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authorDamien Miller <>1999-10-27 13:42:43 +1000
committerDamien Miller <>1999-10-27 13:42:43 +1000
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+This document is inteded 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 <>
+Updated 17 Nov 1995.
+Updated 19 Oct 1999 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 basic buffer manipulation functions are in
+ buffer.c (header buffer.h), and additional code to manipulate specific
+ data types is in bufaux.c.
+ 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 in libc.
+ Multiple Precision Integer Library
+ - Uses the SSLeay BIGNUM sublibrary.
+ - Some auxiliary functions for mp-int manipulation are in mpaux.c.
+ 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. CRC code comes from crc32.c.
+ - The code in packet.c calls the buffer manipulation routines
+ (buffer.c, bufaux.c), compression routines (compress.c, 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.
+ - The client is suid root. It tries to temporarily give up this
+ rights while reading the configuration data. The root
+ privileges are only used to make the connection (from a
+ privileged socket). Any extra privileges are dropped before
+ calling ssh_login.
+ 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)
+ getput.h byte-order independent storage of integers
+ includes.h includes most system headers. Lots of #ifdefs.
+ tildexpand.c expand tilde in file names
+ uidswap.c uid-swapping
+ xmalloc.c "safe" malloc routines