for further specifications.

The avahi RPM package provides the /usr/sbin/avahi-daemon daemon and its configuration files.

Service Control

To manage the avahi-daemon service on demand, use the service(8) tool or run the /etc/init.d/avahi-daemon script directly:

# service avahi-daemon Usage: /etc/init.d/avahi-daemon
# /etc/init.d/avahi-daemon Usage: /etc/init.d/avahi-daemon

The available commands are:

Command	Description
start	Start the avahi-daemon daemon.
stop	Stop the avahi-daemon daemon.
status	Report if the avahi-daemon daemon is running.
restart	Equivalent to a stop and then a start command sequence.
condrestart	If the avahi-daemon daemon is currently running, this is the same as a restart command. If the daemon is not running, no action is taken. Often used in RPM package installation to avoid starting a service not already running.

To manage the avahi-daemon service at boot time, use chkconfig command:

# chkconfig --list avahi-daemon avahi-daemon 0:off 1:off 2:off 3:off 4:off 5:off 6:off

# chkconfig --list avahi-daemon avahi-daemon 0:off 1:off 2:on 3:on 4:on 5:on 6:off

Configuration

Below is the sample configuration file /etc/avahi/avahi-daemon.conf:

# cat /etc/avahi/avahi-daemon.conf # $Id: avahi-daemon.conf 1155 2006-02-22 22:54:56Z lennart $ # # This file is part of avahi. # # avahi is free software; you can redistribute it and/or modify it # under the terms of the GNU Lesser General Public License as # published by the Free Software Foundation; either version 2 of the # License, or (at your option) any later version. # # avahi is distributed in the hope that it will be useful, but WITHOUT # ANY WARRANTY; without even the implied warranty of MERCHANTABILITY # or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public # License for more details. # # You should have received a copy of the GNU Lesser General Public # License along with avahi; if not, write to the Free Software # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 # USA. # See avahi-daemon.conf(5) for more information on this configuration # file! [server] #host-name=foo #domain-name=local browse-domains=0pointer.de, zeroconf.org use-ipv4=yes use-ipv6=yes #check-response-ttl=no #use-iff-running=no #enable-dbus=yes #disallow-other-stacks=no #allow-point-to-point=no [wide-area] enable-wide-area=yes [publish] #disable-publishing=no #disable-user-service-publishing=no #add-service-cookie=yes #publish-addresses=yes #publish-hinfo=yes #publish-workstation=yes #publish-domain=yes #publish-dns-servers=192.168.50.1, 192.168.50.2 #publish-resolv-conf-dns-servers=yes [reflector] #enable-reflector=no #reflect-ipv=no [rlimits] #rlimit-as= rlimit-core=0 rlimit-data=4194304 rlimit-fsize=0 rlimit-nofile=30 rlimit-stack=4194304 rlimit-nproc=3

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Avahi

Avahi is a free Zero-configuration networking (zeroconf) implementation, including a system for multicast DNS/DNS-SD service discovery. It allows programs to publish and discover services and hosts running on a local network with no specific configuration. For example you can plug into a network and instantly find printers to print to, files to look at and people to talk to. It is licensed under the GNU Lesser General Public License (LGPL).

Installation

You can manage the Avahi daemon with avahi-daemon.service using systemd.

Note: systemd-resolved has a built-in mDNS service, make sure to disable systemd-resolved’s multicast DNS resolver/responder (refer to resolved.conf(5) ) or disable systemd-resolved.service entirely before using Avahi.

Using Avahi

Hostname resolution

Avahi provides local hostname resolution using a «hostname.local» naming scheme. To enable it, install the nss-mdns package and start/enable avahi-daemon.service .

Then, edit the file /etc/nsswitch.conf and change the hosts line to include mdns_minimal [NOTFOUND=return] before resolve and dns :

hosts: mymachines mdns_minimal [NOTFOUND=return] resolve [!UNAVAIL=return] files myhostname dns

• If you experience slowdowns in resolving .local hosts (or you do not want to use IPv6) try to use mdns4_minimal and mdns4 instead of mdns_minimal and mdns respectively.
• The line above makes nss-mdns authoritative for the .local domain, unless your unicast DNS server responds to SOA queries for the top level local name, or if the request has more than two labels. See nss-mdns activation notes.
  • systemd-resolved responds to these queries even if its mDNS support is disabled. See #systemd-resolved prevents nss-mdns from working.
  • More info about nss configuration can be found on nss-mymachines(8) and in Avahi docs.

  Configuring mDNS for custom TLD

  The mdns_minimal module handles queries for the .local TLD only. Note the [NOTFOUND=return] , which specifies that if mdns_minimal cannot find *.local , it will not continue to search for it in dns , myhostname , etc.

  In case you want Avahi to support other TLDs, you should:

  • replace mdns_minimal [NOTFOUND=return] with the full mdns module. There also are IPv4-only and IPv6-only modules mdns[46](_minimal)
  • customize /etc/avahi/avahi-daemon.conf with the domain-name of your choice
  • whitelist Avahi custom TLDs in /etc/mdns.allow

  Tools

  Avahi includes several utilities which help you discover the services running on a network. For example, run

  $ avahi-browse --all --ignore-local --resolve --terminate

  to discover services in your network.

  The Avahi Zeroconf Browser ( avahi-discover – note that it needs Avahi’s optional dependencies gtk3 , dbus-python and python-gobject ) shows the various services on your network. You can also browse SSH and VNC Servers using bssh and bvnc respectively.

  Firewall

  Be sure to open UDP port 5353 if you are using a firewall.

  Link-Local (Bonjour/Zeroconf) chat

  Avahi can be used for Bonjour protocol support under Linux. Check Wikipedia:Comparison of instant messaging clients or List of applications#Instant messaging clients for a list of clients supporting the Bonjour protocol.

  Obtaining IPv4LL IP address

  This article or section is a candidate for merging with dhcpcd.

  The dhcpcd client can attempt to obtain an IPv4LL address if it failed to get one via DHCP. By default this option is disabled. To enable it, comment noipv4ll string:

  Alternatively, run avahi-autoipd :

  Adding services

  Avahi advertises the services whose *.service files are found in /etc/avahi/services . Files in this directory must be readable by the avahi user/group.

  If you want to advertise a service for which there is no *.service file, it is very easy to create your own. As an example, let us say you wanted to advertise a quote of the day (QOTD) service operating per RFC:865 on TCP port 17 which you are running on your machine

  The first thing to do is to determine the . avahi.service(5) indicates that the type should be «the DNS-SD service type for this service. e.g. ‘_http._tcp'». Since the DNS-SD register was merged into the IANA register in 2010, we look for the service name on the IANA register or in /etc/services file. The service name shown there is qotd . Since we are running QOTD on tcp, we now know the service is _qotd._tcp and the port (per IANA and RFC 865) is 17 .

  For more complicated scenarios, such as advertising services running on a different server, DNS sub-types and so on, consult avahi.service(5) .

  Keep in mind that Avahi does not support arbitrary strings in the field, you can only set values known in service database of Avahi. If you want to register something custom you will likely have to edit the database definition, build an updated version and distribute it to your hosts.

  SSH

  Avahi comes with an example service file to advertise an SSH server. To enable it:

  # cp /usr/share/doc/avahi/ssh.service /etc/avahi/services/

  File sharing

  NFS

  If you have an NFS share set up, you can use Avahi to be able to automount them in Zeroconf-enabled browsers (such as Konqueror on KDE and Finder on macOS) or file managers such as GNOME/Files.

  Create a .service file in /etc/avahi/services with the following contents:

  /etc/avahi/services/nfs_Zephyrus_Music.service

     NFS Music Share on %h _nfs._tcp 2049 path=/data/shared/Music   

  The port is correct if you have insecure as an option in your /etc/exports ; otherwise, it needs to be changed (note that insecure is needed for macOS clients). The path is the path to your export, or a subdirectory of it. For some reason the automount functionality has been removed from Leopard, however a script is available. This was based upon this post.

  Samba

  With the Avahi daemon running on both the server and client, the file manager on the client should automatically find the server.

  Vsftpd

  You can also auto-discover regular FTP servers, such as vsftpd. Install the vsftpd package and change the settings of vsftpd according to your own personal preferences (see this thread on ubuntuforums.org or vsftpd.conf(5) ).

  Create a .service file in /etc/avahi/services with the following contents:

  /etc/avahi/services/ftp.service

     FTP file sharing _ftp._tcp 21   

  The FTP server should now be advertised by Avahi. You should now be able to find the FTP server from a file manager on another computer in your network. You might need to enable #Hostname resolution on the client.

  Troubleshooting

  Hostname changes with appending incrementing numbers

  This is a known bug that is caused by a hostname race condition. One possible workaround is disabling IPv6 to attempt to prevent the race condition. If multiple interfaces are present use allow-interfaces to limit Avahi to a single interface. Another possible workaround is to disable the cache to prevent Avahi from checking for host name conflicts altogether, but this prevents Avahi from performing lookups.

  systemd-resolved prevents nss-mdns from working

  nss-mdns only works if the DNS server listed in /etc/resolv.conf returns NXDOMAIN to SOA queries for the «local» domain.[1]

  Check if your configured DNS server answers the SOA query for the «local» domain with NXDOMAIN first. For example:

  If the DNS server responds with NXDOMAIN , you do not need to follow the steps below. Avahi should be able to find resources in the network normally, even if using systemd-resolved.

  In older versions of systemd-resolved the global setting for MulticastDNS=no in resolved.conf(5) lead to Avahi-incompatible response codes for the «local» domain. This resulted in Avahi not finding resources (printers) correctly. See systemd issue 21659 for reference.

  However, if the DNS query above fails to return NXDOMAIN for the «local» domain, you can use the full mdns NSS module instead of mdns_minimal and create /etc/mdns.allow to allow only the «local» domain. For example:

  hosts: mymachines mdns [NOTFOUND=return] resolve [!UNAVAIL=return] files myhostname dns

  Note: The above solution will break reverse lookups in networking utilities such as mtr and traceroute . These applications will attempt to do a reverse lookup via Avahi and time out instead of falling back to other DNS services. This seems similar to Debian issue 433943 where the recommendation is, unfortunately, to use mdns_minimal instead of mdns .

  ECONNREFUSED (Connection refused) on avahi socket

  If your Avahi instance starts and operates correctly, but nss does not seem to forward requests to mdns, this may be caused by stuck socket /run/avahi-daemon/socket . This can be verified e.g. with strace. In this case you may have to restart both avahi-daemon.service and avahi-daemon.socket to make it work correctly.

  See also

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