This is the first part for Linux machines. The second part for Windows machines will follow soon.

The Graylog documentation unfortunately reads very contradictory and the Graylog error messages are hard to understand.‌‌‌‌The Graylog documentation unfortunately reads very contradictory and the Graylog error messages are hard to understand.‌‌To save others the trouble, here is a small howto:

Create shadow CA:

‌

apt install openssl 
mkdir /etc/graylog/server/ssl 
cd /etc/graylog/ssl 
# Create CA Keys 
openssl genrsa -aes256 -out graylog-ca-key.pem 2048 
openssl req -x509 -new -nodes -extensions v3_ca \
-key graylog-ca-key.pem -days 1024 \
-out graylog-ca-root.crt -sha512

Activate your shadow root CA.‌‌Local CA repo on Debian/Ubuntu is /usr/local/share/ca-certificates

mv graylog-ca-root.crt \
/usr/local/share/ca-certificates/ 
# Activate your root CA: 
update-ca-certificates

Activa root CA

Graylog needs a private key in PKCS8 format

openssl pkcs8 -in graylog.key -topk8 -out graylogPKCS8.key

Copy your JAVA keystore to /etc/graylog/server/cacerts.jks

[ -f /usr/lib/jvm/java-11-openjdk-amd64/lib/security/cacerts  ] \
&& cp /usr/lib/jvm/java-11-openjdk-amd64/lib/security/cacerts \
/etc/graylog/server/cacerts.jks

Please note that in this example Java Open JDK 11 is used. The paths have to be adapted for 14 or later.

Import your shadow root CA to Elasticsearch:

keytool -importcert -alias graylogCA \
-keystore /etc/graylog/server/cacerts.jks \
-storepass changeit \
-file \
/usr/local/share/ca-certificates/graylog-ca-root.crt

Create Graylog Input

Create a new SSL TLS input in Graylog. Enclosed are the settings as text and as graphic.

    allow_override_date: true
    bind_address: 0.0.0.0
    expand_structured_data: false
    force_rdns: false
    max_message_size: 2097152
    number_worker_threads: 2
    override_source: <empty>
    port: 1515
    recv_buffer_size: 1048576
    store_full_message: false
    tcp_keepalive:     false
    tls_cert_file: /etc/graylog/ssl/graylog.crt
    tls_client_auth: disabled
    tls_client_auth_cert_file: <empty>
    tls_enable: true
    tls_key_file: /etc/graylog/ssl/graylogPKCS8.key
    tls_key_password: ********
    use_null_delimiter: false

Client configuration

Copy the created Shadow Root CA to each machine that should send logs to Graylog (e.g. with SCP).

Install rsyslog and rsyslog-gnutls

apt install rsyslog rsylog-gnutls

Configure graylog as a remote syslog server.

Create a new configuration file:

touch /etc/rsyslog.d/graylog.conf

Add the following settings.
Replace GRAYLOGSERVERIP with your own server IP.:

*.*  action(
   Action.resumeInterval="10"
   # cycling TCP connections allows for load balancing
   RebindInterval="10000"            
   Queue.Size="100000"
   Queue.DiscardMark="97500"
   Queue.HighWaterMark="80000"
   Queue.Type="LinkedList"
   Queue.FileName="rsyslogqueue"
   Queue.CheckpointInterval="100"
   Queue.MaxDiskSpace="2g"
   Action.ResumeRetryCount="-1"
   Queue.SaveOnShutdown="on"
   Queue.TimeoutEnqueue="10"
   Queue.DiscardSeverity="0"
   type="omfwd"
   target="GRAYLOGSERVERIP"
   protocol="tcp"
   port="1515"
   template="RSYSLOG_SyslogProtocol23Format"
   StreamDriver="gtls"
   # run driver in TLS-only mode
   StreamDriverMode="1"
   # host TLS cert CN will be used for authentication
   StreamDriverAuthMode="x509/name"
   # only allowed hosts 
   StreamDriverPermittedPeers="GRAYLOGSERVERIP"
  )

Restart rsyslog:

systemctl restart rsyslog.service