Network Troubleshooting

tcpdump tutorial with Examples

https://danielmiessler.com/study/tcpdump/

Quick Testings

In Windows: ping target_ip -S source_ip

In Linux: ping -I source source_ip target target_ip

verify arp table

arp -a DESTINATION_IP

If the ARP broadcast is blocked in the next hop device, and you know the destination MAC, add the static ARP entry arp -s destination_ip MAC_address hello

Verify Data Network Connectivity

When the system is having a 2nd NIC that requires direct communication to another system, static route need to be added with required network interface

             |   Default Gateway 192.168.1.1 (0.0.0.0 Global Route)
             |   IP: 192.168.1.10/24
             |   Management-NIC (eth0) 
             |   Interface (Idx=1)
          System (A) --------------------------- Data-NIC (eth1)  <-------------->  System-B
                                             IP: 10.1.1.1/30                        IP: 10.1.1.2/30
                                             Interface (Idx=2)
# On Windows server to list all the interfaces
netsh interface ipv4 show interfaces
        # Note down "Idx" relates to the interface name, such Idx (2) relates to "Ethernet 3" for the data link

netsh interface ipv4 show config name=<Idx>     # Show TCP/IP configuration of <Idx>

# Add permenant static route for data NIC
route add -p 10.1.1.0 mask 255.255.255.252 192.168.1.1 if 2

## Testing and Verification
# ping -S source-ip dest
ping -S 10.1.1.1  10.1.1.2      # ping from 10.1.1.1 to 10.1.1.2

# ping from interface
# ping -I interface dest
ping -I eth1 10.1.1.2