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Obsolete:Samba 4 Storing SID in String Format

Obsolete Documentation

Please note that this content was marked as obsolete. We left the content here for study and archaeological purposes.

Please check our Documentation for a recent list of topics.

Overview

In order to guarantee uniqueness among multiple Samba instances backed by DS, DNA plugin will be used to allocate SID. See also this page.

Currently the DNA plugin can only support generating integer or string values whereas Samba is storing the SID in binary format in the DS. In order to utilize the plugin without any changes, Samba code needs to be changed such that it stores the SID in the string format. The SID will be stored in sambaSID attribute which is part of Samba 3 schema.

Current Code

In Samba code the SID may exist in 3 different formats:

  • SID structure (struct dom_sid)
  • Binary format (NDR-encoded DATA_BLOB)
  • String format (LDIF-encoded char*)

The mapping code always converts the SID into binary format before storing it into the DS.

SID Structure

The SID structure is defined in librpc/gen_ndr/security.h:

struct dom_sid {
    uint8_t sid_rev_num;
    int8_t num_auths;
    uint8_t id_auth[6];
    uint32_t sub_auths[15];
};

Conversion Methods

The binary conversion methods are defined in librpc/ndr/ndr.c:

// convert SID structure to binary
_PUBLIC_ enum ndr_err_code ndr_push_dom_sid(
    struct ndr_push *ndr, int ndr_flags, const struct dom_sid *r);

// convert binary to SID structure
_PUBLIC_ enum ndr_err_code ndr_pull_dom_sid(
    struct ndr_pull *ndr, int ndr_flags, struct dom_sid *r);

The string conversion methods are defined in libcli/security/dom_sid.c:

// convert string to SID structure
bool dom_sid_parse(const char *sidstr, struct dom_sid *ret);

// convert SID structure to string
char *dom_sid_string(TALLOC_CTX *mem_ctx, const struct dom_sid *sid);

Attribute Syntax

The SID syntax is defined in source4/lib/ldb-samba/ldif_handlers.c:

static const struct ldb_schema_syntax samba_syntaxes[] = {
    {
        .name		  = LDB_SYNTAX_SAMBA_SID,
        .ldif_read_fn	  = ldif_read_objectSid,
        .ldif_write_fn	  = ldif_write_objectSid,
        .canonicalise_fn  = ldif_canonicalise_objectSid,
        .comparison_fn	  = ldif_comparison_objectSid
    }
}

The handlers are defined in source4/lib/lib-samba/ldif_handlers.c:

static int ldif_read_objectSid(
    struct ldb_context *ldb, void *mem_ctx,
    const struct ldb_val *in, struct ldb_val *out)
{
    // convert string to SID structure
    sid = dom_sid_parse_length(mem_ctx, in);

    // convert SID structure to binary
    ndr_err = ndr_push_struct_blob(out, mem_ctx, NULL, sid,
        (ndr_push_flags_fn_t)ndr_push_dom_sid);
}

static int ldif_write_objectSid(
    struct ldb_context *ldb, void *mem_ctx,
    const struct ldb_val *in, struct ldb_val *out)
{
    // convert binary to SID structure
    ndr_err = ndr_pull_struct_blob_all(in, sid, NULL, sid,
        (ndr_pull_flags_fn_t)ndr_pull_dom_sid);

    // convert SID structure to string
    *out = data_blob_string_const(dom_sid_string(mem_ctx, sid));
}

The ldif_canonicalise_objectSid() makes sure that the SID is converted into binary.

static int ldif_canonicalise_objectSid(
    struct ldb_context *ldb, void *mem_ctx,
    const struct ldb_val *in, struct ldb_val *out)
{
    // if SID is string
    if (ldif_comparision_objectSid_isString(in)) {

        // convert string to binary
        if (ldif_read_objectSid(ldb, mem_ctx, in, out) != 0) {

            // in case of error return a copy
            return ldb_handler_copy(ldb, mem_ctx, in, out);
        }
        return 0;
    }

    // if not string return a copy
    return ldb_handler_copy(ldb, mem_ctx, in, out);
}

Attribute Mapping

The SID mapping is defined in source4/dsdb/samdb/ldb_modules/simple_ldap_map.c:

// mapping for OpenLDAP
static const struct ldb_map_attribute entryuuid_attributes[] = {
    {
        .local_name = "objectSid",
        .type = MAP_CONVERT,
        .u = {
            .convert = {
                .remote_name = "objectSid", 
                .convert_local = sid_always_binary,
                .convert_remote = val_copy,
            },
        },
    }
}

// mapping for DS
static const struct ldb_map_attribute nsuniqueid_attributes[] = {
    {
        .local_name = "objectSid",
        .type = MAP_CONVERT,
        .u = {
            .convert = {
                .remote_name = "objectSid", 
                .convert_local = sid_always_binary,
                .convert_remote = val_copy,
            }
        }
    },
}

The handlers are also defined in source4/dsdb/samdb/ldb_modules/simple_ldap_map.c:

static struct ldb_val sid_always_binary(
    struct ldb_module *module, TALLOC_CTX *ctx, const struct ldb_val *val)
{
    // get SID syntax
    a = ldb_schema_attribute_by_name(ldb, "objectSid");

    // canonicalize SID
    a->syntax->canonicalise_fn(ldb, ctx, val, &out);

    return out;
}

static struct ldb_val val_copy(
    struct ldb_module *module, TALLOC_CTX *ctx, const struct ldb_val *val)
{
    // no conversion because SID is already in binary format
    return ldb_val_dup(ctx, val);
}

Attribute Dereferencing

The handler function in extended_dn_out_fds module reads the binary SID value.

static int handle_dereference_fds(struct ldb_dn *dn,
    struct dsdb_openldap_dereference_result **dereference_attrs, 
    const char *attr, const DATA_BLOB *val)
{
    sid_blob = ldb_msg_find_ldb_val(&fake_msg, "objectSID");
    if (sid_blob) {
        ldb_dn_set_extended_component(dn, "SID", sid_blob);
    }
}

Schema

The provisioning tool generates the objectSid attribute in 99_ad.ldif. The attribute uses Octet String (binary) syntax.

attributeTypes: (
  1.2.840.113556.1.4.146
  NAME 'objectSid'
  EQUALITY octetStringMatch
  SYNTAX 1.3.6.1.4.1.1466.115.121.1.40
  SINGLE-VALUE
  )

Proposed Changes

One option is to change the ldif_canonicalise_objectSid() to convert from binary to string. However, this method is used in many places and by different backends as well.

To minimize the risks, the changes should be done specifically for DS only.

Attribute Mapping

The mapping for DS should be changed as follows:

static const struct ldb_map_attribute nsuniqueid_attributes[] = {
    {
        .local_name = "objectSid",
        .type = MAP_CONVERT,
        .u = {
            .convert = {
                .remote_name = "sambaSID", 
                .convert_local = sid_always_string,
                .convert_remote = sid_always_binary,
            }
        }
    },
}

Then the following method should be added:

static struct ldb_val sid_always_string(
    struct ldb_module *module, TALLOC_CTX *ctx, const struct ldb_val *val)
{
    // if SID is string
    if (ldif_comparision_objectSid_isString(in)) {

        // return a copy
        return ldb_handler_copy(ldb, mem_ctx, in, out);

    } else {

        // convert binary to string
        if (ldif_write_objectSid(ldb, mem_ctx, in, out) != 0) {

            // in case of error return a copy
            return ldb_handler_copy(ldb, mem_ctx, in, out);
        }
        return 0;
    }
}

Attribute Dereferencing

The handler function in extended_dn_out_fds module should be changed to read the string SID value and convert it into SID structure.

static int handle_dereference_fds(struct ldb_dn *dn,
    struct dsdb_openldap_dereference_result **dereference_attrs, 
    const char *attr, const DATA_BLOB *val)
{
    sidBlob = ldb_msg_find_ldb_val(&fake_msg, "sambaSID");
    if (sidBlob) {
        // convert string into SID structure
        sid = dom_sid_parse_length(NULL, sidBlob);

        // convert SID structure into binary
        ndr_push_struct_blob(&sid_blob, NULL, NULL, sid,
            (ndr_push_flags_fn_t)ndr_push_dom_sid);

        ldb_dn_set_extended_component(dn, "SID", sid_blob);
    }
}

Schema

The provisioning tool should be configured such that it doesn't generate the objectSid attribute but instead it uses the sambaSID attribute. The schema conversion is located at source4/setup/schema-map-fedora-ds-1.0:

objectSid
objectSid:sambaSID

Issues

Attribute Dereferencing

In order to change the storage format in DS without affecting the format in OpenLDAP, a new attribute deferencing module needs to be created for the DS. See also this page.

Schema Mapping

Samba 3 schema has a dependency on InetOrgPerson schema which is conflicting with AD schema. To solve this the AD schema needs to be renamed. See also this page.

Patches

The following patch has been applied to the source repository: