sfs_users - user-authentication database

The sfs_users file, maintained and used by the sfsauthd program, maps public keys to local users and groups. It is roughly analogous to the Unix /etc/passwd and /etc/group files. Each line of sfs_users can specify a user or a group. Users are specified as follows (split into two lines here only for clarity of presentation):

USER:user:uid:version:gid:owner:pubkey:privs :srp:privkey:srvprivkey:audit

Note that the first USER is just the literal string USER. The rest of the fields have the following meanings:

user

user is the unique name of a public key in the database. Ordinarily it is the same as a username in the local password file. However, it is also possible to add SFS users who do not have local Unix accounts. It is also possible map multiple public keys to the same local Unix account, as when several people have an account with root privileges. In such cases, each key should be given a unique name (e.g., dm/root, kaminsky/root, etc.).

uid

uid is the user's user ID on the given server.

version

version is the version number of this record in the users database. Upon registration, this value is set to 1. Upon every subsequent update, this value is incremented by 1.

gid

gid is the users's group ID on the given server.

owner

This field is currently ignored, but in a future version may be used to allow users to create ``guest'' accounts.

pubkey

pubkey is an ASCII, human-readable representation of the user's public key. Can be either a Rabin or 2-Schnorr public key.

privs

The privs field contains a comma-separated list of properties of the account. Possible properties are as follows:

unix=account

This property states that an SFS user corresponds to the local Unix account account. In many settings, it is common to use the unix= property to map every SFS user to a local Unix user of the same name. The unix= property has several consequences. First, if there is no local Unix user named account, this SFS user will not be allowed to log in. Second, when the SFS user logs in, SFS will search /etc/group for additional groups the user might belong to. Third, the rexd remote login daemon will allow remote login access to this account, using the shell and home directory specified in /etc/passwd. Finally, on some operating systems, SFS enforces account expiration dates specified by /etc/shadow or /etc/spwd.db.

admin

Indicates that this particular users has administrative privileges in SFS. The option has no effect unless the Userfile directive in sfsauthd_config specifies the -admin option. For sfs_users files with the -admin option, the admin privilege allows users to create and modify other user records remotely, though currently client-side support for doing this is limited.

refresh

timeout

These properties are mostly of use with sfsaclsd, an experimental server that is not part of the mainline SFS distribution yet.

srp

srp is the server-side information for the SRP protocol. Unlike the previous fields, this information must be kept secret. If the information is disclosed, an attacker may be able to impersonate the server by causing the sfskey add command to fetch the wrong HostID. Note also that srp is specific to a particular hostname. If you change the Location of a file server, users will need to register new SRP.

privkey

privkey is actually opaque to sfsauthd. It is private, per-user data that sfsauthd will return to users who successfully complete the SRP protocol. Currently, sfskey users this field to store an encrypted copy of a user's private key, allowing the user to retrieve the private key over the network.

srvprivkey

If a user has chosen 2-Schnorr proactive signatures, the server's half of the private key is kept in this field.

audit

audit contains the time, source IP address, and description of the last update to this field. Useful in recovering from a compromised key.

Each group in sfs_users is specified by a line with the following format:

GROUP:group:gid:version:owners:members:properties:audit

Here again the first GROUP is just the literal string GROUP, while the remaining fields have the following meanings:

group

The name of the group.

gid

The numeric group ID.

version

version is the version number of this record in the database. The number increments when people edit groups through the sfskey interface.

owners

List of users who are allowed to edit the group membership list.

members

List of users who are in the group.

properties

Properties of the group, mostly of use with sfsaclsd, an experimental server that is not part of the mainline SFS distribution yet.

audit

Information about the last time this record was modified through the sfskey interface.

sfs_users files can be stored in one of three formats: plain ASCII, database directories, and database files. (The latter two require SFS to have been compiled with Sleepycat BerkeleyDB support.) The format is determined by the extension of the file name. File names ending .db/ are considered database directories; file names ending .db are considered database files; everything else is considered ASCII. Only read-only and exported public databases can be database files; read-write databases must be directories, ending .db/. (The reason is that read-write database files require write-ahead logging, which relies on auxiliary files.)

You should always edit sfs_users files using the vidb(8) command, for two reasons. First, whenever editing files by hand, you run the risk of overwriting concurrent updates by sfsauthd. vidb acquires the necessary locks to prevent this from happening. Second, when editing a database directory or file, vidb translates from the binary database format into the ASCII format described above; when committing updates, it also atomically modifies various secondary indexes that SFS relies upon.

/etc/sfs/sfs_users

/usr/local/share/sfs/sfs_users

user-authentication database

(Files in /etc/sfs supersede default versions in /usr/local/share/sfs.)

dirsearch(1), newaid(1), rex(1), sfsagent(1), sfskey(1), ssu(1), sfs_config(5), sfs_hosts(5), sfs_srp_params(5), sfsauthd_config(5), sfscd_config(5), sfsrosd_config(5), sfsrwsd_config(5), sfssd_config(5), sfs_environ(7), funmount(8), nfsmounter(8), sfsauthd(8), sfscd(8), sfsrosd(8), sfsrwcd(8), sfsrwsd(8), sfssd(8), vidb(8)

The full documentation for SFS is maintained as a Texinfo manual. If the info and SFS programs are properly installed at your site, the command info SFS should give you access to the complete manual.

For updates, documentation, and software distribution, please see the SFS website at http://www.fs.net/.

sfsdev@redlab.lcs.mit.edu