165 lines
6.5 KiB
Plaintext
165 lines
6.5 KiB
Plaintext
This document is intended for those who wish to read the ssh source
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code. This tries to give an overview of the structure of the code.
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Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>
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Updated 17 Nov 1995.
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Updated 19 Oct 1999 for OpenSSH-1.2
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The software consists of ssh (client), sshd (server), scp, sdist, and
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the auxiliary programs ssh-keygen, ssh-agent, ssh-add, and
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make-ssh-known-hosts. The main program for each of these is in a .c
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file with the same name.
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There are some subsystems/abstractions that are used by a number of
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these programs.
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Buffer manipulation routines
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- These provide an arbitrary size buffer, where data can be appended.
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Data can be consumed from either end. The code is used heavily
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throughout ssh. The basic buffer manipulation functions are in
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buffer.c (header buffer.h), and additional code to manipulate specific
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data types is in bufaux.c.
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Compression Library
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- Ssh uses the GNU GZIP compression library (ZLIB).
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Encryption/Decryption
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- Ssh contains several encryption algorithms. These are all
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accessed through the cipher.h interface. The interface code is
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in cipher.c, and the implementations are in libc.
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Multiple Precision Integer Library
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- Uses the SSLeay BIGNUM sublibrary.
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- Some auxiliary functions for mp-int manipulation are in mpaux.c.
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Random Numbers
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- Uses arc4random() and such.
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RSA key generation, encryption, decryption
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- Ssh uses the RSA routines in libssl.
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RSA key files
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- RSA keys are stored in files with a special format. The code to
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read/write these files is in authfile.c. The files are normally
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encrypted with a passphrase. The functions to read passphrases
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are in readpass.c (the same code is used to read passwords).
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Binary packet protocol
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- The ssh binary packet protocol is implemented in packet.c. The
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code in packet.c does not concern itself with packet types or their
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execution; it contains code to build packets, to receive them and
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extract data from them, and the code to compress and/or encrypt
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packets. CRC code comes from crc32.c.
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- The code in packet.c calls the buffer manipulation routines
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(buffer.c, bufaux.c), compression routines (compress.c, zlib),
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and the encryption routines.
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X11, TCP/IP, and Agent forwarding
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- Code for various types of channel forwarding is in channels.c.
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The file defines a generic framework for arbitrary communication
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channels inside the secure channel, and uses this framework to
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implement X11 forwarding, TCP/IP forwarding, and authentication
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agent forwarding.
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The new, Protocol 1.5, channel close implementation is in nchan.c
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Authentication agent
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- Code to communicate with the authentication agent is in authfd.c.
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Authentication methods
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- Code for various authentication methods resides in auth-*.c
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(auth-passwd.c, auth-rh-rsa.c, auth-rhosts.c, auth-rsa.c). This
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code is linked into the server. The routines also manipulate
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known hosts files using code in hostfile.c. Code in canohost.c
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is used to retrieve the canonical host name of the remote host.
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Code in match.c is used to match host names.
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- In the client end, authentication code is in sshconnect.c. It
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reads Passwords/passphrases using code in readpass.c. It reads
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RSA key files with authfile.c. It communicates the
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authentication agent using authfd.c.
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The ssh client
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- The client main program is in ssh.c. It first parses arguments
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and reads configuration (readconf.c), then calls ssh_connect (in
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sshconnect.c) to open a connection to the server (possibly via a
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proxy), and performs authentication (ssh_login in sshconnect.c).
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It then makes any pty, forwarding, etc. requests. It may call
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code in ttymodes.c to encode current tty modes. Finally it
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calls client_loop in clientloop.c. This does the real work for
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the session.
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- The client is suid root. It tries to temporarily give up this
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rights while reading the configuration data. The root
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privileges are only used to make the connection (from a
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privileged socket). Any extra privileges are dropped before
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calling ssh_login.
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Pseudo-tty manipulation and tty modes
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- Code to allocate and use a pseudo tty is in pty.c. Code to
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encode and set terminal modes is in ttymodes.c.
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Logging in (updating utmp, lastlog, etc.)
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- The code to do things that are done when a user logs in are in
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login.c. This includes things such as updating the utmp, wtmp,
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and lastlog files. Some of the code is in sshd.c.
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Writing to the system log and terminal
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- The programs use the functions fatal(), log(), debug(), error()
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in many places to write messages to system log or user's
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terminal. The implementation that logs to system log is in
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log-server.c; it is used in the server program. The other
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programs use an implementation that sends output to stderr; it
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is in log-client.c. The definitions are in ssh.h.
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The sshd server (daemon)
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- The sshd daemon starts by processing arguments and reading the
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configuration file (servconf.c). It then reads the host key,
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starts listening for connections, and generates the server key.
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The server key will be regenerated every hour by an alarm.
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- When the server receives a connection, it forks, disables the
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regeneration alarm, and starts communicating with the client.
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They first perform identification string exchange, then
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negotiate encryption, then perform authentication, preparatory
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operations, and finally the server enters the normal session
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mode by calling server_loop in serverloop.c. This does the real
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work, calling functions in other modules.
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- The code for the server is in sshd.c. It contains a lot of
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stuff, including:
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- server main program
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- waiting for connections
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- processing new connection
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- authentication
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- preparatory operations
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- building up the execution environment for the user program
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- starting the user program.
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Auxiliary files
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- There are several other files in the distribution that contain
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various auxiliary routines:
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ssh.h the main header file for ssh (various definitions)
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getput.h byte-order independent storage of integers
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includes.h includes most system headers. Lots of #ifdefs.
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tildexpand.c expand tilde in file names
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uidswap.c uid-swapping
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xmalloc.c "safe" malloc routines
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