====== RH124 SSH configuration ======

===== Overview =====

Both client and server side configuration and usage of SSH involves three packages:

  * ''openssh-server''
  * ''openssh-clients''
  * ''openssh''

We will examine these packages shortly.

===== Starting and stopping the SSH service =====

Check the status of ''sshd.service'':

<code>
$ systemctl status sshd.service
● sshd.service - OpenSSH server daemon
     Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; preset: enabled)
     Active: active (running) since Fri 2026-10-23 02:00:51 EDT; 5h 49min ago
 Invocation: 9f19b671aa534042a2785fca1a1078c5
       Docs: man:sshd(8)
             man:sshd_config(5)
   Main PID: 918 (sshd)
      Tasks: 1 (limit: 48852)
     Memory: 7.5M (peak: 24.8M)
        CPU: 119ms
     CGroup: /system.slice/sshd.service
             └─918 "sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups"

Oct 23 02:00:51 localhost.localdomain systemd[1]: Starting sshd.service - OpenSSH server daemon...
Oct 23 02:00:51 localhost.localdomain sshd[918]: Server listening on 0.0.0.0 port 22.
Oct 23 02:00:51 localhost.localdomain sshd[918]: Server listening on :: port 22.
Oct 23 02:00:51 localhost.localdomain systemd[1]: Started sshd.service - OpenSSH server daemon.
Oct 23 02:04:09 localhost.localdomain sshd-session[1924]: Accepted password for rpjday from 192.168.122.1 port 52956 ssh2
Oct 23 02:04:09 localhost.localdomain sshd-session[1924]: pam_unix(sshd:session): session opened for user rpjday(uid=1000) by rpjday(uid=0)
Oct 23 07:12:23 localhost.localdomain sshd-session[2972]: Accepted password for rpjday from 192.168.122.1 port 43614 ssh2
Oct 23 07:12:23 localhost.localdomain sshd-session[2972]: pam_unix(sshd:session): session opened for user rpjday(uid=1000) by rpjday(uid=0)
$
</code>

Stop, check and restart the service:

<code>
$ sudo systemctl stop sshd.service
$ systemctl status sshd.service
$ sudo systemctl start sshd.service
</code>

**NOTE**: Stopping ''sshd.service'' does not kill existing connections, just attempts at new ones.

===== (Abbreviated) package contents and directory layout =====

References to man/doc pages and build IDs deleted for brevity.

**NOTE**: Executables under ''/usr/libexec'' are not meant to be invoked directly, but are typically "helper" utilities for other programs.

==== openssh-server ====

<code>
/etc/pam.d/sshd
/etc/ssh/sshd_config
/etc/ssh/sshd_config.d
/etc/ssh/sshd_config.d/40-redhat-crypto-policies.conf
/etc/ssh/sshd_config.d/50-redhat.conf
/etc/sysconfig/sshd
/usr/lib/systemd/system/ssh-host-keys-migration.service
/usr/lib/systemd/system/sshd-keygen.target
/usr/lib/systemd/system/sshd-keygen@.service
/usr/lib/systemd/system/sshd.service
/usr/lib/systemd/system/sshd.socket
/usr/lib/systemd/system/sshd@.service
/usr/lib/sysusers.d/openssh-server.conf
/usr/libexec/openssh/sftp-server
/usr/libexec/openssh/ssh-host-keys-migration.sh
/usr/libexec/openssh/sshd-keygen
/usr/libexec/openssh/sshd-session
/usr/sbin/sshd
/var/lib/.ssh-host-keys-migration
</code>

==== openssh-client ====

<code>
/etc/ssh/ssh_config
/etc/ssh/ssh_config.d
/etc/ssh/ssh_config.d/50-redhat.conf
/usr/bin/scp
/usr/bin/sftp
/usr/bin/ssh
/usr/bin/ssh-add
/usr/bin/ssh-agent
/usr/bin/ssh-copy-id
/usr/bin/ssh-keyscan
/usr/lib/systemd/user/ssh-agent.service
/usr/lib/systemd/user/ssh-agent.socket
/usr/libexec/openssh/ssh-pkcs11-helper
/usr/libexec/openssh/ssh-sk-helper
</code>

==== openssh ====

<code>
/etc/ssh
/etc/ssh/moduli
/usr/bin/ssh-keygen
/usr/libexec/openssh
</code>

Both higher-level packages depend on ''openssh'':

<code>
$ rpm -qR openssh-server
$ rpm -qR openssh-clients
</code>
===== /etc/ssh/ system-wide SSH configuration =====

Both system-wide client and server configuration.

<code>
$ ls -lF
total 564
-rw-r--r--. 1 root root 541716 May 19 20:00 moduli
-rw-r--r--. 1 root root   1916 May 19 20:00 ssh_config
drwxr-xr-x. 2 root root     28 Oct 17 12:18 ssh_config.d/
-rw-------. 1 root root   3674 May 19 20:00 sshd_config
drwx------. 2 root root     66 Oct 17 12:18 sshd_config.d/
-rw-------. 1 root root    480 Oct 17 12:19 ssh_host_ecdsa_key
-rw-r--r--. 1 root root    162 Oct 17 12:19 ssh_host_ecdsa_key.pub
-rw-------. 1 root root    387 Oct 17 12:19 ssh_host_ed25519_key
-rw-r--r--. 1 root root     82 Oct 17 12:19 ssh_host_ed25519_key.pub
-rw-------. 1 root root   2578 Oct 17 12:19 ssh_host_rsa_key
-rw-r--r--. 1 root root    554 Oct 17 12:19 ssh_host_rsa_key.pub
$
</code>

<code>
$ sudo tree -F
./
├── moduli
├── ssh_config
├── ssh_config.d/
│   └── 50-redhat.conf
├── sshd_config
├── sshd_config.d/
│   ├── 40-redhat-crypto-policies.conf
│   └── 50-redhat.conf
├── ssh_host_ecdsa_key
├── ssh_host_ecdsa_key.pub
├── ssh_host_ed25519_key
├── ssh_host_ed25519_key.pub
├── ssh_host_rsa_key
└── ssh_host_rsa_key.pub
$
</code>

===== The SSH host keys =====

Created in ''/etc/ssh'' at installation time:

<code>
$ ls -l ssh_host*
-rw-------. 1 root root  480 Oct 17 12:19 ssh_host_ecdsa_key
-rw-r--r--. 1 root root  162 Oct 17 12:19 ssh_host_ecdsa_key.pub
-rw-------. 1 root root  387 Oct 17 12:19 ssh_host_ed25519_key
-rw-r--r--. 1 root root   82 Oct 17 12:19 ssh_host_ed25519_key.pub
-rw-------. 1 root root 2578 Oct 17 12:19 ssh_host_rsa_key
-rw-r--r--. 1 root root  554 Oct 17 12:19 ssh_host_rsa_key.pub
$
</code>

Every SSH server has a static, unique cryptographic key pair consisting of a private host key (kept strictly secret on the server) and a public host key (shared with clients).

===== sshd_config file =====

<code>
# This is the sshd server system-wide configuration file.  See
# sshd_config(5) for more information.

# This sshd was compiled with PATH=/usr/local/bin:/usr/bin:/usr/local/sbin:/usr/sbin

# The strategy used for options in the default sshd_config shipped with
# OpenSSH is to specify options with their default value where
# possible, but leave them commented.  Uncommented options override the
# default value.

# To modify the system-wide sshd configuration, create a  *.conf  file under
#  /etc/ssh/sshd_config.d/  which will be automatically included below
Include /etc/ssh/sshd_config.d/*.conf

# If you want to change the port on a SELinux system, you have to tell
# SELinux about this change.
# semanage port -a -t ssh_port_t -p tcp #PORTNUMBER
#
#Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

#HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
#HostKey /etc/ssh/ssh_host_ed25519_key

# Ciphers and keying
#RekeyLimit default none

# Logging
#SyslogFacility AUTH
#LogLevel INFO

# Authentication:

#LoginGraceTime 2m
#PermitRootLogin prohibit-password
#StrictModes yes
#MaxAuthTries 6
#MaxSessions 10

#PubkeyAuthentication yes

# The default is to check both .ssh/authorized_keys and .ssh/authorized_keys2
# but this is overridden so installations will only check .ssh/authorized_keys
AuthorizedKeysFile      .ssh/authorized_keys

#AuthorizedPrincipalsFile none

#AuthorizedKeysCommand none
#AuthorizedKeysCommandUser nobody

# For this to work you will also need host keys in /etc/ssh/ssh_known_hosts
#HostbasedAuthentication no
# Change to yes if you don't trust ~/.ssh/known_hosts for
# HostbasedAuthentication
#IgnoreUserKnownHosts no
# Don't read the user's ~/.rhosts and ~/.shosts files
#IgnoreRhosts yes

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no

# Change to no to disable s/key passwords
#KbdInteractiveAuthentication yes

# Kerberos options
#KerberosAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no
#KerberosUseKuserok yes

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
#GSSAPIStrictAcceptorCheck yes
#GSSAPIKeyExchange no
#GSSAPIEnablek5users no

# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the KbdInteractiveAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via KbdInteractiveAuthentication may bypass
# the setting of "PermitRootLogin prohibit-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and KbdInteractiveAuthentication to 'no'.
# WARNING: 'UsePAM no' is not supported in this build and may cause several
# problems.
#UsePAM no

#AllowAgentForwarding yes
#AllowTcpForwarding yes
#GatewayPorts no
#X11Forwarding no
#X11DisplayOffset 10
#X11UseLocalhost yes
#PermitTTY yes
#PrintMotd yes
#PrintLastLog yes
#TCPKeepAlive yes
#PermitUserEnvironment no
#Compression delayed
#ClientAliveInterval 0
#ClientAliveCountMax 3
#UseDNS no
#PidFile /var/run/sshd.pid
#MaxStartups 10:30:100
#PermitTunnel no
#ChrootDirectory none
#VersionAddendum none

# no default banner path
#Banner none

# override default of no subsystems
Subsystem       sftp    /usr/libexec/openssh/sftp-server

# Example of overriding settings on a per-user basis
#Match User anoncvs
#       X11Forwarding no
#       AllowTcpForwarding no
#       PermitTTY no
#       ForceCommand cvs server
</code>