Automated certreq for GNU/Linux

Last updated 2018-11-14

Background

Microsoft provides the certreq utility for its non-free operating system. This tool makes it easy to get certificates from the Microsoft sub-CA on your network.

GNU Linux hosts do not get that tool, so a viable alternative is to script the interaction with the website. My content in this post is shamelessly ripped from a StackOverflow post and beefed up.

In my research, I came across a question: “How to submit certificate request from red hat to windows ca“.

The solution

Dependencies

Certreq needs the framework.sh shell library available from my bgscripts package. At the very least, you need framework.sh, which you can place either in the same directory as certreq.sh or you can modify the framework lookup paths in the script to point to where you placed it.
This will solve the issue “certreq.sh: validateparams: not found.”

The script

I present my shell script, certreq.sh.
This shell script:

  • Generates CSR and submits it to the Microsoft Sub-CA.
  • Saves private key, public key (the certificate), and cert chain to a temporary directory
  • Removes the temp directory after 5 minutes automatically to remove the private key
  • Sends to standard out the file names and purposes, for consumption by automation tool, e.g., ansible

Code walkthrough

Instead of copying and pasting the whole code here, I will discuss only snippets.
Here is the usage block.

usage: certreq.sh [-dhV] [-u username] [-p password] [-w tempdir] [-t template] [--cn CN] [--ca ]
version ${certreqversion}
 -d debug   Show debugging info, including parsed variables.
 -h usage   Show this usage block.
 -V version Show script version number.
 -u username User to connect via ntlm to CA. Can be "username" or "domain\\username"
 -p password
 -w workdir  Temp directory to work in. Default is a (mktemp -d).
 -t template Template to request from CA. Default is "ConfigMgrLinuxClientCertificate"
 --cn        CN to request. Default is the \$( hostname -f )
 --ca        CA hostname or base URL. Example: ca2.example.com
Return values under 1000: A non-zero value is the sum of the items listed here:
 0 Everything worked
 1 Cert file is still a CSR
 2 Cert file is html, probably due to permissions/credentials issue
 4 Return code of curl statement that saves cert file is non-zero
 8 Cert file does not contain whole certificate
16 Cert does not contain an issuer
Return values above 1000:
1001 Help or version info displayed
1002 Count or type of flaglessvals is incorrect
1003 Incorrect OS type
1004 Unable to find dependency
1005 Not run as root or sudo

All the magic happens at line 239, the main loop. These blocks perform the different web requests, and are the real meat of this script.

Block GENERATE PRIVATE KEY makes the csr and saves in to the file that will eventually hold the cert.

   # GENERATE PRIVATE KEY
   openssl req -new -nodes \
      -out "${CERTREQ_WORKDIR}/${CERTREQ_CNPARAM}.crt" \
      -keyout "${CERTREQ_WORKDIR}/${CERTREQ_CNPARAM}.key" \
      -subj "${CERTREQ_SUBJECT}"
   CERT="$( cat "${CERTREQ_WORKDIR}/${CERTREQ_CNPARAM}.crt" | tr -d '\n\r' )"
   DATA="Mode=newreq&CertRequest=${CERT}&C&TargetStoreFlags=0&SaveCert=yes"
   CERT="$( echo ${CERT} | sed -e 's/+/%2B/g' | tr -s ' ' '+' )"
   CERTATTRIB="CertificateTemplate:${CERTREQ_TEMPLATE}"

SUBMIT CERTIFICATE SIGNING REQUEST submits the CSR to the website

   # SUBMIT CERTIFICATE SIGNING REQUEST
   OUTPUTLINK="$( curl -k -u "${CERTREQ_USER}:${CERTREQ_PASS}" --ntlm \
      "${CERTREQ_CA}/certsrv/certfnsh.asp" \
      -H 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' \
      -H 'Accept-Encoding: gzip, deflate' \
      -H 'Accept-Language: en-US,en;q=0.5' \
      -H 'Connection: keep-alive' \
      -H "Host: ${CERTREQ_CAHOST}" \
      -H "Referer: ${CERTREQ_CA}/certsrv/certrqxt.asp" \
      -H 'User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko' \
      -H 'Content-Type: application/x-www-form-urlencoded' \
      --data "Mode=newreq&CertRequest=${CERT}&CertAttrib=${CERTATTRIB}&TargetStoreFlags=0&SaveCert=yes&ThumbPrint=" | grep -A 1 'function handleGetCert() {' | tail -n 1 | cut -d '"' -f 2 )"
   CERTLINK="${CERTREQ_CA}/certsrv/${OUTPUTLINK}"

FETCH SIGNED CERTIFICATE downloads the cert that the previous page links to.

   # FETCH SIGNED CERTIFICATE
   curl -k -u "${CERTREQ_USER}:${CERTREQ_PASS}" --ntlm $CERTLINK \
      -H 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' \
      -H 'Accept-Encoding: gzip, deflate' \
      -H 'Accept-Language: en-US,en;q=0.5' \
      -H 'Connection: keep-alive' \
      -H "Host: ${CERTREQ_CAHOST}" \
      -H "Referer: ${CERTREQ_CA}/certsrv/certrqxt.asp" \
      -H 'User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko' \
      -H 'Content-Type: application/x-www-form-urlencoded' > "${CERTREQ_WORKDIR}/${CERTREQ_CNPARAM}.crt"
   finaloutput=$?

My additions to this secret sauce start with GET NUMBER OF CURRENT CA CERT. I needed the cert chain, so I automated fetching it from the server.
You have to find out how many different CA certs are being offered by this server, and then use the latest.

   # GET NUMBER OF CURRENT CA CERT
   RESPONSE="$( curl -s -k -u "${CERTREQ_USER}:${CERTREQ_PASS}" --ntlm \
      "${CERTREQ_CA}/certsrv/certcarc.asp" \
      -H 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' \
      -H 'Accept-Encoding: gzip, deflate' \
      -H 'Accept-Language: en-US,en;q=0.5' \
      -H 'Connection: keep-alive' \
      -H "Host: ${CERTREQ_CAHOST}" \
      -H "Referer: ${CERTREQ_CA}/certsrv/certrqxt.asp" \
      -H 'User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko' \
      -H 'Content-Type: application/x-www-form-urlencoded' )"
   CURRENTNUM="$( echo "${RESPONSE}" | grep -cE 'Option' )"

   # GET LATEST CA CERT CHAIN
   CURRENT_P7B="$( curl -s -k -u "${CERTREQ_USER}:${CERTREQ_PASS}" --ntlm \
      "${CERTREQ_CA}/certsrv/certnew.p7b?ReqID=CACert&Renewal=${CURRENTNUM}" \
      -H 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' \
      -H 'Accept-Encoding: gzip, deflate' \
      -H 'Accept-Language: en-US,en;q=0.5' \
      -H 'Connection: keep-alive' \
      -H "Host: ${CERTREQ_CAHOST}" \
      -H "Referer: ${CERTREQ_CA}/certsrv/certrqxt.asp" \
      -H 'User-Agent: Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko' \
      -H 'Content-Type: application/x-www-form-urlencoded' )"

   # CONVERT TO PEM
   echo "${CURRENT_P7B}" | openssl pkcs7 -print_certs -out "${CERTREQ_TEMPFILE}"

I like having the domain name in the filename, so this last part renames the cert chain.

   # RENAME TO PROPER FILENAME
   # will read only the first cert, so get domain of issuer of it.
   CA_DOMAIN="$( openssl x509 -in "${CERTREQ_TEMPFILE}" -noout -issuer 2>&1 | sed -r -e 's/^.*CN=[A-Za-z0-9]+\.//;' )"
   CHAIN_FILE="chain-${CA_DOMAIN}.crt"
   mv -f "${CERTREQ_TEMPFILE}" "${CERTREQ_WORKDIR}/${CHAIN_FILE}" 1>/dev/null 2>&1

The ansible role

I needed this task deployed to my whole environment, so I rolled it into an ansible role saved to gitlab and also added another feature, where it converts the generated cert files into a pcks12 (pfx) file for a specific application’s need.

References

Weblinks

  1. https://technet.microsoft.com/en-us/library/dn296456(v=ws.11).aspx
  2. https://serverfault.com/questions/538270/how-to-submit-certificate-request-from-red-hat-to-windows-ca
  3. https://stackoverflow.com/questions/31283476/submitting-base64-csr-to-a-microsoft-ca-via-curl/39722983#39722983
  4. https://gitlab.com/bgstack15/certreq/blob/master/files/certreq.sh
  5. https://gitlab.com/bgstack15/certreq
  6. Manipulating ssl certificates
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