Network debugging….
A) Tcpdump:- is a packet analyzer
Note: tcpdump uses || (or), &&(and), !(not) concept
i) will show source and destination for 80 port through eth0 10 times:
# tcpdump -c 10 port 80 -i eth0

ii) # tcpdump dst or dst or dst and tcp port 80

iii) You can use wireshark to get clear picture of tcpdump (using -w)
# tcpdump -w savingfile.txt
You can read the file using
# tcpdump -r savingfile.txt

iv) # tcpdump -n host and src and ! port 22
(-n = print IP not resolving.)


B) Network issues:
i) Trace the complete route:
# traceroute
ii) add default gateway if not added (route -n):
# route add default gw eth0
ii) add ip address/netmask:
# ifconfig eth0 netmask
iv) Lookup local ip address (equivalent to host `hostname`)
# hostname -i


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