s6: the accessrules library interface

The `accessrules` library interface

The following functions and structures are declared in the s6/accessrules.h header, and implemented in the libs6.a or libs6.so library.

General information

accessrules is an access control library. It looks up a key in a user-specified database, then returns a code depending on whether the database allows access (in which case additional information can also be returned), denies access, or does not contain the key.

accessrules has been designed to be easily extensible to any database format and any key format.

Check the s6/accessrules.h header for the exact definitions.

Data structures

A s6_accessrules_result_t is a scalar that can have the following values: S6_ACCESSRULES_ERROR, S6_ACCESSRULES_DENY, S6_ACCESSRULES_ALLOW or S6_ACCESSRULES_NOTFOUND.
A s6_accessrules_params_t is a structure containing two strallocs, .env and .exec, used to return data contained in the database when a key has been allowed. The interpretation of this data is application-defined.

Function types

Backend lookups

A s6_accessrules_backend_func_t is the type of a function that takes a single key, looks it up in a database, and returns the result. Namely:

s6_accessrules_result_t f (char const *key, size_t keylen, void *handle, s6_accessrules_params_t *params)

f looks up key key of length keylen in the database represented by handle in an implementation-defined way. It returns a number that says the key has been allowed, denied or not found, or an error occurred. If the key has been allowed, f stores additional information from the database into *params.

Two s6_accessrules_backend_func_t functions are natively implemented:

s6_accessrules_backend_fs takes a char const * handle and interprets it as a base directory to look up key under, in the format understood by s6-accessrules-cdb-from-fs.
s6_accessrules_backend_cdb takes a struct cdb * handle and looks up key in the CDB it points to. handle must already be mapped to a CDB file. Such a file can be built with the s6-accessrules-cdb-from-fs utility.

Frontend key checking

A s6_accessrules_keycheck_func_t is the type of a function that takes a user-level key, makes a list of corresponding backend-level keys and calls a s6_accessrules_backend_func_t function until it finds a match. Namely:

s6_accessrules_result_t f (void const *key, void *handle, s6_accessrules_params_t *params, s6_accessrules_backend_func_t *backend)

f derives a list of low-level keys to check from key. Then, for each key k of length klen in this list, it calls (*backend)(k, klen, handle, params), returning *backend's result if it is not S6_ACCESSRULES_NOTFOUND. If no match can be found in the whole list, f finally returns S6_ACCESSRULES_NOTFOUND.

Five s6_accessrules_keycheck_func_t functions are natively implemented:

s6_accessrules_keycheck_uidgid interprets key as a pointer to a structure containing an uid u and a gid g. The following checks are performed, in this order (i.e. subsequent checks are not performed if a match is found):
- If u is equal to the effective uid of the process, look for a uid/self match.
- Look for a uid/u match.
- If g is equal to the effective gid of the process, look for a gid/self match.
- Look for a gid/g match.
- Look for a uid/default match.

s6_accessrules_keycheck_reversedns interprets key as a string containing a FQDN. Then for each suffix k of key, starting with key itself and ending with key's TLD, it looks up reversedns/k. The final dot is excluded from k. If no match can be found, the function checks reversedns/@ and returns the result. For instance, if key is "foo.bar.com", the following strings are looked up, in this order:
- reversedns/foo.bar.com
- reversedns/bar.com
- reversedns/com
- reversedns/@

s6_accessrules_keycheck_ip4 interprets key as 4 network-byte-order characters containing an IPv4 address. Then for each netmask mask from 32 to 0, it constructs the IPv4 network prefix addr corresponding to that address, and looks up ip4/addr_mask. For instance, if key is "\300\250\001\007", representing the 192.168.1.7 address, the following strings are looked up, in this order:
- ip4/192.168.1.7_32
- ip4/192.168.1.6_31
- ip4/192.168.1.4_30
- ip4/192.168.1.0_29
- ip4/192.168.0.0_28
- ip4/192.168.0.0_27
and so on, down to:
- ip4/192.0.0.0_3
- ip4/192.0.0.0_2
- ip4/128.0.0.0_1
- ip4/0.0.0.0_0
Note that the ip4/0.0.0.0_0 string is a catch-all key that matches everything.

s6_accessrules_keycheck_ip6 interprets key as 16 network-byte-order characters containing an IPv6 address. Then for each netmask mask from 128 to 0, it constructs the IPv6 network prefix addr corresponding to that address, in canonical form, and looks up ip6/addr_mask. For instance, if key is "*\0\024P@\002\b\003\0\0\0\0\0\0\020\006", representing the 2a00:1450:4002:803::1006 address, the following strings are looked up, in this order:
- ip6/2a00:1450:4002:803::1006_128
- ip6/2a00:1450:4002:803::1006_127
- ip6/2a00:1450:4002:803::1004_126
- ip6/2a00:1450:4002:803::1000_125
- ip6/2a00:1450:4002:803::1000_124
- ip6/2a00:1450:4002:803::1000_123
- ip6/2a00:1450:4002:803::1000_122
- ip6/2a00:1450:4002:803::1000_121
- ip6/2a00:1450:4002:803::1000_120
- ip6/2a00:1450:4002:803::1000_119
- ip6/2a00:1450:4002:803::1000_118
- ip6/2a00:1450:4002:803::1000_117
- ip6/2a00:1450:4002:803::1000_116
- ip6/2a00:1450:4002:803::1000_115
- ip6/2a00:1450:4002:803::1000_114
- ip6/2a00:1450:4002:803::1000_113
- ip6/2a00:1450:4002:803::_112
- ip6/2a00:1450:4002:803::_111
and so on, down to:
- ip6/2a00::_11
- ip6/2800::_10
- ip6/2800::_9
- ip6/2000::_8
- ip6/2000::_7
- ip6/2000::_6
- ip6/2000::_5
- ip6/2000::_4
- ip6/2000::_3
- ip6/::_2
- ip6/::_1
- ip6/::_0
Note that the ip6/::_0 string is a catch-all key that matches everything.

s6_accessrules_keycheck_ip46 interprets key as a pointer to an ip46_t, and behaves either as s6_accessrules_keycheck_ip6 or s6_accessrules_keycheck_ip4, depending on the type of address *key contains.

Ready-to-use functions

Those functions are mostly macros; they're built by associating a frontend function with a backend function.

s6_accessrules_result_t s6_accessrules_uidgid_cdb (uid_t u, gid_t g, struct cdb *c, s6_accessrules_params_t *params)
Checks the *c CDB database for an authorization for uid u and gid g. If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_uidgid_fs (uid_t u, gid_t g, char const *dir, s6_accessrules_params_t *params)
Checks the dir base directory for an authorization for uid u and gid g. If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_reversedns_cdb (char const *name, struct cdb *c, s6_accessrules_params_t *params)
Checks the *c CDB database for an authorization for the name FQDN. If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_reversedns_fs (char const *name, char const *dir, s6_accessrules_params_t *params)
Checks the dir base directory for an authorization for the name FQDN. If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_ip4_cdb (char const *ip4, struct cdb *c, s6_accessrules_params_t *params)
Checks the *c CDB database for an authorization for the ip4 IPv4 address (4 network byte order characters). If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_ip4_fs (char const *ip4, char const *dir, s6_accessrules_params_t *params)
Checks the dir base directory for an authorization for the ip4 IPv4 address (4 network byte order characters). If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_ip6_cdb (char const *ip6, struct cdb *c, s6_accessrules_params_t *params)
Checks the *c CDB database for an authorization for the ip6 IPv6 address (16 network byte order characters). If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_ip6_fs (char const *ip6, char const *dir, s6_accessrules_params_t *params)
Checks the dir base directory for an authorization for the ip6 IPv6 address (16 network byte order characters). If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_ip46_cdb (ip46_t *ip, struct cdb *c, s6_accessrules_params_t *params)
Checks the *c CDB database for an authorization for the ip IP address. If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.

s6_accessrules_result_t s6_accessrules_ip46_fs (ip46_t const *ip, char const *dir, s6_accessrules_params_t *params)
Checks the dir base directory for an authorization for the ip IP address. If the result is S6_ACCESSRULES_ALLOW, additional information may be stored into params.