Standard Blue Light sshd configuration

/etc/ssh/sshd_config

Warning: if doing this remotely, when implementing configuration changes, keep the existing ssh session open and test by starting a new one.

If a keyword value is set twice, the first value may be effective!  Experiment showed that was true for PermitRootLogin and not for UsePAM.  For simplicity we decided not to set any keywords twice (outside conditional sections).

A pro-forma file is available in the Blue Light git at conf/ssh/sshd_config.

The parameters we sometimes change:

• AllowUsers tightens security.  We set this to root and add others as the need arises.
  
• GSSAPIAuthentication no turns off several authentication methods which are not needed when using private/public keys or passwords.  Reference: http://en.wikipedia.org/wiki/Generic_Security_Services_Application_Program_Interface
• PasswordAuthentication no or without-password disables login via password.
• PermitRootLogin no or without-password disables login via password.  We always use without-password, sometimes globally and sometimes within Match Address conditional section(s).
• UsePrivilegeSeparation yes allows ssh to function in lxc containers (otherwise we use the as-installed no).
  
• UsePAM no avoids messages like "PAM service(sshd) ignoring max retries; 6 > 3".  
  The message is caused by PAM's compiled-in retry limit being less than sshd's.
• UseDNS no disables reverse DNS lookups to see if your hostname matches the IP address you are connecting from.  Does not make sense with dynamic IP addresses.

Match Address stanzas

We are still evolving how we use these.  TODO: does the order matter (does sshd stop processing these stanzas after the first match?)?  TODO: do these PermitRootLogin values override any global setting?

Some samples:

# Allow root login from the BLUE OpenVPN addresses
Match Address 10.42.23.0/24
    PermitRootLogin without-password

# Allow root login from the BL OpenVPN server
Match Address 10.42.0.1
    PermitRootLogin without-password

# Allow root login from blav2
Match Address 148.251.233.235
    PermitRootLogin without-password
Match Address 10.42.0.2
    PermitRootLogin without-password

/etc/default/ssh

root@localhost:~# diff /etc/default/ssh{.org,}
5c5
< SSHD_OPTS=
---
> SSHD_OPTS=-u0

Warning: if doing this remotely, keep the existing ssh session open and test by starting a new one.

Enable the new configuration by service ssh restart

authorized_keys

command=

Security can be tightened by specifying the command that an authorised key can run by inserting command=<command> before the key itself.  Details in the sshd man page, in the command="command" section.

Sometimes this is not convenient because several commands are to be run.  A solution is to specify a script in authorized_keys and for the script to validate the commands.  For example:

#!/bin/bash
# Purpose: validates the command a remote host is attempting to execute by ss

df_regex='^df '
rsync_server_regex='^rsync --server '
stat_regex='^stat --format=%F '

if [[ $SSH_ORIGINAL_COMMAND =~ $df_regex \
    || $SSH_ORIGINAL_COMMAND =~ $rsync_server_regex \
    || $SSH_ORIGINAL_COMMAND =~ $stat_regex \
]]; then
    exec $SSH_ORIGINAL_COMMAND
else
    echo "${0##*/}: command did not pass validation ($SSH_ORIGINAL_COMMAND)" >&2
    exit 1
fi

TODO: enhance the script to read the regexes from a config file.