netcdf-c/docs/esg.md

Accessing ESG Data Through netCDF

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Introduction

It is possible to access Earth Systems Grid (ESG) datasets from ESG servers through the netCDF API. This requires building the netCDF library with DAP2 protocol support using the "--enable-dap" flag to the configure program.

In order to access ESG datasets, however, it is necessary to register as a user with ESG and to setup your environment so that proper authentication is established between a netCDF client program and the ESG data server. Specifically, it is necessary to use what is called "client-side keys" to enable this authentication. Normally, when a client accesses a server in a secure fashion (using "https"), the server provides an authentication certificate to the client. With client-side keys, the client must also provide a certificate to the server so that the server can know with whom it is communicating.

It is possible to set up the netCDF library to use client-side keys, although the process is somewhat complicated. The DAP2 support in netCDF uses the curl library and it is that underlying library that must be properly configured.

Terminology

The key elements for client-side keys requires the constructions of two "stores" on the client side.

• Keystore - a repository to hold the client side key.
• Truststore - a repository to hold a chain of certificates that can be used to validate the certificate sent by the server to the client.

The server actually has a similar set of stores, but the client need not be concerned with those.

Initial Steps

The first step is to obtain authorization from ESG. Note that this information may evolve over time, and may be out of date. This discussion is in terms of BADC ESG. You will need to substitute the ESG site for BADC in the following.

1. Register at http://badc.nerc.ac.uk/register to obtain access to badc and to obtain an openid, which will looks something like:

   https://ceda.ac.uk/openid/Firstname.Lastname
   

2. Ask BADC for access to whatever datasets are of interest.

3. Obtain short term credentials at http://grid.ncsa.illinois.edu/myproxy/MyProxyLogon/ You will need to download and run the MyProxyLogon program. This will create a keyfile in, typically, the directory globus. The keyfile will have a name similar to this: x509up_u13615 The other elements in ~/.globus are certificates to use in validating the certificate your client gets from the server.

4. Obtain the program source ImportKey.java from this location: http://www.agentbob.info/agentbob/79-AB.html (read the whole page, it will help you understand the remaining steps).

Building the KeyStore

You will have to modify the keyfile in the previous step and then create a keystore and install the key and a certificate. The commands are these:

    openssl pkcs8 -topk8 -nocrypt -in x509up_u13615 -inform PEM -out key.der -outform DER

    openssl x509 -in x509up_u13615 -inform PEM -out cert.der -outform DER

    java -classpath  -Dkeypassword="" -Dkeystore=./ key.der cert.der

Note, the file names "key.der" and "cert.der" can be whatever you choose. It is probably best to leave the .der extension, though.

Building the TrustStore

Building the truststore is a bit tricky because as provided, the certificates in "globus" need some massaging. See the script below for the details. The primary command is this, which is executed for every certificate, c, in globus. It sticks the certificate into the file named "truststore"

  keytool -trustcacerts -storepass "password" -v -keystore "truststore"  -importcert -file "${c}"

Running the C Client

The file ".dodsrc" is used to configure curl. This file must reside either in the current directory or in your home directory. It has lines of the form

• KEY=VALUE, or

• http//x.y/

The first form provides a configuration parameter that applies to all DAP2 accesses. The second form only applies to accesses to the server at "x.y".

The following keys must be set in ".dodsrc" to support ESG access.

• HTTP.SSL.VALIDATE=1
• HTTP.COOKIEJAR=.dods_cookies
• HTTP.SSL.CERTIFICATE=esgkeystore
• HTTP.SSL.KEY=esgkeystore
• HTTP.SSL.CAPATH=.globus

For ESG, the HTTP.SSL.CERTIFICATE and HTTP.SSL.KEY entries should have same value, which is the file path for the certificate produced by MyProxyLogon. The HTTP.SSL.CAPATH entry should be the path to the "certificates" directory produced by MyProxyLogon.

Running the Java Client

If you are using the Java netCDF client, then you need to add some parameters to the "java" command. Specifically, add the following flags.

   -Dkeystore="path to keystore file" -Dkeystorepassword="keystore password"

Script for creating Stores

The following script shows in detail how to actually construct the key and trust stores. It is specific to the format of the globus file as it was when ESG support was first added. It may have changed since then, in which case, you will need to seek some help in fixing this script. It would help if you communicated what you changed to the author so this document can be updated.

#!/bin/sh -x
KEYSTORE="esgkeystore"
TRUSTSTORE="esgtruststore"
GLOBUS="globus"
TRUSTROOT="certificates"
CERT="x509up_u13615"
TRUSTROOTPATH="$GLOBUS/$TRUSTROOT"
CERTFILE="$GLOBUS/$CERT"
PWD="password"

D="-Dglobus=$GLOBUS"
CCP="bcprov-jdk16-145.jar"
CP="./build:${CCP}"
JAR="myproxy.jar"

# Initialize needed directories
rm -fr build
mkdir build
rm -fr $GLOBUS
mkdir $GLOBUS
rm -f $KEYSTORE
rm -f $TRUSTSTORE

# Compile MyProxyCmd and ImportKey
javac -d ./build -classpath "$CCP" *.java
javac -d ./build ImportKey.java

# Execute MyProxyCmd
java -cp "$CP myproxy.MyProxyCmd

# Build the keystore
openssl pkcs8 -topk8 -nocrypt -in $CERTFILE -inform PEM -out key.der -outform DER
openssl x509 -in $CERTFILE -inform PEM -out cert.der -outform DER
java -Dkeypassword=$PWD -Dkeystore=./${KEYSTORE} -cp ./build ImportKey key.der cert.der

# Clean up the certificates in the globus directory
for c in ${TRUSTROOTPATH}/*.0 ; do
    alias=`basename $c .0`
    sed -e '0,/---/d' <$c >/tmp/${alias}
    echo "-----BEGIN CERTIFICATE-----" >$c       
    cat /tmp/${alias} >>$c
done

# Build the truststore
for c in ${TRUSTROOTPATH}/*.0 ; do
    alias=`basename $c .0`
    echo "adding: $TRUSTROOTPATH/${c}"
    echo "alias: $alias"
    yes | keytool -trustcacerts -storepass "$PWD" -v -keystore ./$TRUSTSTORE -alias $alias -importcert -file "${c}"
done
exit

Change Log

Version 1.0:

• 10/17/2013 – Initial Release

Document Information

---

Created: 10/17/2013

Last Revised: 10/17/2013\

Version: 1.0

Author: Dennis Heimbigner

Affiliation: Unidata/UCAR

email: dmh@unida.ucar.edu