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Damien Millerd4d11932001-09-25 12:55:37 +10001[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
Damien Miller431f66b1999-11-21 18:31:57 +11006This document is intended for those who wish to read the ssh source
Damien Millerd4a8b7e1999-10-27 13:42:43 +10007code. This tries to give an overview of the structure of the code.
Damien Millera8e06ce2003-11-21 23:48:55 +11008
Damien Millerd4a8b7e1999-10-27 13:42:43 +10009Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>
10Updated 17 Nov 1995.
11Updated 19 Oct 1999 for OpenSSH-1.2
Damien Millerd4d11932001-09-25 12:55:37 +100012Updated 20 May 2001 note obsolete for > OpenSSH-1.2
Damien Millerd4a8b7e1999-10-27 13:42:43 +100013
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
Damien Millera8e06ce2003-11-21 23:48:55 +110023
Damien Millerd4a8b7e1999-10-27 13:42:43 +100024 - 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
Damien Millera8e06ce2003-11-21 23:48:55 +110031
Damien Millerd4a8b7e1999-10-27 13:42:43 +100032 - 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 - Some auxiliary functions for mp-int manipulation are in mpaux.c.
44
45 Random Numbers
46
47 - Uses arc4random() and such.
48
49 RSA key generation, encryption, decryption
50
51 - Ssh uses the RSA routines in libssl.
52
53 RSA key files
54
55 - RSA keys are stored in files with a special format. The code to
56 read/write these files is in authfile.c. The files are normally
57 encrypted with a passphrase. The functions to read passphrases
58 are in readpass.c (the same code is used to read passwords).
59
60 Binary packet protocol
61
62 - The ssh binary packet protocol is implemented in packet.c. The
63 code in packet.c does not concern itself with packet types or their
64 execution; it contains code to build packets, to receive them and
65 extract data from them, and the code to compress and/or encrypt
66 packets. CRC code comes from crc32.c.
67
68 - The code in packet.c calls the buffer manipulation routines
69 (buffer.c, bufaux.c), compression routines (compress.c, zlib),
70 and the encryption routines.
71
72 X11, TCP/IP, and Agent forwarding
73
74 - Code for various types of channel forwarding is in channels.c.
75 The file defines a generic framework for arbitrary communication
76 channels inside the secure channel, and uses this framework to
77 implement X11 forwarding, TCP/IP forwarding, and authentication
78 agent forwarding.
79 The new, Protocol 1.5, channel close implementation is in nchan.c
80
81 Authentication agent
82
83 - Code to communicate with the authentication agent is in authfd.c.
84
85 Authentication methods
86
87 - Code for various authentication methods resides in auth-*.c
88 (auth-passwd.c, auth-rh-rsa.c, auth-rhosts.c, auth-rsa.c). This
89 code is linked into the server. The routines also manipulate
90 known hosts files using code in hostfile.c. Code in canohost.c
91 is used to retrieve the canonical host name of the remote host.
Damien Millera8e06ce2003-11-21 23:48:55 +110092 Code in match.c is used to match host names.
Damien Millerd4a8b7e1999-10-27 13:42:43 +100093
94 - In the client end, authentication code is in sshconnect.c. It
95 reads Passwords/passphrases using code in readpass.c. It reads
96 RSA key files with authfile.c. It communicates the
97 authentication agent using authfd.c.
98
99 The ssh client
100
101 - The client main program is in ssh.c. It first parses arguments
102 and reads configuration (readconf.c), then calls ssh_connect (in
103 sshconnect.c) to open a connection to the server (possibly via a
104 proxy), and performs authentication (ssh_login in sshconnect.c).
105 It then makes any pty, forwarding, etc. requests. It may call
106 code in ttymodes.c to encode current tty modes. Finally it
107 calls client_loop in clientloop.c. This does the real work for
108 the session.
109
110 - The client is suid root. It tries to temporarily give up this
111 rights while reading the configuration data. The root
112 privileges are only used to make the connection (from a
113 privileged socket). Any extra privileges are dropped before
114 calling ssh_login.
115
116 Pseudo-tty manipulation and tty modes
117
118 - Code to allocate and use a pseudo tty is in pty.c. Code to
119 encode and set terminal modes is in ttymodes.c.
120
121 Logging in (updating utmp, lastlog, etc.)
122
123 - The code to do things that are done when a user logs in are in
124 login.c. This includes things such as updating the utmp, wtmp,
125 and lastlog files. Some of the code is in sshd.c.
126
127 Writing to the system log and terminal
128
129 - The programs use the functions fatal(), log(), debug(), error()
130 in many places to write messages to system log or user's
131 terminal. The implementation that logs to system log is in
132 log-server.c; it is used in the server program. The other
133 programs use an implementation that sends output to stderr; it
134 is in log-client.c. The definitions are in ssh.h.
135
136 The sshd server (daemon)
137
138 - The sshd daemon starts by processing arguments and reading the
139 configuration file (servconf.c). It then reads the host key,
140 starts listening for connections, and generates the server key.
141 The server key will be regenerated every hour by an alarm.
142
143 - When the server receives a connection, it forks, disables the
144 regeneration alarm, and starts communicating with the client.
145 They first perform identification string exchange, then
146 negotiate encryption, then perform authentication, preparatory
147 operations, and finally the server enters the normal session
148 mode by calling server_loop in serverloop.c. This does the real
149 work, calling functions in other modules.
Damien Millera8e06ce2003-11-21 23:48:55 +1100150
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000151 - The code for the server is in sshd.c. It contains a lot of
152 stuff, including:
Damien Millera8e06ce2003-11-21 23:48:55 +1100153 - server main program
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000154 - waiting for connections
155 - processing new connection
156 - authentication
157 - preparatory operations
158 - building up the execution environment for the user program
159 - starting the user program.
160
161 Auxiliary files
162
163 - There are several other files in the distribution that contain
164 various auxiliary routines:
Damien Millera8e06ce2003-11-21 23:48:55 +1100165 ssh.h the main header file for ssh (various definitions)
166 getput.h byte-order independent storage of integers
167 includes.h includes most system headers. Lots of #ifdefs.
Damien Millerd4a8b7e1999-10-27 13:42:43 +1000168 tildexpand.c expand tilde in file names
169 uidswap.c uid-swapping
170 xmalloc.c "safe" malloc routines