1[Note: This file has not been updated for OpenSSH versions after
2OpenSSH-1.2 and should be considered OBSOLETE.  It has been left in
3the distribution because some of its information may still be useful
4to developers.]
5
6This document is intended for those who wish to read the ssh source
7code.  This tries to give an overview of the structure of the code.
8
9Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>
10Updated 17 Nov 1995.
11Updated 19 Oct 1999 for OpenSSH-1.2
12Updated 20 May 2001 note obsolete for > OpenSSH-1.2
13
14The software consists of ssh (client), sshd (server), scp, sdist, and
15the auxiliary programs ssh-keygen, ssh-agent, ssh-add, and
16make-ssh-known-hosts.  The main program for each of these is in a .c
17file with the same name.
18
19There are some subsystems/abstractions that are used by a number of
20these programs.
21
22  Buffer manipulation routines
23
24    - These provide an arbitrary size buffer, where data can be appended.
25      Data can be consumed from either end.  The code is used heavily
26      throughout ssh.  The basic buffer manipulation functions are in
27      buffer.c (header buffer.h), and additional code to manipulate specific
28      data types is in bufaux.c.
29
30  Compression Library
31
32    - Ssh uses the GNU GZIP compression library (ZLIB).
33
34  Encryption/Decryption
35
36    - Ssh contains several encryption algorithms.  These are all
37      accessed through the cipher.h interface.  The interface code is
38      in cipher.c, and the implementations are in libc.
39
40  Multiple Precision Integer Library
41
42    - Uses the SSLeay BIGNUM sublibrary.
43
44  Random Numbers
45
46    - Uses arc4random() and such.
47
48  RSA key generation, encryption, decryption
49
50    - Ssh uses the RSA routines in libssl.
51
52  RSA key files
53
54    - RSA keys are stored in files with a special format.  The code to
55      read/write these files is in authfile.c.  The files are normally
56      encrypted with a passphrase.  The functions to read passphrases
57      are in readpass.c (the same code is used to read passwords).
58
59  Binary packet protocol
60
61    - The ssh binary packet protocol is implemented in packet.c.  The
62      code in packet.c does not concern itself with packet types or their
63      execution; it contains code to build packets, to receive them and
64      extract data from them, and the code to compress and/or encrypt
65      packets.  CRC code comes from crc32.c.
66
67    - The code in packet.c calls the buffer manipulation routines
68      (buffer.c, bufaux.c), compression routines (compress.c, zlib),
69      and the encryption routines.
70
71  X11, TCP/IP, and Agent forwarding
72
73    - Code for various types of channel forwarding is in channels.c.
74      The file defines a generic framework for arbitrary communication
75      channels inside the secure channel, and uses this framework to
76      implement X11 forwarding, TCP/IP forwarding, and authentication
77      agent forwarding.
78      The new, Protocol 1.5, channel close implementation is in nchan.c
79
80  Authentication agent
81
82    - Code to communicate with the authentication agent is in authfd.c.
83
84  Authentication methods
85
86    - Code for various authentication methods resides in auth-*.c
87      (auth-passwd.c, auth-rh-rsa.c, auth-rhosts.c, auth-rsa.c).  This
88      code is linked into the server.  The routines also manipulate
89      known hosts files using code in hostfile.c.  Code in canohost.c
90      is used to retrieve the canonical host name of the remote host.
91      Code in match.c is used to match host names.
92
93    - In the client end, authentication code is in sshconnect.c.  It
94      reads Passwords/passphrases using code in readpass.c.  It reads
95      RSA key files with authfile.c.  It communicates the
96      authentication agent using authfd.c.
97
98  The ssh client
99
100    - The client main program is in ssh.c.  It first parses arguments
101      and reads configuration (readconf.c), then calls ssh_connect (in
102      sshconnect.c) to open a connection to the server (possibly via a
103      proxy), and performs authentication (ssh_login in sshconnect.c).
104      It then makes any pty, forwarding, etc. requests.  It may call
105      code in ttymodes.c to encode current tty modes.  Finally it
106      calls client_loop in clientloop.c.  This does the real work for
107      the session.
108
109    - The client is suid root.  It tries to temporarily give up this
110      rights while reading the configuration data.  The root
111      privileges are only used to make the connection (from a
112      privileged socket).  Any extra privileges are dropped before
113      calling ssh_login.
114
115  Pseudo-tty manipulation and tty modes
116
117    - Code to allocate and use a pseudo tty is in pty.c.  Code to
118      encode and set terminal modes is in ttymodes.c.
119
120  Logging in (updating utmp, lastlog, etc.)
121
122    - The code to do things that are done when a user logs in are in
123      login.c.  This includes things such as updating the utmp, wtmp,
124      and lastlog files.  Some of the code is in sshd.c.
125
126  Writing to the system log and terminal
127
128    - The programs use the functions fatal(), log(), debug(), error()
129      in many places to write messages to system log or user's
130      terminal.  The implementation that logs to system log is in
131      log-server.c; it is used in the server program.  The other
132      programs use an implementation that sends output to stderr; it
133      is in log-client.c.  The definitions are in ssh.h.
134
135  The sshd server (daemon)
136
137    - The sshd daemon starts by processing arguments and reading the
138      configuration file (servconf.c).  It then reads the host key,
139      starts listening for connections, and generates the server key.
140      The server key will be regenerated every hour by an alarm.
141
142    - When the server receives a connection, it forks, disables the
143      regeneration alarm, and starts communicating with the client.
144      They first perform identification string exchange, then
145      negotiate encryption, then perform authentication, preparatory
146      operations, and finally the server enters the normal session
147      mode by calling server_loop in serverloop.c.  This does the real
148      work, calling functions in other modules.
149
150    - The code for the server is in sshd.c.  It contains a lot of
151      stuff, including:
152	- server main program
153	- waiting for connections
154	- processing new connection
155	- authentication
156	- preparatory operations
157	- building up the execution environment for the user program
158	- starting the user program.
159
160  Auxiliary files
161
162    - There are several other files in the distribution that contain
163      various auxiliary routines:
164	ssh.h	     the main header file for ssh (various definitions)
165	uidswap.c    uid-swapping
166	xmalloc.c    "safe" malloc routines
167
168$OpenBSD: OVERVIEW,v 1.11 2006/08/03 03:34:41 deraadt Exp $
169