SSH Tunneling

You can create SSH Tunnels using different kinds of forwarding like
a) Local Port Forwarding, 
b) Remote Port Forwarding,
c) Dynamic Port Forwarding
c) X Forwarding

For a full command syntax check the online man pages for ssh  here  

Rate Limit on Cisco 3550

I have a Cisco 3550 and  want to apply rate limit on the interfaces for both ingress and egress. 
As per Cisco documentation in
Understanding QoS Policing and Marking on the Catalyst 3550
"Policing is not to be confused with traffic shaping, although both make sure the traffic stays within the profile or contract.
Policing does not buffer the traffic, so policing does not affect the transmission delay. Instead of buffering out-of-profile packets, policing drops them or marks them with different QoS levels (DSCP markdown).

Traffic shaping buffers out-of-profile traffic and smoothes the traffic bursts, but affects the delay and delay variation. Shaping can only be applied on the outgoing interface, while policing can be applied on both the incoming and outgoing interface.

The Catalyst 3550 supports policing for both incoming and outgoing directions. Traffic shaping is not supported."

Calculations on Linux

Command Line calculator

how to perform standard calculations on the command line including addition, subtraction, multiplication, division, powers, square root, conversion.


From man pages

bc - An arbitrary precision calculator language

How to add source code in Blogspot

I was googling around to find a way to post source code in the blog pages that look better compare to the default.

I run into these :

1. google-code-prettify
2. syntaxhighlighter
3. Format source code for blogspot

For now I'm using the last one.

Basic DHCP on Cisco Router

Configuring DHCP IOS DHCP server

Sequence of DHCP messages :

1. Client sends a broadcast DHCPDISCOVER message to the server
2. The DHCP server replies with a DHCPOFFER message containing the IP address, subnet mask , gateway and other parameters.
3. The client sends a broadcast DHCPREQUEST message to the server, requesting the offered address.
4. The DHCP server replies with a broadcast DHCPACK packet to the client.

DHCP server was designed to work within the same broadcast domain as the clients.
If DHCP server is behind another router , that router must become a DHCP Relay Agent. The DHCP relay will intercept the broadcast DHCP messages from the client and will forward them to the server as unicast messages. See at the end of this post a setup based on this scenario.

My network setup in GNS3 shown below 

SSH useful commands

I post below some useful SSH commands


Secure Copy File Transfer

SCP copies files between hosts on a network. It uses ssh for data transfer, and uses the same authentication and provides the same security as ssh. Some examples here

From a remote host to  local host

scp -P10022 192.168.2.10:/home/stelios/Desktop/Etherchannel.zip /home/stelios/Desktop/  
 Etherchannel.zip                                            100% 137KB 136.6KB/s  00:00   

More that one file from localhost to remote host

scp -P10022 test01.txt test02.txt 192.168.2.10:/home/stelios/Desktop/  
 test01.txt                                                 100%  0   0.0KB/s  00:00    
 test02.txt                                                 100%  0   0.0KB/s  00:00    

From the local host to a remote host

scp -P10022 /home/stelios/Desktop/Multicast.zip 192.168.2.10:/home/stelios/Desktop/  
 Multicast.zip                                               100%  75MB  1.5MB/s  00:49    

Secure Copy  directory from local to remote host

scp -P10022 -r /home/stelios/Desktop/scp_test 192.168.2.10:/home/stelios/Desktop/  
 WHowe_gns3_vbox.pdf                                       100% 1902KB  1.9MB/s  00:00    

Installing GNS3 0.8.3 on Linux Mint 13 Maya

Below are the basic steps for installing GNS3 0.8.3 on Linux Mint 13 Maya (and Linux Mint 14)
Following same steps should work on Ubuntu Linux as well.

Installing GNS3 v0.8.2 on Ubuntu 12.04

Below are the basic steps for installing GNS3 0.8.2 on Ubuntu 12.04 and get it started. Video tutorials on  gns3 website .

(check out the three step installation for GNS3 v0.8.6 Installing GNS3 v0.8.6 on Linux)

Cisco Routers/Switches - Command alias

You can create command alias on Cisco routers/swithces by using the command alias in the global configuration mode

See the origal documentation by Cisco Systems :
Using Aliases for CLI Commands

Cisco IOS Configuration Fundamentals Command Reference - alias

Syntax :
alias mode command-alias original-command
no alias mode [command-alias]