Posts Tagged ‘Single’

How to clear ARP cache on Linux / Windows for a single IP address / Flush All IPs ARP cache

Wednesday, December 11th, 2019

linux-how-to-delete-modify-arp-cache-entries-after-IP-is-migrated-from-one-server-or-VPN-host-to-another-resized

On times of Public Internet IP migration or Local IPs between Linux servers or especially in clustered Linux Application Services running on environments like Pacemaker / Corosync / Heartbeat with services such as Haproxy.
Once an IP gets migrated due to complex network and firewall settings often the Migrated IP from Linux Server 1 (A) to Linux Server 2 (B) keeps time until a request to reload the Internet server IP ARP cache with to point to the new IP location, causing a disruption of accessibility to the Newly configured IP address on the new locations. I will not get much into details here what are the ARP (Address Resolution protocol) and Network ARP records on a Network attached Computer and how they correspond uniquely to each IP address assigned on Ethernet or Aliased network Interfaces (eth0 eth0:1 eth0:2) . But in this article, I'll briefly explain once IP Version 4 address is migrated from one server Data Center location to another DC, how the unique corresponding ARP record kept in OS system memory should be flushed in the ARP corresponding Operating System so called ARP table (of which you should think as a logical block in memory keeping a Map of where IP addresses are located physically on a Network recognized by the corresponding Unique MAC Address.
 

1. List the current ARP cache entries do

Arp is part of net-tools on Debian GNU / Linux and is also available and installed by default on virtually any Linux distribution Fedora / CentOS / RHEL / Ubuntu / Arch Linux and even m$ Windows NT / XP / 2000 / 10 / whatever, the only difference is Linux tool has a bit of more functionality and has a bit more complex use.
Easiest use of arp on GNU / Linux OS-es is.
 

# arp -an 

sample-IP-address-list-with-the-assigned-ARP-cache-mac-addresses
The -a lists all records and -n flag is here to omit IP resolving as some IPs are really slow to resolve and output of command could get lagged.

2. Delete one IP entry from the cache


Assuming only one IP address was migrated, if you want to delete the IP entry from local ARP table on any interface:
 

# arp -d 192.168.0.8


It is useful to delete an ARP cached entry for IP address only on a certain interface, to do so:
 

# /usr/sbin/arp -i eth1 -d 10.0.0.1

3. Create ARP entry MAC address with a static one for tightened security


A useful Hack is to (assign) / bind specific Static MAC addresses to be static in the ARP cache, this is very useful to improve security and fight an ARP poisoning attacks.
Doing so is pretty easy, to do so:

Above will staticly make IP 192.168.0.8 to always appear in the ARP cache table to the MAC 00:50:ba:85:85:ca. So even if we have another system with the same MAC
trying to spoof our location and thus break our real record location for the Hostname in the network holding in reality the MAC 00:50:ba:85:85:ca, poisoning us
trying to make our host to recognize 192.168.0.8 to a different address this will not happen as the static ARP will be kept unchanged in ARP caching table.

 

 # arp -s 192.168.0.8 00:50:ba:85:85:ca

4. Flush all ARP records only for specific Ethernet Interface


After the IP on interface was migrated run:

# ip link set arp off dev eth0 ; ip link set arp on dev eth0

5. Remove a set of few IPs only migrated ARP cache entries

# for i in 192.168.0.1 10.0.0.1 172.168.0.3; do sudo arp -d $i; done


Once old ARP entries are removed the arp command would return as:

linux:~$ arp
? (192.168.0.8) at <incomplete>  on eth1
? (172.168.0.3) at <incomplete>  on eth2


The 192.168.0.8 / 172.168.0.3 entry now shows as incomplete, which means the ARP entry will be refreshed when it is needed again, this would also depend
on the used network switches / firewalls in the network settings so often could take up to 1 minute or so..

 

6. Flush all ARP table records on Linux

flush-all-arp-cache-addresses-on-linux-howto-with-ip-command

# ip -s -s neigh flush all

7. Delete ARP Cache on FreeBSD and other BSDs

# arp -d -a 

8.  Flush arp cache on Windows

Run command prompt as Administrator -> (cmd.exe)  and do:

C:\> ipconfig /all
netsh interface ip delete arpcache

9. Monitoring the arp table


On servers with multiple IP addresses, where you expect a number of IP addresses migrated to change it is useful to use watch + arp like so:
 

# watch -n 0.1 'arp -an'

The -n 0.1 will make the arp -an be rerun every 10 miliseconds and by the way is a useful trick to monitor stuff returned by commands that needs a higher refresh frequency.
 

Conclusion


In short in this article, was explained how to list your arp cache table.The arp command is also available both on Linux and Windows) and as integral part of OS networking it is useful to check thoroghfully to its man page (man arp).
Explained was how to create Static ARP table records to prevent ARP poisoning attacks on a server.
I went through how to delete only a single ARP records (in case if) only certain IPs on a host are changed and an ARP cache entry reload is needed, as well as how to flush the complete set of ARP records need to get refreshed, sometimes useful on networks with Buggy Network Switches or when completely changing the set of IP-addresses assigned on a server host.

Top AIX UNIX Performance tracking commands every Linux admin / user should know

Monday, March 16th, 2015

IBM_AIX_UNIX-Performance-Tracking-every-commands-Linux-sysadmin-and-user-should-know-AIX_logo

Though IBM AIX is basicly UNIX OS and many of the standard Linux commands are same or similar to AIX's if you happen to be a Linux sysadmin and you've been given some 100 AIX servers,  you will have to invest some time to read on AIX, however as a starter you should be aware to at least be able to do performance tracking on system to prevent system overloads. If that's the case I advise you check thoroughfully below commands documentation.

fcstat – Displays statistics gathered by the specified Fibre Channel device driver

filemon – Performance statistics for files, logical/physical volumes and virtual memory segments

fileplace – Displays the placement of file blocks within logical or physical volumes.

entstat – Displays the statistics gathered by the specified Ethernet device driver

iostat – Statistics for ttys, disks and cpu ipcs – Status of interprocess communication facilities

lsps – Statistics about paging space

netstat – Shows network status

netpmon – Performance statistics for CPU usage, network device-driver I/O, socket calls & NFS

nfsstat – Displays information about NFS and RPC calls

pagesize – Displays system page size ps – Display status of current processes

pstat – Statistics about system attributes

sar – System Activity Recorder

svmon – Captures a snapshot of the current contents of both real and virtual memory

traceroute – intended for use in network testing, measurement, and management.

tprof – Detailed profile of CPU usage by an application vmstat – Statistics about virtual memory and cpu/hard disk usage

topas – AIX euqivalent of Linux top command

Here are also useful examples use of above AIX performance tracking commands

To display the statistics for Fiber Channel device driver fcs0, enter:

fcstat fcs0

To monitor the activity at all file system levels and write a verbose report to the fmon.out file, enter:

filemon -v -o fmon.out -O all

To display all information about the placement of a file on its physical volumes, enter:

fileplace -piv data1

To display a continuous disk report at two second intervals for the disk with the logical name disk1, enter the following command:

iostat -d disk1 2

To display extended drive report for all disks, enter the following command:

iostat -D

To list the characteristics of all paging spaces, enter:

lsps -a

List All Ports (both listening and non listening ports)

netstat -a | more

The netpmon command uses the trace facility to obtain a detailed picture of network activity during a time interval.

netpmon -o /tmp/netpmon.log -O all;

netpfmon is very much like AIX Linux equivalent of tcpdump To print all of the supported page size with an alphabetical suffix, enter:

pagesize -af

To display the i-nodes of the system dump saved in the dumpfile core file

pstat -i dumpfile

To report current tty activity for each 2 seconds for the next 40 seconds, enter the following command:

sar -y -r 2 20

To watch system unit for 10 minutes and sort data, enter the following command:

sar -o temp 60 10

To report processor activity for the first two processors, enter the following command:

sar -u -P 0,1

To display global statistics for virtual memory in a one line format every minute for 30 minutes, enter the following command:

svmon -G -O summary=longreal -i 60 30

The traceroute command is intended for use in network testing, measurement, and management. While the ping command confirms IP network reachability, you cannot pinpoint and improve some isolated problems

traceroute aix1

Basic global program and thread-level summary / Reports processor usage

prof -x sleep 10

Single process level profiling

tprof -u -p workload -x workload

Reports virtual memory statistics

vmstat 10 10

To display fork statistics, enter the following command:

vmstat -f

To display the count of various events, enter the following command: vmstat -s To display the count of various events, enter the following command:

vmstat -s

To display time-stamp next to each column of output of vmstat, enter the following command:

vmstat -t

To display the I/O oriented view with an alternative set of columns, enter the following command:

vmstat -I

To display all the VMM statistics available, enter the following command:

vmstat -vs


If you already have some experience with some BSD (OpenBSD or FreeBSD) you will feel much more confortable with AIX as both operating system share common ancestor OS (UNIX System V), actually IBM AIX is U. System V with 4.3 BSD compatible extensions. As AIX was the first OS to introduce file system journalling, journalling capabilities on AIX are superb. AIX was and is still widely used by IBM for their mainframes, on IBM RS/6000 series (in 1990s), nowdays it runs fine on PowerPC-based systems and IA-64 systems.
For GUI loving users which end up on AIX try out SMIT (System Management Interface tool for AIX). AIX was using bash shell in prior versions up to AIX 3 but in recent releases default shell is Korn Shell (ksh88).
Nowdays AIX just like HP-UX and rest of commercial UNICes are loosing ground as most of functionalities is provided by commercial Linux distributions like RHEL so most of clients including Banks and big business clients are migrating to Linux.


Happy AIX-ing ! 🙂

How to add sidebar to single.php (Single Posts) to your wordpress blog default theme

Tuesday, April 12th, 2011

Until recently, I have used a default wordpress theme which historically is a bit old and used to be a default theme to the older versions of wordpress.
Since however, I’ve went to many updates and on the other hand I do like and enjoy the template I have decided to continue use it for my blog until this very day.

However this nice theme’s default behaviour is a bit weird, since by default the Single opened posts are configured in a way that the usual index page sidebar is missing.
As in the sidebar there are plenty of navigation buttons and search in the blog button, at a certain moment I have realized it’s probably not a good idea that the single.php (single blog posts) open up with the right sidebar missing.

Thus I’ve decided to put back the missing sidebar in the single posts, with a hope that this will be helpful to my readers and hence have positive impact on the overall blog user experience.

Doing so prooved to be rather easy, here is how I added back the right sidebar to my wordpress single posts :

1. Edit blog/wp-content/themes/default/single.php

debian:~# vim /var/www/blog/wp-content/themes/default/single.php

2. In the single.php look up for the code:

<div id="content" class="widecolumn">

Substitute this html code with:

<div id="content" class="narrowcolumn">

3. Next find the code: <?php get_footer(); ?>

Right before the get_footer(); php function add in the function;

<?php get_sidebar(); ?>

Tadam! Refresh a single post in Firefox and you should see your blog index.php sidebar to show up.