ssh.service startup failure; port 22 already in use

Brand new ubuntu installation:

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.1 LTS
Release: 20.04
Codename: focal

ssh.service fails to startup due to port 22 already in use:

$ sudo service ssh status
● ssh.service - OpenBSD Secure Shell server Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enabled) Active: failed (Result: exit-code) since Thu 2021-02-04 14:10:27 CST; 11min ago Docs: man:sshd(8) man:sshd_config(5) Process: 2041 ExecStartPre=/usr/sbin/sshd -t (code=exited, status=0/SUCCESS) Process: 2042 ExecStart=/usr/sbin/sshd -D $SSHD_OPTS (code=exited, status=255/EXCEPTION) Main PID: 2042 (code=exited, status=255/EXCEPTION)
Feb 04 14:10:27 ubu20a systemd[1]: Starting OpenBSD Secure Shell server...
Feb 04 14:10:27 ubu20a sshd[2042]: error: Bind to port 22 on 0.0.0.0 failed: Address already in use.
Feb 04 14:10:27 ubu20a sshd[2042]: error: Bind to port 22 on :: failed: Address already in use.
Feb 04 14:10:27 ubu20a sshd[2042]: fatal: Cannot bind any address.
Feb 04 14:10:27 ubu20a systemd[1]: ssh.service: Main process exited, code=exited, status=255/EXCEPTION
Feb 04 14:10:27 ubu20a systemd[1]: ssh.service: Failed with result 'exit-code'.
Feb 04 14:10:27 ubu20a systemd[1]: Failed to start OpenBSD Secure Shell server.

Only thing I see listening on port 22 is 1/init:

$ sudo netstat -tulpen
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State User Inode PID/Program name
tcp 0 0 127.0.0.53:53 0.0.0.0:* LISTEN 101 34051 556/systemd-resolve
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 0 37066 745/cupsd
tcp6 0 0 :::22 :::* LISTEN 0 34718 1/init
udp 0 0 0.0.0.0:5353 0.0.0.0:* 115 37068 743/avahi-daemon: r
udp 0 0 0.0.0.0:52172 0.0.0.0:* 115 37070 743/avahi-daemon: r
udp 0 0 127.0.0.53:53 0.0.0.0:* 101 34047 556/systemd-resolve
udp 0 0 0.0.0.0:631 0.0.0.0:* 0 39715 822/cups-browsed
udp6 0 0 :::5353 :::* 115 37069 743/avahi-daemon: r
udp6 0 0 :::39012 :::* 115 37071 743/avahi-daemon: r 

While I can get ssh.service to startup on an alternate port (eg, port 2020) I'd like to understand, and fix (if possible), this current issue with port 22.

NOTES:

• linux sysadmin is not my strong point so if I'm missing something stupidly obvious ... go ahead and shoot the messenger
• if the answer is RTFM then a link to which FMTR would be appreciated
• I looked through the first dozen or so suggested 'duplicate' questions but didn't find anything applicable to this issue

4 Answers

A bit long for a comment ...

I took @steeldriver's suggestion and ran the suggested systemctl call:

$ systemctl -all list-sockets
LISTEN UNITS ACTIVATES
... snip ...
/run/udev/control systemd-udevd-control.socket systemd-udevd.service
/run/uuidd/request uuidd.socket uuidd.service
[::]:22 tinysshd.socket
audit 1 systemd-journald-audit.socket systemd-journald.service
kobject-uevent 1 systemd-udevd-kernel.socket systemd-udevd.service 

Of interest is the 3rd line up from the bottom ... something called tinysshd listening on port 22.

I couldn't find any explanation for what tinysshd does (ie, doesn't appear to be a system required service) so I removed tinysshd (sudo apt remove tinysshd) and was able to successfully start the ssh service:

$ sudo service ssh status
● ssh.service - OpenBSD Secure Shell server Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enabled) Active: active (running) since Thu 2021-02-04 15:14:34 CST; 3s ago Docs: man:sshd(8) man:sshd_config(5) Process: 3887 ExecStartPre=/usr/sbin/sshd -t (code=exited, status=0/SUCCESS) Main PID: 3888 (sshd) Tasks: 1 (limit: 7032) Memory: 1.3M CGroup: / └─3888 sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups
Feb 04 15:14:34 ubu20a systemd[1]: Starting OpenBSD Secure Shell server...
Feb 04 15:14:34 ubu20a sshd[3888]: Server listening on 0.0.0.0 port 22.
Feb 04 15:14:34 ubu20a sshd[3888]: Server listening on :: port 22.
Feb 04 15:14:34 ubu20a systemd[1]: Started OpenBSD Secure Shell server.

I can also see sshd listening on port 22:

$ sudo netstat -tulpen
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State User Inode PID/Program name
...snip ...
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 0 66119 3888/sshd: /usr/sbi
tcp6 0 0 :::22 :::* LISTEN 0 66121 3888/sshd: /usr/sbi
... snip ...

try to identify which app allocate port via this command:

 sudo lsof -i -P -n | grep LISTEN 

and make design what to do next

Another thought as I was having the same issue. When I enabled ssh on my router it automatically took port 22 for ssh to the router and that wasn't changable. I could never make ssh to a computer work until I realized make a port trigger so another port say for example 2222 went to 22 local of that computer and voila I had ssh.

I'm using WSL2 & Ubuntu 20.04.

I found the answer by kbulgrien at Unix StackExchange to be my issue: that systemd-user-sessions.service isn't being called automatically. The only way I have figured out to get it to run automatically is to add the line

ExecStartPost=systemctl start systemd-user-sessions.service

in the [Service] section of /lib/systemd/system/ssh.service . But I've yet found a way that will allow SSH Server to start automatically... :( I still have to start it manually via sudo systemctl start sshd.service