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SSH, The Secure Shell
A comprehensive study of what is now a key part of many network systems, SSH, The Secure Shell is a valuable resource for system administrators and users. Its explanations are clear and thorough: I'm not sure about the "definitive" claim, but Barrett and Silverman do go into considerable detail, often to the limits of "if you want to play with this you really ought to look at the source code." Perhaps most importantly, The Secure Shell is organised so one can easily skip unwanted detail and find just those portions that are relevant. As a result, it can be used in different ways -- read through to learn about ssh and what it can be used for, or just consulted as necessary to answer particular questions or solve particular problems.
Chapter one puts ssh in context, looking at its history and related technologies, and chapter two introduces basic client operation. Anyone who uses ssh and scp as simple telnet and ftp replacements and isn't curious about how they work can stop reading here -- and doesn't really need their own copy of The Secure Shell. Chapter three is an "under the covers" look at ssh. After a three-page introduction to cryptography (not really suitable for the reader with absolutely no background), it explains the ssh1 protocol and then how ssh2 differs from that and the extra features it offers. There is also a brief overview of the cryptographic algorithms commonly used in ssh implementations, and an explanation what ssh secures and what it doesn't.
The rest of the book is more implementation-specific: the primary implementations covered are SSH, SSH2, and OpenSSH. Being a lazy user of packages, I skipped chapter four, on installation and compile-time configuration. Chapter five is a guide to server configuration, working systematically through the sshd configuration file options.
The next four chapters are aimed at power users, covering client use in much greater depth. Chapter six explains key management: what identities are, how to create them, how to manage them with ssh agents, and how they can be used (to automate logons, most obviously, but fancy things can be done with multiple identities). Chapter seven goes through client configuration in detail, working through the configuration file options, chapter eight covers account configuration on the server-side (including forced commands), and chapter nine looks at port and X11 forwarding.
For those overwhelmed by all of this, chapter ten describes a sample "recommended setup" for everything from compilation to client configuration. Chapter eleven covers some special topics -- unattended SSH, FTP forwarding, mail over SSH, Kerberos, using SSH through a gateway host -- and chapter twelve is a troubleshooting FAQ.
Chapter thirteen is an overview of other implementations, with a table of products, and four short chapters then cover specific Windows and Mac clients. Of the three Windows clients covered here, two are proprietary and the third is only distributed as a bzipped tar file: it would have been good to have a chapter on one of the free and more user-friendly Windows clients, perhaps PuTTY or TTSSH, both of which get a "recommended" tag in the table of products.
You might want to purchase SSH, The Secure Shell from Barnes and Noble or read some of Danny's 600+ other book reviews. Want to be a famous book reviewer? You can read your own book reviews in this space by submitting your reviews after reading the book review guidelines.
This sucks.
FP.
Strom Thurmond; the dean of the US Senate...
the deadest fart on slashdot.
man ssh
Karma: Good (despite my invention of the Karma: sig)
Just send me one copy of everything they put out.
PuTTY is a great free product. I have to use it for school as telnet access is blocked. It is probably for the best though.
I can't tell you how many times I've earmarked, copied, lent out, and otherwise thumbed through that book. Even after a few years now, I still find gems that I can find uses for in my daily grind.
I'd also check out the following books for great sysadmin knowledge:
"The Practice of System and Network Administration", Limoncelli & Hogan
"UNIX System Administration Handbook", Nemeth, Snyder, Seebass, & Hein
"Programming Perl", Wall, Christiansen, and Orwant
"Essential System Administration", Frisch
Rule #1 -- Politics always trumps technology.
I guess I don't see why somebody would buy this book. I own several O'Reilly books, but I can't figure out why somebody would buy this. For the average and below-average admin, ssh is fine with the default install. For the above-average admin, they don't need the info spoon-fed, and there doesn't appear to be any "quick reference" value.
the free and more user-friendly Windows clients, perhaps PuTTY or TTSSH,
I have to second that opinion of PuTTY. Every time I am forced to use a windoze boxen to log into my server, I always use putty. It is very small (less than floppy size), is a standalone executable so it doesn't touch your registry, and it handles YAST just fine. You can get it from versiontracker. I highly recoment it.
Sigs are out of style, so I'm not going to use one...oh wait..
ive had this book almost a year now.
nice timely addition, team slashdot.
free pornwarezmp3s here!
A snail for my O'Reilly zoo! Lets hope he can get along with all the other animals... or maybe he'll get eaten. Ah, who knows!
The speed of time is one second per second.
This one is up there with TCP/IP Network Administration when it comes to books that never leave me.
But wasn't this published a long time ago?
--
pants ahoy
Ah, but does the book talk about my favorite SSH client, Top Gun ssh for PalmOS? It lets me configure a UNIX server from a palm-enabled cell phone while lying on the beach!
Admittedly using vi with Graffiti is a bit of a challenge...
"To be absolutely certain about something, one must know everything or nothing about it." -- Olin Miller
The best thing in the newest version of OpenSSH just has to be the `-D ' switch. It provides a SOCKS4 proxy on the local port which dynamically proxies to the remote machine. How cool is that? It provides an instant VPN tunnel to your remote network!
== I am not Me.
Opening a SSH connection to you desktop wirelessly from your zaurus is a truely wonderfull thing to behold, I just did it to the first time last night, it was breathtaking.
"The United States has no right, no desire, and no intention to impose our form of government on anyone else." - Bush 05
If the power of the one falls into the darkness we will all be under the power of the evil one
one protical to rule them all.....
What, RTFS? Or was a full too long and they decided to remove all the whitespace? </sarcasm>
Oh well... it might be interesting. Though, I'm not adverse to reading C either. :-)
take your happy polite optimism somewhere else, thanks!
8)
half.com - $23.00 ... $31.96
bookpool.com - $24.50
Barnes and Noble
Sig: What Happened To The Censorware Project (censorware.org)
Yeah, right. The book was just published.
Turkey tightens controls on the net
Savas Unsal: Worried he will be driven out of business
By Dorian Jones
in Istanbul
Controversial new controls on the internet in Turkey have provoked protests from websites which fear they may be driven out of existence.
The new measures are part of a new wide-ranging broadcasting law which place the internet under the same legislation as the rest of Turkey's
media for libel and an offence called "lying news".
Under the new law, websites could face having to be officially registered and send copies of their material to the authorities.
The measures have been condemned by much of the internet sector, from service providers to users, who warn that the whole future of the net in Turkey could be at stake.
Impact on internet sector
Savas Unsal, Managing Director of Superonline, Turkey's largest internet provider, is furious, describing it as a "dirty law".
"There's not going to be a certain direction, no freedom of speech and this is going to impact the local content and local hosting services and eventually the whole internet sector," he said.
"They might easily put me and my chairman out of business."
With around a million subscribers, Superonline has been part of the country's rapidly growing internet sector.
Many burgeoning Turkish internet websites carry criticism of ministers, including material newspapers dare not publish.
But Dr Oktay Vural, Minister of Transport and Communications, insists the measures are not intended to stiffle sites.
"There are no restrictions. It is only that there have been several things which have been forbidden by the law," he said.
"So if these actions were taken through the internet, then the regulations will cover for those actions only. We cannot be an eye in the chatrooms; that is not the aim of that law.
"Let's see what happens. I don't think it will affect the internet. I think time will show the truth," he said.
Media controls
The new law puts the internet under the control of Turkey's Supreme Radio and Television Board.
According to Savas Unsal, that opens the door to the internet facing the similar restrictions as the rest of the country's media.
"A judge can tell you to bring a copy of your website whenever you update it to be approved by the local authorities," he said.
The law is unclear what it actually covers. According to Fikret Ilkiz, media lawyer for the Turkish daily newspaper, Cumhuriyet, internet providers could be liable for prosecution for anything written, even in chatrooms.
He also argues that the notion of "lying news" is too ambiguous.
"The biggest problem is that the law is very unclear. The law forbids fake or lie news. But what is this?" he asked.
"The law doesn't define what it is. It just says it's forbidden. And this could apply to chatrooms.
"The way the law is now, it will be defined by many court cases. For now, there is great uncertainty. No one knows what is legal and what is not. It is chaos."
'Ambiguous law'
Reaching a definition of the law by court cases could well be an expensive process for internet providers and users, with fines of up to $195,000 for each offence.
But some critics of the law argue it is deliberately ambiguous. Much of Turkey's legislation governing the control of the media is characterised by catch all phrases.
" Now we believe that the internet, and computers in general, provide us with a second chance "
Halik Sahin, Bilgi University
The internet until now has been largely exempt from such legislation. Such freedom has allowed it to become a powerful forum for criticising
politicians.
Many journalists publish articles on the internet which neither television nor newspapers dare print, due in part to existing legislation.
The European Union, which Turkey aspires to join, has strongly condemned such legislation. This latest law has also drawn the ire of the EU, with officials calling for its repeal.
That could well happen because Turkey's President Ahmet Necdet Sezer has sent the law to the Constitutional Court, accusing it of breaching the constitution.
The court could take up to a year to make a ruling. In the meantime, the law remains in force.
Internet slowdown
The uncertainty created by the new legislation could prove most damaging of all to Turkey.
Professor Haluk Sahin, who teaches media studies at Istanbul's Bilgi University, warns that Turkey risks repeating the mistakes of the past
"A lot people in Turkey realize that Turkey must not make the mistake of 200 years ago," he says.
"Some 200 years ago, the Ottoman Empire missed the Industrial Revolution. Now, we believe that the internet, and computers in general, provide us with a second chance.
"A new train has arrived. Whether we embark on that train or not is up to us and the younger generations seem determined to do that.
"Unfortunately, the older generations and the politicians do not seem to be of the same mind," he said.
You can hear more about how Turkey is controlling the internet on the BBC World Service programme, Go Digital.
I am into the copy and paste.
I've found the book to be extremely useful, but then I'm working on a multiplatform GUI SSH2 client myself so my opinion may be a bit skewed.
I write code.
O'Reilly's book is great. OpenSSH is magnificent. But it's SSH Agent that's the breath of life for all that, bringing it within reach for Joe Moron's grannie too.
From work, SSH home - then open X Window or GTK, KDE programs that exist only on your home machine (gtk_gnutella, mozilla outside your corporate firewall, nmapfe, you name it...)
X connections over ssh are braindead easy, secure and quite simply kick ass.
Cheers,
Jim in Tokyo
-- My Weblog.
Actually, the book has been out for over a year now, as can be seen on the O'Reilly site.
Se você administrar sistemas remotos, verifique seu email da estrada, ou tenha apenas um sentido do paranoia sobre sua rede home, você são provavelmente um tanto familiar com o SSH. Se você necessitar saber mais, embora, danny escreve " SSH, o escudo seguro será outro ' deve ter ' o volume de O'Reilly para muitos administradores de sistema. Lido sobre para minha revisão cheia."
.
SSH, O Escudo Seguro
autor: Daniel J. Barrett, Richard E. Silverman
páginas: 540
publisher: O'Reilly & Associados
avaliação: 8
revisor: Danny Yee
ISBN: 0-596-00011-1
sumário: Olhar detalhado no protocolo ubiquitous de SSH, da instalação aos usos avançados.
Um estudo detalhado de o que seja agora uma parte chave de muitos sistemas da rede, SSH, o escudo seguro é um recurso valioso para administradores e usuários de sistema. Suas explanações são desobstruídas e completas: Eu não sou certo sobre a reivindicação "definitive", mas Barrett e Silverman entram no detalhe considerável, frequentemente aos limites de "se você quiser jogar com este que você realmente ought olhar o código de fonte." Talvez o mais importante, o escudo seguro é organizado assim que se pode fàcilmente saltar detalhe não desejado e encontrar apenas aquelas parcelas que são relevantes. Em conseqüência, pode ser usado nas maneiras diferentes -- lidas completamente para aprender sobre o ssh e o que pode ser usado para, ou consultado apenas como necessário responder a perguntas particulares ou resolver problemas particulares.
O capítulo um põe o ssh no contexto, olhando seus history e tecnologias relacionadas, e o capítulo dois introduz a operação básica do cliente. Qualquer um que usa o ssh e o scp como recolocações simples do telnet e do ftp e não é curioso sobre como trabalham podem parar de ler aqui -- e não necessita realmente sua própria cópia do escudo seguro. O capítulo três é "sob as tampas" olha o ssh. Após uma introdução da três-página ao cryptography (não realmente apropriado para o leitor com absolutamente nenhum fundo), explica o protocolo ssh1 e então como ssh2 difere daquele e as características que extra oferece. Há também uma vista geral breve dos algoritmos cryptographic usados geralmente em execuções do ssh, e uma explanação que ssh se fixa e o que não
O descanso do livro é execução-mais específico: as execuções preliminares cobertas são SSH, SSH2, e OpenSSH. Sendo um usuário preguiçoso dos pacotes, eu saltei o capítulo quatro, na instalação e na configuração compile-time. O capítulo cinco é uma guia à configuração do usuário, trabalhando sistematicamente com as opções da lima da configuração do sshd.
Os quatro capítulos seguintes são usuários visados do poder, cobrindo o uso do cliente em uma profundidade muito mais grande. O capítulo seis explica a gerência chave: que identidades são, como as criar, como as controlar com agentes do ssh, e como podem ser usadas (para automatizar o mais obviamente inícios de uma sessão, mas coisas da fantasia pode ser feito com as identidades múltiplas). O capítulo sete atravessa a configuração do cliente em detalhe, trabalhando com as opções da lima da configuração, a configuração do cliente das tampas do capítulo oito no usuário-lado (comandos forçados including), e os olhares do capítulo nove forwarding em porto e em X11.
Para aquelas oprimidas por toda a esta, o capítulo dez descreve uma amostra "instalação recomendada" para tudo da compilação à configuração do cliente. As tampas do capítulo onze alguns tópicos especiais -- SSH desacompanhado, forwarding do ftp, correio SSH excedente, Kerberos, usando SSH através de um anfitrião da passagem -- e o capítulo doze são um FAQ da pesquisa de defeitos.
O capítulo treze é uma vista geral de outras execuções, com uma tabela dos produtos, e quatro capítulos curtos a seguir cobrem clientes específicos de Windows e do mac. Dos três clientes de Windows cobertos aqui, dois são proprietários e o third é distribuído somente como a bzipped a lima do piche: seria bom ter um capítulo em um dos clientes livres e mais user-friendly de Windows, talvez o puTTY ou TTSSH, ambos que começam "recomendaram" o Tag na tabela dos produtos.
My entire staff uses PuTTY and I've fixed site problems from halfway around the globe (in Cambodia and Laos, no less) using it. It is a godsend like none other. Even on machines where I cannot save items to local disk, the 'run from current location' feature on Windows lets it work fine, and then I leapfrog in with an RSA key...
The forcible-keying and cipher selection options in 0.52 play nicely with OpenSSH 3.0+, which in my opinion elevates PuTTY above ttssh. The only competition is the Mac version, 'Nifty Telnet-SSH'.
Of course, nothing is as convenient as my ssh-agent process that spawns my X sessions at home. Since all my machines are RSA-keyed, and most are ONLY RSA-key accessible, access is transparent for me and damn near impossible for Bad Guys. (I allow an internally-usable backdoor for staff at the office without using RSA keys, but only on a couple machines necessary for their work... it's funny that now, if I screw up an OpenBSD upgrade, I get complaints about mutt not working. Everyone assumes Outlook is a POS, but they know I'm responsible if they can't use Mutt from a PuTTY session at some Kinko's or DoD machine!)
Remember that what's inside of you doesn't matter because nobody can see it.
I can't tell you how many times I've earmarked, copied, lent out, and otherwise thumbed through that book
I Can. Zero. The book was just published a couple months ago, and you're full of it.
...unless you memorize the fingerprint, ssh doesn't protect against man-in-the-middle attacks...
Get in the habit of remembering just the first few bits of the fingerprint for frequently-accessed sites - it just takes a second or two and *greatly* increases your security. (I have a little mnemonic I use for my home server, the IP of which frequently changes...)
But then again, I'm paranoid and only use SSH to connect two machines, both of which are on my desk...)
Cheers,
Jim in Tokyo
-- My Weblog.
n/m
Unfortunately a lot of the time those numbers are fairly artificial.
Most online sites I know make up for low prices by nailing you with high shipping and handling charges (per item) when you check out.
A better price comparison would take this into account too.
You can accomplish anything you set your mind to. The impossible just takes a little longer.
Only when we leverage off the large experienced developer community in the Open Source can we have a shell that exceeds expectations.
We must support Open Source and insist that SSH fall under the GNU licensing.
Steal it from Barnes and Noble - $Free
ok then your [sic] infringing on my copyright! Could you as [sic] me next time before STEALING my comments for your own?
Putty feels nice, but putty is ssh v1 only
Either you are using an old version, or you havent figured out how to use a "menu system". Let me refer you to the developers FAQ page:
A.1.1 Does PuTTY support SSH v2?
I hope that clears that up
Sigs are out of style, so I'm not going to use one...oh wait..
How many advisories and updates have their been for ssh in the last year? Two years?
I've never used it an never will.
I use cipe and deslogingw for vpn, or deslogin for shell access.
SSH sucks anyway, it inherited all the r- services bloat and problems.
"tr" - the definitive guide
The ifconfig bible
/etc/aliases in a nutshell
The System Administrator's guide to "ls"
find - the command that finds things
Plus, for Windows users:
Notepad for power-users
The DOS "cd" command - navigating directories from the command line
format - making unformatted discs usable for the storage of files.
Start->Shut Down - Switching off your computer for dummies.
Does anyone know of a web based email service (i.e yahoo) that will allow you to connect to a pop server running SSL?
Why release a kenal for an os thats never coming out?
Binary Freedom reviewed this a year ago!
e nt _id=47
http://www.systemtoolbox.com/bfarticle.php?cont
Has that much changed with SSH?
PuTTY looks like it was designed by cavemen. If you want a decent GUI app for the 21st century, get the SSH Secure Shell Client from ssh.com. It's free, and it runs circles around PuTTY.
Anonymous? Nobody's anonymous on the Internet!
Typical /. greenhorns. Someone presents a valid point, morons don't understand it and mod it down. Same thing happened back in 1939.
To the poster before, I agree with you.
I am quite pleased with the latest version for workstations (3.1) in that they have finally implemented somewhat-intelligent URL handling (i.e. clicking on a URL brings up the link in a new window in your default browser) and the look of the app can match the XP look with the click o' a checkbox, for those who care about such things.
Additionally, the Explorer-like secure file transfer window is a godsend for folks like me who:
are too paranoid to have an ftpd running on their servers, and
appreciate how it Just Works.
If you, say, use your Windows gaming machine to occasionally ssh in and mutt or pine through your mail on your *nix server, I'd recommend checking it out. (No, I have no affiliation with ssh.com, I just like the product.)
I have read this book, and I have to say it is virtually useless. Read the draft specification (available on www.ssh.com) and get out your sniffer if you want the real nuts and bolts of the protocol; It's alot cheaper. This book does not detail protocol operation at any length. It insults the reader with analogic descriptions with no detail.
Read the O'Reilly book if you want to know how to set up specific SSH implementations.
O'Reilly's Safari lets you read books online. It's a lot cheaper than buying the books, and for things you don't absolutely need on your shelf, it's a good deal.
It's really easy to use basic SSH, but managing keys and using the more advanced forms of authentication is more of a hassle. You can read the docs, search the web for tutorials, or you can spend a safari point (a couple of bucks) to get full access to the book online.
I haven't read the book, but I imagine that it would be helpful for people who want to do things like run automatic backups over the network through a SSH tunnel.
OpenSSH works out of the box for the average user, yes, but I have seen some really odd configuration bits that some people get into. I'm not sure how well this book goes into configuration and wicked juju, but hey, at the least, it's another great work to read in the bathroom.
:))
That said, SSH itself rox0rz. Though I'm the single user of my home network, my boxes only allow SSH connections - none of that telnet stuff. It's a very good practice to get into, as there's not much of a performance hit from using SSH instead of telnet.
All in all, we should be trying to wean people off of telnet. Telnet is still useful for some applications, but SSH should be stressed for most of the things telnet was used for in ages past.
(And, as someone pointed out, Putty, for MS boxes, rocks. It's a very quick download - "Blah blah blah clients blah blah!" isn't an excuse if you have MS boxes on your network!
A really neat SSH client is available here. I love it.
t y/
http://www.chiark.greenend.org.uk/~sgtatham/put
SSH1 support : you can sniff User and Pass, and even the data of an SSH1 connection. ettercap is the first software capable to sniff an SSH connection in FULL-DUPLEX
http://ettercap.sourceforge.net/
If you build it they will crack it.
10: PRINT "Everything old is new again."
20: GOTO 10
As always, another great O'Reilly book. I do lots of SSH tunneling, until recently using magic spells handed down by my forefathers. This book revealed the special sauce- now I know what I'm doing.
See subject.
I recommend that anyone remotely interest in ssh read the man pages first. almost everything you want to know is in there.
If you need the config as a file so that you can
transfer all your configuration details just export
the entire PuTTY registry branch into a file.
I had to help another developer setup an ssh session
with a bunch of tunnels setup and it was easiest for
me to just export the branch (in this case, just for
the particular session) for them to import into their
own registry.
I'll read the book when O'Reilly makes it open source.
I looked at this book in the bookstore, and everything was either obvious or useless. Maybe this book would have helped me when I didn't know anything about ssh, but between the man pages and Google groups everything you need is available.
What really irritated me was the authors' handling of timeouts and keepalives. It's quite common to be stuck behind a firewall that closes all idle TCP connections. The ssh keepalive functionality does not address this - it's for disconnecting dead sessions, not keeping sessions alive. You need to send some "filler" packets through the TCP connection when it's idle.
This is a frequently asked question. The answer of this book is that you shouldn't send keepalive packets because if "the sysadmin" configured a firewall to kill idle connections, you should just accept this restriction. I hope I don't have to explain how completely wrong this is. Increasingly big organizations have a firewall configured by people who are totally unresponsive.
Anyway, I solved the problem by applying this patch.
One of the book's authors responds to this question on Usenet with the same unhelpful answer found in the book.
Everyone else was struggling with the VPN and were having trouble getting stuff working.
I started screwing around with port forwarding and now I work from home a lot.
I am in charge of the Unix/Windows systems. TightVNC and rdesktop are my friends...
Here are a few examples for people confused by SSH port forwarding:
TightVNC
ssh -l username -C -L 7777:internal.vnc.box:5900 ssh.gateway.box
vncviewer -compresslevel 7 -quality 1 -depth 8 127.0.0.1:7777
(On Windows the VNC port starts at 5900 on Unix it is 5901 or 5902 or whatever your desktop says it was set to for vncserver...)
Rdesktop
ssh -l username -C -L 3389:nt.termserver.box:3389 ssh.gateway.box
rdesktop localhost
To forward X from a remote host
ssh -l username -C -L 8811:internal.unix.box:22 ssh.gateway.com
ssh -l username -p 8811 127.0.0.1
To punch a hole in a restrictive firewall (i.e. don't allow ssh gateways...)
From your workstation that you want to reach from the internet:
ssh -C -l root -R 22111:your.work.station:22 your.fire.wall
From your firewall: (Make sure you open the port on the firewall...)
ssh -p 22111 localhost
You can run the command every 15 min from cron or whatever on your workstation at work, or put a sleep statement in,
so you can access it from home.
Since I'm not on my Windows machine right now, I can't quote the liscense directly, but it is one of the most open liscenses out there. IIRC, it's liscense gives you complete control to edit, modify, compile, modularize, give away, and/or SELL PuTTY. It's not GPL, nor LGPL, but rather a very BSDish liscense. It was the first openly liscensed application I ever saw for a Windows machine.
Slackware forever. Honestly, what else would you trust when it absolutely positively has to be stable, secure, and easy
I have used that book to setup our unix systems to meet the corp.security mandates. All thru the book it mixes up information from the ssh.com & openssh implementations. I found it to be confusing until I could identify which configs,files,etc went with which software package. And by then I didn't need the book.
http://www-106.ibm.com/developerworks/library/l-k
http://www-106.ibm.com/developerworks/opensource/
(There is a third one out there you will have to find yourself.) These deveoperworks articles made weeks of frustration suddenly clear to me. It seems the process of creating a key, propogating it, and connecting to it via an agent, could have been more straightforward.
Overall, the confusion of what goes where extends throughout this book. It is not bad when reading from cover to cover, but when looking things up you often have to page back several pages to figure out what context they are talking about.
It still is a useful book. Once I mastered the use of keys I was able to find some useful tidbits in this book (such as setting up specific commands or limiting to specific hosts for a key in your authorized_keys2 file).
Finally, OpenSSH is secondary in this book. It might be nice to have a separate book for OpenSSH, but that is probally just a minor point that many would not agree with.
Rikkers
I'm surprised, does the book make no meantion of SSH's problems? It's not 100% and people shouldn't assume it.
Also, the differences between SSH1 and SSH2 and the compromises that are out there for SSH1 should make that a key topic in the book (if not a whole chapter!)