Category Archives: Technology

Response for preflight does not have HTTP ok status

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Issuing an AJAX request is more complex than you might think.

Issue

When you issue an AJAX request to a server in another domain (CORS), you may get the following error message:

Response for preflight does not have HTTP ok status.

Problem

The server is configured to allow CORS. The Apache configuration includes

Header set Access-Control-Allow-Origin "*"

The response header of the service contains the correct header value. With this set you can access the service via CORS.

Solution

Now, why does it not work?

You have to be aware that this works for simple CORS requests. For more complex requests that set custom headers, etc, the service may not work. This is due to the preflight mechanism of the browser that checks if the service accepts the request. Before issuing an AJAX request (e.g. GET or POST), an OPTIONS is triggered to check what the service is accepting. You can see this in the network tab of Chrome.

The request includes two headers:

  • Access-Control-Request-Headers
  • Access-Control-Request-Method

Before issuing the actual GET request, the browser is checking if the service is correctly configured for CORS. This is done by checking if the service accepts the methods and headers going to be used by the actual request. Therefore, it is not enough to allow the service to be accessed from a different origin, but also the additional requisites must be fulfilled.

Solution

To configure Apache to set the headers, add the missing headers. Include in your HTTP service configuration the header set directive.

Header always set Access-Control-Allow-Origin "*"
Header always set Access-Control-Allow-Methods "POST, GET, OPTIONS, DELETE, PUT"
Header always set Access-Control-Allow-Headers "append,delete,entries,foreach,get,has,keys,set,values,Authorization"

Still not working!

After setting the values, you may still not able to call the service, as the browser still reports an error. The response code is not 2xx. Returning a 200 HTTP code can be enforced in Apache config using a rewrite rule.

RewriteEngine On
RewriteCond %{REQUEST_METHOD} OPTIONS
RewriteRule ^(.*)$ $1 [R=200,L]

With this configuration, the service will now work with CORS. The first OPTIONS request will pass:

The following GET request will also pass:

Links

 

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Automount share

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The example used in this blog is a CIFS share from a Samba server running on a Raspberry Pi mounted on demand by a client running Debian.

Goal

The goal is to have a share on a client that is dynamically mounted. The share should only be mounted when an app needs to access.

In my case I do have a server with a data storage share connected. The storage is made available to clients in the network. To not have the share being mounted by the clients all the time, the share should be mounted only when real demand exists. For instance, an app needs to read data from the share. If the share is not needed by the client, it should not be mounted.

Process

To understand the scenario better, take a look at the picture below. The process can be separated into 4 steps.

  • Step 1: the client is configured, but the share is inactive.
  • Step 2: An app is accessing the share. This can be an app as simple as ls /mnt/share
  • Step 3: The client is connecting to the server and mounting the share to the local mount point /mnt/share. The data is now available to the app.
  • Step 4: The app is not using the data from the share any longer. After a timeout, the client disconnects the share.

The example is using for the server a Raspberry Pi with Raspbian and for the client a Debian based system (Proxmox). As share type, CIFS is used. On the client, Samba is running and configured to give access to a named user to the data storage.

Installation

Server

Install Samba and configure access for a named user. This is not part of this blog.

Client

Autofs is the package and tool taking of mounting shares automatically. Install it using apt-get.

apt-get update
apt-get install autofs

Configuration of autofs

Autofs needs to be configured. To make it easier, the packages comes with templates. I am going to use the autofs master template as my starting point. Take a look at the master template as it contains an explanation of what is needed.

cd /etc/auto.master
more /etc/auto.master

To add an auto mount share a new line must be added to the file. The format is: mount-point file options.

Before adding the line, you first must understand how the template and autofs works and what you want to achieve. First parameter is for the local mount point. The directory informed here is the parent. The actual shares mounted are created as sub-folders in that directory. For instance, if you choose /mnt/server and the remote share is data, the final mount point will be /mnt/server/data. I am going to mount the remote share to /mnt/server.

The seconds parameter is for the configuration file for that mount point. The third parameter specifies how autofs is treating the mount point. To unmount the share after 1 minute of inactivity, use option –timeout=60. The ghost option will create the subfolder even in case the server is not reachable.

Edit master template

Add a new configuration line for mounting the server share

/media/net /etc/autofs/auto.net --timeout=60 --ghost

Mount configuration

The actual mount configuration for the share is specified in the file /etc/auto.server. Create file /etc/auto.server and edit it.

touch /etc/auto.server
vim /etc/auto.server

Insert mount options.

  • [username] – name of user used to connect to Samba share
  • [password] – password of the user
  • [remote_server] – IP / server name of the Samba server
  • /data – name of the share configured at Samba. In case you have a share configured named Music, or Fotos, or Work, substitute data with the correct share name.
data -fstype=cifs,username=[username],password=[password] ://[remote_server]/data

Save file

Change permission

chmod 0644 auto.server

Start autofs as service

Stop autofs and start it in debug mode to see if the configuration works.

If it worked, exit and start the service.

systemctl start autofs.service

Test

To test, go through the 4 steps described in the picture at the top of this blog.

Step 1

Client is ready. Check the mounted volumes. You will see that no CIFS volume is available.

Using mount, you can see that the mount point is available.

/etc/auto.server on /mnt/server type autofs (rw,relatime,…

In the parent mount point for the CIFS share, autofs created the folder data.

Step 2 & 3

Run an app that is accessing the share

ls /mnt/share/data

Accessing now the content of /mnt/server/data will automatically mount the CIFS share.

df -h

Mount

//192.168.0.164/data on /mnt/server/data type cifs (rw,relatime

Step 4

Assure that no app is using the share and wait until the specified timeout occurs. Check with mount and df to see that the share is umounted.

Additional information

Start / stop autofs service

Start service

systemctl stop autofs.service

Stop service

systemctl stop autofs.service

Links

https://wiki.archlinux.org/index.php/autofs

 

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OpenVPN Assign static IP to client

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After configuring the overall OpenVPN client and server infrastructure, my clients can connect to a VPN. The client can access server resources and vice versa. While the server gets normally always the same IP assigned, the client IP address is assigned dynamically from a pool of IP addresses. Meaning: there is no guarantee that the client always gets the same IP address. Normally, this is not a problem, as the client connects to consume server resources. Such like a web site, or git repository. In my case, the architecture is that the OpenVPN server acts as a proxy to internal services. The web site, git repository, etc are running on the client. Therefore, the server must be able to connect to the client using a fix address.

To make this work, each time a client connects, the same IP must be assigned to. OpenVPN allows to assign a static IP to a client.

Configuration

  1. In /etc/openvpn create folder ccd. Ccd stands for client config directory, meaning: it contains the configuration for a client.
  2. Edit file server.conf and add line “client-config-dir ccd“
# EXAMPLE: Suppose the client
# having the certificate common name "Thelonious"
# also has a small subnet behind his connecting
# machine, such as 192.168.40.128/255.255.255.248.
# First, uncomment out these lines:
client-config-dir ccd

3. Create a configuration file for each client and put into directory ccd. As file name, use the same name for the client as used in the CN field of the client certificate.

ifconfig-push IP MASK

Example:

ifconfig-push 10.8.0.2 255.255.255.255

CLI steps

sudo mkdir /etc/openvpn/ccd
sudo touch /etc/openvpn/ccd/client1
sudo vim /etc/openvpn/server.conf
Uncomment the line containing client config parameter
client-config-dir ccd

sudo vim /etc/openvpn/ccd/client1
Insert:
ifconfig-push 10.8.0.2 255.255.255.255
Restart OpenVPN service on server
sudo /etc/init.d/openvpn restart

Client with automatic assignment of IP: 10.8.0.6

After restart of OpenVPN server: IP is now 10.8.0.2

Server log

 

Additional information can be found in OpenVPN documentation.

client-config-dir

“This file can specify a fixed IP address for a given client using –ifconfig-push, as well as fixed subnets owned by the client using –iroute.” https://openvpn.net/index.php/open-source/documentation/manuals/65-openvpn-20x-manpage.html

ifconfig-push

„Push virtual IP endpoints for client tunnel, overriding the –ifconfig-pool dynamic allocation.” https://openvpn.net/index.php/open-source/documentation/manuals/65-openvpn-20x-manpage.html

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ERR_CONTENT_DECODING_FAILED

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Configuring a reverse proxy is not an easy task. It involves some trial and error and dealing with unexpected errors. One of those errors is ERR_CONTENT_DECODING_FAILED. The web site won’t load in your browser will show this error message:

Error ERR_CONTENT_DECODING_FAILED may show up in your browser when a resource is configured on your reverse proxy, and the backend communication is working. That is: the backend is returning data, but not in a form the browser expects.

Example: browser expects a GZIP response, but receives plain text. Therefore the hint from your browser about content decoding failed. The content is received, but the browser is not able to decode / understand the data. If a plain text response is expected, but the received response from the backend is zipped, the browser cannot read the content.

To solve this error, reset the Accept-Encoding request header in your Reverse Proxy configuration.

Apache

Documentation

RequestHeader unset Accept-Encoding

Example

<Location /test>
    RequestHeader unset Accept-Encoding
    ProxyPass https://0.0.0.0:443
    ProxyPassReverse https://0.0.0.0:443/
    Order allow,deny
    Allow from all
</Location>

NGINX

Documentation

proxy_set_header Accept-Encoding "";
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How to use find to sort files across folders

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Short version

You have files named File0.txt to File100.txt in different folders and want to move the first 30 files in a separate directory (command for Mac users. Linux users can use find and mv):

For sorting FileNN.txt (character + number)

gfind -type f -printf "%f %p\n" | sort -n -k 1.5 | sed 's/.* //' | head -30 | xargs gmv -t ./A/

For sorting NN.txt (numeric filename)

gfind -type f -printf "%f %p\n" | sort -n | sed 's/.* //' | head -30 | xargs gmv -t ./A/

Preparation

For the below commands to work, you’ll need to use GNU find. If you are using a Mac, you’ll need to install the GNU version of find and mv via homebrew.

brew install findutils coreutils

Create a test folder structure. There will be 3 folders and several files in them.

mkdir 1
mkdir 2
mkdir 3

Create 100 files with name TestNN.txt with sample content and place them in one of the three directories randomly.

for i in {000..100}
  do
    Num=$((1 + RANDOM % 3))
    echo hello > "$Num/File${i}.txt"
done

After running the above script, the folder will look like this (running ls -R)

Also create the target directory A:

mkdir A

Commands

After the initial setup is done, we have several files in 3 directories. If you use find to get a list of all files, you’ll see that the output is not sorted.

gfind ./ -type f

A Unix command to sort files is sort. Applying sort in this scenario won’t help, as the files are sorted by the folder name:

gfind ./ -type f | sort -n

The output is now sorted by folder name and then by file name, but not only by file name. Copying the first 50 elements won’t result in the File1 – File 50. The files are not distributed across the directories as needed.

It is possible to see a solution to the problem: sort only on the filename, while still having the complete path in the output for piping the parth to the copy command. Find includes exactly this possibility: print a specific field. To control the output, parameter -printf is available, and %f prints the filename, while %p includes the folder.

gfind -type f -printf "%f\n"

The output of the command only prints the filename.

To output the file with path, use %p. In both cases \n is used to have each file in a new line.

gfind -type f -printf "%p\n"

Both output parameters can be combined. %f %p\n will first print the filename, then space, then the path.

gfind -type f -printf "%f %p\n"

Applying sort on this output will sort on the file name only.

gfind -type f -printf "%f %p\n" | sort -n

Close, but not exactly how it should be. In case your filename consists only of numbers, this will already work. In the example however, the filename contains characters. Therefore, sorting is not working correctly. It starts with File0.txt, then File1.txt, but then comes File10.txt and not File2.txt. To sort by the number, add to sort an additional parameter: -k 1.5. As the filename contains a fixed value (File), the parameter will instruct sort to ignore this part when sorting and focus only on the number.

Note: you may apply the same sort parameter without using find, just ls. As long as your path has the same size, it will work. For folders named 1..9 it’s ok, but when your folder has two or more chars (like 10, or 213, or test), the parameter needs to be adjusted.

List all files with directory name using ls:

ls -d1 */*

Sort by number in filename:

ls -d1 */* | sort -n -k 1.7

gfind -type f -printf "%f %p\n" | sort -n -k 1.5

With the last command, the output is correctly sorted based on the filename. Now, how to use this output to move the files to the target directory? Just piping the output to mv won’t work. The first part with the filename is not needed, only the second part. Both parts are separated by blank, and using sed, it’s possible to eliminate the part before the blank from the output.

gfind -type f -printf "%f %p\n" | sort -n -k 1.5 | sed 's/.* //'

The last step is now to use mv to move the files to the target directory. To not have to move all files, let’s take only the first 30 files. Gnu mv is needed to move the files, as the default MacOS BSD mv does not include the -t parameter. To pass the files line by line, xargs is used together with gmv.

gfind -type f -printf "%f %p\n" | sort -n -k 1.5 | sed 's/.* //' | head -30 | xargs gmv -t ./A/

Result

Now there are the first 30 files in folder A.

gls -1v ./A

 

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Download resources from SAP Cloud for your CI job

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When running a CI job you may need to use some SAP tools. For instance, the MTA builder or Neo tools. Many CI servers include integration to build tools or plugins are provided by the community or vender. Jenkins offers plugins for Maven, Ant or Node that let you easily integrate these into a CI jobs. If you have a CI job for SAP, it is your task to make the necessary tools available. There are not many plugins for SAP available for Jenkins.

Some tools you may need can be found on SAP’s tool site. For instance, the MTA builder. A simple JAR file that is available for download and needed in case you are working with MTA apps.

Before you can download the JAR file, you need to agree to the EUL.

This means that you cannot download the JAR using cli:

wget https://tools.hana.ondemand.com/additional/mta_archive_builder-1.1.0.jar

Solution

Running the above wget command will not download the tool, but a web site. Some may know that this is very close to how Oracle protected it’s Java download. And the “solution” here is the same: send the right cookie via wget.

wget --header "Cookie: eula_3_1_agreed=tools.hana.ondemand.com/developer-license-3_1.txt" https://tools.hana.ondemand.com/additional/mta_archive_builder-1.1.0.jar

Works for downloading other tools from the download page like the Neo SDK too:

wget --header "Cookie: eula_3_1_agreed=tools.hana.ondemand.com/developer-license-3_1.txt" https://tools.hana.ondemand.com/sdk/neo-javaee6-wp-sdk-2.137.0.1.zip

Let’s hope SAP provides some Jenkins plugins that take care of downloading these automatically.

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Clone a SCP git repository from command line

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I have a git repository on SCP that I want to clone using git on my laptop. I thought this should be easy to do. The source code of my project is available in the git repo at SCP. Cloning the repo using git clone from this URL should work.

git clone https://git.hanatrial.ondemand.com/p539123trial/cisample

The clone fails with “service not enabled.” Looking at SAP’s documentation, this should not have happened. Here SAP Cloud Platform documentation for the git service differs from reality.

SAP Help

I did a), and b) did not apply, as I wasn’t asked for my SCN user ID nor password. SAP’s git troubleshooting guide contains a section about the error message. Good to know that there is a possible solution, but I already did already what the proposed solution to the error is:

„Ensure that you have the correct repository URL. Copy it from the Source Location section of the repository’s details page in the SAP Cloud Platform cockpit.“

As it is possible to access the repository in SAP Web IDE, it should also be possible to access it from outside SCP. I know that the git repository is protected. Maybe the requests from git cli is blocked by SCP? After all, I was not asked to authenticate. Maybe I can force SCP to ask me for my password? Changing the URL to include my SCN user ID did just that: I was asked to provide my password.

git clone https://p539123@git.hanatrial.ondemand.com/p539123trial/cisample

SCP is now asking for my password and – magic happening – the git service is now accessible and the repo can be cloned. Would be nice if the git service would ask me to authenticate instead of failing directly.

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Solving reverse proxy error ERR_CONTENT_DECODING_FAILED

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Configuring a reverse proxy is not an easy task. It involves some trial and error and dealing with unexpected errors. One of those errors is ERR_CONTENT_DECODING_FAILED. The web site won’t load in your browser and Chrome will show this error message:

Error ERR_CONTENT_DECODING_FAILED may show up in your browser when a resource is configured on your reverse proxy, and the backend communication is working. That is: the backend is returning data, but not in a form the browser expects. Like: browser expects a GZIP response, but receives plain text. Therefore the hint content decoding failed. Content received, but the browser is not able to decode / understand the data.

To solve this error, reset the Accept-Encoding request header in your Reverse Proxy configuration.

Apache

RequestHeader unset Accept-Encoding

http://httpd.apache.org/docs/current/mod/mod_headers.html 

Example Apache configuration section for a location named test.

<Location /test>
  RequestHeader unset Accept-Encoding
  ProxyPass https://0.0.0.0:443
  ProxyPassReverse https://0.0.0.0:443/
  Order allow,deny
  Allow from all
</Location>

NGINX

proxy_set_header Accept-Encoding "";

http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_set_header

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Setup OpenVPN troubleshooting

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While setting up OpenVPN I came accross some common errors or workarounds that make life easier. To make it easier to remember these I have documented them in this blog. Maybe they are useful for others as well.

Remove pass phrase

In case you want to remove the pass phrase from the server key to make it easiert to start the OpenVPN server part, use the following command:

mv server.key server.key.orig
openssl rsa -in server.key.orig -out server.key

You’ll have to enter one more time the pass phrase of the key, and then a new server.key file is written without the pass phrase. You can see this when looking into the key files.

With pass phrase:

Note: file starts with: BEGIN ENCRYPTED PRIVATE KEY

Without pass phrase:

Note: file starts with: BEGIN RSA PRIVATE KEY

Run OpenVPN as a service on Linux

After installing openvpn via yum on AWS AMI Linux, a service script is also installed. How the file works and can be activated is written in the file itself:

more /etc/init.d/openvpn

The file should already be copied by yum to /etc/rc.d/init.d/openvpn

Activate the service

chkconfig

Check whether or not openvpn is already configured to run as a service. For each run level, the status is either on or off. In case of on, openvpn is already configured to run as a service. In this example, opevpn is not configured to run as a service in any runlevel.

sudo chkconfig --add openvpn

sudo chkconfig openvpn on

OpenVPN will now be started as a service in the run levels 2, 3, 4 and 5. Output of openvpn is then written to /var/log/messages

sudo tail -f /var/log/messages

Systemd

To start and control openvpn via systemd. Check status of openvpn.

sudo systemctl status openvpn

Edit service configuration

sudo vim /etc/default/openvpn

Insert the client configuration to start automatically. Here, I am going to start client1.conf:

AUTOSTART=”client1”

Start service

sudo systemctl start openvpn
sudo systemctl status openvpn

Solving common OpenVPN connection error message

Some information on how to solve common OpenVPN error message on the server and client. Most occur when trying to start OpenVPN for the first time.

TA.KEY

Client starts connecting but no connection is established.

Error message

TLS Error: cannot locate HMAC in incoming packet from [AF_INET]

Cause

Server is configured to use ta.key.

Solution

Copy the ta.key into the openvpn configuration directory and specify its location in the conf file.

Cipher final failed

OpenVPN server accepts a client connection, but communication fails.

Error message

Authenticate/Decrypt packet error: cipher final failed

Cause

Server and client are using different algorithms for encryption and decryption. On the server, the log gives more information:

WARNING: 'cipher' is used inconsistently, local='cipher AES-256-CBC', remote='cipher BF-CBC'

Solution

Server uses AES-256-CBC, while the client is using BF-CBC. Adjust the client configuration in client.conf. Insert cipher AES-256-CBC in client.conf

Other parameters to adjust

During first startup, some warning message may be written on the server log. Most common they refer to link-mtu, cipher, keysize or comp-lzo.

WARNING: 'link-mtu' is used inconsistently, local='link-mtu 1557', remote='link-mtu 1542'
WARNING: 'keysize' is used inconsistently, local='keysize 256', remote='keysize 128'
WARNING: 'comp-lzo' is present in remote config but missing in local config, remote='comp-lzo'

Solution

Adjust the parameters in the client.conf file so that they match the server configuration. Also good to check this way if a not controlled/configured client is connecting to your server.

Link-mtu

Configure the client to use the same mtu size as the server. Insert parameter link-mtu into client.conf.

link-mtu 1557

Keysize

Keysize used by client and server should be the same. Insert parameter keysize into client.conf.

keysize 256

Comp-lzo

Uncomment the parameter in server.conf.

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OpenVPN connection test

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After configuring and running both the OpenVPN server and client, it’s a good idea to test if the VPN is working. This involves some tests on both the server and client.

OpenVPN Server

Network Device

After the server is started, a new interface should be created. Run ifconfig to get a list of all available interfaces. In case tun is configured in the conf file as device type, a new interface with name tun0 is created.

ifconfig

Check server log for client connection

In case OpenVPN is started as a service, the log can be found at /var/log/messages. If you start it directly on the command line, the log will be shown on the shell. When a client connects, the log of the server shows the connection information.

tail -f /var/log/messages

The last lines show client1, meaning that the client not only connected, but is also correctly identified as client1. The connection is working.

OpenVPN client

Start OpenVPN and the client will try to connect to the server specified in the client.conf file. Client connecting and receiving IP.

openvpn /etc/openvpn/client.conf
tail -f /var/log/messages

After the connection was established, the client is also creating a new interface named tun0. Here a client named client1 connects and receives the IP 10.8.0.6.

ifconfig

Connection test

Easiest way to test that client and server can talk to each other is to ping both. Just run a ping from the server to the client IP, and from the client to the server IP. For this, the VPN IP address must be used (e.g. 10.8.0.x).

OpenVPN server

Ping client1 from server.

ping 10.8.0.6

OpenVPN client

Ping server from client.

ping 10.8.0.1

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