To troubleshoot enabling swap space in Linux

In linux sometimes we face the issue of enabling swap space

The most easy way to troubleshoot is to rewrite the contents of etc/hosts in single user mode

For example: The contents of etc/hosts file is somewhat like this.


vim /etc/hosts

#please do not remove the following thing or required line
#that required network functionality will fail

127.0.0.1      localhost.localdomain   localhost

:wq

The simple way to trobleshoot is just rewrite the whole contents including the commented line as it is and the problem of enabling swap space would be solved.

Note: *There should not be any mistake while rewriting otherwise it would not work. If you forgot the contents see the etc/hosts file of other linux system. The default contents of file remains same in all linux with similar versions.

For Example: If you are performing in Redhat linux system then see /etc/hosts file of other system of redhat linux.

System connected to network & pinging with the gateway & other systems in the network but unable to access nothing

                           Fig: Location of  lsass file in Windows
This type of network problem is most often caused by a virus called lsass. The original file is present in window\system 32\folder. In normal circumstances only one service of lsass should be executed i.e; default of windows. But moment when more than one service of lsass starts executing it would result in anetwork problem. This is nothing but a virus in the system similar to lsass file in windows/ system32 folder. In this situation, system behaves normally if anyone checks the ping response but internet would not open, unable to access mail server, other shared data & network printer. To identify this see window task manager ->processes if more than one prcocesses of lsass is executing  then it is a virus. Another way, see the behaviour of icon of  two smalll computers  in startupbar resembling connectivity. If both are glowing simultaneously & constantly without any changes & problem is occuring then it means that it is affected by lsass virus. To resolve use systemrestore & update antivirus. Even third party combofix can also remove this virus but it cannot give gurantee because it deletes all files which it considers as virus which can result in loss of users useful data. To use combofix take backup of all important files & data.

Fig: Under normal circumstances only one process of lsass should be running

VLAN Trunking

What is VLAN Trunking

VLAN Trunking among cascaded switched network

To establish trunking using VTP among cascaded switched network

To establish trunking using VTP among cascaded switched network

                  Fig: cascading between  two switches
Now, consider the  figure. In this figure there are two switches and  both are connected to each other. Let us assume as per the diagram interface 12 of switch 1 is connected to interface 18 of switch 2. Now, we have to establish vlan trunking so that switch 2 can access vlan information of switch 1

//To configure switch 1 as VTP server:

switch# conf t
switch(config)#vtp mode server
switch(config)#vtp domain iop
switch(config)#vtp password 123456#
switch(config)#int fa0/12
switch (config-if)#switchport mode trunk
switch(config-if)#^Z

//To configure switch 2 as VTP client:
switch#conf t
switch(config)#vtp mode client
switch(config)#vtp domain iop
switch(config)#vtp password 123456#
switch(config)#int fa0/18
switch(config-if)#switchport mode trunk
switch(config-if)#^z

//To see vtp status:
switch#sh vtp status

This command would give information about VTP domain, mode(whether sever or client mode) & many more

//To see trunk link
switch# sh vtp counter
This command would give the information that vtp trunk link is established between interface 12 of switch 1 & interface 18 of switch 2

Now, members of switch 2 can access vlan information of switch 1 and can be put on the same vlan created on switch 1. But switch 2 can't create or add vlan.
Through  this we can manage vlan centrally

Now, Consider Fig 2:

fig 2: Two switches connected with each other parallely

The fig 2 is the best scenario of cascading switched network. People think that this would result in a loop but this is managed by Spanning Tree Protocol. Spanning Tree Protocol is by default supported by cisco switches.

To explain this further let us assume that interface 11 of switch1 is connected to the interface 19 of switch 2

Server switch(Switch 1)
switch# conf t
switch(config)#int fa0/13
switch (config-if)#switchport mode trunk
switch(config-if)#^Z

Client switch(Switch 2)
switch# conf t
switch(config)#int fa0/19
switch (config-if)#switchport mode trunk
switch(config-if)#^Z

Now, As per the scenario interface 12 of switch 1is coonected to interface 18 of switch 2 & interface 13 of switch 1 is connected to switch interface 19 of switch 2. Now, in Cisco switches Spanning Tree Protocol by default blocks one port of anyone switch to avoid loop formation & if one line fails then second line automatically takes up whole thing. The algorithim which supports spanning Tree Protocol is
called Spanning Tree Algorithim

VLAN Trunking

What is VLAN Trunking?

VLAN Trunking is the method by which we can share the vlan information accross switched network. The biggest benefit of this method is that there is no need of having different management of vlans and when created centrally are more managed. One more benefit of vlan trunking is that computers across the switched network comes under same vlan if there is cascading between switches.

*Trunking does not share member information.

Requirement for Trunking:

To establish trunking between vlans either tagging method or trunking protocol is required

Tagging method:  Two types of tagging method:

1>ISL: This tagging method is used between CISCO switches.

2> IEEE 802.1Q or DOT1Q: This tagging method is used between CISCO & non - CISCO switches.

Trunking Protocol:  Two types of trunking protocol:

1> DTP(Dynamic Trunking Protocol): DTP establishes automatic trunking when two switches having same tagging method are connected together.

2>VTP(Virtual Trunking Protocol): VTP is used to configure vlan trunking manually. To configure VTP three tings are required:
VTP mode
Domain
password 

VTP mode:

Server:  This is a default mode. This mode switch can  create, delete & add the virtual Lan Information and that virtual lan information is then shared with trunk link.

Client: Client mode switch can receive the virtual Lan information from trunk link and accordingly changes its databases and then also shares with another trunk link. It cannot create or add vlan information. 

VTP Domain: It is a logical boundry in which all the switches exists. VTP domain name is case sensitive.

Password: Password is required for secure transcations of vlans.

Exchange server services

Exchange 2000 logs a critical state when any of the following Services stops running:

 
Web Storage System
Message Transfer Agent (MTA) Stacks
Routing Engine
System Attendant
SMTP
World Wide Web Publishing Service* Exhange 2003 services.

If one of the service stop then exchange 2003 stops working
Micosoft exchange event
Microsoft exchange IMAP4
Microsoft exchange Information store
Microsoft echange Management
Microsoft Exchange MTA stacks
Micosoft Exchange POP3
Microsoft Exchange Routing Engine
Microsoft Exchange site Replication service
Microsoft Exchange System Attendant
　
*Exchange server 2007 services
Total 18 services:
Microsoft Exchange Active Directory Topolgy service
Microsoft Exchange Anti-spam Update
Microsoft Echange Edgesync
Micosoft Exchange File distribution
Microsoft Exchange IMAP4
Microsoft Exchange Information store
Microsoft Exchange Mail submission
Microsoft Exchange Mailbox Assistants
Microsoft Exchange Monitoring
Microsoft Exchange POP3
Microsoft Exchange Replication service
Microsoft Exchange Search Indexer
Microsoft Exchange Service host
Microsoft Exchange Speech Engine
Microsoft Exchange System Attendant
Microsoft Exchange Transport
Microsoft Exchange Transport log search
Microsoft search (Exchange)

DNS Resource Records

A-(Address record):  It returns a 32-bit IPv4 address and is used to map hostnames to IP address of the host.

CNAME(Canonical name record): Alias of one name to another i.e; DNS lookup will continue by retrying the lookup with new name.

MX(Mail Exchange record): It maps the domain name to the list of message transfer agents for that domain

PTR(pointer record): It is used for reverse  DNS lookup i.e; to map IP address to hostnames.

SOA(Start of Authority record): It specifies the authoritative information about the DNS zone, including the primary name server, email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone. The SOA resource record indicates that this DNS name server is the best source of information for the data within this DNS domain.

To identify whether the cogestion is at Lan port end or switch port end

To identify whether the congestion is at switch port end or LAN port end just see the flow of packets.

Packets received should not be much larger the packet send . If the packet received is much larger than packet sent then it would result in congestion at Lan port end. Its effects are : slow webbrowsing, connectivity loss for short duration(in min) etc. This is may be due to the behaviour of operating system. To resolve: 1>disable the LAN card for minute to discard incoming packets if not then , 2>shut the system for 5-10 minutes so that incoming packets would be discarded. Repeat it once or twice if necessary.

Fig : Congestion at Lan port