Sshd Configuration
Internal
Overview
The system-wide sshd server configuration file is /etc/ssh/sshd_config (or /etc/sshd_config for cygwin).
Change the Default Port
Uncomment and/or update the default "Port" value in /etc/ssh/sshd_config:
#Port 22 Port 12345
Change the Default Port on a SELinux System
If SELinux is enable, you have to tell SELinux about the port change:
semanage port -a -t ssh_port_t -p tcp 12345
Also see How to install SELinux semanage.
Update the Firewall Rules
If iptables is enabled, there's a firewall rule that allows ssh access, and it usually mentions the port. You may want to check and change that: Iptables_Command_Line_Tool_Examples#Allow_SSH_Only_From_the_Internal_Network_on_a_Non-Standard_Port
Change the Network Interface to Listen On
ListenAddress 192.168.1.10
Turn Off Client Name DNS Verification
sshd can be configured with a "UseDNS" option, which specifies whether sshd should look up the remote host name and check that the resolved host name for the remote IP address maps back to the same IP address. The default is “yes” but in some case this causes the initial connection setup to take a long time, so it is best to turn this verification off:
... UseDNS no ...
The service needs to be restarted after reconfiguration.
Allow root To Connect with Password
In /etc/ssh/sshd_config:
PermitRootLogin yes
TODO: research what the following means:
PermitRootLogin without-password
Logging Verbosity
By default, sshd logs at INFO level:
LogLevel INFO
Options: DEBUG, DEBUG1, DEBUG2, DEBUG3
Increased log output will be available in /var/log/secure.
Allow Port Forwarding
AllowTcpForwarding yes
Configuration Reference
ClientAliveInterval
ClientAliveCountMax
UsePrivilegeSeparation
Specifies whether sshd separates privileges by creating an unprivileged child process to deal with incoming network traffic. After successful authentication, another process will be created that has the privilege of the authenticated user. The goal of privilege separation is to prevent privilege escalation by containing any corruption within the unprivileged processes. If UsePrivilegeSeparation is set to "sandbox"' then the pre-authentication unprivileged process is subject to additional restrictions. The default is "sandbox".