Linux commands for server troubles - karate405 @Wiki

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Log and Warning

• real time log file checking

checking last 10 lines 
# tail -f -n 10 /var/log/messages

• finding hidden file

find "file name" from top directory
# find / -name "file name" -print
but if directory path is known
# find / "directory path" "file name" -print

• finding file with keyword

# grep "keyword" *

• finding file with keyword iterately

same as the grep option above, but working iterately 
# find / -type f -exec grep -H "keyword" {} \;
# find / -type f -print | xargs grep "keyword"
in addition. if grep equipped "-r" option
# grep -r "keyword" *

Device

''obtain device information''
there are file with current device information and
process information under "/proc" directory

• device recognization when starting OS

# dmesg

• CPU information

# cat /proc/cpuinfo

• I/O information

# cat /proc/ioports

• device interruption address

# /proc/interrupts

• disk pertition ratio

# /proc/partitions 
# fdisk -l

• connection device list and vender name, type number PCI bus address

# lspci

• connection device list and vender name type number

# scanpci

• loaded module confirmation

# lsmod

Network

• check if Linux recognizes erthanet card (NIC)

# ifconfig 
expected result if working properly includes
eth0 and UP to display

Using "ping" command

General ping command
# ping "host name or IP address"
ping command with packet size
# ping -l "packet size" "host name or IP address"

• no responce
• respond sometimes
• average responce time changes extream
• proper responce from DNS server but no reply for
  "dig"command
• no responce from default gateway
• sharp responce to small packet but creeping to large
  packet

Using "traceroute" command
checking till where packet is sent

# traceroute "host name or IP address"

Using "dig or nslookup" command

Using "netstat" command

• routing table confirmation

# netstat -rn

• checking server condition and port condition

# netstat -a

• interface status confirmation

# netstat -i

1. troubles with cable or hub can be assumed if diplaying
   lots of errors(*-ERR)
2. netork trouble is occured if ratio of received byte
   (RX-OK) / (RX-ERR) exceeds 20%

Server

"telnet" command is used to check service with port
number

• mail(SMTP) checing

# telnet localhost 25
SMTP protocol command
# EHLO "domain name"
specifies client side host name and IP address
and those information indicated on receiver's mail
header "received"
# VRFY user
checkes user existence
# TURN
swops role in between client and server
# MAIL FROM:<"sender">
specifies mail sender
# RCPT TO:<"receipient">
specifies mail receiver
# DATA
specifies where to start writing mail body
# RSET
clear prompted commands before typing RSET command
# HELP command
displays available command 
# NOOP
works as "keep alived packet", using as a prevention 
of connection time out 
# QUIT
disconnects connection

Port number list