OVERVIEW revision 1.7
1 [Note: This file has not been updated for OpenSSH versions after 2 OpenSSH-1.2 and should be considered OBSOLETE. It has been left in 3 the distribution because some of its information may still be useful 4 to developers.] 5 6 This document is intended for those who wish to read the ssh source 7 code. This tries to give an overview of the structure of the code. 8 9 Copyright (c) 1995 Tatu Ylonen <ylo (a] cs.hut.fi> 10 Updated 17 Nov 1995. 11 Updated 19 Oct 1999 for OpenSSH-1.2 12 Updated 20 May 2001 note obsolete for > OpenSSH-1.2 13 14 The software consists of ssh (client), sshd (server), scp, sdist, and 15 the auxiliary programs ssh-keygen, ssh-agent, ssh-add, and 16 make-ssh-known-hosts. The main program for each of these is in a .c 17 file with the same name. 18 19 There are some subsystems/abstractions that are used by a number of 20 these 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 buffer manipulation functions are in 27 sshbuf*.c (header sshbuf.h). 28 29 Compression Library 30 31 - Ssh uses the GNU GZIP compression library (ZLIB). 32 33 Encryption/Decryption 34 35 - Ssh contains several encryption algorithms. These are all 36 accessed through the cipher.h interface. The interface code is 37 in cipher.c, and the implementations are in libc. 38 39 Multiple Precision Integer Library 40 41 - Uses the SSLeay BIGNUM sublibrary. 42 43 Random Numbers 44 45 - Uses arc4random() and such. 46 47 RSA key generation, encryption, decryption 48 49 - Ssh uses the RSA routines in libssl. 50 51 RSA key files 52 53 - RSA keys are stored in files with a special format. The code to 54 read/write these files is in authfile.c. The files are normally 55 encrypted with a passphrase. The functions to read passphrases 56 are in readpass.c (the same code is used to read passwords). 57 58 Binary packet protocol 59 60 - The ssh binary packet protocol is implemented in packet.c. The 61 code in packet.c does not concern itself with packet types or their 62 execution; it contains code to build packets, to receive them and 63 extract data from them, and the code to compress and/or encrypt 64 packets. 65 66 - The code in packet.c calls the buffer manipulation routines 67 (buffer.c, bufaux.c), compression routines (zlib), and the 68 encryption routines. 69 70 X11, TCP/IP, and Agent forwarding 71 72 - Code for various types of channel forwarding is in channels.c. 73 The file defines a generic framework for arbitrary communication 74 channels inside the secure channel, and uses this framework to 75 implement X11 forwarding, TCP/IP forwarding, and authentication 76 agent forwarding. 77 The new, Protocol 1.5, channel close implementation is in nchan.c 78 79 Authentication agent 80 81 - Code to communicate with the authentication agent is in authfd.c. 82 83 Authentication methods 84 85 - Code for various authentication methods resides in auth-*.c 86 (auth-passwd.c, auth-rh-rsa.c, auth-rhosts.c, auth-rsa.c). This 87 code is linked into the server. The routines also manipulate 88 known hosts files using code in hostfile.c. Code in canohost.c 89 is used to retrieve the canonical host name of the remote host. 90 Code in match.c is used to match host names. 91 92 - In the client end, authentication code is in sshconnect.c. It 93 reads Passwords/passphrases using code in readpass.c. It reads 94 RSA key files with authfile.c. It communicates the 95 authentication agent using authfd.c. 96 97 The ssh client 98 99 - The client main program is in ssh.c. It first parses arguments 100 and reads configuration (readconf.c), then calls ssh_connect (in 101 sshconnect.c) to open a connection to the server (possibly via a 102 proxy), and performs authentication (ssh_login in sshconnect.c). 103 It then makes any pty, forwarding, etc. requests. It may call 104 code in ttymodes.c to encode current tty modes. Finally it 105 calls client_loop in clientloop.c. This does the real work for 106 the session. 107 108 Pseudo-tty manipulation and tty modes 109 110 - Code to allocate and use a pseudo tty is in pty.c. Code to 111 encode and set terminal modes is in ttymodes.c. 112 113 Logging in (updating utmp, lastlog, etc.) 114 115 - The code to do things that are done when a user logs in are in 116 login.c. This includes things such as updating the utmp, wtmp, 117 and lastlog files. Some of the code is in sshd.c. 118 119 Writing to the system log and terminal 120 121 - The programs use the functions fatal(), log(), debug(), error() 122 in many places to write messages to system log or user's 123 terminal. The implementation that logs to system log is in 124 log-server.c; it is used in the server program. The other 125 programs use an implementation that sends output to stderr; it 126 is in log-client.c. The definitions are in ssh.h. 127 128 The sshd server (daemon) 129 130 - The sshd daemon starts by processing arguments and reading the 131 configuration file (servconf.c). It then reads the host key, 132 starts listening for connections, and generates the server key. 133 The server key will be regenerated every hour by an alarm. 134 135 - When the server receives a connection, it forks, disables the 136 regeneration alarm, and starts communicating with the client. 137 They first perform identification string exchange, then 138 negotiate encryption, then perform authentication, preparatory 139 operations, and finally the server enters the normal session 140 mode by calling server_loop in serverloop.c. This does the real 141 work, calling functions in other modules. 142 143 - The code for the server is in sshd.c. It contains a lot of 144 stuff, including: 145 - server main program 146 - waiting for connections 147 - processing new connection 148 - authentication 149 - preparatory operations 150 - building up the execution environment for the user program 151 - starting the user program. 152 153 Auxiliary files 154 155 - There are several other files in the distribution that contain 156 various auxiliary routines: 157 ssh.h the main header file for ssh (various definitions) 158 uidswap.c uid-swapping 159 xmalloc.c "safe" malloc routines 160 161 $OpenBSD: OVERVIEW,v 1.14 2018/07/27 03:55:22 dtucker Exp $ 162 $NetBSD: OVERVIEW,v 1.7 2018/08/26 07:46:36 christos Exp $ 163
Indexes created Thu Sep 10 00:26:39 UTC 2026