libnftables - Man Page

These functions aid in nft context management. In order to make use of the library, at least one context object has to be allocated. The context holds temporary data such as caches, library configuration and (if enabled) output and error buffers.

The nft_ctx_new() function allocates and returns a new context object. The parameter flags is unused at this point and should be set to zero. For convenience, the macro NFT_CTX_DEFAULT is defined to that value.

The nft_ctx_free() function frees the context object pointed to by ctx, including any caches or buffers it may hold.

Dry-run setting controls whether ruleset changes are actually committed on kernel side or not. It allows one to check whether a given operation would succeed without making actual changes to the ruleset. The default setting is false.

The nft_ctx_get_dry_run() function returns the dry-run setting’s value contained in ctx.

The nft_ctx_set_dry_run() function sets the dry-run setting in ctx to the value of dry.

The flags setting controls the input format.

enum {
        NFT_CTX_INPUT_NO_DNS = (1 << 0),
        NFT_CTX_INPUT_JSON   = (1 << 1),
};

NFT_CTX_INPUT_NO_DNS

Avoid resolving IP addresses with blocking getaddrinfo(). In that case, only plain IP addresses are accepted.

NFT_CTX_INPUT_JSON: When parsing the input, first try to interpret the input as JSON before falling back to the nftables format. This behavior is implied when setting the NFT_CTX_OUTPUT_JSON flag.

The nft_ctx_input_get_flags() function returns the input flags setting’s value in ctx.

The nft_ctx_input_set_flags() function sets the input flags setting in ctx to the value of val and returns the previous flags.

The flags setting controls the output format.

enum {
        NFT_CTX_OUTPUT_REVERSEDNS     = (1 << 0),
        NFT_CTX_OUTPUT_SERVICE        = (1 << 1),
        NFT_CTX_OUTPUT_STATELESS      = (1 << 2),
        NFT_CTX_OUTPUT_HANDLE         = (1 << 3),
        NFT_CTX_OUTPUT_JSON           = (1 << 4),
        NFT_CTX_OUTPUT_ECHO           = (1 << 5),
        NFT_CTX_OUTPUT_GUID           = (1 << 6),
        NFT_CTX_OUTPUT_NUMERIC_PROTO  = (1 << 7),
        NFT_CTX_OUTPUT_NUMERIC_PRIO   = (1 << 8),
        NFT_CTX_OUTPUT_NUMERIC_SYMBOL = (1 << 9),
        NFT_CTX_OUTPUT_NUMERIC_TIME   = (1 << 10),
        NFT_CTX_OUTPUT_NUMERIC_ALL    = (NFT_CTX_OUTPUT_NUMERIC_PROTO |
                                         NFT_CTX_OUTPUT_NUMERIC_PRIO  |
                                         NFT_CTX_OUTPUT_NUMERIC_SYMBOL |
                                         NFT_CTX_OUTPUT_NUMERIC_TIME),
        NFT_CTX_OUTPUT_TERSE          = (1 << 11),
};

NFT_CTX_OUTPUT_REVERSEDNS

Reverse DNS lookups are performed for IP addresses when printing. Note that this may add significant delay to list commands depending on DNS resolver speed.

NFT_CTX_OUTPUT_SERVICE

Print port numbers as services as described in the /etc/services file.

NFT_CTX_OUTPUT_STATELESS

If stateless output has been requested, then stateful data is not printed. Stateful data refers to those objects that carry run-time data, e.g. the counter statement holds packet and byte counter values, making it stateful.

NFT_CTX_OUTPUT_HANDLE

Upon insertion into the ruleset, some elements are assigned a unique handle for identification purposes. For example, when deleting a table or chain, it may be identified either by name or handle. Rules on the other hand must be deleted by handle, because there is no other way to uniquely identify them. This flag makes ruleset listings include handle values.

NFT_CTX_OUTPUT_JSON

If enabled at compile-time, libnftables accepts input in JSON format and is able to print output in JSON format as well. See libnftables-json(5) for a description of the supported schema. This flag enables JSON output format. If the flag is set, the input will first be tried as JSON format, before falling back to nftables format. This flag implies NFT_CTX_INPUT_JSON.

NFT_CTX_OUTPUT_ECHO

The echo setting makes libnftables print the changes once they are committed to the kernel, just like a running instance of nft monitor would. Amongst other things, this allows one to retrieve an added rule’s handle atomically.

NFT_CTX_OUTPUT_GUID

Display UID and GID as described in the /etc/passwd and /etc/group files.

NFT_CTX_OUTPUT_NUMERIC_PROTO

Display layer 4 protocol numerically.

NFT_CTX_OUTPUT_NUMERIC_PRIO

Display base chain priority numerically.

NFT_CTX_OUTPUT_NUMERIC_SYMBOL

Display expression datatype as numeric value.

NFT_CTX_OUTPUT_NUMERIC_TIME

Display time, day and hour values in numeric format.

NFT_CTX_OUTPUT_NUMERIC_ALL

Display all numerically.

NFT_CTX_OUTPUT_TERSE

If terse output has been requested, then the contents of sets are not printed.

The nft_ctx_output_get_flags() function returns the output flags setting’s value in ctx.

The nft_ctx_output_set_flags() function sets the output flags setting in ctx to the value of val.

Libnftables supports separate debugging of different parts of its internals. To facilitate this, debugging output is controlled via a bit mask. The bits are defined as such:

enum nft_debug_level {
        NFT_DEBUG_SCANNER               = 0x1,
        NFT_DEBUG_PARSER                = 0x2,
        NFT_DEBUG_EVALUATION            = 0x4,
        NFT_DEBUG_NETLINK               = 0x8,
        NFT_DEBUG_MNL                   = 0x10,
        NFT_DEBUG_PROTO_CTX             = 0x20,
        NFT_DEBUG_SEGTREE               = 0x40,
};

NFT_DEBUG_SCANNER

Print LEX debug output.

NFT_DEBUG_PARSER

Print YACC debug output.

NFT_DEBUG_EVALUATION

Print debug information about evaluation phase.

NFT_DEBUG_NETLINK

Print netlink debug output.

NFT_DEBUG_MNL

Print libmnl debug output.

NFT_DEBUG_PROTO_CTX

Print protocol context debug output.

NFT_DEBUG_SEGTREE

Print segtree (i.e. interval sets) debug output.

The nft_ctx_output_get_debug() function returns the debug output setting’s value in ctx.

The nft_ctx_output_set_debug() function sets the debug output setting in ctx to the value of mask.

By default, any output from the library (e.g., after a list command) is written to stdout and any error messages are written to stderr. To give applications control over them, there are functions to assign custom file pointers as well as having the library buffer what would be written for later retrieval in a static buffer. This buffer is guaranteed to be null-terminated and must not be freed. Note that the retrieval functions rewind the buffer position indicator. Further library output will probably overwrite the buffer content and potentially render it invalid (due to reallocation).

The nft_ctx_set_output() and nft_ctx_set_error() functions set the output or error file pointer in ctx to the value of fp. They return the previous value to aid in temporary file pointer overrides. On error, these functions return NULL. This happens only if fp is NULL or invalid (tested using ferror() function).

The nft_ctx_buffer_output() and nft_ctx_buffer_error() functions enable library standard or error output buffering. The functions return zero on success, non-zero otherwise. This may happen if the internal call to fopencookie() failed.

The nft_ctx_unbuffer_output() and nft_ctx_unbuffer_error() functions disable library standard or error output buffering. On failure, the functions return non-zero which may only happen if buffering was not enabled at the time the function was called.

The nft_ctx_get_output_buffer() and nft_ctx_get_error_buffer() functions return a pointer to the buffered output (which may be empty).

The include command in nftables rulesets allows one to outsource parts of the ruleset into a different file. The include path defines where these files are searched for. Libnftables allows one to have a list of those paths which are searched in order. The default include path list contains a single compile-time defined entry (typically /etc/).

The nft_ctx_add_include_path() function extends the list of include paths in ctx by the one given in path. The function returns zero on success or non-zero if memory allocation failed.

The nft_ctx_clear_include_paths() function removes all include paths, even the built-in default one.

The define command in nftables ruleset allows one to define variables.

The nft_ctx_add_var() function extends the list of variables in ctx. The variable must be given in the format key=value. The function returns zero on success or non-zero if the variable is malformed.

The nft_ctx_clear_vars() function removes all variables.

These functions perform the actual work of parsing user input into nftables commands and executing them.

The nft_run_cmd_from_buffer() function passes the command(s) contained in buf (which must be null-terminated) to the library, respecting settings and state in nft.

The nft_run_cmd_from_filename() function passes the content of filename to the library, respecting settings and state in nft.

Both functions return zero on success. A non-zero return code indicates an error while parsing or executing the command. This event should be accompanied by an error message written to library error output.