Unix - Symbolic links

A symbolic link or soft link is very much like a shortcut in Windows. Unlike a hard link, a symbolic link does not contain the data in the target file, and instead points to another file in the system.

• Let's create a hard link of new directory as the source file
  # mkdir /usr/test
  


• Let's create a symbolic link  
  # ln -s /usr/test/ /test        ( # ln –s sourceFile symbolicLinkFile )
  


• # ls –al /
  …
  lrwxr-xr-x   1 root  wheel       10 Sep 25 11:18     test -> /usr/test/
  …
  


• # touch /usr/test/sample.txt
  


• # ls /test
  sample.txt
  
  Note: sample.txt IS listed.
  


• # ls -al /test
  lrwxr-xr-x  1 root  wheel  10 Sep 25 11:18 /test -> /usr/test/
  
  Note: sample.txt IS NOT listed during detailed listing.
  


• # ls -al /usr/test/
  total 6
  drwxr-xr-x   2 root  wheel  512 Sep 25 11:21 .
  drwxr-xr-x  18 root  wheel  512 Sep 25 11:18 ..
  -rwxr-xr-x   1 root  wheel   61 Sep 25 11:22 sample.txt
  


• # touch /test/sample2.txt
  


• # ls -al /test
  lrwxr-xr-x  1 root  wheel  10 Sep 25 11:18 /test -> /usr/test/
  
  Note: sample2.txt is NOT listed either.
  


• # ls /test
  sample.txt       sample2.txt
  


• # rm /test
  
  Note: Symbolic link is deleted without any complain about files within directory
  


• # ls /usr/test/
  sample.txt       sample2.txt
  


• # rm /usr/test
  rm: /usr/test: is a directory

Unix - Z File System (ZFS)

The Z file system (ZFS) developed by Sun Microsystems is a feature rich file system for server solutions. It is designed for high storage volumes, mirror and RAID options for redundancy, data snapshots, integrity checking, and automatic repairs.

Here are some ZFS troubleshooting commands and typical outputs:

• # zpool status
  

    pool: zroot
   state: ONLINE
    scan: resilvered 30.6M in 0h0m with 0 errors on Sep 16 10:18:02 2013
  config:
  
   NAME        STATE     READ WRITE CKSUM
   zroot       ONLINE       0     0     0
     mirror-0  ONLINE       0     0     0
     gpt/disk0 ONLINE       0     0     0
     gpt/disk1 ONLINE       0     0     3
  
  errors: No known data errors 



• # zpool list
  

  NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
  zroot   228G   124G   104G    54%  1.00x  ONLINE  - 



• # zfs list
  

  NAME                        USED  AVAIL  REFER  MOUNTPOINT
  zroot                      31.8G   417G    31K  /zroot
  zroot/ROOT                 3.38G   417G  3.38G  /
  zroot/scratch              4.94G   417G  4.94G  /scratch
  zroot/tmp                  58.9M   417G  58.9M  /tmp
  zroot/usr                  21.0G   417G    31K  /usr
  zroot/usr/home             17.9G   417G  17.9G  /usr/home
  zroot/usr/local             813M   417G   813M  /usr/local
  zroot/usr/obj               997M   417G   997M  /usr/obj
  zroot/usr/ports             965M   417G   896M  /usr/ports
  zroot/usr/ports/distfiles  67.6M   417G  67.6M  /usr/ports/distfiles
  zroot/usr/ports/packages   1.30M   417G  1.30M  /usr/ports/packages
  zroot/usr/src               359M   417G   359M  /usr/src
  zroot/var                  2.46G   417G   109M  /var
  zroot/var/crash            2.15G   417G  2.15G  /var/crash
  zroot/var/db                206M   417G   204M  /var/db
  zroot/var/db/pkg           1.90M   417G  1.90M  /var/db/pkg
  zroot/var/empty              31K   417G    31K  /var/empty
  zroot/var/log               706K   417G   706K  /var/log
  zroot/var/mail             59.5K   417G  59.5K  /var/mail
  zroot/var/run              56.5K   417G  56.5K  /var/run
  zroot/var/tmp              1.79M   417G  1.79M  /var/tmp
  



• # ls -l /etc/zfs
  

  total 1
  -rw-------  1 root  wheel  181 Jun 10 12:59 exports



• # ls -l /boot/zfs /boot/zfsboot /boot/zfsloader
  

  -r--r--r--  1 root  wheel   66048 Dec  4  2012 /boot/zfsboot
  -r-xr-xr-x  1 root  wheel  303104 Oct 16  2013 /boot/zfsloader
  
  /boot/zfs:
  total 3
  -rw-r--r--  1 root  wheel  1588 Sep 13  2013 zpool.cache



• # ls -l /boot/gptboot /boot/gptzfsboot
  

  -r--r--r--  1 root  wheel  16723 Dec  4  2012 /boot/gptboot
  -r--r--r--  1 root  wheel  43443 Dec  4  2012 /boot/gptzfsboot

Unix - Hardware Timer Selection

• To find the timer counter being used by the computer:
  
  myhost# sysctl kern.timecounter.hardware
  kern.timecounter.hardware: HPET



• List of available timers:
  
  myhost# sysctl kern.timecounter.choice
  kern.timecounter.choice: TSC(-100) HPET(900)

The integer value within brackets defines the quality of the time counter compared to others. A negative value means this time counter is broken and should not be used.



• Selecting the timer of your choice:
  myhost# sysctl kern.timecounter.hardware=TSC
  
  Note: This selection does not persist after a reboot. To make this change persistent add the following line to the file /etc/sysctl.conf:
  sysctl kern.timecounter.hardware=HPET

Network Troubleshooting Commands

The following are the basic commands that helps to locate network issues. To open the command line interface or consoles:
* Ubuntu : 'Ctrl + Alt + T'
* Windows Systems : 'Window key + R' --> type 'cmd' and click 'OK/Run'


ifconfig / ipconfig

It is generally the very first to check whether the computer's network interface has got a valid IP address to establish communication with other devices in the network. MAC (physical) address of the network can also be found using these commands.

On Unix based systems,

e.g.1: ifconfig ==> Displays all the network interfaces, such as Ethernet and Loopback, MAC addresses, IP addresses, and subnet masks. Depending on the flavour of Unix and type of interface, some additional details such as number of packets transmitted and received, errors, active/disabled status, and size of the maximum transmittable unit (MTU) can also be found.

e.g.2: ifconfig eth0 down ==> Disables the Ethernet interface 'eth0'. Super user (root) only.

e.g.3: ifconfig eth0 up ==> Enables the Ethernet interface 'eth0'. Super user (root) only.

e.g.4: ifconfig eth0 192.168.1.1 netmask 255.255.255.0 ==> Assigns IP address and netmask manually. Super user (root) only.

On Windows systems,

e.g.1: ipconfig ==> Displays the IP address, netmask, and default gateway of all the network interfaces.

e.g.2: ipconfig /all ==> Displays more information about the network interfaces, including MAC address, DHCP status, and DNS servers used.

e.g.3: ipconfig /renew ==> Re-establishes TCP/IP connections on all network adapters and refreshes IP address with the DHCP server.

e.g.4: ipconfig /flushdns ==> Flushes the DNS cache stored locally (URL to IP address conversion)


ping

It is the basic command to check network reachability based on echos received from a network interface. It works in both Unix and Windows systems. However, command switches for a given additional option, such continuous ping, may vary.

On both Unix and Windows based systems,

e.g.1: ping 127.0.0.1 ==> reveals whether the IP protocol is up on you own computer (localhost). On Unix, continuously sends echo requests until terminated with 'Ctrl + C'.

e.g.2: ping www.yahoo.com ==> shows whether you can reach yahoo.com's server over the internet

On Windows,

e.g.1: ping -t www.google.com ==> On windows, continuously sends echo requests until terminated with 'Ctrl + C'.

Note:

• If no response for a well known server, then ping to the gateway IP addess found from the ifconfig / ipconfig command above. This reveals the connectivity to your network gateway.
• When URL is used with the ping command instead of an IP address, and it does not get resolved into an IP address, then the Domain Name System (DNS) resolution could be tested. 

traceroute / tracert

A command to identify the hops in the network, and in case of failures the output indicates the intermediate hops which packets travel successfully.

On Unix based systems,

e.g.1: traceroute www.yahoo.com ==> Names of hops along the network path and time taken to receive an echo response from them

On Windows, 

e.g.1: tracert www.yahoo.com ==> Names of hops along the network path and time taken to receive an echo response from them


nslookup

Returns the corresponding IP address for a given URL (also vice-versa).

On both Unix and Windows based systems,

e.g.1: nslookup www.yahoo.com 


netstat

A command to visualize the details of the ongoing network communication.

On both Unix and Windows based systems,

e.g.1: netstat ==> Details of all the established TCP connections

e.g.2: netstat -r ==> The routing table of your computer, including the default route and gateway details.


arp

Displays the IP address to MAC address conversion, and vice versa.

e.g.1: arp -a ==> Displays all the IP addresses and corresponding MAC addresses of the known network interfaces in the local broadcast domain


hostname

Gives the name assigned to your computer.

e.g.1: hostname ==> Hostname of your system


whoami / set U

Reveals the username currently used by you.

On Unix systems:

e.g.1: whoami ==> Shows your username (or 'root' in the case of superuser)

On Windows systems,

e.g.2: set U ==> Shows your username, domain, and user profile location.

IPv4 Martian Addresses

Martian addresses are the blocks of IPv4 addresses that are reserved for special use by the Internet Assigned Numbers Authority (IANA), and are NOT meant to be reached over the Internet. The following are the IANA reserved category addresses which are generally considered as Martian addresses:

Address block	Use
0.0.0.0/8	"This" network
100.64.0.0/10	Carrier-grade NAT
127.0.0.0/8	Loopback
169.254.0.0/16	Link local
192.0.0.0/24	IETF protocol assignments
192.0.2.0/24	TEST-NET-1
198.18.0.0/15	Network interconnect device benchmark testing
198.51.100.0/24	TEST-NET-2
203.0.113.0/24	TEST-NET-3
224.0.0.0/4	Multicast
240.0.0.0/4	Reserved for future use

Depending on the manufacturer of routers used at the customer premises or at the network service providers end these Martian addresses may also be blocked within Layer 3 Virtual Private Networks (IP VPNs) or Local Area Networks (LANs). E.g., Juniper Network routers block Martian addresses within IP VPNs.

Private Network IP Address Blocks

IP address blocks that are dedicated for private LANs are also included within the Martian Address blocks according to some definitions.

Subnet Address block	Use
10.0.0.0/8	Class A private networks (used as /8 subnets)
172.16.0.0/12	Class B private networks (used as /16 subnets)
192.168.0.0/16	Class C private networks (used as /24 subnets)

Bogon IP Address Blocks

IP address blocks that are yet to be allocated to any user or delegated to any regulator agency by the IANA are called Bogon IP addresses. These blocks change with time.

Unix - SSH/SCP

• Check SSH server running:
  

  # cat /etc/rc.conf | grep ssh
  sshd_enable="YES"



• SSH access to a remote machine:
  

  #/$ ssh username@remoteMachine

  
  'remoteMachine' is the DNS resolvable hostname of the remote host. IP address can also be used instead of the remote machine's hostname.


• Securely transfer file over SSH:
  

  #/$ scp -q -C -l 80000 username@remoteMachine:/home/username/test.txt \

  /home/username/. 



• Enable root access over SSH (only if it is essential):
  

  # vi /etc/ssh/sshd_config
  PermitRootLogin yes
  
  # service sshd restart  (or, # /etc/rc.d/sshd restart) 



• Generate and copy SSH keys:
  

  #/$ cd ~
  #/$ ssh-keygen -t rsa
  #/$ scp ~/.ssh/id_rsa.pub remoteMachine:.ssh/authorized_keys 

  
  'RSA' is used here as authentication mechanism. Similarly 'DSA' can also be used.


• Verify whether SSH key is working:
  

  #/$ ssh remoteMachine hostname

  
  Displays remote machine's hostname without a password prompt.


• Monitoring/tracing SSH/SCP activities:
  

  # cat /var/log/auth.log

  Displays detailed log.

Note: Unix commands and file locations used here have been tested on FreeBSD systems.

Crimping an Ethernet LAN cable

Steps:

1. Insert the cat 5e/6 cable into the boot/cap.
2. Remove approximately 1 inch of the sleeve of the wire.
3. Straighten the wires (ie, untwist the twisted pairs).
4. Arrange the wires in the right order as shown below. Choose either (EIA/TIA 568) A or B type, and if it is a straight cable follow the colour sequence of the first image on both the ends of the cable.
5. Cutoff excess length of the wire (to make it levelled inside the RJ-45 clip).
6. Insert the wire into the RJ-45 clip while maintaining the order of the wires all the way in until you can see all the wires touching on the right side wall of the clip as shown by the red line (ie, you have to insert the beyond the end of arrows shown here.)
7. Crimp the RJ-45 clip firmly with the crimping tool.
8. Fix the boot.