SSH(4) SSH(4)
NAME
ssh, netssh - serve SSH v2 over TCP
SYNOPSIS
netssh [ -9dkv ] [ -m mntpt ] [ -s srvpt ]
cd /net/ssh ./n/status
./clone ./n/tcp
./ctl ...
./keys ./n/ch
./n ./n/ch/ctl
./n/clone ./n/ch/data
./n/ctl ./n/ch/listen
./n/data ./n/ch/request
./n/listen ./n/ch/status
./n/local ...
./n/remote
DESCRIPTION
The netssh file server implements SSH v2 over TCP. All of
the encryption, authentication, and SSH protocol are handled
by a server managing SSH tunnels that appears as a protocol
directory, /net/ssh, similar to those of ip(3) but with an
extra level of hierarchy for SSH channels within connec-
tions. Options are:
-d Increase the amount of debugging output.
-k Use keyfs(4) for password validation.
-m Mount point for the SSH protocol directory; defaults to
/net.
-s Name to post in /srv. If -s is not given, no file is
posted to /srv.
-v Do not verify the remote system's host key (which is
somewhat pedantic anyway). This lets us talk to SSH v1
systems. The presence of /env/nosshkeyverify is equiva-
lent to specifying this option.
/net/ssh contains a set of numbered directories, each of
which is an SSH connection that is currently active or has
been used in the past, per ip(3). Opening clone reserves an
SSH connection, reading from it returns the connection num-
ber reserved, and writing to it writes to the ctl file in
the numbered connection directory. Reading the ctl file
returns the most active state of any connection.
/net/ssh/ctl currently implements no commands. Finally, the
keys file is used by ssh2(1) to relay information about keys
and passwords between a user and the SSH server.
Each of the numbered connection directories contains a set
of numbered directories, one for each channel used on that
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SSH(4) SSH(4)
connection (see Channel Directories below). Similar to the
top-level clone file, opening a connection's clone file
reserves a channel and gives access to its ctl file. Read-
ing from the ctl file returns the connection number (also
the name of that directory). Commands may be written to a
connection's ctl file:
connect Dial the remote system and perform the initial
handshake to exchange versions, lists of supported
algorithms, and to establish the encryption keys to
use.
ssh-userauth
Attempt to authenticate a user with the remote sys-
tem, with either public key authentication or a
password.
ssh-connection
Currently unsupported.
hangup Shut down a connection and all of its channels.
announce /net/ssh will accept connection requests from
remote systems.
accept Do the initial connection handshake with the call-
ing system.
reject Send back a connection rejection message to the
caller and shut down the connection.
Because data is always carried over a channel, the connec-
tion data file is not used for usual data. However, reads
from the connection data file do return the capability
needed for sshsession to change identity to the user logging
in. As with other protocol directories, opens on listen
block until a remote system establishes a connection, at
which point, a server should write either an accept or
reject message to the ctl file. The local and remote files
give the IP addresses and port numbers of the local and
remote systems. The connection status file gives the status
of the most-recently established channel.
Channel Directories
In each channel directory, /net/ssh/conn/chan, reads from
channel ctl files return the channel number. Commands that
may be written to a channel ctl file include:
connect Create a new channel over this connection. SSHv2
defines session, x11, forwarded-tcpip, and
direct-tcpip channels. The connect command
defaults to a session channel if no argument is
given. (This implementation correctly handles only
session channel requests.)
global Reserved for future development. In particular,
this is necessary to support TCP/IP forwarding.
hangup Shut down a channel. If this is the last open
channel on this connection, then shut down the
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connection too.
announce Announce willingness to accept new channel requests
from the calling system.
The channel data file is the file over which all application
data is carried. Opens of the channel listen file block
until a channel is opened by the remote end. Unlike the
connection listen file, the listening program should not
write an accept or reject message to the ctl file.
SSHv2 defines a number of out-of-band channel requests, sent
and received through the request file. Among these are env,
exec, exit-signal, exit-status, pty-req, shell, signal,
subsystem, window-change, x11-req, and xon-xoff. Sshsession
only fully handles the shell and exec requests. Others are
blithely acknowledged, rejected or ignored, depending on
whether they are expected to be available by the remote sys-
tem.
The channel status file contains one of: Empty, Allocated,
Initting, Listening, Opening, Negotiating, Authing,
Established, Eof, Closing, or Closed.
Cryptographic Algorithms
During the initial connection exchange, both parties send
lists of supported algorithms. The first list is for key
exchange; we support diffie-hellman-group1-sha1 and
diffie-hellman-group14-sha1. The second list is the set for
which corresponding host keys exist; we support ssh-rsa and
ssh-dss. The next lists are encryption algorithms, which
may be negotiated independently for the server-to-client and
client-to-server directions; we support aes128-cbc,
aes192-cbc, aes256-cbc, 3des-cbc, and arcfour with prefer-
ence given in that order. The final list is message authen-
tication code algorithms; we only support hmac-sha1.
Keys and Management
Various keys are used by the SSH server. Most of them are
expected to be stored in the instance of factotum(4) running
in the namespace of that server instance. Sometimes there
are alternative locations.
The first key needed is the host key for server operation.
In the case of the keys being stored in factotum(4), these
keys will be the first ones listed with proto=rsa and
proto=dss. Alternatively, these keys can be specified in
the environment variables rsakey and dsskey or in files of
the same names in the directory where netssh is started.
The next set of keys are the public host keys used by
clients to verify the identities of servers. As with the
original Plan 9 SSH implementation, there is a system-wide
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list of these in /sys/lib/ssh/keyring and each user may have
a list in $home/lib/keyring. If a public key for a remote
server is listed and matches the one offered by the server,
the connection proceeds. If a public key for a remote
server is listed but does not match the one offered by the
server, or if no public key is listed for a remote server,
ssh (see ssh2(1)) presents the key to the user and asks
whether to reject the key, accept the key only for that ses-
sion, or accept the key permanently. The last option causes
the key to be written to the user's keyring. In the case of
a mismatching key, the accept option can either be to add to
or replace the old key.
An SSH daemon, such as sshsession in ssh2(1), must also have
a list of public keys for public key authentication. Again,
these keys must be stored in the factotum instance running
in the name space of the daemon's SSH server. Each such key
must have the attributes role=verify, proto=rsa, and either
user= or sys=.
The next key is a user's private key to be used for public
key authentication. We only support RSA keys for this, and
the key must be in the factotum instance running in the
namespace of the ssh(4) instance. Creating a key and put-
ting it in factotum can be done by:
auth/rsagen >key; read -m key >/mnt/factotum/ctl
The key file will normally be loaded when factotum is
started, either by way of secstore(1) or directly in the
user's lib/profile. See ssh2(1) for rsa2ssh2.
The command
auth/pemdecode 'RSA PRIVATE KEY' id_rsa | auth/asn12rsa >key
will translate a private key used with OpenSSH to one suit-
able for loading into factotum.
To disambiguate when a user has more than one private key
stored in factotum, the following selection criteria are
applied:
1. The selected key must have both proto=rsa and !dk=
attributes present.
2. Among those keys, the attributes user=, sys=, and any
attribute/value pair specified in a -z option to ssh are
examined. The value of the user attribute is expected to
be the user name being authenticated on the remote sys-
tem, and the value of the sys attribute is expected to be
the remote system as specified in the ssh invocation.
3. The key with the greatest number of matches (most
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specific match) is selected. Among keys with equal num-
ber of matches, the first is chosen.
For password-based user authentication, netssh can run in
one of two modes. If given -k, it will validate passwords
against those stored in /mnt/keys provided by keyfs(4). If
run without -k, it will validate passwords with an authenti-
cation server using auth_userpasswd in auth(2).
FILES
/sys/lib/ssh/keyring System-wide known host public keys.
$home/lib/keyring Per-user known host public keys.
/env/nosshkeyverify
SOURCE
/sys/src/cmd/ssh2
SEE ALSO
ssh2(1), ip(3), factotum(4), keyfs(4), authsrv(6), dsa(8),
rsa(8)
RFCs 4250, 4251, 4252, 4253, 4254, and 4419
BUGS
Netssh shouldn't have to run as the host owner and using
factotum(4) correctly would permit this.
Netssh should be simplified by removing the top (connection)
level of the /net/ssh hierarchy and multiplexing a single
network connection after the initial negotiation. This
would fit better with dial(2), permit transports other than
TCP, and allow co-existence of v1 and v2 implementations on
a single TCP port. Better still, use only a single channel
(since we don't use X11) and eliminate the top 2 levels.
Netssh authentication via keyfs and public keys uses
`#ยค/caphash', which isn't normally present after cpurc runs,
so netssh needs to be converted to use factotum.
netssh -k assumes that keyfs is mounted, which is typically
only true of authentication servers.
The keys file protocol should be documented.
Only capable of using TCP for underlying connections.
Can't coexist with SSH v1 on the same port.
Several aspects of key management still need some work.
TCP/IP forwarding and some potentially useful channel
requests have not been implemented.
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Zlib compression is not supported and probably not needed.
The SSH v2 protocol is a classic second system: over-
engineered, overly complicated, misdesigned and jammed full
of pointless goodies.
Host key verification is broken, so it's disabled temporar-
ily until it's fixed.
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