Networking

iperf

This link is quite nice: https://www.golinuxcloud.com/linux-monitor-network-traffic/

How to use iperf:

Whitelist port 5201 on your security group and firewall.

Then start the server first:

iperf3 -i 5 -s

Then the client:

iperf3 -i 5 -t 60 -c <IP/hostname of the server>

It gives a nice report but apparently isn’t good for latency checks, only bandwidth.

UDP Client (Python 3)

import socket
import sys

# Create a UDP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

server_address = ('127.0.0.1', 10000)
message = b'This is the message. It will be repeated.'

try:
    # Send data
    print(f'sending "{message!r}"', file=sys.stderr)
    sent = sock.sendto(message, server_address)

    # Receive response
    print('waiting to receive', file=sys.stderr)
    data, server = sock.recvfrom(4096)
    print(f'received "{data!r}"', file=sys.stderr)

finally:
    print('closing socket', file=sys.stderr)
    sock.close()

UDP Server (Python 3)

import socket
import sys

# Create a UDP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

# Bind the socket to the port
server_address = ('localhost', 10000)
print(f'starting up on {server_address[0]} port {server_address[1]}', file=sys.stderr)
sock.bind(server_address)

while True:
    print('\nwaiting to receive message', file=sys.stderr)
    data, address = sock.recvfrom(4096)

    print(f'received {len(data)} bytes from {address}', file=sys.stderr)
    print(data, file=sys.stderr)

    if data:
        sent = sock.sendto(data, address)
        print(f'sent {sent} bytes back to {address}', file=sys.stderr)

...

Force NTP sync

Modern systems often use chrony. To force a sync:

sudo chronyc -a makestep

On older systems using ntpdate:

sudo systemctl stop ntpd
sudo ntpdate -s time.nist.gov
sudo systemctl start ntpd

- To add a header in the outgoing request, use -H:

curl --header "X-MyHeader: 123" www.google.com

• Follow redirects with -l

• Disable security check with -k

• Send content with -d:

curl -X POST --header "Content-Type: application/json" -d '{"test": true}' http://localhost/blah

If more than 10 telnet sessions to a server fail

per_source = 10 in /etc/xinetd.d/telnet or /etc/xinetd.conf

Start xinetd with debugs turned on

/usr/sbin/xinetd -f /etc/xinetd.conf -d

Check duplicate ip with arping

bash-4.2 ~$ arping 192.168.0.58 -D -c 3 -I ens32
ARPING 192.168.0.58 from 0.0.0.0 ens32
Unicast reply from 192.168.0.58 [18:E7:28:2E:92:9C]  1.747ms
Sent 1 probes (1 broadcast(s))
Received 1 response(s)
bash-4.2 ~$ echo $?
1
bash-4.2 ~$

• exit status of 0 confirms a duplicate ip

Test tcp connections with nc

(from http://unix.stackexchange.com/questions/73767/how-to-check-whether-firewall-opened-for-a-port-but-not-listening-on-the-port)

nc -vz targetServer portNum

For example:

bash-3.2 ~$ nc -vz erdos 22
Connection to erdos 22 port [tcp/ssh] succeeded!
bash-3.2 ~$

Force NTP sync

sudo systemctl stop ntpd
sudo ntpdate -s time.nist.gov
sudo systemctl start ntpd