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diff --git a/doc/developer/vtysh.rst b/doc/developer/vtysh.rst new file mode 100644 index 0000000..160676a --- /dev/null +++ b/doc/developer/vtysh.rst @@ -0,0 +1,201 @@ +.. _vtysh: + +***** +VTYSH +***** + +.. seealso:: :ref:`command-line-interface` + +.. _vtysh-architecture: + +Architecture +============ + +VTYSH is a shell for FRR daemons. It amalgamates all the CLI commands defined +in each of the daemons and presents them to the user in a single shell, which +saves the user from having to telnet to each of the daemons and use their +individual shells. The amalgamation is achieved by +:ref:`extracting <vtysh-command-extraction>` commands from daemons and +injecting them into VTYSH at build time. + +At runtime, VTYSH maintains an instance of a CLI mode tree just like each +daemon. However, the mode tree in VTYSH contains (almost) all commands from +every daemon in the same tree, whereas individual daemons have trees that only +contain commands relevant to themselves. VTYSH also uses the library CLI +facilities to maintain the user's current position in the tree (the current +node). Note that this position must be synchronized with all daemons; if a +daemon receives a command that causes it to change its current node, VTYSH must +also change its node. Since the extraction script does not understand the +handler code of commands, but only their definitions, this and other behaviors +must be manually programmed into VTYSH for every case where the internal state +of VTYSH must change in response to a command. Details on how this is done are +discussed in the :ref:`vtysh-special-defuns` section. + +VTYSH also handles writing and applying the integrated configuration file, +:file:`/etc/frr/frr.conf`. Since it has knowledge of the entire command space +of FRR, it can intelligently distribute configuration commands only to the +daemons that understand them. Similarly, when writing the configuration file it +takes care of combining multiple instances of configuration blocks and +simplifying the output. This is discussed in :ref:`vtysh-configuration`. + +.. _vtysh-command-extraction: + +Command Extraction +------------------ + +When VTYSH is built, a Perl script named :file:`extract.pl` searches the FRR +codebase looking for ``DEFUN``'s. It extracts these ``DEFUN``'s, transforms +them into ``DEFSH``'s and appends them to ``vtysh_cmd.c``. Each ``DEFSH`` +contains the name of the command plus ``_vtysh``, as well as a flag that +indicates which daemons the command was found in. When the command is executed +in VTYSH, this flag is inspected to determine which daemons to send the command +to. This way, commands are only sent to the daemons that know about them, +avoiding spurious errors from daemons that don't have the command defined. + +The extraction script contains lots of hardcoded knowledge about what sources +to look at and what flags to use for certain commands. + +.. _vtysh-special-defuns: + +Special DEFUNs +-------------- + +In addition to the vanilla ``DEFUN`` macro for defining CLI commands, there are +several VTYSH-specific ``DEFUN`` variants that each serve different purposes. + +``DEFSH`` + Used almost exclusively by generated VTYSH code. This macro defines a + ``cmd_element`` with no handler function; the command, when executed, is + simply forwarded to the daemons indicated in the daemon flag. + +``DEFUN_NOSH`` + Used by daemons. Has the same expansion as a ``DEFUN``, but ``extract.pl`` + will skip these definitions when extracting commands. This is typically used + when VTYSH must take some special action upon receiving the command, and the + programmer therefore needs to write VTYSH's copy of the command manually + instead of using the generated version. + +``DEFUNSH`` + The same as ``DEFUN``, but with an argument that allows specifying the + ``->daemon`` field of the generated ``cmd_element``. This is used by VTYSH + to determine which daemons to send the command to. + +``DEFUNSH_ATTR`` + A version of ``DEFUNSH`` that allows setting the ``->attr`` field of the + generated ``cmd_element``. Not used in practice. + +.. _vtysh-configuration: + +Configuration Management +------------------------ + +When integrated configuration is used, VTYSH manages writing, reading and +applying the FRR configuration file. VTYSH can be made to read and apply an +integrated configuration to all running daemons by launching it with ``-f +<file>``. It sends the appropriate configuration lines to the relevant daemons +in the same way that commands entered by the user on VTYSH's shell prompt are +processed. + +Configuration writing is more complicated. VTYSH makes a best-effort attempt to +combine and simplify the configuration as much as possible. A working example +is best to explain this behavior. + +Example +^^^^^^^ + +Suppose we have just *staticd* and *zebra* running on the system, and use VTYSH +to apply the following configuration snippet: + +.. code-block:: frr + + ! + vrf blue + ip protocol static route-map ExampleRoutemap + ip route 192.168.0.0/24 192.168.0.1 + exit-vrf + ! + +Note that *staticd* defines static route commands and *zebra* defines ``ip +protocol`` commands. Therefore if we ask only *zebra* for its configuration, we +get the following:: + + (config)# do sh running-config zebra + Building configuration... + + ... + ! + vrf blue + ip protocol static route-map ExampleRoutemap + exit-vrf + ! + ... + +Note that the static route doesn't show up there. Similarly, if we ask +*staticd* for its configuration, we get:: + + (config)# do sh running-config staticd + + ... + ! + vrf blue + ip route 192.168.0.0/24 192.168.0.1 + exit-vrf + ! + ... + +But when we display the configuration with VTYSH, we see:: + + ubuntu-bionic(config)# do sh running-config + + ... + ! + vrf blue + ip protocol static route-map ExampleRoutemap + ip route 192.168.0.0/24 192.168.0.1 + exit-vrf + ! + ... + +This is because VTYSH asks each daemon for its currently running configuration, +and combines equivalent blocks together. In the above example, it combined the +``vrf blue`` blocks from both *zebra* and *staticd* together into one. This is +done in :file:`vtysh_config.c`. + +Protocol +======== + +VTYSH communicates with FRR daemons by way of domain socket. Each daemon +creates its own socket, typically in :file:`/var/run/frr/<daemon>.vty`. The +protocol is very simple. In the VTYSH to daemon direction, messages are simply +NUL-terminated strings, whose content are CLI commands. Here is a typical +message from VTYSH to a daemon: + +:: + + Request + + 00000000: 646f 2077 7269 7465 2074 6572 6d69 6e61 do write termina + 00000010: 6c0a 00 l.. + + +The response format has some more data in it. First is a NUL-terminated string +containing the plaintext response, which is just the output of the command that +was sent in the request. This is displayed to the user. The plaintext response +is followed by 3 null marker bytes, followed by a 1-byte status code that +indicates whether the command was successful or not. + +:: + + Response + + 0 1 2 3 + 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Plaintext Response | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | Marker (0x00) | Status Code | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + +The first ``0x00`` byte in the marker also serves to terminate the plaintext +response. |