Subject: Re: how to (cross)connect two (physical) eth ports for ping test?

From: Roman Mashak <[email-masked]>
Date: Sun, 19 Aug 2018 11:55:25 -0400

"Robert P. J. Day" <[email-masked]> writes:

>   (i'm sure this has been explained many times before, so a link
> covering this will almost certainly do just fine.)
>
>   i want to loop one physical ethernet port into another, and just
> ping the daylights from one to the other for stress testing. my fedora
> laptop doesn't actually have two unused ethernet ports, so i just want
> to emulate this by slapping a couple startech USB/net adapters into
> two empty USB ports, setting this up, then doing it all over again
> monday morning on the actual target system, which does have multiple
> ethernet ports.

[...]

I used this in the past to test dual-port NIC over loopback cable, you
will need to ajust the script:

#!/bin/bash -x

ip="sudo $HOME/bin/ip"
eth1=192.168.2.100
eth2=192.168.2.101

dev1=eth1
dev2=eth2
dev1mac=00:1b:21:9b:24:b4
dev2mac=00:1b:21:9b:24:b5

# fake client interfaces and addresses
dev=dummy0
dev_mac=00:00:00:00:00:11

# max fake clients supported for simulation
maxusers=3

## Create dummy device
## Accepted parameters:
##    $1 - devname
##    $2 - devmac
##    $3 - subnet (e.g. 10.10.10)
##    $4 - max number of IP addresses to create on interface
setup_dummy()
{
#   sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
   # Enable tc hardware offload
#   ethtool -K $SGW_DEV hw-tc-offload on

   $ip link add $1 address $2 type dummy
   $ip link set $1 up
   for i in `seq 1 $4`;
   do
      $ip addr add $3.$i/32 dev $1
   done
}

## Delete dummy device
## Accepted parameters:
##    $1 - devname
delete_dummy()
{
  $ip link del $1 type dummy
}

setup_network()
{
  # Send traffic eth3 <-> eth4 over loopback cable, where both interfaces
  # eth3 and eth4 are in the same subnet.
  #
  # We assume that NetworkManager is not running and eth3/eth4 are configured
  # via /etc/network/interfaces:
  #
  # 192.168.1.100/32 dev eth3
  # 192.168.1.101/32 dev eth4
  #
  # Specify source IP address when sending the traffic:
  # ping -I 192.168.1.100 192.168.1.101
  #
  #
  $ip neigh add $eth2 lladdr $dev2mac nud permanent dev $dev1
  $ip neigh add $eth1 lladdr $dev1mac nud permanent dev $dev2
  $ip route add table main $eth1 dev $dev2
  $ip route add table main $eth2 dev $dev1
  $ip rule add from all lookup local pref 100
  $ip rule del pref 0
  $ip rule add from $eth2 to $eth1 iif $dev1 lookup local pref 1
  $ip rule add from $eth1 to $eth2 iif $dev2 lookup local pref 2
  $ip rule add from $eth2 to $eth1 lookup main pref 3
  $ip rule add from $eth1 to $eth2 lookup main pref 4

#  $ip rule add from 10.10.10.0/24 to $eth1 iif $dev1 lookup local pref 5
#  $ip rule add from 10.10.10.0/24 to $eth2 iif $dev2 lookup local pref 6
#  $ip rule add from $eth1 to 10.10.10.0/24 iif $dev2 lookup local pref 7
#  $ip rule add from $eth2 to 10.10.10.0/24 iif $dev1 lookup local pref 8
}

restore_network()
{
  # FIX: hangs connections
  $ip rule flush
  $ip rule add priority 32767 lookup default
}

#delete_dummy dummy0
#delete_dummy dummy1

#setup_dummy dummy0 00:00:00:00:00:11 10.10.10 3
#setup_dummy dummy1 00:00:00:00:00:22 20.20.20 3
setup_network