Introduction to Networking IP Configuration Troubleshooting

Network Connection Details When the network isn’t working we may need to collect data in order to troubleshoot the network and see what is happening and why it isn’t working In Windows, to do this we can click on the connection in the Network and Sharing Center, then click on details to get our information We can also use IPconfig from the command prompt

IPconfig IPconfig will give us our basic internet properties including IP address, subnet mask and default gateway. This is useful to diagnose basic problems, but we may need more info. In that case, we can use IPconfig /all

IPconfig /release and /renew IPconfig /release forces our NIC to give up its IP address. This can be useful if our IP address is misconfigured and we need to give up the old data in order to be assigned new IPconfig /renew forces the NIC to send out a request to the DHCP server for an IP address If we do /release and /renew when everything is working right, our IP address usually won’t change since the lease won’t be up yet, but when something is wrong we can be given a new proper IP address and information

Example of APIPA address If we have a computer set up to use DHCP, but it is having problems accessing the internet, or it is not connected to parts of its subnet, it may have been assigned an IP address from APIPA Signs APIPA gave us our IP address No default gateway Network address starting with 169.254 Subnet mask of 255.255.0.0

Setting Up Alternate Configuration To set up how our interface gets its IP address when the DHCP server is down, click on the connection in network and sharing center. Go to properties, then IPv4, properties, alternate configuration We can let it automatically assign an IP address or we can have a static IP assigned in the case of DHCP outage The benefit of a static alternate IP address is that we can assign a default gateway and DNS, allowing us access to the internet

IFconfig Linux version of IPconfig Need to be root to change, any user can view it To change the IP address and subnet mask for example, you would write ipconfig ens32 (NEW IP) netmask (NEW MASK)

ENS32 (or eth0) and LO ENS32 (Or eth0 if you’re using an older version of Linux) will be the wired or wireless interface LO will be the loop-back adapter to the local host. This will exist even if you don’t have an ethernet interface, since it is used by various services running on the system to send info to each other

Anatomy of IFconfig HWaddr - Hardware address, or Mac address Inet addr - Internet address, or IP address Bcast - Broadcast IP for our subnet Mask - Subnet mask for our subnet RX Packets - number of packets received TX Packets - number of packets sent

Recognizing Different States Static IP - DHCP Enabled - no, No DCHP server line DHCP configuration - DHCP Enabled - yes, IP address on DCHP server line Rogue DHCP - DHCP Enabled - yes, wrong DCHP IP address. Possibly multiple hosts with incorrect information with the same DHCP address APIPA - DHCP Enabled -yes, No DHCP server line, IP address between 169.254.0.1 and 169.254.254.254 and subnet mask of 255.255.255.0 Alternate configuration DHCP Enabled -yes, No DHCP server line, IP address other than APIPA

Netstat Netstat stands for network statistics Netstat without modifiers will show me a list of all active incoming and outgoing connections, the port they use, the IP address they are connected to and their status Netstat -a will also include any listening ports Netstat -es Netstat -r

Netstat -es Netstat -es gives the ethernet network statistics. This includes packets sent and received, packets discarded and errors caught and current connections

Netstat -r Netstat -r gives us the routing table for our computer A netmask (another name for subnet mask) of 0.0.0.0 will show us our default gateway, and a gateway of “On-Link” shows us what we are locally connected to

Nslookup and Dig Nslookup and dig both allow us to test our DNS server We can enter nslookup with no options to use our default DNS server, or we can use nslookup -[SERVER IP ADDRESS] to use a different server After entering nslookup we can enter website names to have the DNS server process. To quit, we type “exit” When we get an answer, it will specify authoritative if our server is authoritative for the website we lookup, and non-authoritative if the server needs to lookup the site from a root server Dig is available by default on Linux, but not Windows. It has similar uses

Dig