3

After swapping a faulty router, trying to ssh from PC "A" (Ubuntu 20.04.6) to PC "A", produces the error message: ssh: Could not resolve hostname pca.local: Name or service not known. ping and other such commands can't find the host either.

However, hostname connections from other devices like "pcb" to pca.local, "pca" to pcb.local, "pcb" to pcb.local work. Same with "pca" to "pca" via IP address. Interestingly, connections from "pca" to pca, without the .local, actually work!

But that is useless to me because it is already known in scripts and whatnot as pca.local. All devices in the LAN preserved their IP addresses across router changes.

What I have tried:

  • cat /etc/hostname shows its own name as expected.
  • systemctl restart systemd-resolved.
  • /etc/init.d/networking restart.
  • dhclient -r eth0.
  • Can't reboot that machine for a few days.

The output of service avahi-daemon status

● avahi-daemon.service - Avahi mDNS/DNS-SD Stack
     Loaded: loaded (/lib/systemd/system/avahi-daemon.service; enabled; vendor preset: enabled)
     Active: active (running) since Wed 2024-06-12 12:44:47 CST; 1h 49min ago
TriggeredBy: ● avahi-daemon.socket
   Main PID: 163913 (avahi-daemon)
     Status: "avahi-daemon 0.7 starting up."
      Tasks: 2 (limit: 4383)
     Memory: 1.3M
     CGroup: /system.slice/avahi-daemon.service
             ├─163913 avahi-daemon: registering [pca.local]
             └─163914 avahi-daemon: chroot helper

jun 12 12:44:47 pca avahi-daemon[163913]: Server startup complete. Host name is pca.local. Local service cookie is 2927830272.
jun 12 12:44:47 pca avahi-daemon[163913]: Failed to parse address 'fe80::1%eth0', ignoring.
jun 12 13:08:25 pca avahi-daemon[163913]: Withdrawing address record for fe80::342a:68ff:fec8:fa4e on veth2964dfb.
jun 12 13:08:28 pca avahi-daemon[163913]: Registering new address record for fe80::a881:e7ff:fec7:ead3 on veth6ddcaf7..
jun 12 13:34:33 pca avahi-daemon[163913]: Withdrawing address record for fe80::a881:e7ff:fec7:ead3 on veth6ddcaf7.
jun 12 13:34:35 pca avahi-daemon[163913]: Registering new address record for fe80::5c62:9eff:fe29:543e on veth31f1578..
jun 12 13:53:37 pca avahi-daemon[163913]: Withdrawing address record for fe80::5c62:9eff:fe29:543e on veth31f1578.
jun 12 13:53:40 pca avahi-daemon[163913]: Registering new address record for fe80::c0eb:52ff:fe22:656d on veth6d79f34..
jun 12 14:26:11 pca avahi-daemon[163913]: Withdrawing address record for fe80::c0eb:52ff:fe22:656d on veth6d79f34.
jun 12 14:26:13 pca avahi-daemon[163913]: Registering new address record for fe80::2ce3:93ff:fe4e:8b6f on vethd1fb373..

The output of ip addr show dev eth0

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 6a:55:c4:0f:59:ba brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.94/24 brd 192.168.1.255 scope global noprefixroute eth0
       valid_lft forever preferred_lft forever
    inet6 2806:107e:15:413d:bdc8:3b3e:88ec:370e/64 scope global temporary dynamic 
       valid_lft 540279sec preferred_lft 21774sec
    inet6 2806:107e:15:413d:c92f:8f13:fca8:2576/64 scope global temporary deprecated dynamic 
       valid_lft 453986sec preferred_lft 0sec
    inet6 2806:107e:15:413d:7893:65ce:cb5a:a28c/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 2441181sec preferred_lft 2441181sec
    inet6 fe80::4455:f83b:22ee:5769/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

Output of avahi-resolve-host-name pca.local

pca.local  192.168.1.94

Output of cat /etc/avahi/avahi-daemon.conf | grep -v "#"

[server]
use-ipv4=yes
use-ipv6=no
ratelimit-interval-usec=1000000
ratelimit-burst=1000

[wide-area]
enable-wide-area=yes


[publish]
publish-hinfo=no
publish-workstation=no


[reflector]


[rlimits]

The output of cat /etc/nsswitch.conf

# /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:         files
group:          files
shadow:         files
gshadow:        files


hosts:          files mymachines dns myhostname
networks:       files


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


netgroup:       nis
eridani
  • 1,584

1 Answers1

3

The reason you cannot ssh from PC "A" to PC "A" with ssh pca.local is because you don't have any mdns service defined in your /etc/nsswitch.conf file.

First, you need to ensure avahi-daemon and libnss-mdns are installed.

  • You've confirmed that avahi-daemon is already installed.
  • To check if libnss-mdns is installed, run apt-cache policy libnss-mdns. If it's not installed, install with sudo apt install libnss-mdns.

Next, you need to check that the hosts line in /etc/nsswitch.conf has mdns4_minimal [NOTFOUND=return] added to it.

  • If libnss-mdns was not installed, then installation will update /etc/nsswitch.conf appropriately.

  • Otherwise, edit /etc/nsswitch.conf and change the hosts line from this:

     hosts:          files mymachines dns myhostname

    ...to this:

     hosts:          files mdns4_minimal [NOTFOUND=return] mymachines dns myhostname

To quote the man-page for nsswitch.conf(5), the purpose of the hosts line in /etc/nsswitch.conf is the following:

The order of the services on the line determines the order in which those services will be queried, in turn, until a result is found.

Here is a brief explanation of the services:

  • The files service searches /etc/hosts first
  • The mdns4_minimal service is searched next using Multicast DNS (mDNS) for domains ending in .local
  • [NOTFOUND=return] indicates processing should not continue after an error
  • mymachines is a service providing hostname resolution for the names of containers running locally that are registered with systemd-machined.service(8).
  • The dns service will reference the DNS servers defined in /etc/resolv.conf
  • myhostname is a service that primarily provides hostname resolution for the locally configured system hostname as returned by gethostname(2).

This AskUbuntu answer has some good info as does this one.

mpboden
  • 3,046