Cis Bind V100 4 1

CIS 4.1 — Use TSIG Keys 256 Bits in Length

Profile Applicability

• Authoritative Name Server Level 1
• Caching Only Name Server Level 1

Description

The TSIG secret keys used by the name server should be generated from a good source of entropy and should be at least 256 bits in length.

Rationale

Weak cryptographic keys may allow cryptographic attacks to discover the key value, through repeated guesses. A strong HMAC key requires a good source of entropy and at least 256 bits in length.

Impact

Not specified in the PDF.

Audit Procedure

To check if the secret key is at least 256 bits long before encoding then the base64 encoded value should be 44 characters long or longer with the padding and a trailing =. To easily count the number of characters, copy the key value into the quoted value in the command below.

$ echo -n "ezoZopbE4Q73HShuFYlf3FRvLWjtNXI5fd0TeQAYOug=" | wc -c
44

Remediation

For remediation, replace any keys which are too short with a securely generated key with a length of 256 or 512. The tsig-keygen command below can be used to generate a key.

# tsig-keygen -a sha256 ns1-ns4.example.net > ns1-ns4.example.net.key
# cat ns1-ns4.example.net.key
key "ns1-ns4.example.net" {
        algorithm hmac-sha256;
        secret "ezoZopbE4Q73HShuFYlf3FRvLWjtNXI5fd0TeQAYOug=";
};

Ensure the key file has the appropriate file permissions, and include the file in the named configuration file.

Default Value

The rndc key is generated as 256 bits during bind-utils package install, and the nsupdate session key is dynamically generated with a length of 256 bits.

References

Not specified in the PDF.

CIS Controls

MITRE ATT&CK Mappings

Profile

• Level 1 - Authoritative Name Server
• Level 1 - Caching Only Name Server