Yum is software installation tool for Red hat linux and Fedora Linux. It is a complete software management system. Other option is to use up2date utility. yum is designed to use over network/internet. It does not use CDROM to install packages. If you are using fedora you don't have to install it, it is part of fedora itself.
If you don't have yum then download it from project home page http://linux.duke.edu/projects/yum/download.ptml And then install it
rpm -ivh yum*
Step # 1: Configure yum
You need to edit /etc/yum.conf and modify/add following code to it:
vi /etc/yum.conf
Append or edit code as follows: Code:
[base] name=Fedora Core $releasever - $basearch - Base baseurl=http://apt.sw.be/fedora/$releasever/en/$basearch/dag baseurl=http://mirrors.kernel.org/fedora/core/$releasever/$basearch/os
[dag] name=Dag RPM Repository for Red Hat Enterprise Linux baseurl=http://www.city-fan.org/ftp/contrib/yum-repo/rhel4/ #baseurl=http://www.city-fan.org/ftp/contrib/yum-repo/rhel4/i386/ gpgcheck=0 rpm --import http://dag.wieers.com/rpm/packages/RPM-GPG-KEY.dag.txt
All web servers been connected to the Internet subjected to DoS (Denial of Service) or DDoS (Distrubuted Denial of Service) attacks in some kind or another, where hackers or attackers launch large amount connections consistently and persistently to the server, and in advanced stage, distributed from multiple IP addresses or sources, in the hope to bring down the server or use up all network bandwidth and system resources to deny web pages serving or website not responding to legitimate visitors.
You can detect the ddos using the following command
It will shows the number of connections from all IPs to the server.
There are plenty of ways to prevent, stop, fight and kill off DDoS attack, such as using firewall. A low cost, and probably free method is by using software based firewall or filtering service. (D)DoS-Deflate is a free open source Unix/Linux script by MediaLayer that automatically mitigate (D)DoS attacks. It claims to be the best, free, open source solution to protect servers against some of the most excruciating DDoS attacks.
(D)DoS-Deflate script basically monitors and tracks the IP addresses are sending and establishing large amount of TCP network connections such as mass emailing, DoS pings, HTTP requests) by using “netstat” command, which is the symptom of a denial of service attack. When it detects number of connections from a single node that exceeds certain preset limit, the script will automatically uses APF or IPTABLES to ban and block the IPs. Depending on the configuration, the banned IP addresses would be unbanned using APF or IPTABLES (only works on APF v 0.96 or better).
Installation and setup of (D)DOS-Deflate on the server is extremely easy. Simply login as root by open SSH secure shell access to the server, and run the the following commands one by one:
Users can change any of these settings to suit the different need or usage pattern of different servers. It’s also possible to whitelist and permanently unblock (never ban) IP addresses by listing them in /usr/local/ddos/ignore.ip.list file. If you plan to execute and run the script interactively, users can set KILL=0 so that any bad IPs detected are not banned
iptables is the userspace command line program used to configure the Linux 2.4.x and 2.6.x IPv4 packet filtering ruleset. It is targeted towards system administrators.
Since Network Address Translation is also configured from the packet filter ruleset, iptables is used for this, too.
The iptables package also includes ip6tables. ip6tables is used for configuring the IPv6 packet filter. Dependencies
iptables requires a kernel that features the ip_tables packet filter. This includes all 2.4.x and 2.6.x kernel releases. Main Features
* listing the contents of the packet filter ruleset * adding/removing/modifying rules in the packet filter ruleset * listing/zeroing per-rule counters of the packet filter ruleset
Rules
* If you create a set of rules in iptables during one session and then reboot your computer, all the rules that were added will be lost.
* If you want the rules to persist, you should put the commands to add them into a startup script.
* To check what rules are already implemented:
o Type into a terminal window:
ComputerName:~# iptables -L o A list of the present rules will appear on the screen under a variety of headings.
Rule Components
* There are three basic components to each rule:
1. Where to apply the rule during the process of sending and receiving network traffic (packets). There are three different places, or chains:
1. INPUT: Applies rules to packets being received from the network. 2. OUPUT: Applies rules to packets being sent from your computer. 3. FORWARD: Applies rules to packets that your machine is forwarding to others on the network.
2. What type of effect the rule has, regardless of where it is applied. The 3 effects are:
1. ACCEPT: Accepts a given packet and allows it to pass either in or out. 2. DENY: Does not allow a packet to pass but sends an error message back to its sender. 3. DROP: Completely ignores a packet without sending an error message to its sender.
Each chain also has a default policy (usually ACCEPT) that is applied if a specific packet does not match any rules.
3. The location you want to block packets from or going to, usually called the source can be written as either an IP address or a DNS name (such as www.yahoo.com).
* Each of these three components are used to create a rule through command line arguments.
Back to top
Adding a Rule
* To add a rule:
o Use the argument -A to tell iptables to add a rule to the chain Chain_Name.
o Add the source with the option -s . We can also specify a range of IPs with the '/' character (200.200.200.1/24 specifies 200.200.200.*) as well as use the wildcard character '*' (Find further information on t;/##" notation called CIDR blocks).
o Specify the desired effect with the -j argument. For example, if we wanted to block information coming from 200.200.200.1 we would enter:
ComputerName:~# iptables -A INPUT -s 200.200.200.1 -j DROP o Typing # iptables -L again will now show the new rule under the INPUT chain heading. It should look like this:
Chain INPUT (policy ACCEPT) target prot opt source destination DROP all -- 200.200.200.1 anywhere
Back to top
Removing a Rule
* Removing a rule:
o Type the argument -D where Rule_Num starts at 1 and counts down from the top of each list of rules. To remove our rule, we simply type in (assuming that the new rule is the first in the list):
ComputerName:~# iptables -D INPUT 1
o Now, typing:
# iptables -L
should show that the rule has been deleted.
Back to top
Advanced Rule Examples:
* There are many other advanced options for these rules, one of the most important of which is the ability to specify what "type" of packets to block by blocking specific ports on which certain services operate. For example, we could specify that we wanted to block only packets going into port 23, named telnet packets, coming into your computer from 200.200.200.1 by writing the rule:
ComputerName:~# iptables -A INPUT -s 200.200.200.1 -j DROP -p tcp --destination-port telnet
* Other ports can be specified. For a full list of ports being used on your computer and the name or type attached to each, go to your /etc/services file.
* Other common ports to block are:
o HTTP (port 80) o (port 21) o SSH (22)
* There are also a wide variety of other command line arguments that can be used, but these simple rules so far introduced allow for a wide variety of applications. o If you wanted to block all incoming telnet connections to your computer:
ComputerName:~# iptables -A INPUT -j DROP -p tcp --destination-port telnet
Since there is no defined source, any telnet request to your computer will be blocked. o If you have two or more network connections, you can specify which of these connections you would like to apply your rule with -i command for input rules and -o command for output rules. For example, if we would like to block any incoming tcp packets on your second Ethernet connection (eth1):
ComputerName:~# iptables -A INPUT -j DROP -p tcp -i eth1
This rule is not very useful since all incoming ports are blocked. We would not hear any tcp packet replies to our outbound requests, thus rendering our connection for the most part useless. o We can specify ports that we want open while the rest would remain closed by implementing two rules:
1. Explicitly accept packets on the port we want to open, and 2. Block all of the ports.
o For the web server example above, the first rule would accept tcp packets on port 80 through eth1 and the second would block all incoming tcp traffic. These two rules are given below:
+ ComputerName:~# iptables -A INPUT -j DROP -p tcp -i eth1
This combination of rules works because iptables implements the rules in order. When a new incoming tcp packet bound for port 80 arrives, iptables will see the accept rule first and admit the packet before the all-encompassing deny rule takes effect. o For blocking only incoming tcp transactions but allowing our computer to start new transactions with other web servers or the like, we can use the --syn option in the following rule:
ComputerName:~# iptables -A INPUT -p tcp --syn -j DROP
Since all tcp connections must first be initialized, we can block all incoming packets that take the task of initializing the connection, the SYN tcp packets. This basically tells our computer to ignore anything it did not speak to first. o While the solution above will work, a better implementation is to put the following as the first rule in your list:
ComputerName:~# iptables -A INPUT -m star --state ESTABLISHED,RELATED -S ACCEPT o You could block specific, rowdy users on your network from accessing your computer by blocking their IP, but if his or her IP ever changed they would be able to access your computer once again. Blocking the hardware address or MAC address of their Ethernet card is more efficient. This address is a set of six two-digit hexadecimal numbers separated by colons (ex: 00:0B:DB:45:56:42). The option for specifying a MAC address, --mac-source , could be used as follows:
ComputerName:~# iptables -A INPUT --mac-source 00:0B:DB:45:56:42 -j DROP
For more information on other command line options for iptables, please refer to the man page.
fstab consists of a number of lines (one for each filesystem) seperated into six fields. Each field is seperated from the next by whitespace (spaces/tabs).
For Example:
/dev/hdc /mnt/cdrom iso9660 noauto,ro,user 0 0
The first field (/dev/hdc) is the physical device/remote filesystem which is to be described.
The second field (/mnt/cdrom) specifies the mount point where the filesystem will be mounted.
The third field (iso9660) is the type of filesystem on the device from the first field.
The fourth field (noauto,ro,user) is a (default) list of options which mount should use when mounting the filesystem.
The fifth field (0) is used by dump (a backup utility) to decide if a filesystem should be backed up. If zero then dump will ignore that filesystem.
The sixth field (0) is used by fsck (the filesystem check utility) to determine the order in which filesystems should be checked. If zero then fsck won't check the filesystem. (as the example line above is a cdrom there is very little point in doing a fsck on it, so the value is zero).
This article will guide you through the installation of Ruby on Rails in a linux machine. As you are aware Ruby on Rails made a splash with its simplicity and ease of use for devoloping web applications.
What is Ruby?
Ruby is a pure object-oriented programming language with a super clean syntax that makes programming elegant and fun. Ruby successfully combines Smalltalk's conceptual elegance, Python's ease of use and learning, and Perl's pragmatism. Ruby originated in Japan in the early 1990s, and has started to become popular worldwide in the past few years as more English language books and documentation have become available.
What is Rails?
Rails is an open source Ruby framework for developing database-backed web applications. Rails is designed from the ground up to create dynamic Web sites that use a relational database backend. It adds key words to the Ruby programming language that make Web applications easier to configure. In addition, it’s designed to automatically generate a complete, if somewhat crude, Web application from an existing database schema. The latter is both Ruby’s greatest strength and its Achilles’ heel. Rails makes assumptions about database schema naming conventions that, if followed, make generating a basic Web site a matter of executing single command.
Installing the Software on CentOS .
1. Install Ruby
Need to install the testing repository so add a file named "testing" to the directory /etc/yum.repos.d/ That will allow you to rock ruby 1.8.4.
cd /usr/local/src wget http://rubyforge.org/frs/download.php/5207/rubygems-0.8.11.tgz tar -xvzf rubygems-0.8.11.tgz cd rubygems-0.8.11 ruby setup.rb cd ..
3. Install fast-cgi
cd /usr/local/src wget http://www.fastcgi.com/dist/fcgi-2.4.0.tar.gz tar xzvf fcgi-2.4.0.tar.gz cd fcgi-2.4.0 ./configure make make install cd ..
4. Install fast-cgi Bindings
cd /usr/local/src wget http://sugi.nemui.org/pub/ruby/fcgi/ruby-fcgi-0.8.6.tar.gz tar zxvf ruby-fcgi-0.8.6.tar.gz cd ruby-fcgi-0.8.6 ruby install.rb config ruby install.rb setup ruby install.rb install cd ..
5. Install Rails
gem install rails --include-dependencies
Ruby and Rails on Red Hat Enterprise Linux
Make sure you have installed zlib-devel installed otherwise Gem will fail.
up2date zlib-devel
First you need to install ruby installed using rpm's from the machine.
Now configuring mod_fastcgi Apache (1.3) config file httpd.conf
1. Install mod_fcgi module
curl -O http://fastcgi.com/dist/mod_fastcgi-2.4.2.tar.gz or wget http://fastcgi.com/dist/mod_fastcgi-2.4.2.tar.gz
tar xvfz mod_fastcgi-2.4.2.tar.gz cd mod_fastcgi-2.4.2 /usr/local/apache/bin/apxs -cia mod_fastcgi.c
2. Configuring httpd.conf
LoadModule fastcgi_module modules/mod_fastcgi.so
AddHandler fastcgi-script .fcgi .fcg .fpl
service httpd restart
3. Edit the .htaccess file
change /dispatch.cgi to /dispatch.fcgi
4. This server has been upgraded to MySQL 4.1
The default Ruby mysql driver will not connect because it is running in old_password compatibility mode (otherwise Ensim cannot connect). In order to fix it we need to reinstall the mysql-ruby client
wget http://www.tmtm.org/en/mysql/ruby/mysql-ruby-2.5.tar.gz tar vxzf mysql-ruby-2.5.tar.gz cd mysql-ruby-2.5 ruby extconf.rb --with-mysql-config=/usr/bin/mysql_config
5. Edit your .htaccess with following entries
#Set to development, test, or production DefaultInitEnv RAILS_ENV production
dig is a command-line tool for querying DNS name servers for information about host addresses, mail exchanges, name servers, and related information. Understanding the default output
The most typical, simplest query is for a single host. By default, however, dig is pretty verbose. You probably don’t need all the information in the default output, but it’s probably worth knowing what it is. Below is an annotated query.
This article explains you how to do the data recovery from a crashed windows-plesk server.
$ dig www.yahoo.com
That’s the command-line invocation of dig I used
; <<>> DiG 9.2.3 <<>> www.yahoo.com ;; global options: printcmd
The opening section of dig’s output tells us a little about itself (version 9.2.3) and the global options that are set (in this case, printcmd). This part of the output can be quelled by using the +nocmd option, but only if it’s the very first argument on the command line (even preceeding the host you’re querying).
Here, dig tells us some technical details about the answer received from the DNS server. This section of the output can be toggled using the +[no]comments option—but beware that disabling the comments also turns off many section headers.
;; QUESTION SECTION: ;www.yahoo.com. IN A
In the question section, dig reminds us of our query. The default query is for an Internet address (A). You can turn this output on or off using the +[no]question option.
;; ANSWER SECTION: www.yahoo.com. 600 IN A 203.23.184.88
Finally, we get our answer: the address of www.yahoo.com is 204.152.184.88. I don’t know why you’d ever want to turn off the answer, but you can toggle this section of the output using the +[no]answer option.
;; AUTHORITY SECTION: yahoo.com. 2351 IN NS ns1.nis.tc.org. yahoo.com. 2351 IN NS ns1.gnac.com. yahoo.com. 2351 IN NS ns2.nis.tc.org.
The authority section tells us what DNS servers can provide an authoritative answer to our query. In this example, yahoo.com has three name servers. You can toggle this section of the output using the +[no]authority option.
;; ADDITIONAL SECTION: ns1.gnac.com. 171551 IN A 203.23.34.21 ns-int.yahoo.com. 2351 IN A 211.52.18.65 ns-int.yahoo.com. 2351 IN AAAA 2001:4f8:0:2::15
The final section of the default output contains statistics about the query; it can be toggled with the +[no]stats option. Some useful options with dig
dig will let you perform any valid DNS query, the most common of which are A (the IP address), TXT (text annotations), MX (mail exchanges), NS name servers, or the omnibus ANY. # get the address(es) for yahoo.com
dig yahoo.com A +noall +answer
# get a list of yahoo's mail servers
dig yahoo.com MX +noall +answer
# get a list of DNS servers authoritative for yahoo.com
dig yahoo.com NS +noall +answer
# get all of the above
dig yahoo.com ANY +noall +answer
#Short answer
dig yahoo.com +short
#To get the TTL values
dig +nocmd yahoo.com mx +noall +short
#To get a long answer
dig +nocmd yahoo.com any +multiline +noall +answer
#To reverselookup
dig -x 216.109.112.135 +short
To bulk lookups # do full lookups for a number of hostnames
#dig -f /path/to/host-list.txt
#the same, with more focused output
dig -f /path/to/host-list.txt +noall +answer
Tracing dig's path
dig yahoo.com +trace
How to interpret TTL value
If you ask your local DNS server for an Internet address, the server figures out where to find an authoritative answer and then asks for it. Once the server receives an answer, it will keep the answer in a local cache so that if you ask for the same address again a short time later, it can give you the answer quickly rather than searching the Internet for it all over again.
When domain administrators configure their DNS records, they decide how long the records should remain in remote caches. This is the TTL number (usually expressed in number of seconds).
When domain administrators configure their DNS records, they decide how long the records should remain in remote caches. This is the TTL number (usually expressed in number of seconds). For example, as of this writing, the TTL for the MX records for the gmail.com domain is 300 seconds. The gmail.com admins are asking that remote servers cache their MX records for no more than five minutes. So when you first ask for that record set, dig will report a TTL of 300.
$ dig +nocmd gmail.com MX +noall +answer gmail.com. 300 IN MX 20 gsmtp57.google.com. gmail.com. 300 IN MX 10 gsmtp171.google.com.
If you ask a few seconds later, you’ll see the TTL number reduced by approximately the number of seconds you waited to ask again.
$ dig +nocmd gmail.com MX +noall +answer gmail.com. 280 IN MX 10 gsmtp171.google.com. gmail.com. 280 IN MX 20 gsmtp57.google.com.
If your timing is good, you can catch the record at the very end of its life.
$ dig +nocmd gmail.com MX +noall +answer gmail.com. 1 IN MX 10 gsmtp171.google.com. gmail.com. 1 IN MX 20 gsmtp57.google.com.
After that, the DNS server you’re querying will “forget" the answer to that question, so the whole cycle will start over again (in this example, at 300 seconds) the next time you perform that query.
vmstat helps you to see, among other things, if your server is swapping. Take a look at the following run of vmstat doing a one second refresh for two iterations.
root@sexy [~]# vmstat 1 2 procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 0 0 1172 1689332 333588 663092 0 0 19 113 1 2 3 1 95 1 0 0 1172 1690320 332920 663100 0 0 352 256 355 681 5 3 91 2
The first row shows your server averages. The si (swap in) and so (swap out) columns show if you have been swapping (i.e. needing to dip into 'virtual' memory) in order to run your server's applications. The si/so numbers should be 0 (or close to it). Numbers in the hundreds or thousands indicate your server is swapping heavily. This consumes a lot of CPU and other server resources and you would get a very significant benefit from adding more memory to your server.
Some other columns of interest: The r (runnable) b (blocked) and w (waiting) columns help see your server load. Waiting processes are swapped out. Blocked processes are typically waiting on I/O. The runnable column is the number of processes trying to something. These numbers combine to form the 'load' value on your server. Typically you want the load value to be one or less per CPU in your server.
The bi (bytes in) and bo (bytes out) column show disk I/O (including swapping memory to/from disk) on your server.
The us (user), sy (system) and id (idle) show the amount of CPU your server is using. The higher the idle value, the better. PS
This command is used to know all the processes running in the server. It can be also used to find out process which is using most of the memory and cpu. To find out top 3 memory consuming processes.
ps -auxf | sort -nr -k 4 | head -3
To find out top 3 cpu consuming processes
ps -auxf | sort -nr -k 3 | head -3
TOP
Say the system is slow and you want to find out who is gobbling up all the CPU and/or memory. To display the top processes, you use the command top.
Note that unlike other commands, top does not produce an output and sits still. It refreshes the screen to display new information. So, if you just issue top and leave the screen up, the most current information is always up. Top runs until you press "q" to quit top. Let's examine the different types of information produced. The first line:
shows the current time (18:46:13), that system has been up for 11 days; that the system has been working for 21 hours 50 seconds. The load average of the system is shown (0.11, 0.19, 0.18) for the last 1, 5 and 15 minutes respectively. (By the way, you can also get this information by issuing the uptime command.) If the load average is not required, press the letter "l" (lowercase L); it will turn it off. To turn it back on press l again. The second line: 151 processes: 147 sleeping, 4 running, 0 zombie, 0 stopped shows the number of processes, running, sleeping, etc. The third and fourth lines:
show the CPU utilization details. The above line shows that user processes consume 12.5% and system consumes 6.7%. The user processes include the Oracle processes. Press "t" to turn these three lines off and on. If there are more than one CPU, you will see one line per CPU. The next two lines: Mem: 1026912k av, 1000688k used, 26224k free, 0k shrd, 113624k buff 758668k actv, 146872k in_d, 14460k in_c Swap: 2041192k av, 122476k used, 1918716k free 591776k cached
show the memory available and utilized. Total memory is "1026912k av", approximately 1GB, of which only 26224k or 26MB is free. The swap space is 2GB; but it's almost not used. To turn it off and on, press "m". The rest of the display shows the processes in a tabular format. Here is the explanation of the columns:
Column Description PID The process ID of the process USER The user running the process PRI The priority of the process NI The nice value: The higher the value, the lower the priority of the task SIZE Memory used by this process (code+data+stack) RSS The physical memory used by this process SHARE The shared memory used by this process STAT The status of this process, shown in code. Some major status codes are: R – Running S –Sleeping Z – Zombie T – Stopped You can also see second and third characters, which indicate: W – Swapped out process N – positive nice value %CPU The percentage of CPU used by this process %MEM The percentage of memory used by this process TIME The total CPU time used by this process CPU If this is a multi-processor system, this column indicates the ID of the CPU this process is running on. COMMAND The command issued by this process
While the top is being displayed, you can press a few keys to format the display as you like. Pressing the uppercase M key sorts the output by memory usage. (Note that using lowercase m will turn the memory summary lines on or off at the top of the display.) This is very useful when you want to find out who is consuming the memory.
Now that you learned how to interpret the output, let's see how to use command line parameters.
The most useful is -d, which indicates the delay between the screen refreshes. To refresh every second, use top -d 1.
The other useful option is -p. If you want to monitor only a few processes, not all, you can specify only those after the -p option. To monitor processes 13609, 13608 and 13554, issue: top -p 13609 -p 13608 -p 13554 This will show results in the same format as the top command, but only those specific processes.
SKILL & SNICE From the previous discussion you learned how to identify a CPU consuming resource. What if you find that a process is consuming a lot of CPU and memory, but you don't want to kill it?
$ skill -STOP 1
The process is effectively frozen. After some time, you may want to revive the process from coma:
$ skill -CONT 16514
The command is very versatile. If you want to stop all processes of the user "test"
$ skill -STOP test>
You can use a user, a PID, a command or terminal id as argument. The following stops all rman commands. $ skill -STOP rman
As you can see, skill decides that argument you entered—a process ID, userid, or command—and acts appropriately. This may cause an issue in some cases, where you may have a user and a command in the same name. The best example is the "test" process, which is typically run by the user "test". So, when you want to stop the process called "test" and you issue:
$ skill -STOP test
all the processes of user "test" stop, including the session you may be on. To be completely unambiguous you can optionally give a new parameter to specify the type of the parameter. To stop a command called test, you can give: $ skill -STOP -c test
The command snice is similar. Instead of stopping a process it makes its priority a lower one
lsof
The command lsof shows a list of processes attached to open files or network ports. List processes attached to a given file: lsof filenmame List all open files on system:
#lsof
To kill the processes
kill killall
This will perform an orderly shutdown of the process. If it hangs give a stronger signal with:
kill -9 .
This method is not as sanitary and thus less preferred.
A signal may be given to the process. The program must be programmed to handle the given signal. See /usr/include/bits/signum.h for a full list.
To restart a process after updating it's configuration file, issue the command
kill -HUP
The process attached to an open file can be killed using the command fuser:
fuser -ki filename
Now I am going indroduce you to a set of commands that may come handy FIND
find -perm 777 -type d -exec chmod 755 {} \; #Command to change all the folders under present directory with 777 to 755
find -perm 755 -type f -exec chmod 644 {} \; #Command to change all the folders under present directory with 755 to 644
find -type d -maxdepth 3 -exec cp file {} \; #Copy file to 3 levels of directories below the present directory
find . -name "*.trn" -ctime +3 -exec rm -f {} \; #Forcible remove files with .trn extension and 3 days old.
find . -cmin -5 #Find all files created or updated in the last five minutes: (Great for finding effects of make install)
LS
ls -lSh #List files by their size
ls -ltr #List files by date
ls -F #Appends a symbol after files and directories
RSYNC
rsync -e ssh -az /currentdirectory IP:/remotedirectory #Sync remote directory with our current directory.
rsync --bwlimit=1000 fromfile tofile #Locally copy with rate limit
GPG
gpg -c file #Encrypt file
gpg file.gpg #Decrypt file
DF
du -h --max-depth 1 #Show disk space used by all the files and directories.
du -s * | sort -k1,1rn | head #Show top disk users in current dir.
df -h #Show free disk space
df -i #Show free inodes
Add system swap space for virtual memory paging
Swap space may be a swap partition, a swap file or a combination of the two. One should size swap space to be at least twice the size of the computer's RAM. (but less than 2GB)
dd if=/dev/zero of=/swapfile bs=1024 count=265032 - #Create file filled with zeros of size 256Mb
mkswap /swapfile #Create swap file
swapon /swapfile #Begin use of given swap file. Assign a priority with the "-p" flag.
swapon -s #List swap files
scat /proc/swaps #Same as above
This example refers to a swap file. One may also use a swap partition. Make entry to /etc/fstab to permanently use swap file or partition.
/swapfile swap swap defaults 0 0
Note: To remove the use of swap space, use the command swapoff. If using a swap partition, the partition must be unmounted. Debuggin Tools
strace -c ls >/dev/null #Summarise/profile system calls made by command
strace -f -e open ls >/dev/null #List system calls made by command
ltrace -f -e getenv ls >/dev/null #List library calls made by command
lsof -p $$ #List paths that process id has open
lsof -p PID #List paths PID has open
lsof ~ #List processes that have specified path open
last reboot #Indicates last reboot time
renice +15 PID #To give lower priority for a PID -19 is highest and +20 is lowest
tcpdump not port 22 #To show network traffic except on port 22
Perl Administration
Installation of perl module can be done from tar file.
tar xzf yourmodule.tar.gz #Untar Module
perl Makefile.PL #Build with PERL makefile: make make install #Install
You can also do this from cpan shell
perl -MCPAN -e shell #First time through it will ask questions Answer "no" to the first question for autoconfigure
cpan> install URI
cpan> i /PerlMagick/ #Inquire about module. (Search by keyword) Distribution J/JC/JCRISTY/PerlMagick-5.36.tar.gz Module Image::Magick (J/JC/JCRISTY/PerlMagick-5.36.tar.gz)
cpan> install Image::Magick
cpan> force install Image::Magick #Install a module forcefully.
YUM :RPM Updater
YUM (Yellowdog Updater, Modified) is a client command line application for updating an RPM based system from an internet repository (YUM "yum-arch" server) accessible by URL (http://xxx, ftp://yyy or even file://zzz local or NFS)
yum -y install package-name #To install a package along with its dependencies
yum remove package-name #To remove package
yum list #To list available packages version and state
yum list extras #To list packages not available in repositories but listed in config file
yum list obsoletes #To list packages which are obsoleted by repositories
yum clean all #To list packages which are obsoleted by packages in yum repository
yum update #Update all packages on your system
yum update package-name #Update a package
yum update package-name-prefix\* #Update all with same prefix
You can add new repos in /etc/yum.repos.d with files named file.repo For the option "gpgcheck=1" to work, use the "rpm --import GPG-KEY
rpm --import /usr/share/rhn/RPM-GPG-KEY
rpm --import /usr/share/rhn/RPM-GPG-KEY-fedora
File: /etc/yum.repos.d/fedora.repo with following entry
