MySQL and Postgres command equivalents

Task: list existing databases, connect to one of the databases, then list existing tables and finally show the structure of one of the tables.

In Mysql:

show databases;
use database;
show tables;
describe table;
exit

In PostgreSQL:

\l
\c database
\dt
\d+ table
\q

Test imap using telnet

For added security, you can encrypt your IMAP connection. This requires that your server supports SSL or TLS and that you have access to an SSL/TLS client program, for example OpenSSL, to use instead of telnet.

As the port-number normally is 993, an example OpenSSL command would be openssl s_client -connect imap.example.com:993 -quiet. (If you would like to see the public key of the server, as well as some other encryption-related information, omit -quiet.) The server should then start an IMAP session, displaying a greeting such as the * OK Dovecot ready example below.

telnet imap.example.com 143

#output: Trying 193.136.28.29...
#output: Connected to imap.example.com.
#output: Escape character is '^]'.
#output: * OK Dovecot ready.

a1 LOGIN MyUsername MyPassword
#output: a1 OK Logged in.

a2 LIST "" "*"
#output: * LIST (\HasNoChildren) "." "INBOX"
#output: a2 OK List completed.

a3 EXAMINE INBOX
#output: * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
#output: * OK [PERMANENTFLAGS ()] Read-only mailbox.
#output: * 1 EXISTS
#output: * 1 RECENT
#output: * OK [UNSEEN 1] First unseen.
#output: * OK [UIDVALIDITY 1257842737] UIDs valid
#output: * OK [UIDNEXT 2] Predicted next UID
#output: a3 OK [READ-ONLY] Select completed.

a4 FETCH 1 BODY[]
#output: * 1 FETCH (BODY[] {405}
#output: Return-Path: sender@example.com
#output: Received: from client.example.com ([192.0.2.1])
#output:    by mx1.example.com with ESMTP
#output:    id <20040120203404.CCCC18555.mx1.example.com@client.example.com>
#output:    for <recipient@example.com>; Tue, 20 Jan 2004 22:34:24 +0200
#output: From: sender@example.com
#output: Subject: Test message
#output: To: recipient@example.com
#output: Message-Id: <20040120203404.CCCC18555.mx1.example.com@client.example.com>
#output:
#output: This is a test message.
#output: )
#output: a4 OK Fetch completed.

a5 LOGOUT
#output: * BYE Logging out
#output: a5 OK Logout completed.

Source IP based reverse proxy

If you want to proxy some source IP ranges/subnets to one server and onther subnets to go to a different server, you can do this using mod_rewrite for proxying. You will have to setup a rewrite condition based on the source IP and a rewrite rule with the [P] flag. Something like this should work:

RewriteCond %{REMOTE_ADDR} ^10\.2\.
RewriteRule ^/(.*) http://old-app/$1 [P]
ProxyPassReverse / http://serverA/

RewriteCond %{REMOTE_ADDR} ^10\.3\.
RewriteRule ^/(.*) http://new-app/$1 [P]
ProxyPassReverse / http://serverB/

One possible scenario for this if you want to migrate your users from server A to server B but you want to migrate one IP range at the time.

If you're using Apache 2.4 or newer you can also achieve this with the following configuration:

<If "-R '10.2.0.0/16'">
  ProxyPassReverse / /http://serverA/
</If>
<ElseIf "-R '10.3.0.0/16'">
  ProxyPassReverse / /http://serverB/
</ElseIf>
<Else>
  ProxyPassReverse / /http://serverC/
</Else>

Preserving client ip with apache reverse proxy

The first thing that I thought of was the “X-Forwarded-For” headers, which is an HTTP header inserted into the original HTTP GET request whose value is equal to the client’s public IP. Turns out apache reverse proxy inserts this header by default. So we somehow need to instruct the backend server itself to provide the application with the correct client IP.

If your backend server is a Tomcat server the solution cam be using the RemoteIP tomcat valve.

It’s quite simple to configure in that all that needs to be done is to modify tomcat server.xml to recognise original client IP rather than the proxy IP by adding the following to server.xml:

<Valve className="org.apache.catalina.valves.RemoteIpValve" internalProxies="127\.0\.0\.1" />

make sure to change 127.0.0.1 to the address of the apache reverse proxy.

The application could now recognise the original client IP.

The apache equivalent of the above method is using mod_rpaf for Apache 1.3 & 2.2.x and mod_remoteip for Apache 2.4 and 2.5. 

These apache modules can be used to preserve both remote IP/HOST. Internally they use X-Forwarded-For header to detect a proxy in it’s list of known proxies and reset the headers accordingly. This works with any proxy server in the front end provided that the proxy server sets X-Forwarded-For header. 

To use mod_rpaf, install and enable it in the backend server and add following directives in the module’s configuration:

RPAFenable On
RPAFsethostname On
RPAFproxy_ips 127.0.0.1

Remote IP is automatically preserved when RPAFenable On directive is used. RPAFsethostname On directive should be used to preserve host and RPAFproxy_ips is the list of known proxy ips.

Restart backend apache server and you are good to go.

For mod_remoteip, it’s a bit similar, the configuration should look something lke this:

RemoteIPHeader X-Real-IP
RemoteIPInternalProxy 1.2.3.4
RemoteIPInternalProxy 5.6.7.8

mod_remoteip however has a lot more configuration options.

When the proxy server is an Apache, ProxyPreserveHost directive in mod_proxy can be used to preserve the remote host not the remote ip. This is useful for situations where name based virtual hosting is used and the backend server needs to know the virtual name of host.

Open mod_proxy configuration file of your proxy server and enter directive, ProxyPreserveHost On, and restart your apache instance.

Finding external IP using the command line

The easiest way is to use an external service via a commandline browser or download tool. Since wget is available by default in Ubuntu, we can use that.

To find your ip, use:

wget -qO- http://ipecho.net/plain ;

You can do the same using curl:

curl ipecho.net/plain ; echo

How to test a listening TCP/UDP port through nc

Netcat (nc) can also be used for a lot of other purposes. It can also be used as a very fast basic port scanner, you can scan a port or a range.

To scan a range of UDP ports 1 to 1000:

nc -vzu destination_ip 1-1000

To scan a range of TCP ports 1 to 1000

nc -vz destination_ip 1-1000

Install Vsftpd on Ubuntu

On my last post I've talked about enabling the userdir module on Apache, you can use vsftpd to give your users FTP access to their own pages, this is hoe you can install it:

aptitude -y install vsftpd

Then edit it's configuration file:

vi /etc/vsftpd.conf

And make the following changes:

# line 29: uncomment
write_enable=YES
# line 97,98: uncomment ( allow ascii mode transfer )
ascii_upload_enable=YES
ascii_download_enable=YES
# line 120: uncomment ( enable chroot )
chroot_local_user=YES
# line 121: uncomment ( enable chroot list )
chroot_list_enable=YES
# line 123: uncomment ( enable chroot list )
chroot_list_file=/etc/vsftpd.chroot_list
# line 129: uncomment
ls_recurse_enable=YES
# add at the last line
# specify root directory ( if don't specify, users' home directory equals FTP home directory)
#local_root=public_html
# turn off seccomp filter
seccomp_sandbox=NO

Edit the list of users that can access your server.

vi /etc/vsftpd.chroot_list

Add the users you allow to move over their home directory

Finally restart the FTP service:

service vsftpd restart