encrypting/decrypting files using gpg

Typically you should never transmit any SSH keys over the network or emails. However, in some cases, you may need to encrypt them and move them into a vault or software. In such cases, you could use GnuPG package.

Encrypt:-

$tar cvz keys.tar.gz *.pem | gpg -c -o keys.tar.gz.gpg
tar: keys.tar.gz: Cannot stat: No such file or directory
test.pem
gpg: WARNING: using insecure memory!
gpg: please see http://www.gnupg.org/faq.html for more information
Enter passphrase: tar: Exiting with failure status due to previous errors

Decrypt:-

$gpg -d keys.tar.gz.gpg | tar xvz
gpg: WARNING: using insecure memory!
gpg: please see http://www.gnupg.org/faq.html for more information
gpg: CAST5 encrypted data
gpg: encrypted with 1 passphrase
gpg: WARNING: message was not integrity protected
test.pem

Purging MySQL active DB connections

In some cases when you are trying to truncate a table, there could be active DB connections that could prevent the query from executing quickly. In order to purge the active DB connections. Here is an example from Percona folks

mysql> select concat('KILL ',id,';') from information_schema.processlist where user='root';
+------------------------+
| concat('KILL ',id,';') |
+------------------------+
| KILL 3101;             |
| KILL 2946;             |
+------------------------+
2 rows in set (0.00 sec)
mysql> select concat('KILL ',id,';') from information_schema.processlist where user='root' into outfile '/tmp/a.txt';
Query OK, 2 rows affected (0.00 sec)
mysql> source /tmp/a.txt;
Query OK, 0 rows affected (0.00 sec)

Openjdk bug that results in NullPointerException when taking a dump from jmap utility

When taking a heap dump using jmap utility, you might see a NPE thrown such as below:-

$jmap -dump:format=b,file=/tmp/dump <pid>
Debugger attached successfully.
Server compiler detected.
JVM version is 24.51-b03
Dumping heap to /tmp/asgportaldump ...
Exception in thread "main" java.lang.reflect.InvocationTargetException
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at sun.tools.jmap.JMap.runTool(JMap.java:197)
        at sun.tools.jmap.JMap.main(JMap.java:128)
Caused by: java.lang.NullPointerException
        at sun.jvm.hotspot.utilities.HeapHprofBinWriter.writeSymbolID(HeapHprofBinWriter.java:905)
        at sun.jvm.hotspot.utilities.HeapHprofBinWriter.writeFieldDescriptors(HeapHprofBinWriter.java:743)
        at sun.jvm.hotspot.utilities.HeapHprofBinWriter.writeClassDumpRecord(HeapHprofBinWriter.java:508)
        at sun.jvm.hotspot.utilities.HeapHprofBinWriter.access$000(HeapHprofBinWriter.java:297)
        at sun.jvm.hotspot.utilities.HeapHprofBinWriter$1.visit(HeapHprofBinWriter.java:446)
        at sun.jvm.hotspot.memory.SystemDictionary$2.visit(SystemDictionary.java:179)

Openjdk defect id - JDK-8023105

Ingress rules on multi-AZ RDS security group needs RDS instance restart to take effect

When we add a new ingress rule to the security group to which a running RDS instance belongs, then it would need an instance restart to take effect. In case of MySQL RDS instance, you will see an error on mysql client

>mysql -h <rds instance url> -u <userid> -p
ERROR 2003 (HY000): Can't connect to MySQL server on '<rds instance>' (10060)

After restarting the instance, you will see that inbound connections from the EIP address that you have allowed will start working. 

If you try to install Virtualbox on top of a RHEL instance you will likely see an error

When you try to install a virtualization layer on top of an EC2 instance, you will likely see an error like below:-

As per Amazon Support, "It's not possible to run VirtualBox within an EC2 instance since we trying to run a hypervisor within Xen based hypervisor which is unsupported. The required VT-x CPU extensions simply aren't exposed at the guest layer either. You can try to convert your appliance (into a format supported by Xen which is very similar to EC2) and then to import the VM per the instructions provided at instances of your vm

Simple grep command to output lines before and after the search string

$grep -C 3 "NullPointerException" *.log --color

The option "-C 3" will output 3 lines before and 3 lines after the occurrence of NPE in the logs. 

Using jmap utility to take heap dump on an application running in VPC

If you have secure environments in which there are strict ingress/egress rules and cannot open up jmx ports for tools like VisualVM or set JVM parameters like -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/tmp/heapdump

In such cases you can use jmap utility such as

$ jmap -dump:format=b,file=/tmp/heapdump <pid>
Dumping heap to /tmp/heapdump ...
Heap dump file created

Once the dump has been generated, you can analyze the heap using Eclipse memory analyzer (MAT)