gratuitous ARP

what is ARP?

Arp is a protocol that is used to map ip addresses to corresponding MAC addresses. This is referred to as neighbour discovery. (ipv6 uses icmpv6 instead of ARP).
Arp is used by a machine when it wants to send an ip packet to another machine on the same LAN segment (physical LAN).

• The sender issues a 'Who has' ARP broadcast query. The machine that owns the IP address responds with the corresponding MAC address.
• The sender uses this MAC address as the destination address in the datalink packet and injects the packet on to the wire.

gratuitous ARP

gratuitous ARP is an ARP reply that is sent when there is no request. And it is a broadcast while normal ARP replies are not broadcast. This results in all machines in the segment updating their ARP cache.


Why is gratuitous ARP important

gratuitous ARP is useful to let other machines on the same subnet know any change in IP address configurations. This is particularly relevant in High Availability scenario where the active machine goes down and the standby machine takes over the new IP. If a gratuitous ARP is not sent here, the gateway(of this subnet) will continue to forward the IP packets to previously active machine.

Generally when an IP address is configured on a machine, the machine's network stack will send a gratuitous ARP.But some OSes(Like Linux) don't do that. This can be overcome on Linux by manually sending a gratuitous arp using arping command's '-A' option as follows.

arping -q -c 3 -A -I

AIX issues a gratuitous ARP when ip is configured on one of it's interfaces using ifconfig.

SQLite

I got to know about SQLite when I tried using a tool called "almanah" which is a simple diary.
"almanah" stores all the information in a file /Your/home/.local/share/diary.db.
did some search and found that this was a SQLite database file. Also figured how SQLite works. it's pretty simple.

SQLite is a set of libraries that you can link with your application. It gives a set of API's so that you can execute SQL statements as you would do with any DBMS. The difference here is that there is no other DBMS process, and everything is stored in the SQLite database file. check out this link, it's straightforward.

I was looking for a tool that would help me extract the contents of the diary into a text file.
Found this tool called "sqlite3" on linux. This is like a shell for SQLite.
run it
$ sqlite3 /Your/home/.local/share/diary.db
sqlite> .tables
entries entry_attachments entry_links
sqlite>

It shows 3 tables entries,entry_attachments and entry_links

doing a "select * from entries" gives all the entries in the diary.

the below command can be run to do it with a single command.
$sqlite3 ./diary.db "select * from entries"