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 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 either in libc or 38 LibreSSL. 39 40 Multiple Precision Integer Library 41 42 - Uses the LibreSSL 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. 66 67 - The code in packet.c calls the buffer manipulation routines 68 (buffer.c, bufaux.c), compression routines (zlib), and the 69 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 Pseudo-tty manipulation and tty modes 110 111 - Code to allocate and use a pseudo tty is in pty.c. Code to 112 encode and set terminal modes is in ttymodes.c. 113 114 Logging in (updating utmp, lastlog, etc.) 115 116 - The code to do things that are done when a user logs in are in 117 login.c. This includes things such as updating the utmp, wtmp, 118 and lastlog files. Some of the code is in sshd.c. 119 120 Writing to the system log and terminal 121 122 - The programs use the functions fatal(), log(), debug(), error() 123 in many places to write messages to system log or user's 124 terminal. The implementation that logs to system log is in 125 log-server.c; it is used in the server program. The other 126 programs use an implementation that sends output to stderr; it 127 is in log-client.c. The definitions are in ssh.h. 128 129 The sshd server (daemon) 130 131 - The sshd daemon starts by processing arguments and reading the 132 configuration file (servconf.c). It then reads the host key, 133 starts listening for connections, and generates the server key. 134 The server key will be regenerated every hour by an alarm. 135 136 - When the server receives a connection, it forks, disables the 137 regeneration alarm, and starts communicating with the client. 138 They first perform identification string exchange, then 139 negotiate encryption, then perform authentication, preparatory 140 operations, and finally the server enters the normal session 141 mode by calling server_loop in serverloop.c. This does the real 142 work, calling functions in other modules. 143 144 - The code for the server is in sshd.c. It contains a lot of 145 stuff, including: 146 - server main program 147 - waiting for connections 148 - processing new connection 149 - authentication 150 - preparatory operations 151 - building up the execution environment for the user program 152 - starting the user program. 153 154 Auxiliary files 155 156 - There are several other files in the distribution that contain 157 various auxiliary routines: 158 ssh.h the main header file for ssh (various definitions) 159 uidswap.c uid-swapping 160 xmalloc.c "safe" malloc routines 161 162$OpenBSD: OVERVIEW,v 1.15 2018/10/23 05:56:35 djm Exp $ 163$NetBSD: OVERVIEW,v 1.8 2019/04/20 17:16:40 christos Exp $ 164