Avahi

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From Wikipedia:Avahi (software):

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

Install the avahi package.

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

Note: systemd-resolved has a built-in multicast DNS service, make sure to disable systemd-resolved's mDNS 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
Note:
  • If you experience slowdowns in resolving .local hosts try to use mdns4_minimal instead of mdns_minimal.
  • 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.

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

Merge-arrows-2.pngThis article or section is a candidate for merging with dhcpcd.Merge-arrows-2.png

Notes: should be merged into the main page (Discuss in Talk:Avahi)

By default, if you are getting IP using DHCP, you are using the dhcpcd package. It 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: 

/etc/dhcpcd.conf
...
#noipv4ll
...

Alternatively, run avahi-autoipd: 

# avahi-autoipd -D

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 <type>. 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.

Our service file is thus: 

qotd.service
<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">

<service-group>

  <name replace-wildcards="yes">%h</name>

  <service>
    <type>_qotd._tcp</type>
    <port>17</port>
  </service>

</service-group>

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

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
<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <name replace-wildcards="yes">NFS Music Share on %h</name>
  <service>
    <type>_nfs._tcp</type>
    <port>2049</port>
    <txt-record>path=/data/shared/Music</txt-record>
  </service>
</service-group>

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
<?xml version="1.0" standalone='no'?>
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <name>FTP file sharing</name>
  <service>
    <type>_ftp._tcp</type>
    <port>21</port>
  </service>
</service-group>

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.

AirPrint from Mobile Devices

Merge-arrows-2.pngThis article or section is a candidate for merging with AirPort#Printing .Merge-arrows-2.png

Notes: AirPrint is a feature provided by the Apple AirPort Express and by some printers. The manual configuration technique (using JetDirect) can also apply here and may be easier. (Discuss in Talk:Avahi)

Avahi along with CUPS also provides the capability to print to just about any printer from airprint compatible mobile devices. In order to enable print capability from your device, simply create an Avahi service file for your printer in /etc/avahi/services/. An example of a generic services file for an HP-Laserjet printer would be similar to the following with the name, rp, ty, adminurl and note fields changed. 

/etc/avahi/services/airprint.service
<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
  <name>yourPrnterName</name>
  <service>
    <type>_ipp._tcp</type>
    <subtype>_universal._sub._ipp._tcp</subtype>
    <port>631</port>
    <txt-record>txtver=1</txt-record>
    <txt-record>qtotal=1</txt-record>
    <txt-record>rp=printers/yourPrnterName</txt-record>
    <txt-record>ty=yourPrnterName</txt-record>
    <txt-record>adminurl=http://198.168.7.15:631/printers/yourPrnterName</txt-record>
    <txt-record>note=Office Laserjet 4100n</txt-record>
    <txt-record>priority=0</txt-record>
    <txt-record>product=(GPL Ghostscript)</txt-record>
    <txt-record>printer-state=3</txt-record>
    <txt-record>printer-type=0x801046</txt-record>
    <txt-record>Transparent=T</txt-record>
    <txt-record>Binary=T</txt-record>
    <txt-record>Fax=F</txt-record>
    <txt-record>Color=T</txt-record>
    <txt-record>Duplex=T</txt-record>
    <txt-record>Staple=F</txt-record>
    <txt-record>Copies=T</txt-record>
    <txt-record>Collate=F</txt-record>
    <txt-record>Punch=F</txt-record>
    <txt-record>Bind=F</txt-record>
    <txt-record>Sort=F</txt-record>
    <txt-record>Scan=F</txt-record>
    <txt-record>pdl=application/octet-stream,application/pdf,application/postscript,image/jpeg,image/png,image/urf</txt-record>
    <txt-record>URF=W8,SRGB24,CP1,RS600</txt-record>
  </service>
</service-group>

Alternatively, https://raw.github.com/tjfontaine/airprint-generate/master/airprint-generate.py can be used to generate Avahi service files. It depends on python and python-pycups. The script can be run using: 

# python3 airprint-generate.py -d /etc/avahi/services
Note: If your printer under http://localhost:631/printers is "Not Shared", this python script will not output any file to /etc/avahi/services; in that case, you will need to "Modify Printer" under one of the CUPS drop-down menus to turn sharing on. If that does not work, check out the ArchWiki on CUPS printer sharing.

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] Even if systemd-resolved is configured with MulticastDNS=no in resolved.conf(5), it will not return NXDOMAIN for these queries. See systemd issue 21659.

A solution is to use the full mdns NSS module instead of mdns_minimal and create /etc/mdns.allow to allow only the "local" domain. For example: 

/etc/nsswitch.conf
hosts: mymachines mdns [NOTFOUND=return] resolve [!UNAVAIL=return] files myhostname dns
/etc/mdns.allow
.local.
.local

See also