Network troubleshooting and tools

Ping, traceroute, nameserver lookups, SNMP and more…

This page is about the tools used for troubleshooting and day-to-day management of networks. Be it loss of connection, devices not responding properly or other problems arising in larger scale networks or just simple configuration. This page is all about the day-to-day tools you can use to troubleshoot and maintain your a network and solve problems. Or, as we should call it, challenges.

Ping

Ping is a software utility employed by basically any and all network brands and types of operating systems to test the connectivity of an host on an IP network.

Ping tests the both the total time it takes for a client host to reach a server host and the server host to respond back to the client host. Along with that it also tests for the fact if said host is actually turned on or not. In other words: If it’s actually present or not.

A execution of the 'ping' command on a Windows host.

Ping, by default, operates on ICMP (Internet Control Message Protocol), when sending a ping it sends so called ‘echo’s’. This means it has no ports. It’s using ICMP types. Type 8 would be the echo and type 0 the echo reply.

Though, ping can be done of UDP and TCP this can bring drawbacks regarding latency and potential loss of replies.

Traceroute

A traceroute to Google DNS

Traceroute (Also known as tracert or trace) is a software utility similar to ping but instead traceroute sends a packet with a set time-to-live (TTL). This TTL basically tells the receiving hop (which is a router) how many times it is allowed to be passed. Once this limit is reached the router will reply to the initial source host.

Traceroute can be used to see where the traffic will end when there’s no connection and can thus show where there could be a routing problem for instance.

Nameserver lookup (with or without ping)

Nameserver lookup is used to resolve and FQDN (Fully Qualified Domain Name) to an IP address. Be it an public domain or an hostname of a server.

Using this in combination with ping can be useful to troubleshoot a lot of things. If you would ping the domain of a website and it will reply you both test the DNS and the fact if the internet connection works since you need to have a gateway in order to ping the IP of the website you resolved the domain for.

Pinging Google.nl both tests your own network, your gateway and DNS.

If you would for instance not be able to ping Google.nl but you can ping 8.8.8.8 (Google DNS) for instance. This tells you your gateway works. But DNS is not.

Console cables

In the real world, when you want to configure a Cisco device offline and locally, you got to use a console cable.

A console cable is used to communicate between two devices, in IT mostly between your computer and network equipment for configuration. These cables are typically flat and light blue, but not always! Console cables can come with a 3.5mm jack or USB connection. Newer networking equipment will have micro-USB ports that simply allow you to use a regular charger/data cable for instance.

There are several programs that allow you to make a console connection. For instance:
PuTTY
KiTTY
mRemoteNG
Serial (for MacOSx)

In combination with using the software, you will need to have drivers on your computer installed, especially for USB adapters.

Ethernet cable tester

Ethernet Cable Tester
A cheap network cable tester can easily tell you if a created cable is terminated correctly

One of the most used ones are ethernet cable testers.
An ethernet cable tester will check whether each wire in the cable is terminated correctly on both sides of an T-568B (straight) cable or an T-568A (cross-over) cable.

Ethernet cable testers like the one in the image on left can be purchased for $10-20. But more advanced ones can cost several hundreds of bucks.

Toner and probe

Toners and probes are used to let a sound signal go through a pair of wires. With a probe you can then hear the sound coming out of the wires on the other side without actually having to connect the probe to anything.

This is very useful to find cables through a building when the distance in tens of meters/feet.

These sets can cost around $50 but can easily be hundreds too.

(Optical) time domain reflectometer

Time domain reflectometers are used to measure the length of cables and impurities in the copper and thus also the breaks in the cable, wiremaps (basically a wire test just like the cable tester mentioned above), continuity and much much more.

There are also optical domain reflectometers, these devices are specifically meant for optical fibre.
The way they work is by sending light at specific wavelengths and waiting for the light to turn back. This way it can measure the length of the fibre.

Good TDR’s can easily costs several thousands of bucks.