DHCP v4 traffic operates on port 67 (Server) and port 68 (Client). So we can capture the appropriate traffic with the following expression. (v4)
tcpdump -i eth0 udp port 67 or port 68 -vvv
tcpdump-examples
tcpdump-examples
/ actimeo.md
Last active
February 6, 2021 04:15
What does actimeo mean in nfs mount
What does actimeo mean in nfs mount
The "actimeo=0" option means turn attribute caching off. The actimeo option actually makes the time duration the nfs client holds the various cache attributes like "acregmin","acregmax","acdirmin" and "acdirmax" to zero seconds, which means that the nfs client will need to look for the attributes everytime from the server.
tcpdump-examples
/ disk-usage-on-linux.md
Last active
February 5, 2021 14:36
Analyzing disk usage is an important task for Linux admin. Disk I/O bottlenecks can bring down applications. Hope this post can give you more ideas on how to troubleshoot disk problems.
Check Disk Space Usage with df
Check Disk Utilization In Linux
Check Disk IOPS and Bandwidth In Linux
Check IO usage by Process
tcpdump-examples
/ tcpdump-cdp.md
Last active
February 15, 2021 09:40
tcpdump -v -s 1500 -c 1 '(ether[12:2]=0x88cc or ether[20:2]=0x2000)'
This will often show you the uplink Cisco chassis switch info like the native vlan, port info, device name, serial name etc.
tcpdump -v -s 1500 -c 1 'ether[20:2] == 0x2000'
tcpdump-examples
/ tcpdump.md
Last active
April 18, 2024 13:14
Tcpdump is a CLI tool to capture raw network packets. This is useful for various forms of network troubleshooting. This cheat sheet covers all the basic and advanced options for tcpdump.
Tcpdump command is a famous network packet analyzing tool that is used to display TCP\IP & other network packets being transmitted over the network attached to the system on which tcpdump has been installed. Tcpdump uses libpcap library to capture the network packets & is available on almost all Linux/Unix flavors.
tcpdump-examples
/ tcpdump-ssh-traffic.md
Last active
February 15, 2021 09:37
tcpdump-examples
/ tcp window scaling on Linux.md
Last active
February 15, 2021 11:42
source: Why Tcp Window Scaling Improves Network Performance?
Tcp window scaling is a key design in tcp protocol to improve network performance. Today we will dive into this topic to see how tcp window scale works in Linux.
Linux high IOwait is a common Linux performance issue. Today we will look at what iowait means and what contributes to this problem. Hope this can give you more ideas about high IOwait issue.
IO wait is related to the CPU resource on the server.
The iowait column on top command output shows the percentage of time that the processor was waiting for I/O to complete. It indicates that the system is waiting on disk or network IO. Because the system is waiting on those resources, it can not fully utilize the CPU.
tcpdump-examples
/ bash-for-loop.md
Created
March 23, 2021 14:05
This type of for loop is characterized by counting. The range is specified by a beginning (#1) and ending number (#5). The for loop executes a sequence of commands for each member in a list of items. A representative example in BASH is as follows to display welcome message 5 times with for loop:
#!/bin/bash
for i in 1 2 3 4 5
do
Each TCP flag corresponds to 1 bit in size. The list below describes each flag in greater detail. Additionally, check out the corresponding RFC section attributed to certain flags for a more comprehensive explanation.
- SYN - The synchronisation flag is used as a first step in establishing a three way handshake between two hosts. Only the first packet from both the sender and receiver should have this flag set. The following diagram illustrates a three way handshake process.
- ACK - The acknowledgment flag is used to acknowledge the successful receipt of a packet. As we can see from the diagram above, the receiver sends an ACK as well as a SYN in the second step of the three way handshake process to tell the sender that it received its initial packet.
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