VTYSH

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 extracting 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 Special DEFUNs section.

VTYSH also handles writing and applying the integrated configuration 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 Configuration Management.

Command Extraction

When VTYSH is built, a Perl script named 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.

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.

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:

!
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 vtysh_config.c.

Protocol

VTYSH communicates with FRR daemons by way of domain socket. Each daemon creates its own socket, typically in /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.