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diff --git a/OVERVIEW b/OVERVIEW new file mode 100644 index 0000000..cec7cd7 --- /dev/null +++ b/OVERVIEW @@ -0,0 +1,162 @@ +[Note: This file has not been updated for OpenSSH versions after +OpenSSH-1.2 and should be considered OBSOLETE. It has been left in +the distribution because some of its information may still be useful +to developers.] + +This document is intended for those who wish to read the ssh source +code. This tries to give an overview of the structure of the code. + +Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi> +Updated 17 Nov 1995. +Updated 19 Oct 1999 for OpenSSH-1.2 +Updated 20 May 2001 note obsolete for > OpenSSH-1.2 + +The software consists of ssh (client), sshd (server), scp, sdist, and +the auxiliary programs ssh-keygen, ssh-agent, ssh-add, and +make-ssh-known-hosts. The main program for each of these is in a .c +file with the same name. + +There are some subsystems/abstractions that are used by a number of +these programs. + + Buffer manipulation routines + + - These provide an arbitrary size buffer, where data can be appended. + Data can be consumed from either end. The code is used heavily + throughout ssh. The buffer manipulation functions are in + sshbuf*.c (header sshbuf.h). + + Compression Library + + - Ssh uses the GNU GZIP compression library (ZLIB). + + Encryption/Decryption + + - Ssh contains several encryption algorithms. These are all + accessed through the cipher.h interface. The interface code is + in cipher.c, and the implementations are either in libc or + LibreSSL. + + Multiple Precision Integer Library + + - Uses the LibreSSL BIGNUM sublibrary. + + Random Numbers + + - Uses arc4random() and such. + + RSA key generation, encryption, decryption + + - Ssh uses the RSA routines in libssl. + + RSA key files + + - RSA keys are stored in files with a special format. The code to + read/write these files is in authfile.c. The files are normally + encrypted with a passphrase. The functions to read passphrases + are in readpass.c (the same code is used to read passwords). + + Binary packet protocol + + - The ssh binary packet protocol is implemented in packet.c. The + code in packet.c does not concern itself with packet types or their + execution; it contains code to build packets, to receive them and + extract data from them, and the code to compress and/or encrypt + packets. + + - The code in packet.c calls the buffer manipulation routines + (buffer.c, bufaux.c), compression routines (zlib), and the + encryption routines. + + X11, TCP/IP, and Agent forwarding + + - Code for various types of channel forwarding is in channels.c. + The file defines a generic framework for arbitrary communication + channels inside the secure channel, and uses this framework to + implement X11 forwarding, TCP/IP forwarding, and authentication + agent forwarding. + The new, Protocol 1.5, channel close implementation is in nchan.c + + Authentication agent + + - Code to communicate with the authentication agent is in authfd.c. + + Authentication methods + + - Code for various authentication methods resides in auth-*.c + (auth-passwd.c, auth-rh-rsa.c, auth-rhosts.c, auth-rsa.c). This + code is linked into the server. The routines also manipulate + known hosts files using code in hostfile.c. Code in canohost.c + is used to retrieve the canonical host name of the remote host. + Code in match.c is used to match host names. + + - In the client end, authentication code is in sshconnect.c. It + reads Passwords/passphrases using code in readpass.c. It reads + RSA key files with authfile.c. It communicates the + authentication agent using authfd.c. + + The ssh client + + - The client main program is in ssh.c. It first parses arguments + and reads configuration (readconf.c), then calls ssh_connect (in + sshconnect.c) to open a connection to the server (possibly via a + proxy), and performs authentication (ssh_login in sshconnect.c). + It then makes any pty, forwarding, etc. requests. It may call + code in ttymodes.c to encode current tty modes. Finally it + calls client_loop in clientloop.c. This does the real work for + the session. + + Pseudo-tty manipulation and tty modes + + - Code to allocate and use a pseudo tty is in pty.c. Code to + encode and set terminal modes is in ttymodes.c. + + Logging in (updating utmp, lastlog, etc.) + + - The code to do things that are done when a user logs in are in + login.c. This includes things such as updating the utmp, wtmp, + and lastlog files. Some of the code is in sshd.c. + + Writing to the system log and terminal + + - The programs use the functions fatal(), log(), debug(), error() + in many places to write messages to system log or user's + terminal. The implementation that logs to system log is in + log-server.c; it is used in the server program. The other + programs use an implementation that sends output to stderr; it + is in log-client.c. The definitions are in ssh.h. + + The sshd server (daemon) + + - The sshd daemon starts by processing arguments and reading the + configuration file (servconf.c). It then reads the host key, + starts listening for connections, and generates the server key. + The server key will be regenerated every hour by an alarm. + + - When the server receives a connection, it forks, disables the + regeneration alarm, and starts communicating with the client. + They first perform identification string exchange, then + negotiate encryption, then perform authentication, preparatory + operations, and finally the server enters the normal session + mode by calling server_loop in serverloop.c. This does the real + work, calling functions in other modules. + + - The code for the server is in sshd.c. It contains a lot of + stuff, including: + - server main program + - waiting for connections + - processing new connection + - authentication + - preparatory operations + - building up the execution environment for the user program + - starting the user program. + + Auxiliary files + + - There are several other files in the distribution that contain + various auxiliary routines: + ssh.h the main header file for ssh (various definitions) + uidswap.c uid-swapping + xmalloc.c "safe" malloc routines + +$OpenBSD: OVERVIEW,v 1.15 2018/10/23 05:56:35 djm Exp $ |