Tips for Network Troubleshooting

Thought I would make this sticky on some basic stuff for troubleshooting your network.

If you are having trouble getting onto the internet, it could be a few different things. Here is a list of possible causes:

  • Router overheating/acting up
  • Invalid IP Configuration
  • Bad Network Cable
  • DNS not responding

Those are just common causes. Here is a couple of things to try:

  • Use a Laptop Cooler or restart the router
  • unplug network cable then plug back in
  • set a static IP
  • Try different network cable

I will add more on as I do more research. I will be checking this often for suggestions.

Some more tips:

Make sure you only have one device acting as a DHCP server, this can cause a lot of headaches and can be difficult to diagnose if you don't realise that you have two DHCP servers running. 

Use your DHCP server to assign static IPs, being able to centrally manage your IP addresses makes things a lot easier and helps reduce the chances of making a mistake or having an IP conflict. 

Do some research, learning a bit about basic networking, ipv4, the difference between a modem/router/access point/switch will really help in figuring out why things aren't working.

 

Always go up the OSI model layers is THE rule for networking troubleshooting. 

1. Physical Layer : Cables, NICs, connectors of said NICs, etc. Make sure these work by trying out with another cable, another NIC, etc. Light indicators on NICs have a purpose ;)

2. Data Link Layer : MAC address conflicts, broadcasting storms, excessive amount of collisions [shouldn't happen on a switched network], MAC filtering, etc. These can get tricky to find and easily overlooked.

3. Network Layer : IP adressing [no DHCP, incorrect adressing, etc.], routing errors [ping from closest to farthest ; 1st your PC [email protected], then the gateway [email protected], then your DNS [email protected], then any random internet website]. Are you sure your PC has a gateway setup [usually your modem/router's [email protected]] ? Does your IP address look like 169.254.x.x [cause if it does, it usually means your router is not responding to requests for giving [email protected] to machines on the network]?

Layers 4, 5 and 6 are virtually never the cause of a home networking issue.

7. Application Layer : Are all your applications not capable of networking or just one ? If so, then check the configuration of the faulty app [you shouldn't have messed with those setting, y'know ?]

 

I could get into much more details if you'd like [but that's gonna take some time, haha!]

Network mapping software can help troubleshoot if you're doing a slightly convoluted soho scheme. Say, something with a computer lab, dmz, server and a few routers.

 

Absolutely this, but it's still worth knowing about layer 4 issues, especially if you're the one hosting the server. Layer 4 issues can either be caused by firewalls, or the service not running on the right port, or at all.

TCP connections are initiated with a three way handshake using a segment containing the SYN flag(Client -> Server), a segment with a SYN-ACK (Server -> Client), and finally an ACK (Client -> Server). If no service is listening on that port, ideally the server will respond with a TCP reset flag and your client should display an (connection reset/refused). If a firewall is blocking it you may receive an ICMP port administratively prohibited packet. UDP isn't connection orientated, so doesn't have a mechanism for resetting connections; instead, you may receive an ICMP port unreachable packet.

Note how I said "may" in the above; normally administrators will silence their routers/firewalls/servers (having a router respond with ICMP administratively prohibited packets isn't a good thing from a security perspective), so you may not get ICMP errors, and it's also why most web servers don't (shouldn't) respond to ping.

For troubleshooting issues, I use a combination of ping, My Traceroute (MTR), something to make TCP connections such as a telnet client, netstat, and a piece of packet capturing software such as Wireshark/Tcpdump. Ping and traceroute are good for spotting delay along the path to the server you're trying to connect to, but it's better to use TCP ("telnet google.com 80" or "wget google.com") to detect connectivity issues. Packet capturing software is good for connection analysis (packet retransmission/packets arriving out of order, or even just making sure you are sending/receiving requests). Netstat is great to check your service is listening on the expected port, and to check for current connections.

In any networking situation it is very helpful to know how to use commands in command line, knowing what to ping, and being able to test your connection from your personal OSI stack to to your DNS, to the site you are trying to connect to. Learning the well known ports can also be beneficial to you.

 

I built a new mashine and installed windows 7. But it won't connect to the internet unless I have the correct drivers. The modem dosent have a driver disc and it addressed that on the manual of the modem. How can I fix this?