3.5.2.6 Ensure base chains exist - hook output

Information

Chains are containers for rules. They exist in two kinds, base chains and regular chains. A base chain is an entry point for packets from the networking stack, a regular chain may be used as jump target and is used for better rule organization.

Rationale:

If a base chain doesn't exist with a hook for input, forward, and delete, packets that would flow through those chains will not be touched by nftables.

Solution

Run the following command to create the base chains:

# nft create chain inet <table name> <base chain name> { type filter hook <(input|forward|output)> priority 0 ; }

Example:

# nft create chain inet filter input { type filter hook input priority 0 ; }

# nft create chain inet filter forward { type filter hook forward priority 0 ; }

# nft create chain inet filter output { type filter hook output priority 0 ; }

Impact:

If configuring nftables over ssh, creating a base chain with a policy of drop will cause loss of connectivity.

Ensure that a rule allowing ssh has been added to the base chain prior to setting the base chain's policy to drop

See Also

https://workbench.cisecurity.org/files/2948

Item Details

Category: SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|SC-7(15), CSCv7|9.4

Plugin: Unix

Control ID: 93e3ce2fcb6a954dad16501ce5611979b6126306c258a31e5044a37ad66b787c