[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
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 <firstname.lastname@example.org>
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
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.
- Ssh uses the GNU GZIP compression library (ZLIB).
- 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.
- 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
The new, Protocol 1.5, channel close implementation is in nchan.c
- Code to communicate with the authentication agent is in authfd.c.
- 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 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
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
- server main program
- waiting for connections
- processing new connection
- preparatory operations
- building up the execution environment for the user program
- starting the user program.
- 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
xmalloc.c "safe" malloc routines