www.allitebooks.com www.allitebooks.com SSH, the Secure Shell The Definitive Guide www.allitebooks.com Other computer security resources from O’Reilly Related titles Security Books Resource Center 802.11 Security Digital Identity Firewall Warrior Internet Forensics Network Security Assessment Network Security with OpenSSL nmap: The Definitive Guide Managing Security with Snort and IDS Tools PGP: Pretty Good Privacy Snort Cookbook security.oreilly.com is a complete catalog of O’Reilly’s books on security and related technologies, including sample chapters and code examples oreillynet.com is the essential portal for developers interested in open and emerging technologies, including new platforms, programming languages, and operating systems Conferences O’Reilly brings diverse innovators together to nurture the ideas that spark revolutionary industries We specialize in documenting the latest tools and systems, translating the innovator’s knowledge into useful skills for those in the trenches Visit conferences.oreilly.com for our upcoming events Safari Bookshelf (safari.oreilly.com) is the premier online reference library for programmers and IT professionals Conduct searches across more than 1,000 books Subscribers can zero in on answers to time-critical questions in a matter of seconds Read the books on your Bookshelf from cover to cover or simply flip to the page you need Try it today with a free trial www.allitebooks.com SECOND EDITION SSH, the Secure Shell The Definitive Guide Daniel J Barrett, Richard E Silverman, and Robert G Byrnes Beijing • Cambridge • Farnham • Köln • Paris • Sebastopol • Taipei • Tokyo www.allitebooks.com SSH, the Secure Shell: The Definitive Guide™ by Daniel J Barrett, Richard E Silverman, and Robert G Byrnes Copyright © 2005, 2001 O’Reilly Media, Inc All rights reserved Printed in the United States of America Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472 O’Reilly books may be purchased for educational, business, or sales promotional use Online editions are also available for most titles (safari.oreilly.com) For more information, contact our corporate/institutional sales department: (800) 998-9938 or corporate@oreilly.com Editor: Mike Loukides Production Editor: Mary Brady Cover Designer: Ellie Volckhausen Interior Designer: David Futato Printing History: February 2001: May 2005: First Edition Second Edition Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc SSH, the Secure Shell: The Definitive Guide, the image of a land snail, and related trade dress are trademarks of O’Reilly Media, Inc Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks Where those designations appear in this book, and O’Reilly Media, Inc was aware of a trademark claim, the designations have been printed in caps or initial caps While every precaution has been taken in the preparation of this book, the publisher and authors assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein This book uses RepKover™, a durable and flexible lay-flat binding ISBN: 0-596-00895-3 ISBN13: 978-0-596-00895-6 [M] [1/07] www.allitebooks.com Table of Contents Preface xi Introduction to SSH 1.1 1.2 1.3 1.4 1.5 1.6 1.7 What Is SSH? What SSH Is Not The SSH Protocol Overview of SSH Features History of SSH Related Technologies Summary 3 10 15 Basic Client Use 16 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 A Running Example Remote Terminal Sessions with ssh Adding Complexity to the Example Authentication by Cryptographic Key The SSH Agent Connecting Without a Password or Passphrase Miscellaneous Clients Summary 16 16 18 21 28 32 33 34 Inside SSH 36 3.1 3.2 3.3 3.4 3.5 Overview of Features A Cryptography Primer The Architecture of an SSH System Inside SSH-2 Inside SSH-1 36 39 43 45 68 v www.allitebooks.com 3.6 3.7 3.8 3.9 3.10 3.11 3.12 Implementation Issues SSH and File Transfers (scp and sftp) Algorithms Used by SSH Threats SSH Can Counter Threats SSH Doesn’t Prevent Threats Caused by SSH Summary 69 81 84 91 93 97 98 Installation and Compile-Time Configuration 99 4.1 4.2 4.3 4.4 4.5 4.6 Overview Installing OpenSSH Installing Tectia Software Inventory Replacing r-Commands with SSH Summary 99 106 111 124 125 127 Serverwide Configuration 128 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 Running the Server Server Configuration: An Overview Getting Ready: Initial Setup Authentication: Verifying Identities Access Control: Letting People In User Logins and Accounts Forwarding Subsystems Logging and Debugging Compatibility Between SSH-1 and SSH-2 Servers Summary 129 132 141 171 184 198 201 206 209 223 226 Key Management and Agents 227 6.1 6.2 6.3 6.4 6.5 6.6 6.7 vi | What Is an Identity? Creating an Identity SSH Agents Multiple Identities PGP Authentication in Tectia Tectia External Keys Summary Table of Contents www.allitebooks.com 227 233 242 260 262 264 265 Advanced Client Use 266 7.1 7.2 7.3 7.4 7.5 7.6 7.7 How to Configure Clients Precedence Introduction to Verbose Mode Client Configuration in Depth Secure Copy with scp Secure, Interactive Copy with sftp Summary 266 276 277 278 313 323 325 Per-Account Server Configuration 326 8.1 8.2 8.3 8.4 8.5 Limits of This Technique Public-Key-Based Configuration Hostbased Access Control The User rc File Summary 326 328 346 348 348 Port Forwarding and X Forwarding 349 9.1 9.2 9.3 9.4 9.5 9.6 What Is Forwarding? Port Forwarding Dynamic Port Forwarding X Forwarding Forwarding Security: TCP-wrappers and libwrap Summary 350 351 373 377 389 395 10 A Recommended Setup 396 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 The Basics Compile-Time Configuration Serverwide Configuration Per-Account Configuration Key Management Client Configuration Remote Home Directories (NFS, AFS) Summary 396 397 397 403 404 404 404 407 11 Case Studies 408 11.1 11.2 11.3 11.4 Unattended SSH: Batch or cron Jobs FTP and SSH Pine, IMAP, and SSH Connecting Through a Gateway Host 408 415 436 444 Table of Contents www.allitebooks.com | vii 11.5 Scalable Authentication for SSH 11.6 Tectia Extensions to Server Configuration Files 11.7 Tectia Plugins 452 468 479 12 Troubleshooting and FAQ 495 12.1 Debug Messages: Your First Line of Defense 12.2 Problems and Solutions 12.3 Other SSH Resources 495 497 513 13 Overview of Other Implementations 515 13.1 Common Features 13.2 Covered Products 13.3 Other SSH Products 515 516 516 14 OpenSSH for Windows 521 14.1 14.2 14.3 14.4 14.5 14.6 Installation Using the SSH Clients Setting Up the SSH Server Public-Key Authentication Troubleshooting Summary 521 522 522 524 525 525 15 OpenSSH for Macintosh 526 15.1 Using the SSH Clients 15.2 Using the OpenSSH Server 526 526 16 Tectia for Windows 531 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9 16.10 16.11 viii | Obtaining and Installing Basic Client Use Key Management Accession Lite Advanced Client Use Port Forwarding Connector File Transfers Command-Line Programs Troubleshooting Server Table of Contents www.allitebooks.com 532 533 534 536 539 542 543 551 552 554 555 idle-timeout keyword, 155, 343, 403, 499 IdPgpKeyFingerprint keyword, 263, 627 IdPgpKeyId keyword, 263, 627 IdPgpKeyName keyword, 263, 627 IgnoreLoginRestrictions.PasswordExpiration keyword, 615 IgnoreLoginRestrictions.Rlogin.AIX keyword, 615 IgnoreRhosts keyword, 76, 175, 176, 197, 198, 405, 412, 478 IgnoreRootRhosts keyword (Tectia), 176 IgnoreUserKnownHosts keyword (OpenSSH), 176 IMAP (Internet Message Access Protocol), 437 authentication, 437 inetd server configuration and debugging, 223 server invocation using, 150 initialization scripts, SSH servers, 200 installation prerequisites, 100 signature verification, 100 software inventory, table, 124 source code, 100 source files extraction, 101 symbolic links created during, 123 Tectia (see Tectia, installation) Unix implementations, 99–101 binary distributions, 99 on Unix systems, 99 integrity, 37 integrity checking, 4, 167–169 interactive sessions, authentication without passwords, 32 International Data Encryption Algorithm (see IDEA) IP attacks, 94 IP spoofing, 91 IPSEC (Internet Protocol Security), 12 J J2SSH Maverick, 517 Java SSH implementations, 517 JavaSSH, 517 JSch, 517 634 | K kadmin command, 413 KDC (Key Distribution Center), 461 KeepAlive keyword, 153 keepalive messages, 152–154 Kerberos, 12, 461–468 batch job authentication using, 413 integration in SSH, 12 OpenSSH and Tectia interoperability, 464–468 OpenSSH implementation, 111 server configuration for, 181–183 support in SSH, 463 tickets, 12 KerberosAuthentication keyword (OpenSSH), 181 KerberosOrLocalPasswd keyword (OpenSSH), 182 KerberosTgtPassing keyword (OpenSSH), 182 KerberosTicketCleanup keyword, 182 Kermit, 518, 519 KEXINIT messages, 51 Key keyword (Tectia), 232, 330 keyboard-interactive authentication, 177–180 one-time passwords, 177 Tectia plugin for, 488–492 KeyRegenerationInterval keyword, 609, 615 keys, 8, 22, 40, 44 changing, 27 host keys, 20, 284–287 implementation dependency, SSH, 69 key exchange, 51 key generators, 45 key management, 227–265 programs for key creation, 227 setup recommendations, 404 key pairs, 228 key-distribution problem, 41 secrecy, 24 session keys, 462 Tectia external keys, 264 troubleshooting, 504 keywords, 134 known hosts, 19–21 known hosts mechanism, 20 known-hosts databases, 45, 284–287 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved L launch-sshd shell script, 139 LDAPServers keyword, 477, 622 limiting simultaneous connections, 157 Linux SSH implementations, 519 ListenAddress, 148 local computers, securing, 29 LocalForward keyword, 355, 356, 357, 593, 622 LoginGraceTime keyword, 155, 156, 477, 615 LogLevel keyword, 312, 313, 615 lsh, 520 M MAC (message authentication code), 53, 167–169 Macintosh OpenSSH, 526–530 SSH clients, 526 SSH server, 526–530 SSH implementations, 517 Macs keyword, 168, 615, 622 MacSFTP, 517 MacSSH, 517 man-in-the-middle attacks, 19, 92 masquerading, 426 Maverick SSHD, 517 Maverick.NET, 517 MaxAuthTries keyword, 156 MaxBroadcastsPerSecond keyword, 159 MaxConnections keyword, 157 MaxStartups keyword, 157 MD5, 90 message authentication code (see MAC) metaconfiguration, 469 Microsoft Windows (see Windows) MindTerm, 517 motd (message of the day), 198 N Nagle Algorithm, 159 name service spoofing, 91 NAT (Network Address Translation), 426–429 masquerading, 426 server-side issues, 427 netgroups, 74 network applications, security issues, Network Information Service (NIS), 11 network interface server settings, 148 networking terminology, NEWKEYS, 57 NFS, recommended settings, 404–407 NiftyTelnet SSH, 517 NIS (Network Information Service), 11 nmap, 161 no-agent-forwarding keyword, 334, 344, 403, 414 NoDelay keyword, 115, 159, 293, 477, 616, 622 NoHostAuthenticationForLocalhost keyword, 287, 622 “none” encryption, 53 no-port-forwarding keyword, 330, 331, 334, 344, 371, 403, 414 no-pty keyword, 296, 334, 345, 346, 403, 414, 506 no-X11-forwarding keyword, 381, 414, 513 NumberOfPasswordPrompts keyword, 295, 622 O one-time pad, 40 one-time passwords, 111, 177 OpenBSD, 5, 10 SSH implementations, 519 OpenSSH, 5, 10, 99 account authorization files, 329 authorization files, 626 configuration, 107–111 access control with TCP-wrappers, 111 command-line flags, 107–111 dependencies, 106 file locations, 107 Kerberos support, 111 networking, 109 PAM authentication, 109 pid file, 108 turning on support for Internet Protocol Version (IPv4), 109 conversion, SSH-1 to SSH-2 keys, 231 environment variables, 627 help command, 278 host access control, 338 host keys implementation, 70 identities, 229, 626 installation, 106–111 build and install, 107 extraction of zipped files, 106 verification with PGP, 107 Macintosh operation, 526–530 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved | 635 OpenSSH (continued) SSH clients, 526 SSH server, 526–530 popularity of, xi prerequisites, 106 privilege separation, 80, 184 public-key installation, 24 quick reference, 612–627 random number generation, 108 random number storage, 79 scp keywords, 620–623 scp options, 619 server configuration, 157 logging and debugging, 211–215 server host-key generation, 130 server protocol version string, 170 serverwide configuration authentication keywords, 171 configuration files, checking, 135 debugging messages, 496 hostbased authentication, 175 Kerberos authentication, 181 password authentication, 173 public-key authentication, 174 recommended settings, 398–401 reverse IP mapping, 158 SSH protocol settings, 169 user welcome, 198 smartcard support, 241 software inventory, 124 SSH configuration directory, key storage file, 24 ssh keywords, 620–623 ssh options, 618 SSH-1, 609 ssh-add options, 625 ssh-agent options, 625 sshd keywords, 613–617 sshd options, 612 ssh-keygen options, 623 SSH_ORIGINAL_COMMAND environment variable, 336 subsystem command syntax, 208 Version 4.0 new features, 591–594 AddressFamily configuration keyword, 591 clients, 592 connection sharing, 592 hostname hashing, 592 KbdInteractiveDevices keyword, 592 logging of access violations, 591 636 | password and account expiration warnings, 591 port forwarding, 592 server, 591 sftp command line, 593 ssh-keygen command-line options, 593 Windows and Cygwin operation, 518, 521–525 agents, 524 Cygwin installation, 521 opening remote windows, 523 public-key authentication, 524 ssh clients, 522 SSH server setup, 522 troubleshooting, 525 OpenSSL, 14, 106 directory path, flagging, 108 Options keyword, 330, 331, 371 OS/2 SSH implementations, 519 P packet filters, stateful, 424 PalmOS SSH implementations, 519 PAM (Pluggable Authentication Modules), 109, 183 OpenSSH authentication, 109 serverwide configuration, 183 passive mode (FTP), 419, 422–426 firewalls and, 424 PASV port theft problem, 423 passphrases, 24 changing, 27 limitations, 28 PasswdPath keyword (Tectia), 174 password authentication, 173–174 batch jobs, issues with, 408 empty passwords, 173 expired passwords, 173 failed password attempts, 173 troubleshooting, 499 password cracking attacks, 93 PasswordAuthentication keyword, 171, 173, 272, 307, 400, 402, 499, 616, 622 PasswordExpireWarningDays keyword, 616 PasswordGuesses keyword (Tectia), 156, 473 PasswordPrompt keyword, 295, 622 passwords one-time passwords, 111 security risks, 21 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved PenguiNet, 518 per-account configuration, 102, 326–348 advantages, 326 authentication, 328 access restriction by host or domain, 338 forced commands, 331 OpenSSH authorization files, 329 public-key based configuration, 328–346 Tectia authorization files, 330 environment variables, setting, 340–343 forwarding, disabling, or limiting, 344 hostbased access control, 346 idle-timeout option, setting (Tectia), 343 limitations, 326 setup recommendations, 403 troubleshooting, 506 TTY allocation, disabling, 345 user’s rc file, 348 Perl modules for SSH implementation, 519 PermitEmptyPasswords keyword, 173 permitopen keyword, 344 PermitUserEnvironment keyword, 199 PGP (Pretty Good Privacy), 11 authentication in Tectia, 262–264 PgpKeyFingerprint keyword, 263, 331, 626 PgpKeyId keyword, 263, 331, 626 PgpKeyName keyword, 263, 331, 626 PGPPublicKeyFile keyword, 478, 616 PgpPublicKeyFile keyword, 263, 331, 626 PgpSecretKeyFile keyword, 264, 627 PidFile keyword, 143, 212, 616 Pine email client, 126, 436–444 connection scripts, 444 mail relaying, 442 remote usernames and, 442 PKI (Public Key Infrastructure), 55, 454 plaintext, 40 PocketPuTTY, 519 PocketTTY, 519 port forwarding, 8, 349, 351–372 dynamic port forwarding, 373–377 firewalls, bypassing, 364 forwarding off-host, 361–364 ftp protocol forwarding, 371 listening port number, 367 local forwarding, 352–356 gateway ports, 355 remote forwarding, compared to, 358–361 multiple connection issues, 357 remote forwarding, 356 remote logins, without, 366–367 server configuration, 201–205, 370 target forwarding address, choosing, 368 TCP-wrappers (see TCP-wrappers) termination, 369 TIME_WAIT problem, 370 troubleshooting, 512 X forwarding (see X forwarding) Port keyword, 148 port number server settings, 148 Pragma Fortress, 518 PreferredAuthentications keyword, 308, 622 PrintLastLog keyword (OpenSSH), 199 PrintMotd keyword, 198, 398, 498, 616 privacy, 37 private keys, 228 privilege separation, issues with, 80 privileged ports, 10 PRNGs (pseudo-random number generators, 79 Protocol keyword (OpenSSH), 223 protocols, ProxyCommand keyword, 445, 450, 451, 452, 622 ProxyServer keyword, 616, 622 pseudo-random number generators (PRNGs), 79 pSSH, 519 pty-req channel request, 66 PubKeyAuthentication keyword (OpenSSH), 174 public key files, 229 Public Key Infrastructure (PKI), 55 PublicHostKeyFile keyword, 142, 477, 616 public-key authentication, 21–32 agents, using (see agents) algorithms, 84–86 authenticator, 22 batch jobs, 409–412 agents, 410–412 filesystem passphrases storage, 409 plaintext keys, 410 client/server interaction, 22 key pair generation, 23 keys, changing, 27 OpenSSH, 27 Tectia systems, 27 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved | 637 public-key authentication (continued) password authentication, compared to, 26 private keys, 22 public keys, 22, 228 installing in remote accounts, 24 OpenSSH installation, 24 Tectia systems, installation, 25 server configuration for, 174 Tectia systems, key generation on, 23 troubleshooting, 501 public-key cryptography, 41 PuTTY, 518, 520, 576–589 batch jobs, 587 configuration and settings authentication, 586 compression, 586 encryption algorithms, 585 logging and debugging, 586 Proxies and SOCKS, 585 pseudo-terminal allocation, 585 configuration and use, 576 host keys, 584 saved sessions, 583 SSH protocol selection, 584 TCP/IP settings, 584 file transfers, 578 PSCP, 579 PSFTP, 579 forwarding, 587 installation, 576 key management, 580–583 agents, 582 key selection, 582 Plink console client, 577 remote commands, 578 TCP/IP settings keepalive messages, 585 Nagle algorithm, 585 remote port selection, 585 Q QuietMode keyword, 211, 313, 478, 616, 622 R random number generation, 78 OpenSSH, 108 random seed, 45 RandomSeed keyword, 616 638 | RandomSeedFile keyword, 143, 477, 616, 622 RC4 (ARCFOUR), 88 r-commands, 10 disabling, 398 insecurity, 11 SSH, replacing with, 125–127 in CVS, 125 in GNU Emacs, 126 in Pine, 126 in rsync and rdist, 127 rcp, 81 rdist, 127 realms, 462 regex syntax, SSH patterns (Tectia), 599–603 character sets, 602 escaped tokens, 601 regular expressions manpage (Tectia), 595–603 egrep patterns, 595 ZSH_FILEGLOB, 597 RekeyIntervalSeconds keyword, 155, 162, 307, 478, 616, 622 remote account name, 279–281 remote program invocation and security, 333 RemoteForward keyword, 356, 357, 593, 622 RemotelyAnywhere, 518 replay attacks, 37 requests, 65 RequiredAuthentications keyword (Tectia), 172 gssapi, 182 hostbased, 175 keyboard interactive, 178 RequireReverseMapping keyword, 158, 189, 193, 477, 616 ResolveClientHostName keyword, 189, 477, 616 restricted shell, 414 reverse IP mappings in server configuration, 158 RhostsRSAAuthentication keyword, 172, 175, 347, 609, 616, 623 RIPEMD-160, 90 Rivest-Shamir-Adleman public-key algorithm (see RSA) RPM packages, 99 RSA (Rivest-Shamir-Adleman) public-key algorithm, 84 RSAAuthentication keyword, 172, 174, 477, 609, 616, 623 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved rsh (restricted shell), 414 rsh suite, 10 rsync, 127 S ScanSSH program, 161 scp (Secure Copy Program), 7, 17, 81, 82, 313–323 authentication through local agents, 31 bandwidth settings, 320 batch mode, 319 Cygwin under Windows, 522 data compression, 320 directories, recursive copying, 316 double-remote copying using agents, 30–32 encryption algorithms, setting, 319 file conversions, 320 file transfers, 17 forced commands and, 338 gateway hosts, using through, 448 help, 322 internal options, 322 keywords, 620–623 Macintosh, 526 optimization, 321 options, 619 original file, automatic removal (Tectia), 317 permissions, 317 safety features, 318 ssh executable, locating, 322 SSH protocol settings, 319 statistics, display of, 321 syntax, 18, 313–316 TCP/IP settings, 319 troubleshooting, 509 user identity, 319 wildcards, 316 scp2, 82, 84 sealed servers, 438 SecPanel, 520 secret-key algorithms, 86–89 secret-key cryptography, 41 SECSH (Secure Shell) working group, 10 secure file transfers, Secure Hash Algorithm (see SHA-1) Secure iXplorer, 518 Secure KoalaTerm, 518 secure remote logins, Secure Shell protocol (see SSH) Secure Socket Layers (SSL), 14 SecureCRT, 518, 563–573 client configuration and use, 568–570 command-line programs, 572 file transfers, 572–573 vcp and vsftp commands, 572 Zmodem over SSH, 573 forwarding, 570–572 port forwarding, 570 X forwarding, 571 key management, 564–567 agents, 567 key generation, 565 key installation, automatic, 565 key installation, manual, 566 multiple identities, 567 session configuration, 564 troubleshooting, 574 SecureFX, 573 SecurID, 180 SecurIdGuesses keyword (Tectia), 180 security agent forwarding and untrusted machines, 206 batch job precautions, 413 carelessness and, 97 compile-time configuration setup recommendations, 397 forced commands and, 333 forwarding and, 205 multiple identities, advantages, 260 network applications and, shell escapes and, 333 Tectia SSH-1 compatibility mode issues, 225 SendEnv keyword, 199, 289, 623 server settings and, 199 server authentication, 38 ServerAliveCountMax keyword, 154, 293, 623 ServerAliveInterval keyword, 154, 293, 623 ServerKeyBits keyword, 609, 616 serverwide configuration, xv, 102, 128–226 access control, 184–198 account access control, 185–191 chroot, restricting directory access with, 195 external access control, 194 group access control, 191 hostname access control, 192 root access control, 194 shosts access control, 193 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved | 639 serverwide configuration (continued) authentication, 171–184 authentication syntax, 171–173 hostbased authentication, 175–177 Kerberos, 181–183 keyboard-interactive authentication, 177–180 login programs, selecting, 184 PAM, 183 password authentication, 173–174 PGP, 181 public-key authentication, 174 configuration files, 133–138 checking, 135 time values in, 155 file locations, 142–146 host-key files, 142 per-account authorization files, 145 process ID file, 143 random seed file, 143 server configuration files, 144 utmp file structure, 145 file permissions, 146 forwarding, 201–206 agent forwarding, 206 port forwarding, 201–205 X forwarding, 205 host-key generation, 130 initial setup, 141–171 data compression, 170 encryption algorithms, 162–167 integrity-checking (MAC) algorithms, 167–169 key regeneration, 161 numeric values, configuration files, 149 protocol version string, 170 restart for each connection, 151 SSH protocol settings, 169 TCP/IP settings (see TCP/IP settings, server) logging and debugging, 209–223 syslog, 210 making changes, 139 metaconfiguration information, 134, 468–479 per-account configuration (see per-account configuration) port forwarding, 370 port selection, 131 reconfiguration example, 141 640 | server compatibility, SSH-1 and SSH-2, 223–226 setup recommendations, 397–403 startup file script, 129 subconfiguration files, 134 subsystems, 206–209 definition syntax, 206 troubleshooting, 506 user logins and accounts, 198–201 client environment variables, setting permissions, 199 initialization scripts, 200 user welcome messages, 198 session keys, 462 sessions, 44 identity-based tailoring, 262 SetRemoteEnv keyword (Tectia) server settings and, 199 SettableEnvironmentVars keyword (Tectia), 200 setup recommendations, 396–407 client configuration, 404 compile-time configuration, 397 key management, 404 per-account configuration, 403 remote home directories, 404–407 serverwide configuration, 397–403 sftp, 33, 81, 84, 323–325 ASCII vs binary transfer, 34 command-line options, 34, 325 Cygwin under Windows, 522 interactive commands, 323–325 Macintosh, 526 vs ftp, 34 SftpSysLogFacility keyword, 211, 617 SHA-1 (Secure Hash Algorithm), 90 shadow files, 110 Shannon, Claude, 40 shell channel request, 66 SHELL environment variable, 28 shell escapes, 333 ShellGuard, 518 SIGHUP signal, 140 signers, 45 single-shell agent invocation, 243–245 S/Key in OpenSSH, 111 SkeyAuthentication keyword, 617 slogin (SSH1), 34 SmartcardDevice keyword, 623 SMTP (Simple Mail Transfer Protocol), 437 sniffing, 37 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved SocksServer keyword, 120, 304, 477, 548, 617, 623 source distributions, 100 SRP (Secure Remote Password), 13 ssh (client), configuration, 266–313 authentication, 307–310 command-line options, 267 configuration files, 268–275 connections, 294–302 data compression, 310 encryption algorithms, 306 environment variables, 275 forwarding, 305 host key types, 307 host keys and known-hosts databases, 284–287 integrity-checking (MAC) algorithms, 306 logging and debugging, 312 precedence, 276 protocol settings, 287–289 proxies, 302, 302–305 random seeds (Tectia), 313 remote account name, 279–281 session rekeying, 307 SOCKS, 302–305 subsystems, 311 TCP/IP settings, 290–294 user identities, 281–283 Cygwin under Windows, 522 debugging messages, 495 escape character, 21 keywords, 620–623 known and unknown hosts, 19–21 Macintosh, 526 remote terminal sessions, 16 client/server channel, establishing, 17 login, 17 ssh options, 618 ssh-add options, 625 ssh-agent options, 625 ssh-keygen options, 623 troubleshooting, 508 unexpected behaviors, handling, 19 verbose mode, 19, 277 SSH Communications Security, 5, SSH (protocol) quick reference, 612–627 SSH (Secure Shell), xii, 1–15, 36–98 address name with multiple numeric address, problems, 71 algorithms, 84–91 hash functions, 89 public-key algorithms, 84–86 secret-key algorithms, 86–89 authentication, 38 supported methodologies, 38 authorization, 39 authorization in hostbased authentication, 71 control file details, 72 hostbased access files, 72 netgroups, 74 netgroups as wildcards, 76 backward compatibility, 78 clients, 16–35, 44 scp (see scp) sftp (see sftp) slogin, 34 ssh (see ssh) client/server architecture, compression algorithms, 91 configuration directory key storage files, 24 configuration (see configuration) cryptography (see cryptography) denotation of protocols, products and clients, features, 5–9, 36–39 keys and agents, port forwarding, remote commands execution, remote logins, scp (see scp) secure file transfers, file transfers, 81–84 flexibility in prosecution of services, 47 forwarding, 39 supported types, 39 function and purpose, history, implementation-dependent features, 48, 69–81 host keys, 69 included component protocols, 46–49 installation (see installation) integrity, 37 keys, 44 known-hosts mechanism, 20 PKI, supported types and supporting implementations, 55 privacy, 37 privilege separation (OpenSSH), 80 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved | 641 SSH (Secure Shell) (continued) pronunciation, protections provided by, 91–93 random number generation, 78 r-commands, replacing, 125–127 in CVS, 125 in GNU Emacs, 126 in Pine, 126 in rsync and rdist, 127 related technologies, 10–15 firewalls, 15 IPSEC and VPNs, 12 Kerberos, 12 PGP and GnuPG, 11 SRP, 13 SSL, 14 SSL-enhanced telnet and FTP, 14 stunnel, 15 security vulnerabilities, 93–98 server, 43 sessions, 44 software inventory, 124 SSH agent (see agents) SSH-1, 36, 68 Tectia compatibility support, 122–123 SSH-1 protocol, SSH-2, 36, 45–67 SSH-1 compared to, 68 SSH-2 protocol, SSH-AUTH, 47, 57–63 authentication request, 57 authentication response, 58 host-based authentication, 62 “none” request, 59 password authentication, 61 public-key authentication, 60 SSH-CONN, 47, 64–67 channel requests, 66 channels, 64 completing the connection process, 67 global requests, 65 requests, 65 SSH-SFTP, 48 SSH-TRANS, 47, 49–57 connection, 49 initialization of encryption, 56 key exchange algorithm, 51 key exchange and server authentication, 54 message authentication code and algorithms, 53 parameter negotiation, 51 642 | protocol version selection, 50 server authentication and anti-spoofing, 56 supported encryption algorithms, 37 system architecture, 43–45 Unix implementations (see OpenSSH; Tectia) Unix versions, xvi SSH Secure Shell product (see Tectia) SSH1 product, Ssh1AgentCompatibility keyword, 611, 623 Ssh1Compatibility keyword, 224, 477, 610, 617, 623 Ssh1InternalEmulation keyword, 610, 623 Ssh1MaskPasswordLength keyword, 611, 623 Ssh1Path keyword, 611, 623 ssh-add command, 28, 247–253 command-line options, 250 listing keys, 29 reading input, 28 troubleshooting, 505 ssh-agent command, 28 locking agents form unauthorized use, 29 troubleshooting, 505 ssh-askpass program, 28 password piping, 409 ssh-copy-id command (for key installation), 26 sshd (server), 129 authentication syntax, 171–173 client environment variables and, 199 command-line options, 138 configuration (see server configuration) debugging messages, 496 hushlogin and, 199 inetd, 223 initialization scripts, 200 key regeneration, 161 keywords, 613–617 launch-sshd shell script, 139 public keys file, 176 running as ordinary user, 129 disadvantages, 131 running as superuser, 129 setup recommendations, 397–403 SIGHUP signal, 140 sshd command options, 612 user SSH directory, 144 user welcome messages, 198 xinetd, 223 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved Sshd1ConfigFile keyword, 225, 477, 610, 617 Sshd1Path keyword, 224, 477, 610, 617 sshd-check-conf program, 136–138, 219 ssh-keyconverter (OpenSSH), 231 ssh-keygen command, 23, 233 command line options to change passphrases, 27 ssh.pid file, path specification, 108 ssh-probe program (Tectia), 160 sshrc files, 200 sshregex (Tectia) manpage, 595–603 egrep patterns, 595 syntax, 595 ZSH_FILEGLOB, 597 SshSignerPath keyword, 311, 623 SSHTerm Professional, 517 SSL (Secure Socket Layers), 14 TCP-based applications, enhanced with, 14 StrictHostKeyChecking keyword, 284, 285, 286, 623 StrictModes keyword, 25, 146, 147, 149, 478, 617 stunnel, 15 subconfiguration files, 471 forbidden keywords, 476 keyword order, 473 sections, 474 subshell agent invocation, 246 subsystem channel request, 66 Subsystem keyword, 207 symbolic links, created by SSH installations, 123 symmetric ciphers, 41 SYN flood attack, 94 SyslogFacility keyword, 210 system administration, xv T tar format, 101 TCP attacks, 94 TCP/IP settings, server ASLR (Address Space Layout Randomization), 151 failed logins, 156 idle connections, 155 invocation by inetd or xinetd, 150 keepalive messages, 152–154 Nagle Algorithm, 159 port number and network interface, 148 reverse IP mappings, 158 server discovery, 159 simultaneous connections, limiting, 157 tcpip-forward request, 65 TCPKeepAlive keyword, 153 TCP_NODELAY bit, 159 TCP-wrappers, 389–395 Tectia, 5, 99 account authorization files, 330 authentication authorization file, 232 external keys, 264 identification files, 232 identities, 230 PGP, using, 262–264 X.509 certificates, 454–461 authorization files, 626 client for FTP, 416 configuration, 113–122 authentication, 117–120 debugging, 120 encryption, 117 file locations and permissions, 113 networking, 115 random number generation, 115 SOCKS proxies, 120 TCP port forwarding, 117 X Window system, 116 configuration extensions, 468–479 configuration files keywords, 477 quoted values, 478 debugging module names, 604–608 environment variables, 627 file-naming conventions, 130 help command, 278 host access control, 340 host keys implementation, 70 host-key generation, 131 identity files, 626 idle-timeout option, setting, 343 installation, 111–113 build and install, 113 file extraction, 112 md5 verification, 112 prerequisites, 112 metaconfiguration, 134, 468–479 plugins, 479–494 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved | 643 Tectia (continued) customized password-change plugin, 487 expired passwords, changing, 479–484 external authorization, 492 general rules, 485 keyboard-interactive authentication, 488–492 Perl package for plugin implementation, 484 public keys, changing, 27 public-key generation, 23 public-key installation, 25 quick reference, 612–627 random number storage, 79 scp keywords, 620–623 scp options, 619 scp2, 84 scp, contrasted with, 82 server debugging messages, 496 serverwide configuration access control files, 196 authentication syntax, 172 authentication techniques, 172 configuration files, checking, 136 hostbased authentication, 175 host-key generation, 130 Kerberos authentication, 182 keyboard-interactive authentication, 178 limiting simultaneous connections, 157 logging and debugging, 215–223 password authentication, 173 public-key authentication, 174 recommended settings, 401–403 rules for quoted strings, 135 server discovery, 159 SSH protocol settings, 170 SSH-1 and SSH-2 compatibility issues, 223–226 ssh-probe, 160 user welcome, 198 software inventory, 124 SSH configuration directory, key storage file, 24 ssh keywords, 620–623 ssh options, 618 SSH Secure Shell product, name change, xi SSH-1, 610–611 644 | client configuration, 610 key management, 611 scp file transfers, 611 serverwide configuration, 610 SSH-1 protocol compatibility and support, 121–123 SSH2_ORIGINAL_COMMAND, 336 ssh-add options, 625 ssh-agent options, 625 sshd keywords, 613–617 sshd options, 612 ssh-keygen options, 623 sshregex manpage, 595–603 egrep patterns, 595 syntax, 595 ZSH_FILEGLOB, 597 subconfiguration files, 471 forbidden keywords, 476 keyword order, 473 sections, 474 subsystem command syntax, 208 Windows operation (see Tectia for Windows) Tectia for Windows, 531–562 Accession Lite, 536–539 client application, 533–534 configuration and profiles, 539–542 command-line programs, 552 Connector, 543–551 file transfers, 551 installation, 532 key management, 534 port forwarding, 542–543 supported Windows platforms, 531 Tectia Servers A and T, 555–562 access control, 559 authentication, 559 commands, 557 configuration, 557 forwarding, 560 logging and debugging, 561 operation, 556 SFTP server, 560 troubleshooting, 554 telnet SSL-enhanced, 14 terminal locking, 29 Terminal.AllowGroups keyword, 617 Terminal.AllowUsers keyword, 617 Terminal.DenyGroups keyword, 617 Terminal.DenyUsers keyword, 617 tickets, 462 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved time values, server configuration files, 155 TIME_WAIT state, 433 TLS (Transport Layer Security), 14 Top Gun SSH, 519 traffic analysis, 95 transparency, 349 transparent proxies, 424 Triple-DES, 88 Trojnara, Micha, 15 troubleshooting, 497–513 TrustX11Applications keyword, 383, 623 TTY allocation, disabling per-account, 345 tunneling, 39, 351 advantages, 451 tunnels, TuSSH, 519 Twofish, 89 U Unix hushlogin convention and SSH, 199 "message of the day" (motd), 198 syslog, 210 UseDNS keyword, 158 UseLogin keyword, 184, 205, 617 UsePAM keyword, 110, 171, 178, 183, 617 UsePrivilegedPort keyword, 292, 623 UsePrivilegeSeparation keyword, 184, 617 user authentication, 38 User keyword, 279, 499, 623 UserConfigDirectory keyword, 137, 144, 145, 269, 539, 559, 617 UserKnownHosts keyword, 176, 617 UserKnownHostsFile keyword, 287, 623 UserSpecificConfig keyword (Tectia), 471 UseSOCKS5 keyword, 477, 617, 623 W Windows OpenSSH on Cygwin, 521–525 agents, 524 enabling remote windows, 523 installation, 521 public-key authentication, 524 ssh clients, 522 SSH server setup, 522 troubleshooting, 525 PuTTY client (see PuTTY) SecureCRT (see SecureCRT) SSH implementations, 517 Tectia (see Tectia for Windows) Windows Pocket PC SSH implementations, 519 WinSSHD, 518 WiSSH, 518 wu-ftpd, 423 X X forwarding, 349, 377–389 limiting or disablig per-account, 344 server configuration, 205 X11DisplayOffset keyword, 381, 617 X11Forwarding keyword, 205, 617 x11-req channel request, 66 X11UseLocalhost keyword, 617 xauth, 108 XAuthLocation keyword, 381, 389, 513, 617, 623 XAuthPath keyword, 617, 623 xinetd server configuration and debugging, 223 server invocation using, 150 Y V VanDyke Software, 563 VerboseMode keyword, 216, 219, 312, 478, 617, 623 VerifyHostDNS keyword, 286 VerifyHostKeyDNS keyword, 623 version-control systems, 125 VMS SSH implementations, 520 VPNs (Virtual Private Networks), 12 VShell, 518, 574 Ylönen, Tatu, 4, 5, Z zlib, 91, 106 ZOC, 518, 519 ZSH_FILEGLOB, sshregex (Tectia), 597 character sets, 598 Index This is the Title of the Book, eMatter Edition Copyright © 2008 O’Reilly & Associates, Inc All rights reserved | 645 About the Authors Daniel J Barrett, Ph.D., has been immersed in Internet technology since 1985 Currently working as a software engineer, Dan has also been a heavy metal singer, Unix system administrator, university lecturer, web designer, and humorist He is the author of O’Reilly’s Linux Pocket Guide, and is the coauthor of Linux Security Cookbook and the first edition of SSH, The Secure Shell: The Definitive Guide He also writes monthly columns for Compute! and Keyboard Magazine, as well as articles for the O’Reilly Network Richard E Silverman has a B.A in computer science and an M.A in pure mathematics Richard has worked in the fields of networking, formal methods in software development, public-key infrastructure, routing security, and Unix systems administration He coauthored the first edition of SSH, The Secure Shell: The Definitive Guide, and he loves to read, study languages and mathematics, sing, dance, and exercise Robert G Byrnes, Ph.D., has been hacking on Unix systems for 20 years, and has been involved with security issues since the original Internet worm was launched from Cornell University, while he was a graduate student and system administrator Currently, he’s a software engineer at Curl Corporation He has worked in the fields of networking, telecommunications, distributed computing, financial technology, and condensed matter physics Colophon Our look is the result of reader comments, our own experimentation, and feedback from distribution channels Distinctive covers complement our distinctive approach to technical topics, breathing personality and life into potentially dry subjects The animal on the cover of SSH, the Secure Shell: The Definitive Guide is a land snail (Mollusca gastropoda) A member of the mollusk family, a snail has a soft, moist body that is protected by a hard shell, into which it can retreat when in danger or when in arid or bright conditions Snails prefer wet weather and, though not nocturnal, will stay out of bright sun At the front of a snail’s long body are two sets of tentacles: its eyes are at the end of one set, and the other set is used for smelling and navigation Land snails are hermaphrodites, each having both female and male sex organs, though a snail must mate with another snail in order for fertilization to occur A snail lays eggs approximately six times a year, with almost 100 eggs each time Young snails hatch in a month and become adults in two years A snail’s life span is approximately 5–10 years Known as a slow mover, a snail moves by muscles on its underside that contract and expand, propelling the snail along at a slow pace It leaves a wet trail of mucus, which protects the snail from anything sharp it may need to crawl over as it searches for food The snail’s diet of plants, bark, and fruits causes it to be a pest in many parts of the world where it is notorious for destroying crops Mary Brady was the production editor for SSH, the Secure Shell: The Definitive Guide Audrey Doyle proofread the book Marlowe Shaeffer and Mary Anne Weeks Mayo provided quality control Lydia Onofrei provided production assistance John Bickelhaupt wrote the index Ellie Volckhausen designed the cover of this book, based on a series design by Edie Freedman The cover image is an original engraving from the book Natural History of Animals by Sanborn Tenney and Abby A Tenney, published by Scribner, Armstrong & Co in 1873 Karen Montgomery produced the cover layout with Adobe InDesign CS using Adobe’s ITC Garamond font David Futato designed the interior layout This book was converted by Keith Fahlgren to FrameMaker 5.5.6 with a format conversion tool created by Erik Ray, Jason McIntosh, Neil Walls, and Mike Sierra that uses Perl and XML technologies The text font is Linotype Birka; the heading font is Adobe Myriad Condensed; and the code font is LucasFont’s TheSans Mono Condensed The illustrations that appear in the book were produced by Robert Romano, Jessamyn Read, and Lesley Borash using Macromedia FreeHand MX and Adobe Photoshop CS The tip and warning icons were drawn by Christopher Bing This colophon was written by Nicole Arigo ... First Edition Second Edition Nutshell Handbook, the Nutshell Handbook logo, and the O’Reilly logo are registered trademarks of O’Reilly Media, Inc SSH, the Secure Shell: The Definitive Guide, the. .. might be named ssh1 or ssh2 OpenSSH The product OpenSSH from the OpenBSD project, http://www.openssh.com Tectia The successor to SSH2 , this refers to the product suite SSH Tectia” from SSH Communications... of OpenSSH This little upstart from the OpenBSD world has become the dominant implementation of SSH on the Internet, snatching the crown from the original, SSH Secure Shell (now called SSH Tectia,