Kernel panic - not syncing: Attempted to kill init!
When the kernel gets into a situation where it does not know how to proceed (most often during booting, but at other times), it issues a kernel panic by calling the panic(msg) routine defined in kernel/panic.c. (Good name, huh?) This is a call from which No One Ever Returns.
The panic() routine adds text to the front of the message, telling you more about what the system was actually doing when the panic occurred ... basically how big and bad the trail of debris in the filesystem is likely to be. This is where the "not syncing" part comes from, and when you see that, it's good. (panic() doessinc() system-call to push all buffered data out to the hard-disks before it goes down.)
The second part of the message is what was provided by the original call to panic(). For example, we find panic("Tried to kill init!") it tries to issue a kernel/exit.c.
So, what does this actually mean? Well, in this case it really doesn't mean that someone tried to kill the magical init process (process #1...), but simply that it tried to die. ( This process is not allowed to die or to be killed. )
When you see this message, it's almost always at boot-time, and the real messages ... the cause of the actual failure ... will be found in the startup messages immediately preceding this one. This is often the case with kernel-panics. init encountered something "really bad," and it didn't know what to do, so it died, so the kernel died too.
BTW, the kernel-panic code is rather cute. It can blink lights and beep the system-speaker in Morse code. It can reboot the system automagically. Obviously the people who wrote this stuff encountered it a lot...
In diagnosing, or at least understanding, kernel-panics, I find it extremely helpful to have on-hand a copy of the Linux source-code, which is usually stored someplace like /usr/src/linux-2.x. You can use the grep utility to locate the actual code which caused the panic to occur.
Things to know about linux
What is the minimum number of partitions you need to install Linux?
What command can you use to review boot messages?
only need 2 partitions to install Linux one is the root partition / and other is the swap partition ,you can install linux without creating /boot ,it will automatically created under /
dmesg will show you the logged messages at boot time .most information is about detected hardware at boot time
what is stored at /lib/modules ? It contains all the kernel modules that needed to be loaded
into kernel (booting etc). there will some .map, .dep
(dependency files) files present.
When the kernel needs a feature that is not resident in the
kernel, the kernel module daemon kmod[1] execs modprobe to
load the module in.
You can see what modules are already loaded into the kernel
by running lsmod, which gets its information by reading the
file /proc/modules
How to Load a Linux kernel module automatically at boot time
When the kernel needs a feature that is not resident in the kernel, the kernel module
daemon kmod[1] execs modprobe to load the module in.
modprobe first searches a files called File /etc/modules.conf is configuration file for loading kernel modules.
what is stored in /proc ?
what is significance of /proc in linux ?
Mainly hardware related information such as CPU
information, Memory (RAM) information stored under /proc
directory
example:
# cat /proc/cpuinfo (show the information of CPU of that
particular hardware)
# cat /proc/meminfo (show the information of Memory i.e.
RAM of that particular hardware)
why the kernel panic error was appering?
Kernel panic error occurs when a Linux OS files system goes curroupt or it cannot read the file system
postfix mail flow
Receiving Mail
When a message enters the Postfix mail system, the first stop is the incoming queue. The figure below shows the main components that are involved with new mail.
Mail is posted locally. The Postfix sendmail program invokes the privileged postdrop program which deposits the message into the maildrop directory, where the message is picked up by the pickup daemon. This daemon does some sanity checks, in order to protect the rest of the Postfix system.
Mail comes in via the network. The Postfix SMTP server receives the message and does some sanity checks, in order to protect the rest of the Postfix system.
Mail is generated internally by the Postfix system itself, in order to return undeliverable mail to the sender. The bounce or defer daemon brings the bad news.
Mail is forwarded by the local delivery agent, either via an entry in the system-wide alias database, or via an entry in a per-user .forward file. This is indicated with the unlabeled arrow.
Mail is generated internally by the Postfix system itself, in order to notify the postmaster of a problem (this path is also indicated with the unlabeled arrow).The Postfix system can be configured to notify the postmaster of SMTP protocol problems, UCE policy violations, and so on.
The cleanup daemon implements the final processing stage for new mail. It adds missing From: and other message headers, arranges for address rewriting to the standard user@fully.qualified.domain form, and optionally extracts recipient addresses from message headers. The cleanup daemon inserts the result as a single queue file into the incoming queue, and notifies the queue manager of the arrival of new mail. The cleanup daemon can be configured to transform addresses on the basis of canonical and virtua table lookups.
On request by the cleanup daemon, the trivial-rewrite daemon rewrites addresses to the standard user@fully.qualified.domain form.
things to know about apache
What is the main difference between Location and Directory sections?
Directory sections refer to file system objects; Location sections refer to elements in the address bar of the Web page
What is the difference between a restart and a graceful restart of a web server?
During a normal restart, the server is stopped and then started, causing some requests to be lost. A graceful restart allows Apache children to continue to serve their current requests until they can be replaced with children running the new configuration.
What is the use of mod_perl module?
mod_perl scripting module to allow better Perl script performance and easy integration with the Web server.
If you have added "loglevel Debug" in httpd.conf file, than what will happen?
It will give you more information in the error log in order to debug a problem.
Can we do automatically roll over the Apache logs at specific times without having to shut down and restart the server?
Yes
Use CustomLog and the rotatelogs programs Add following line in httpd.conf file. CustomLog "| /path/to/rotatelogs /path/to/logs/access_log.%Y-%m-%d 86400" combined
How you will put a limit on uploads on your web server? This can be achieved by LimitRequestBody directive.<Directory "/var/www/html/data_uploads"> LimitRequestBody 100000 </Directory>
Here I have put limit of 100000 Bytes
How to enable PHP scripts on your server? If you have mod_php installed, use AddHandler to map .php and .phtml files to the PHP handler. AddHandler application/x-httpd-php .phtml .php
postfix interview questions
Explain the working of local mail submission for postfix?
Explain default_destination_concurrency_limit Parameter?
When a local email message enters the postfix system. Local messages are deposited into the maildrop directory of the Postfix queue by the postdrop command, usually through the sendmail compatibility program. The pickup daemon reads the message from the queue and feeds it to the cleanup daemon. The cleanup daemon processes all inbound mail and notifies the queue manager after it has placed the cleaned-up message into the incoming queue. The queue manager then invokes the appropriate delivery agent to send the message to its next hop or ultimate destination.
What are the important files for postfix server ?
/etc/postfix/main.cf
/etc/postfix/access
/etc/postfix/aliases
Which command checks for configuration problems?
# postfix check
How you will see the queue of postfix server?
#postqueue -p
or mailq
How can I clear postfix mail server queue?
# postsuper -d ALL
How you will reload the postfix queue?
# postsuper -r ALL
which command is used to find out that postfix is complied with mysql or not?
# postconf -m
nis
regexp
environ
mysql
btree
unix
hash
Explain smtpd_recipient_limit parameter? And what is the default value for this parameter?
The smtpd_recipient_limit parameter can limit the number of recipients allowed in a single incoming message.
The default value for this parameter is 1000.
Explain smtpd_timeout Parameter?
The smtpd_timeout parameter limits the amount of time Postfix waits for an SMTP client request after sending a response. This allows the Postfix administrator to quickly disconnect SMTP servers that "camp out" on the SMTP connection, utilizing system resources for the SMTP connection without actually sending a message.
smtpd_timeout = value
By default, Postfix will assume the value is in seconds.
Explain maximal_queue_lifetime Parameter?
The maximal_queue_lifetime parameter sets the amount of time (in days) that a message remains in the deferred message queue before being returned as undeliverable. The default value is 5 days. Once this value is reached, Postfix returns the message to the sender.
Explain queue_run_delay Parameter?
The queue_run_delay parameter sets the time interval (in seconds) that Postfix scans the deferred message queue for messages to be delivered. The default value for this is 1,000 seconds.
Explain default_destination_concurrency_limit Parameter?
The default_destination_concurrency_limit parameter defines the maximum number of concurrent SMTP sessions that can be established with any remote host. This parameter is related to the SMTP maxprocess parameter in the master.cf configuration file. The maximum number of concurrent SMTP sessions cannot exceed the maxprocess value set for the maximum number of SMTP client processes. Thus, if the default maxprocess value of 50 is used, setting the default_destination_concurrency_limit greater than 50 has no effect.
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