Default Preferences
From v2.9.0 of DiscrimiNAT, certain behavioural preferences can be set at global level per deployment. These are defined in JSON format in a Secret named DiscrimiNAT_<deployment_id> (case-sensitive) in Secret Manager and the default value if not defined or not accessible is:
{
"%default": {
"allowlist": {
"tls": null,
"ssh": null
},
"wildcard_exposure": "prohibit_public_suffix",
"flow_log_verbosity": "full",
"see_thru": null,
"x509_crls": "ignore",
"starttls": {
"smtp": {
"posture": "closed",
"ehlo_entropy_limit": 0
}
}
}
}
Preferences you wish to leave at defaults can be excluded. Also, any additional JSON keys will simply be ignored.
Preferences are refreshed once every five minutes from the Secret.
Preferences
allowlist
This JSON key is the parent for nested, protocol-specific default allowlists. A default allowlist is a list of allowed addresses that applies to workloads not discovered by DiscrimiNAT or that have no Firewall Rule level allowlist specification. The only protocols supported at this time are tls and ssh.
Default
"allowlist":{"tls":null,"ssh":null}
allowlist.tls
This nested key under allowlist sets the default list of allowed addresses (FQDNs or IP addresses) over the TLS protocol for all workloads. The value is supposed to be the full path of a Secret containing a list of addresses. See the Secret Path variant docs for more.
Example
{"tls":"projects/000000000000/secrets/foo-default-allowed-fqdns"}
allowlist.ssh
Same as tls above but for the SSH protocol.
wildcard_exposure
Generally, ? represents one character match exactly which isn't a ., and * matches any number of characters except ..
Options
none: no wildcard characters such as ? or * are allowed at all
prohibit_asterisk: ? is allowed; * is not allowed
prohibit_public_suffix: ? is allowed; * is allowed as long as the possibility of resulting domain names are not publicly registrable (aka Effective TLDs). For example, *.github.com is okay but *.github.io is not. Another example would be *.cloudflare.net being effectively public. This is determined using Mozilla's Public Suffix List (PSL) bundled into DiscrimiNAT. The version of the PSL bundled will be in the Release Notes and will be updated with every release.
If a wildcard pattern is not accepted because it could have led to a Public Suffix List match, a config log message like the following will be emitted:
{addr: "*.github.io", cat: "addr", outcome: "publicsuffix[.]org list matched with `github.io`"}
nuclear: ? is allowed; * is allowed with no public suffix list safeguard
Default
prohibit_public_suffix
flow_log_verbosity
This setting only impacts the flow logs. config logs verbosity and behaviour will not be affected.
Options
full: all allowed and disallowed flows for both the client requests and the server responses are logged in the flow logs
only_disallowed: only disallowed flows will be logged
none: no logs at all
Default
full
see_thru
The see_thru non-blocking, monitoring mode can be enabled for an entire deployment rather than per Firewall Rule from this level. This is only the default and does not override a see-thru mode defined at a more granular level – even if the one the granular level may have expired.
See see-thru mode docs for understanding this non-blocking, monitoring mode in detail.
Options
yyyy-mm-dd formatted date
null to not enable see_thru at the default level. null value to be specified without quotes or this top-level key can be left out completely.
Default
null
x509_crls
Options
auto_allow: Automatically allow plaintext HTTP CRL Endpoints of x509 SSL certificates for all TLS FQDNs allowlisted.
ignore: Do not allow CRL Endpoints automatically.
Default
ignore
starttls
This JSON key is the parent for nested, protocol-specific StartTLS preferences. The only StartTLS protocol supported at this time is smtp.
Default
"smtp":{"posture":"closed","ehlo_entropy_limit":0}
starttls.smtp
See the StartTLS SMTP guide for safely enabling this capability.
This nested key under starttls controls the posture and limits for the SMTP protocol's StartTLS commands, i.e. the plaintext communication before the connection upgrades to TLS.
Default
{"posture":"closed","ehlo_entropy_limit":0}
starttls.smtp.posture
This nested key under starttls.smtp sets the deployment-wide posture for DiscrimiNAT to let StartTLS SMTP plaintext commands pass through or not. This includes messages received from an SMTP server beginning with 220 and 250, and messages sent by a client beginning with EHLO and STARTTLS.
Options
closed: StartTLS SMTP plaintext commands are not allowed.
open: StartTLS SMTP plaintext messages received from an SMTP server beginning with 220 and 250, and messages sent by a client beginning with EHLO and STARTTLS are allowed.
Default
closed
starttls.smtp.ehlo_entropy_limit
This nested key under starttls.smtp sets the deployment-wide Shannon Entropy limit of the argument to the EHLO command in EHLO messages sent by any client to the Internet. For example, in the StartTLS command EHLO mail.example.com\r\n, the argument is mail.example.com of which the Shannon Entropy is 3.20.
EHLO messages sent by a client exceeding this limit will be disallowed by DiscrimiNAT.
Shannon Entropy is rounded to two decimal places with the MidpointAwayFromZero strategy. For example, 3.2028 becomes 3.20, 3.25 remains 3.25 and 3.256 becomes 3.26.
You can calculate the Shannon Entropy of any text input using GCHQ's CyberChef Entropy tool set to the Shannon scale.
Shannon Entropy is a measure of the average amount of information contained in a message. It can range from 0 to 8, where the higher the number the more 'information' it can pack by the way of encoding.
Higher numbers can be indicative of exfiltration via the EHLO argument. See the StartTLS SMTP guide for safely enabling this capability.
Range
Minimum: 0.00
Maximum: 8.00
Default
0
Examples
It may be useful to turn on the non-blocking, monitoring see_thru mode on until a specified date on a new deployment of DiscrimiNAT. This would ensure apps continue to work while data is captured for building an allowlist from eventually:
{"%default":{"see_thru":"2026-11-19"}}
Ultimately, when allowlists are built and enforced, you may want to turn off flow logs for allowed connections, saving on logging costs:
{"%default":{"flow_log_verbosity":"only_disallowed"}}
To also enable automatic allowing of X509 CRL Endpoints:
{"%default":{"flow_log_verbosity":"only_disallowed","x509_crls":"auto_allow"}}
System
System-level preferences are passed directly to VM instance creation, are stored as files on the operating system disk, and are only read on start.
These are placed within the instance because they represent critical customer choices that must be respected whether external APIs are accessible or not and the right IAM permissions on the Service Account are assigned or not.
BYOL
A licence key supplied by Chaser Support if using a BYOL version.
The value can be passed to the byol variable in our Terraform modules. The contents are Base64 decoded and written to the path /etc/chaser/licence-key.der.
ASHR
Automated System Health Reporting.
10 minutes after boot and then at around 0200 UTC every day, each instance of DiscrimiNAT collects its OS internals & system logs since instance creation, config changes & traffic flow information from last two hours and uploads it to a Chaser-owned cloud bucket. This information is encrypted at rest with a certain public key so only relevant individuals with access to the corresponding private key can decrypt it. The transfer is encrypted over TLS.
Access to this information is immensely useful to create a faster and more reliable DiscrimiNAT as we add new features. We also get to learn about how users interact with the product in order to further improve the usability of it as they embark on a very ambitious journey of fully accounted for and effective egress controls.
We understand if certain environments within your deployment would rather not have this turned on. To disable it, a file at the path /etc/chaser/disable_automated-system-health-reporting should exist.
This can be achieved by setting the ashr variable to false in our Terraform modules.