A Tip A Day [:: ATAD ::]

The two main formats for file archiving are tar and cpio. Both a used in tandem with compression utilities like the gzip and bzip. The archive formats were initially used for tape and other sequential access devices for backup purposes, it is now commonly used to collate collections of files into one larger file, for distribution or archiving, while preserving file system information such as user and group permissions, dates, and directory structures.

An important point you need to consider while using these archives to create backups is that, in the unfortunate event where a portion of the archive gets corrupted, tar will skip a corrupted archive portion and proceed to the next whereas cpio is going to quit with an error.

Tar accepts explicit file names to make the archive whereas cpio seems to give better control on the files that need to be archived.

Eg

# find . -type f -name '*.txt' -print | cpio -o | gzip >all_my_txt.cpio.gz

A useful way to use archives and transfer multiple files and directories to a target directory/machine is to create an archive in stdout and then, in the target directory/machine, extracting the tar file from the piped stdin.

Eg

# using cpio

# find . -type f -name '*.txt' -print | cpio -o | ssh target_machine "cd /target_dir && cpio -idum"

# using a tarpipe

# tar -cf - "source_dir" | ( cd "target_dir" && tar -xvf - )

Many software vendors like RedHat and Oracle ship their products in cpio file format. Eg, RPM uses cpio and can be extracted using the rpm2cpio utility

# rpm2cpio rpm_name | cpio -ivd

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-vinaydeep

The Domain Name System that dates back from the era of the ARPAnet can be understood as a distributed database (like a “phone book”) for any network that translates hostnames to IP addresses, and also vice versa translation called reverse lookup.

The DNS namespace/data stored in a nameserver is divided into manageable sets of data called zones. Zones contain name and IP address information about one or more parts of a DNS domain. A server that contains all of the information for a zone is the authoritative server for the domain. The namespace information is stored in zone files that may contain directives and resource records. Directives (optional) tell the nameserver to perform tasks or apply special settings to the zone. Resource records (mandatory) define the parameters of the zone and assign identities to individual hosts.

A nameserver can take one or more of the following roles:

master

Stores original and authoritative zone records for a namespace, and answers queries about the namespace from other nameservers.

slave

Answers queries from other nameservers concerning namespaces for which it is considered an authority. However, slave nameservers get their namespace information from master nameservers.

caching-only

Offers name-to-IP resolution services, but is not authoritative for any zones. Answers for all resolutions are cached in memory for a fixed period of time, which is specified by the retrieved zone record as the TTL value.

forwarding

Forwards requests to a specific list of nameservers for name resolution. If none of the specified nameservers can perform the resolution, the resolution fails.

Furthur Reading: BIND,  named.conf, rndc

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A runlevel is a state or mode of operation of the operating system. Runlevels define what services or processes should be running on the system which are listed in the /etc/rc.d/rc(x).d (where x is the number of the runlevel) directory in linux. After init is invoked as the last step of the kernel boot sequence, it looks for the file /etc/inittab file for the default runlevel.

# Default runlevel. The runlevels used by RHS are:

#   0 – halt (Do NOT set initdefault to this).
#   1 – Single user mode. Only the root user can log in.
#   2 – Multiuser, without NFS (The same as 3, if you do not have networking)
#   3 – Full multiuser mode with networking
#       This is the most common runlevel for server based systems that do not require GUI.
#   4 – unused or user defined.
#   5 – X11
#       Networked, multi-user state with X Window System capability
#   6 – reboot (Do NOT set initdefault to this)
#
id:3:initdefault:

The runlevel program can be used to determing to the current runlevel of the system and the telinit program can be used to change the runlevel of the system on the fly. Note that its also possiblesupply init with the desired runlevel by specifying it as a kernel command line parameter from the grub bootloader (kernel /vmlinuz ro root=/dev/hda1 5)

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