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Half Stack Python

Network connectivity troubleshooting

Networking is sophisticated, and debugging application errors caused by them can be a painful process.

Here is what I’ve used to root cause server issues - feel free to add others in a comment if you know of a helpful utility I haven’t listed.

Gather full hostname, and ip address of source, and destination hosts with nslookup

Section titled “Gather full hostname, and ip address of source, and destination hosts with nslookup”
Terminal window
nslookup <host> - will show ip address

From the source computer - confirm you’re able to resolve the destination ip address

Section titled “From the source computer - confirm you’re able to resolve the destination ip address”
Terminal window
nslookup rhino
# Server:         192.168.1.1
# Address:        192.168.1.1#53
#
# Name:   rhino.localdomain
# Address: 192.168.1.168

Confirm server is online reachable

Section titled “Confirm server is online reachable”
Terminal window
ping <host>
ssh <host>

Confirm server responding to curl commands

Section titled “Confirm server responding to curl commands”
Terminal window
curl -v https://example.com:443

Check if port is listening

Section titled “Check if port is listening”
Terminal window
ss -tuln | grep :PORT
lsof -i :PORT
nmap -p- TARGET

Confirm ports are available and not blocked

Section titled “Confirm ports are available and not blocked”
Terminal window
iperf3 - allows testing host to host on a specific port
iperf3 -s -p 5201 # On server
iperf3 -c <server-ip> -p 5201 # On Client
iperf3 -c rhino -p 5201
# Connecting to host rhino, port 5201
# [ ID] Interval           Transfer     Bitrate
# [  7]   0.00-1.00   sec  20.6 MBytes   172 Mbits/sec


iperf3 -c <server-ip> -p 5201 -u # udp
for port in {5201..5210}; do iperf3 -c <server-ip> -p $port; done # multiple ports

Check route server is using

Section titled “Check route server is using”
Terminal window
traceroute <host> - shows jumps made to get to host


traceroute rhino
# traceroute to rhino.localdomain (192.168.1.168), 64 hops max, 40 byte packets
#  1  rhino.localdomain (192.168.1.168)  3.722 ms  5.200 ms  3.759 ms

Confirm DNS Resolution

Section titled “Confirm DNS Resolution”
Terminal window
dig - allows troubleshooting DNS
dig google.com
dig @8.8.8.8 example.com - uses specific dns server to resolve
dig +trace example.com - adds tracing

Find what process is using a port

Section titled “Find what process is using a port”
Terminal window
lsof -i
lsof -i TCP:22
lsof -i [email protected]

Show Route Address

Section titled “Show Route Address”
Terminal window
route
ip link show # shows mac address
Terminal window
curl -H "Authorization: Bearer <Token>" -T myfile.tar.gz "https://artifactory/artifactory/<reponame>/path/to/file.tar.gz

Tracing traffic through firewalls

Section titled “Tracing traffic through firewalls”
Terminal window
ip route
#  example output
#  DEFAULT GATEWAY
#  default via 10.0.0.1 dev eth0 proto dhcp metric 100


#  LOCAL LAN ROUTE
#  10.0.0.0/24 dev eth0 proto kernel scope link src 10.0.0.10 metric 100


#  Traffic to 172.16.x.x is routed through 10.0.0.254 - likely firewall or router
#  172.16.0.0/16 via 10.0.0.254 dev eth0


traceroute 172.16.0.1 # shows hops to destination
ping 10.0.0.254
nslookup 10.0.0.254
# check arp table
arp -n | grep 10.0.0.254


# scan device
nmap -sV 10.0.0.254
# can tell you if its a firewall

Check DNS Resolution

Section titled “Check DNS Resolution”
Terminal window
less /etc/sssd/sssd.conf # look at avilable ipas
#Confirm you can hit it
nslookup url.ipa.net # lets say this returned 10.0.0.0
# Try resolving to url from ip in dig
# Trying to confirm artifactory.internal.net resolves to ip address
dig @10.0.0.0 artifactory.internal.net