What is linux link local

Link-local address/Linux

A link-local address is an Internet Protocol (IP) unicast address intended to be used only to connect to the hosts on the same network, or link (such as an Ethernet one) as the given host. The activities below are intended to show how to view and test IP link-local addresses in Linux-based systems.

Contents

Readings [ edit | edit source ]

• Link-local address at Wikipedia.
• Manual pages:
  • ip-address(8);
  • ping6(8) (also covers the ping command);
  • avahi-autoipd(8).

Preparation [ edit | edit source ]

To prepare for this activity:

1. ensure that the Linux-based system of choice is started;
2. log into the system.

Conventions [ edit | edit source ]

The commands available to non-privileged users are from now on written as $ command (where $ is the shell prompt, and is not to be typed.)

The commands available only to the privileged users are written as # command (where # is again not to be typed.) Depending on the configuration, you may be able to execute them either of the following ways:

• by using $ sudo command (and entering your own user password);
• by using $ su -c ‘command’ (and entering the “root” password);
• by logging in as root , or switching there with either $ sudo -i or $ su , and entering the command directly into the resulting privileged shell;
• not at all, if you are not authorized to perform administrative tasks on the host in question.

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How to Create Symbolic Links in Linux

Learning Linux symbolic commands is a great way of improving your potential in the Linux terminal. In this tutorial, we’ll cover a few commands to learn symbolic links in a quick and easy way. Before we begin, let’s overview what are symbolic links.

What are Linux Symbolic Links

Symbolic Links are not only helpful in creating shortcuts and file management in operating systems like Linux. They also serve as a way to create various locations for primary user folders, for instance, Documents, Pictures, Downloads, and much more!

Symbolic Links act like a string creating pathways for different files, folders, and directories in the computer system. They are capable of creating and storing multiple files in different places refer to one single file. Thus, increasing efficiency by locating all the specific documents in one command.

These links are stored in the mainframe, so even if the original file is deleted, you’ll have a backup for most of the important files. Symbolic links help create invalid link pathways to store pieces of information as per the requirement of the user.

Due to the user-friendly features in Linux, even Microsoft is following it to create Symbolic Links. Symbolic links, also known as Soft links or Symlinks, are not unique to Linux but they are just like a Search option in Windows where one can search a specific file or directory in a disk by executing various commands.

How to Create Symbolic Links in Linux?

Let’s look at how you can create file and folder links in Linux:

Create Symbolik Link in Linux for Files

Generally, to create links use we use the ln command and the -s option to specify Symbolic links. This is the easiest way to ensure a flexible approach that allows experimenting with the language as much as possible. There is nothing hard in creating Symbolic links in Linux – you just need to follow one simple step.

The ln command in Linux creates links between source files and directories.

• -s – the command for Symbolic Links.
• [target file] – name of the existing file for which you are creating the link
• [Symbolic filename] – name of the symbolic link.

Created links can be verified by directory listing using detailed list command:

However, if you do not specify the [Symbolic filename], the command will automatically create a new link in the existing directory.

Create Symbolic Link in Linux for Folders

Creating symbolic links for folders is not difficult either. The command used to create the folder symbolic link is:

For example, to link the /user/local/downloads/logo directory to /devisers folder, use the following command:

Once a Symbolic link is created and attached to the folder /devisers, it will lead to /user/local/downloads/logo. When the user changes directory – cd – to /devisers, the system will automatically change to the specific file and write it in the command directory.

Symbolic link options are called command line switches. Here are the most common ones and their descriptions:

Command Switch	Description
–backup[=CONTROL]	backup each existing destination file
-d, -F, –directory	superuser is allowed to attempt hard link
-f, –force	existing destination file is removed
-I, –interactive	prompt before removing destination files
-L, –logical	deference targets that are symbolic links
-n, –non-dereference	symbolic links to directory are treated as files
-P, –physical	make hard links directly to symbolic links
-r, –relative	create symbolic links relative to link location
-s, –symbol	make symbolic links instead of hard links
-S, –suffix=SUFFIX	override usual backup suffix
-v, –verbose	print name of each linked file

How to Change or Remove Symbolic Link in Linux?

You can remove existing links attached to files or directories by the unlink or rm command. This is how you can do it with the unlink command:

Removing symbolic link using the rm command is similar to the unlink command which is as under:

Wrapping up

Remember, if the source is no longer in the current location, then you should delete the symbolic files to avoid creating duplicates, which might slow down your work.

Linux is a wonderful platform for creating an interactive and dynamic application, where you can experiment and innovate. A strong foundation is critical. Learn the basic of the language thoroughly to use it to its full potential. We hope this tutorial helped you improve your skills with another useful tool!

Edward is an expert communicator with years of experience in IT as a writer, marketer, and Linux enthusiast. IT is a core pillar of his life, personal and professional. Edward’s goal is to encourage millions to achieve an impactful online presence. He also really loves dogs, guitars, and everything related to space.

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How to: Linux / UNIX create soft link with ln command

Symbolic links can be made to directories as well as to files on different filesystems or different partitions.

Two types of links

• symbolic links (also known as “soft links” or “symlinks”): Refer to a symbolic path indicating the abstract location of another file.
• hard links : Refer to the specific location of physical data.

How do I create soft link / symbolic link under Unix and Linux?

Soft links are created with the ln command. For example, the following would create a soft link named link1 to a file named file1, both in the current directory
$ ln -s file1 link1
To verify new soft link run:
$ ls -l file1 link1
Sample outputs:

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From the above outputs it is clear that a symbolic link named ‘link1’ contains the name of the file named ‘file1’ to which it is linked.

How to use the ln command

So the syntax is as follows to create a symbolic link in Unix or Linux, at the shell prompt:
$ ln -s < source-filename >< symbolic-filename >

For example create a softlink for /webroot/home/httpd/test.com/index.php as /home/vivek/index.php, enter the following command:
$ ln -s /webroot/home/httpd/test.com/index.php /home/vivek/index.php
$ ls -l
Sample outputs:

You can now edit the soft link named /home/vivek/index.php and /webroot/home/httpd/test.com/index.php will get updated:
$ vi /home/vivek/index.php
Your actual file /webroot/home/httpd/test.com/index.php remains on disk even if you deleted the soft link /home/vivek/index.php using the rm command:
$ rm /home/vivek/index.php ## ##
## But original/actual file remains as it is ##
$ ls -l /webroot/home/httpd/test.com/index.php

Creating Symlink to a directory

The syntax remains same:
$ ln -s
For example, create a symbolic link from the /home/lighttpd/http/users/vivek/php/app/ directory to the /app/ directory you would run:
$ ln -s /home/lighttpd/http/users/vivek/php/app/ /app/
Now I can edit files using /app/
$ cd /app/
$ ls -l
$ vi config.php

How to overwrite symlinks/Soft link

Pass the -f to the ln command to overwrite links:
ln -f -s /path/to/my-cool-file.txt link.txt

How to delete or remove symlinks/soft links

Use the rm command to delete a file including symlinks:
rm my-link-name
unlink /app/
rm /home/vivek/index.php

Getting help about the ln command

Type the following ln command:
$ man ln
$ ln —help

ln command option	Description
—backup	make a backup of each existing destination file
-b	like —backup but does not accept an argument
-d	allow the superuser to attempt to hard link directories (note: will probably fail due to system restrictions, even for the superuser)
-f	remove existing destination files
-i	prompt whether to remove destinations
-L	dereference TARGETs that are symbolic links
-n	treat LINK_NAME as a normal file if it is a symbolic link to a directory
-P	make hard links directly to symbolic links
-r	create symbolic links relative to link location
-s	make symbolic links instead of hard links
-S	override the usual backup suffix
-t	specify the DIRECTORY in which to create the links
-T	treat LINK_NAME as a normal file always
-v	print name of each linked file
—help	display this help and exit
—version	output version information and exit

Conclusion

You learned how to create a symbolic link in Linux using the ln command by passing the -s option. See ln command man page here for more information.

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What is the interface scope (global vs. link) used for?

Routing table entries have an attribute scope . I would like to know how the change from global to link (or the other way round) affects the network system.

3 Answers 3

Let look at route scope definition in Linux :

So if you change the scope of a route, your computer probably can not connect to network in that route anymore. The router simply doesn’t forward the packet which is send to destination belongs local network.

Note that the scope does not reflect the distinction between nonroutable (private) and routable (public) addresses.

Both 10.0.0.1 (private — non routeable) and 8.8.8.8 (public — routable) can be given either link or universe (global) scope. It is configured by system administrator.

suppose we have NIC settings with 3 ip’s with different scopes

suppose we have some route for ens160 in the route table

as we see we have scope setting in NIC and in the route.

If a route has src specified in this case linux completely ignores scope settings in the route and in the NIC settings. it ignores it completely. And linux just uses in packets flowing out of NIC src ip = 172.20.0.1

suppose we have another route

if src ip is not specified in the route then linux look what scope the route has. in our case scope = link. Then linux goes to NIC settings and searches IP with the same scope. in our case IP with scope=link = 172.21.0.1/24.

so for dst ip = 4.4.4.4 linux will use src ip = 172.21.0.1

if scope is not specified in a route then it means scope = global

next. lets look at default route

it does not have scope specified , it means scope = global

as default route does not have src specified it means linux will search on ens160 IP with scope=global and use it as src ip.

next. suppose a route has one scope and NIC IP has another scope. example

what happens when we ping 14.14.14.14

the route has scope=link but NIC only has IP with scope=host. the point is that ip with scope=host can be as backend only for a route with scope=host. in other cases linux can not use such ip. so linux will use src ip = 0.0.0.0 for dst ip 14.14.14.14

(actually it also depends if nic is real physical or for instance dummy, if nic is dummy in this case linux will use some other ip from another nic that
has scope=global )

general rule: if a route does not have src specified then

1. ip with scope=host can be as backend only for a route with scope=host
2. ip with scope=link can be as backend only for a route with scope=host or scope=link
3. ip with scope=global can be as backend only for a route with any scope

im quite surprised about such uncomfortable architecture

if you want to forget about all this «scope stuff» — just use src field in a route in route table.

The scope influences source address selection.

For connections/associations where the source address is not yet fixed (e.g. initiating a TCP connection, but not when reacting to an incoming packet), the source address will be selected depending on the scope of the route the packet is about to hit.

This is why addresses also have a scope attribute.

Example where no source address selection occurs: an incoming TCP connection initiation or ping packet will be answered with the IP addresses reversed (source → destination, destination → source), otherwise the other host would not recognize the packet as answer.

Example where source address selection occurs: ping xyz or telnet xyz . Common programs do not tell the operating system which source address to use (and that is a good habit). The OS needs to pick one and is prepared to do so: it tests the potential outgoing packet for the route it would hit (normal routing uses the destination address only, if you use advanced routing, the packet will not have a source address yet!). The resulting scope reduces the selection to addresses from the corresponding scope on the outgoing interface if any are available.

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