INSTANT 9P CONNECTION BETWEEN TWO MACHINES
The following should be used only on secure private networks or by machines with no private data. It provides a read-only public 9p service of the full namespace of the invoking user:
server% aux/listen1 -tv tcp!*!9999 /bin/exportfs -R -r /
To dial and mount this service from another machine:
client% srv tcp!ip.of.server!9999 server /n/server
The client machine will print "post..." and following this, the client may
client% ls /n/server
and use other commands to work with the mounted filesystem. After this connection has been made, you may terminate the listener on the server machine. It is not needed once the mount has been made.
STANDARD 9P SERVICES FROM FILE AND CPU SERVERS
There are two main types of 9p service provided in a typical installation. Disk fileservers such as fossil(4) serve 9p on port 564. This is used for tcp boot of other machines or general purpose disk storage. The Bell Labs "sources" server is an example. The commands srv(4) and the 9fs script connect to this type of 9p server.
CPU servers serve 9p via exportfs(4) on port 17007 and clients connect by means of import(4). Exportfs enables serving arbitrary namespace, not simply on-disk file servers. The cpu(1) command makes use of exportfs to let the cpu server use the devices and filesystem of the client terminal. Cpu export/import traffic is encrypted by default. 9p file service on port 564 is not. The normal usage for import(4) requires specifying what portion of the cpu namespace to attach. An example:
client% import cpuserver /srv /n/cpusrv
This selects only the /srv of the target cpuserver and mounts it to /n/cpusrv. Some of the most important targets for import are /srv and /proc. A grid of cpu servers running from a common file server and which all import /srv and /proc and certain /dev files from each other behaves very much like single large machine. This is the prototypical form of a "Plan 9 Grid" of machines.
SERVICES ON DEMAND: USING AUX/LISTEN1 AND EXPORTFS
In a standard configuration of installed machines, file servers listen on port 564 and cpus listen on port 17007. The file listener is the responsibility of the disk fs, but the cpu listener is started by the aux/listen(8) command which starts listeners for every service found in /rc/bin/service. The aux/listen1 command allows users to start individual service listeners on demand.
Using this effectively requires understanding the additional options for exportfs. There is an important caveat - the -a flag for authentication of exportfs is in conflict with the -r and -S flags for specifying a root or /srv to mount. If you wish to use exportfs to provide secure non-root service exportfs publicly, additional measures such as the use of tlssrv(8) would be necessary.
When exportfs is used with the flag -r to specify a root or -S to specify a /srv, it becomes compatible with the operation of srv(4) and mount(1). An exportfs done with these flags may be used via srv and mount of the address. Srv may not be used to attach to standard exportfs listeners running the full protocol.
The example at the beginning of this page shows the basic formula for using exportfs and srv together in this manner. If you wish to use exportfs for authenticated connections, use import for the client, like this:
server% aux/listen1 -v tcp!*!9999 /bin/exportfs -a client% import tcp!server!9999 /remote/path /local/mountpoint
Note the absence of the -t flag from this listen1 command.
SRV IN DETAIL: MORE THAN JUST 9P
The srv(4) command as used above is actually two commands in one. An initial srv to dial a connection and post a file descriptor in /srv, and a mount command to attach to that file descriptor as a 9p fs. If you use srv in this manner:
server% srv tcp!server!port srvname
Then srv acts to dial server and post that network connection as a file descriptor in /srv. If that file descriptor is a 9p fs, it can be mounted. If it is not a 9p fs, it can still be interacted with. An example to show the principles:
server% aux/listen1 -tv tcp!*!9999 /bin/cat /lib/namespace client% srv tcp!server!9999 remotens client% cat /srv/remotens
The output will be the output of the cat command on the server machine. In the case of a 9p service, you can separate the srv and the mount as follows:
client% srv tcp!server!564 server client% mount /srv/server /n/server
Note that the action of srv simply dials and places a file descriptor. If authentication is required, that occurs when the mount command is issued.
LISTEN1 AND NAMESPACES: THE -T FLAG
Most of the examples on this page use the -t flag for listen1. This means that the service listener is running as the invoking user. Without the -t flag, the listener runs as user "none". This is a significant difference because your user account has a different namespace than the "generic" namespace of "none". When you need to share a namespace that includes mounts and binds that are not part of the generic namespace, the -t flag is necessary.
However, this does make the listener more vulnerable to malicious security threats. As user "none" a listener which is subverted can do much less harm than a user listening under the identity of the invoking user, especially if that user is the hostowner. Be aware of the difference between -t and listening as none. It is usually easier and more convenient to listen as your user account, but this is not recommended for public facing services or services which may be connected to by untrusted users.
Also understand that some processes, such as the server side of cpu(1), may be able to change user identity even when the invoking listener is none.
This page does not address the configuration of ndb(8) and the configuration of factotum(4) and auth(8). It is crucial to have a correct /lib/ndb/local and correct authentication between grid nodes for standard use of these tools as described by the manpages and this article.
Mounting and serving 9p can also be done with Inferno, plan9port, and many other libraries and tools. In general correctly implemented clients and servers should all be compatible, but in practice issues like support for the exportfs protocol may require adjustments and compromises. The initial author of this wiki page recommends u9fs, 9pfuse, plan9port, and Inferno as the best supported non-plan9 9p clients and servers.