ft_ping

Implementation of the ping command in C. This program sends ping probes to a given host and waits for the responses. This is useful to check if a given server is up or down and diagnostic eventual connexion problems from the the probes round trip time values.

Setup

# clone it with the libft submodule
git clone --recurse-submodules https://github.com/Taiwing/ft_ping
# build it
cd ft_ping/ && make
# run it
sudo ./ft_ping example.com

As shown above this program needs sudo rights. This is because ft_ping uses raw sockets for crafting custom ip packets and read responses. If you do not have root access on your machine but docker is available, then execute the following commands to run ft_ping:

# build docker image and run it
./setup-docker.bash
# run ft_ping inside the container
./ft_ping example.com

Usage

ft_ping sends an ICMP ECHO_REQUEST packet and waits for an ECHO_REPLY. It loops and sends one probe per second by default. If the count option is not given to ft_ping it will need to be manually stopped with a Ctrl+C, otherwise it will go on forever.

Usage:
	ft_ping [options] <destination>
Options:
	<destination>		hostname or IPv4 address
	-c count		stop after sending count ECHO_REQUEST packets
	-h			print help and exit
	-p pattern		up to 16 "pad" bytes to fill out the packet
	-s packetsize		number of data bytes to send
	-t ttl			IP time to live
	-v			verbose output
	-W timeout		time to wait for a response, in seconds

This program only handles valid local or remote hostnames and IPv4.

example:

# send 3 pings to google.com and exit
sudo ./ft_ping -c 3 google.com

possible output:

PING google.com (172.217.19.238) 56(84) bytes of data.
64 bytes from google.com (172.217.19.238): icmp_seq=1 ttl=116 time=7.14 ms
64 bytes from google.com (172.217.19.238): icmp_seq=2 ttl=116 time=7.81 ms
64 bytes from google.com (172.217.19.238): icmp_seq=3 ttl=116 time=8.23 ms

--- google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2016ms
rtt min/avg/max/mdev = 7.142/7.727/8.231/0.448 ms

How it works

ft_ping crafts ICMP packets (Internet Control Message Protocol) with the type field set to ECHO_REQUEST. The ICMP protocol is on the the Network layer of the TCP/IP OSI model, on the same level as the IP protocol since it handles errors and diagnostic messages directly related to IP. If the target host is up and not configured to ignore ECHO_REQUESTS it should send back an ECHO_REPLY response.

ECHO_REQUEST example:

IP header:

version	total length	time to live	protocol	source ip	destination ip
4	84	255	ICMP	1.2.3.4	93.184.216.34

Some fields have been removed for clarity. For a complete overview of the IPv4 protocol check this page.

ICMP header:

type	code	checksum	identifier	sequence number
8 (ECHO_REQUEST)	0	0xffff	4321	1

Every field is present but this strucuture is specific to ICMP ECHO packets. For a complete overview of ICMP check this page.

The IP header is 20 bytes long, ICMP is 8 bytes and there is 56 bytes of data by default which is why the total length IP field amounts to 84 bytes. The ICMP identifier is chosen by ft_ping and is simply set to its PID. This is useful to make sure that the response is intended for this process. The sequence number is an increasing counter of the ECHO_REQUEST probes sent by ft_ping to the given host. The 56 bytes of data can be anything and is usually filled with zeroes except if the pattern option is used.

ECHO_REPLY example:

IP header:

version	total length	time to live	protocol	source ip	destination ip
4	84	56	ICMP	93.184.216.34	1.2.3.4

ICMP header:

type	code	checksum	identifier	sequence number
0 (ECHO_REPLY)	0	0xffff	4321	1

The reply is almost identical to the request except that the source and destination IP addresses are reversed and the ICMP type field is set to ECHO_REPLY. The data is also typically identical to the request's data payload.

To compute the RTT (Round Trip Time) - which is also called the ping - ft_ping simply substracts the reply departure timestamp from the request arrival. So this value basically represents how long it takes for a packet to reach the target host and come back.