libnftables

LIBNFTABLES(3)                                                  LIBNFTABLES(3)

NAME
       libnftables - nftables frontend library

SYNOPSIS
       #include <nftables/libnftables.h>

       struct nft_ctx *nft_ctx_new(uint32_t flags);
       void nft_ctx_free(struct nft_ctx *ctx);

       bool nft_ctx_get_dry_run(struct nft_ctx *ctx);
       void nft_ctx_set_dry_run(struct nft_ctx *ctx, bool dry);

       unsigned int nft_ctx_output_get_flags(struct nft_ctx *ctx);
       void nft_ctx_output_set_flags(struct nft_ctx *ctx, unsigned int flags);

       unsigned int nft_ctx_output_get_debug(struct nft_ctx *ctx);
       void nft_ctx_output_set_debug(struct nft_ctx *ctx, unsigned int mask);

       FILE *nft_ctx_set_output(struct nft_ctx *ctx, FILE *fp);
       int nft_ctx_buffer_output(struct nft_ctx *ctx);
       int nft_ctx_unbuffer_output(struct nft_ctx *ctx);
       const char *nft_ctx_get_output_buffer(struct nft_ctx *ctx);

       FILE *nft_ctx_set_error(struct nft_ctx *ctx, FILE *fp);
       int nft_ctx_buffer_error(struct nft_ctx *ctx);
       int nft_ctx_unbuffer_error(struct nft_ctx *ctx);
       const char *nft_ctx_get_error_buffer(struct nft_ctx *ctx);

       int nft_ctx_add_include_path(struct nft_ctx *ctx, const char *path);
       void nft_ctx_clear_include_paths(struct nft_ctx *ctx);

       int nft_run_cmd_from_buffer(struct nft_ctx *nft, const char *buf);
       int nft_run_cmd_from_filename(struct nft_ctx *nft,
                                     const char *filename);

       Link with -lnftables.

DESCRIPTION
       This library was designed with nftables integration into applications
       in mind. Its API is therefore kept as simple as possible, which
       somewhat limits its flexibility. Due to support for JSON markup of
       input and output though, convenience in constructing and parsing of
       input and output data may be achieved by using a third-party library
       such as libjansson.

       At the very basic level, one has to allocate a new object of type
       struct nft_ctx using nft_ctx_new() function, then pass commands via
       nft_run_cmd_from_buffer() or nft_run_cmd_from_filename() functions. By
       default, any output is written to stdout (or stderr for error
       messages). These file pointers may be changed using
       nft_ctx_set_output() and nft_ctx_set_error() functions. On top of that,
       it is possible to have any output buffered by the library for later
       retrieval as a static buffer. See nft_ctx_buffer_output() and
       nft_ctx_buffer_error() functions for details.

   nft_ctx_new() and nft_ctx_free()
       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.

   nft_ctx_get_dry_run() and nft_ctx_set_dry_run()
       Dry-run setting controls whether ruleset changes are actually committed
       on kernel side or not. It allows 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.

   nft_ctx_output_get_flags() and nft_ctx_output_set_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 controls JSON output format, input is auto-detected.

       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 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.

   nft_ctx_output_get_debug() and nft_ctx_output_set_debug()
       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.

   Controlling library standard and error output
       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).

   nft_ctx_add_include_path() and nft_ctx_clear_include_path()
       The include command in nftables rulesets allows to outsource parts of
       the ruleset into a different file. The include path defines where these
       files are searched for. Libnftables allows 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.

   nft_run_cmd_from_buffer() and nft_run_cmd_from_filename()
       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.

EXAMPLE
           #include <stdio.h>
           #include <string.h>
           #include <nftables/libnftables.h>

           int main(void)
           {
                   char *list_cmd = "list ruleset";
                   struct nft_ctx *nft;
                   const char *output, *p;
                   char buf[256];
                   int rc = 0;

                   nft = nft_ctx_new(NFT_CTX_DEFAULT);
                   if (!nft)
                           return 1;

                   while (1) {
                           if (nft_ctx_buffer_output(nft) ||
                               nft_run_cmd_from_buffer(nft, list_cmd)) {
                                   rc = 1;
                                   break;
                           }
                           output = nft_ctx_get_output_buffer(nft);
                           if (strlen(output)) {
                                   printf("\nThis is the current ruleset:\n| ");
                                   for (p = output; *(p + 1); p++) {
                                           if (*p == '\n')
                                                   printf("\n| ");
                                           else
                                                   putchar(*p);
                                   }
                                   putchar('\n');
                           } else {
                                   printf("\nCurrent ruleset is empty.\n");
                           }
                           nft_ctx_unbuffer_output(nft);

                           printf("\nEnter command ('q' to quit): ");
                           fflush(stdout);
                           fgets(buf, 256, stdin);
                           if (strlen(buf))
                                   buf[strlen(buf) - 1] = '\0';

                           if (buf[0] == 'q' && buf[1] == '\0')
                                   break;

                           if (nft_run_cmd_from_buffer(nft, buf)) {
                                   rc = 1;
                                   break;
                           }
                   }

                   nft_ctx_free(nft);
                   return rc;
           }

SEE ALSO
       libnftables-json(5), nft(8)

AUTHOR
       Phil Sutter <phil@nwl.cc>
           Author.

                                  08/10/2022                    LIBNFTABLES(3)