Information for Netatalk Developers
For basic installation instructions, see the Installation chapter in the html manual published on https://netatalk.io and the INSTALL.md file in the root of the source tree.
Netatalk is an implementation of Apple Filing Protocol (AFP) over TCP. The session layer used to carry AFP over TCP is called DSI.
Netatalk also supports the AppleTalk Protocol Suite for legacy Macs, Lisas and Apple IIs via the “atalkd” daemon. It supports EtherTalk Phase I and II, RTMP, NBP, ZIP, AEP, ATP, PAP, and ASP, while expecting the kernel to supply DDP.
The complete stack looks like this on a BSD-derived system:
AFP AFP
| |
ASP PAP DSI
\ / |
ATP RTMP NBP ZIP AEP |
| | | | | |
-+---------------------------------------------------+- (kernel boundary)
| Socket |
+-----------------------+------------+--------------+
| | TCP | UDP |
| DDP +------------+--------------+
| | IP |
+-----------------------+---------------------------+
| Network-Interface |
+---------------------------------------------------+
-
DDP is a socket to socket protocol that all other AppleTalk protocols are built on top of.
-
“atalkd” implements RTMP, NBP, ZIP, and AEP. It is the AppleTalk equivalent of Unix “routed”.
-
There is also a client-stub library for NBP. ATP and ASP are implemented as libraries.
-
“papd” allows Macs to spool to “lpd”, and “pap” allows Unix machines to print to AppleTalk connected printers.
Error checking and logging
We want rigid error checking and concise log messages. This often leads
to significant code bloat where the relevant function call is buried in error
checking and logging statements.
In order to alleviate error checking and code readability, we provide a set
of error checking macros in
- EC_CHECK just checks the CHECK
- EC_CHECK_LOG additionally logs the stringified function call.
- EC_CHECK_LOG_ERR allows specifying the return value
- EC_CHECK_CUSTOM allows custom actions
The macros EC_CHECK* unconditionally jump to a cleanup label where the necessary cleanup can be done alongside controlling the return value. EC_CHECK_CUSTOM doesn’t do that, so an extra “goto EC_CLEANUP” may be performed as appropriate.
Example:
- stat() without EC macro:
static int func(const char *name) {
int ret = 0;
...
if ((ret = stat(name, &some_struct_stat)) != 0) {
LOG(...);
ret = -1; /* often needed to explicitly set the error indicating return value */
goto cleanup;
}
return ret;
cleanup:
...
return ret;
}
- stat() with EC macro:
static int func(const char *name) {
EC_INIT; /* expands to int ret = 0; */
char *uppername = NULL
EC_NULL(uppername = strdup(name));
EC_ZERO(strtoupper(uppername));
EC_ZERO(stat(uppername, &some_struct_stat)); /* expands to complete if block from above */
EC_STATUS(0);
EC_CLEANUP:
if (uppername) free(uppername);
EC_EXIT;
}
A boilerplate function template is:
int func(void)
{
EC_INIT;
...your code here...
EC_STATUS(0);
EC_CLEANUP:
EC_EXIT;
}
Ini Parser
The ini parser is taken from https://github.com/ndevilla/iniparser. It has been slightly modified:
- case-sensitive
- “include” directive added
- atalk_iniparser_getstrdup() to complemnt atalk_iniparser_getstring(), it returns allocated strings which the caller must free as necessary
- the API has been modified such that all atalk_iniparser_get* funcs take a section and a parameter as separate args instead of one string of the form “section:parameter” in the original library
CNID Database Daemons
The CNID database daemons cnid_metad and cnid_dbd are an implementation of the netatalk CNID database support that attempts to put all functionality into separate daemons. There is one cnid_dbd daemon per netatalk volume. The underlying database structure is based on Berkeley DB.
Advantages
-
No locking issues or leftover locks due to crashed afpd daemons any more. Since there is only one thread of control accessing the database, no locking is needed and changes appear atomic.
-
Berkeley DB transactions are difficult to get right with several processes attempting to access the CNID database simultaneously. This is much easier with a single process and the database can be made nearly crash-proof this way (at a performance cost).
-
No problems with user permissions and access to underlying database files, the cnid_dbd process runs under a configurable user ID that normally also owns the underlying database and can be contacted by whatever afpd daemon accesses a volume.
-
If an afpd process crashes, the CNID database is unaffected. If the process was making changes to the database at the time of the crash, those changes will be rolled back entirely (transactions). If the process was not using the database at the time of the crash, no corrective action is necessary. In any case, database consistency is assured.
Disadvantages
- Performance in an environment of processes sharing the database (files) is potentially better for two reasons:
i) IPC overhead. ii) r/o access to database pages is possible by more than one process at once, r/w access is possible for non overlapping regions.
The current implementation of cnid_dbd uses unix domain sockets as the IPC mechanism. While this is not the fastest possible method, it is very portable and the cnid_dbd IPC mechanisms can be extended to use faster IPC (like mmap) on architectures where it is supported. As a ballpark figure, 20000 requests/replies to the cnid_dbd daemon take about 0.6 seconds on a Pentium III 733 MHz running Linux Kernel 2.4.18 using unix domain sockets. The requests are “empty” (no database lookups/changes), so this is just the IPC overhead.
I have not measured the effects of the advantages of simultaneous database access.
Installation and configuration
There are two executables that will be built in etc/cnid_dbd and installed into the systems binaries directories of netatalk cnid_metad and cnid_dbd. cnid_metad should run all the time with root permissions. It will be notified when an instance of afpd starts up and will in turn make sure that a cnid_dbd daemon is started for the volume that afpd wishes to access. The cnid_dbd daemon runs as long as necessary and services any other instances of afpd that access the volume. You can safely kill it with SIGTERM, it will be restarted automatically by cnid_metad as soon as the volume is accessed again.
cnid_dbd changes to the Berkeley DB directory on startup and sets effective UID and GID to owner and group of that directory. Database and supporting files should therefore be writeable by that user/group.
Current shortcomings:
-
The parameter file parsing of db_param is very simpleminded. It is easy to cause buffer overruns and the like. Also, there is no support for blanks (or weird characters) in filenames for the usock_file parameter.
-
There is no protection against a malicious user connecting to the cnid_dbd socket and changing the database.
