DNS Tunneling / Long Subdomain Hunt

KQL detection rule by kyhomelab · MIT

Hunts for DNS queries with abnormally long subdomains, high entropy, or excessive query volume to a single parent domain — indicators of DNS tunneling, C2, or exfiltration.

// Title: DNS Tunneling / Long Subdomain Hunt
// Description: Hunts for DNS queries with abnormally long subdomains, high entropy, or excessive query volume to a single parent domain — indicators of DNS tunneling, C2, or exfiltration.
// Category: Security
// MITRE: T1071.004 (Application Layer Protocol: DNS), T1048.003 (Exfiltration Over Unencrypted Non-C2 Protocol)

DeviceNetworkEvents
| where TimeGenerated > ago(24h)
| where ActionType == "DnsConnectionInspected" or RemotePort == 53
| extend QueriedDomain = tostring(parse_json(AdditionalFields).query)
| where isnotempty(QueriedDomain)
// Extract the parent domain (last two labels — naive but works for most cases)
| extend Labels = split(QueriedDomain, ".")
| extend ParentDomain = strcat(tostring(Labels[array_length(Labels)-2]), ".", tostring(Labels[array_length(Labels)-1]))
| extend Subdomain = substring(QueriedDomain, 0, strlen(QueriedDomain) - strlen(ParentDomain) - 1)
| extend SubdomainLength = strlen(Subdomain)
// Flag long subdomains (typical tunneling chunks 30+ chars) and high label count
| where SubdomainLength >= 30 or array_length(Labels) >= 6
// Exclude well-known high-entropy services (CDN, telemetry) — tune for your environment
| where ParentDomain !endswith "akamai.net"
    and ParentDomain !endswith "cloudfront.net"
    and ParentDomain !endswith "azureedge.net"
    and ParentDomain !endswith "trafficmanager.net"
    and ParentDomain !endswith "windows.com"
    and ParentDomain !endswith "office.com"
| summarize
    QueryCount = count(),
    UniqueSubdomains = dcount(Subdomain),
    MaxSubdomainLength = max(SubdomainLength),
    AvgSubdomainLength = avg(SubdomainLength),
    SampleQueries = make_set(QueriedDomain, 5),
    FirstSeen = min(TimeGenerated),
    LastSeen = max(TimeGenerated)
    by DeviceName, ParentDomain
| where UniqueSubdomains >= 20 or MaxSubdomainLength >= 50
| extend Suspicion = case(
    UniqueSubdomains >= 100 and MaxSubdomainLength >= 50, "High",
    UniqueSubdomains >= 50 or MaxSubdomainLength >= 60, "Medium",
    "Low"
)
| order by UniqueSubdomains desc

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

2560c671-187c-430d-bfd9-8b3e8a81c879

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Executable code, not a declarative rule

This submission is arbitrary [[ currentRule.format ? currentRule.format.toUpperCase() : '' ]] code, not a declarative detection rule — review it as you would any script before running it, and note that it cannot be safely aggregated or auto-executed alongside YARA/Sigma/Suricata/CRS rules.

Rejected pattern present

[[ reason ]]

What does this mean?

Format & documentation suggestions

Not a security issue — this rule is valid and was accepted as-is. These are optional fields/conventions its own format recommends, mainly affecting its quality score and how well it documents itself for other reviewers.

[[ reason ]]

What do these messages mean?

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

[[ currentRule.title ]]

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