Tag Archives: tcp wrapper

Red Hat Certification – RHCE – Network Services – ssh

1st post – System Management and Configuration

Objectives

Network services

Network services are an important subset of the exam objectives. RHCE candidates should be capable of meeting the following objectives for each of the network services listed below:

  • Install the packages needed to provide the service.
  • Configure SELinux to support the service.
  • Configure the service to start when the system is booted.
  • Configure the service for basic operation.
  • Configure host-based and user-based security for the service.

User should be able to do the following for all these services:

SSH:

To test from windows you can use putty.

But in linux you just need ssh for client and sshd for server.

man 5 sshd_config and this blogpost has an overview.

  • Install the packages needed to provide the service.
    • yum install openssh
  • Configure SELinux to support the service
    • getsebool -a|grep ssh
  • Configure the service to start when the system is booted.
    • chkconfig sshd on
  • Configure the service for basic operation.
    • /etc/ssh/sshd_config
  • Configure host-based and user-based security for the service
    • iptables
      • port 22 (TCP)
    • tcp.wrapper

 

TCP Wrapper

More info in man tcpd and man 5 hosts_access

Check that your daemon supports it:

which sshd
ldd /usr/sbin/sshd|grep wrap

For this test, let’s say that the server you are configuring has IP/netmask 192.168.1.1/24 and that you have a client on 192.168.0.0/24

cat /etc/hosts.allow

sshd: 192.168.0.0/255.255.255.0
sshd: ALL : twist /bin/echo DEATH

The last row sends a special message to a client connecting from a non-allowed network.

cat /etc/hosts.deny

ALL: ALL

If you on the server with these settings try to do “ssh -v root@localhost” or “ssh -v root@192.168.1.1” you’ll get the message from twist.

If you in hosts.allow add:

sshd: KNOWN

You can log on to the localhost, but not if you add “LOCAL”.

If you add

sshd: 192.168.1.

you can log on from localhost to the public IP of the server.

Extra

  • Configure key-based authentication.
    • ssh-keygen
    • ssh-copy-id user@host
    • ssh user@host
    • set PasswordAuthentication to no in sshd_config
    • service sshd restart
  • Configure additional options described in documentation.
    • many things can be done, see “man 5 sshd_config”
    • chrootdirectory looks quite cool but requires a bit of work