This is going to be a contentious one, but here at Anchor we think automatically applying updates to servers is a Good Thing. It’s definitely not for everyone, but in an environment like ours with hundreds of managed servers it’s the only way you’re going to get things done and get any sleep at night.

Sysadmin of note Tom Limoncelli advocates rolling out updates to progressively more machines with prior testing beforehand to mitigate the scope of potential problems (it’s called “one, some, many”). It’s certainly a good strategy for a large number of homogenous computers, but what we’re talking about here is a bit smaller-scale.

Specifically, we have customers with servers that we never touch, we call this Anchor Monitor. These customers often have particular environments that they’re better off supporting themselves, so we monitor the machine to ensure it’s still on the network, and leave it at that. Unfortunately they’re not always kept up to date, so one of the more recent improvements to our process has been to enable automatic updating by default – it’s up to the customer if they want to change this once it’s handed over to them.

We’ve written this into a short procedure if you’re interested. It applies directly to Debian and Redhat distributions, but it’s easily portable to other systems. If you run Windows, it’ll already be hassling you every 20min for updates.

Do you love SSH for the good it has done for mankind, but get annoyed by how long it takes to establish a connection over a high-latency connection? Perhaps you have a process that needs to make thousands of SSH connections, and you’d like a little extra speed from the whole thing. Either way, ControlMaster is your new best friend.

The concept is very simple — rather than each new SSH connection to a particular server opening up a new TCP connection, you instead multiplex all of your SSH connections down one TCP connection. The authentication only happens once, when the TCP connection is opened, and thereafter all your extra SSH sessions are sent down that connection.

If you’re SSHing between machines on the same LAN, or otherwise a short ping away, you probably wouldn’t notice the difference — the round-trip times are negligible. However, when you’re doing transcontinental SSHing (which we do often, when we’re managing customer machines in the US), it’s a godsend. On some trivial benchmarking I did when validating ControlMaster for our use, I found that we were saving nearly 2.5 seconds per connection — a drop from 3.3 seconds to 0.8. Mighty convenient.

It’s simple to use, too. If you just want to enable “opportunistic” multiplexing, you can do something as simple as this in your SSH config:

Host *
ControlMaster auto
ControlPath ~/.ssh/cm_socket/%r@%h:%p

Then mkdir ~/.ssh/cm_socket, and you’re away. Any time a connection to a remote server exists, it’ll be used as the master for any other connections. Perusal of the ssh_config(5) manpage should give you the necessary hints to setup more restrictive configurations. If you need to disable control master for a given connection (the reasons why this might be necessary will be covered shortly), you can pass -S none to ssh (or set ControlPath none).

Whilst this basic setup is undeniable, pure, distilled awesome, there are some limitations and caveats to beware of. The first, and most important, is that SSH session multiplexing isn’t particularly stable when you try to put a lot of data down it from a lot of connections at once. This came to light fairly early on in my testing, when I stress-tested things by doing about 25 concurrent rsync runs all at once. The result was a large number of rsync sessions going “aiee!” and falling over. So, don’t do that.

The second, semi-related problem, is a simple bandwidth issue. For a given connection latency and TCP configuration, there is a hard limit to how fast you can send data, due to the time it takes to acknowledge the packets being received. When you’re multiplexing multiple file transfers down the one TCP connection, therefore, your total transfer speed will be limited by this TCP speed limit. Once again, it’s unlikely that this will cause you problems on a LAN (where round-trip delays are negligible), but in the high-latency world where connection sharing does the most good from a connection setup perspective, the speed limits will cause much wailing and gnashing of teeth. So, the take home message is: if you’re doing a lot of heavy data transfer over SSH, ControlMaster probably isn’t the solution for your problems. Instead, run multiple concurrent SSH connections, as the TCP speed limits are per-connection, so you can still fill your high-latency gigabit pipe — you just need lots of concurrent connections to do it (see also: BitTorrent).

Finally, there is something of an annoyance with ControlMaster, and it’ll probably confuse you mightily when you first come across it. Because all of your SSH sessions are multiplexed down a single TCP connection initiated by the first SSH session, that first session must stay alive until all of the other sessions are complete. This problem will manifest itself as an apparent “hang” when you log out of the remote session that is acting as the master — instead of getting your local prompt back, SSH will just sit there. If you Ctrl-C or otherwise kill this session, all of the other sessions you’ve got setup to that server will drop, so don’t do that. Instead, when you logout of all the other sessions, the master will then return to the local prompt.

If you’re doing a high volume of SSH connections to a particular remote endpoint, consider setting up a dedicated master connection — that way it’ll always be available (and you don’t have to worry about master logout hangs). I use a simple daemontools service, that runs ssh -MNn user@server. Works an absolute treat.

Sampling shamelessly stolen from four fine folk, for your amusement.

We love vi, everyone at Anchor uses it.
‘Cause it’s better than emacs, yo.

.

AWW SNAP, BRO! WE JUST IMPROVED YO GESTURE!!

.

In Brief: Come down to the James Squire Brew house (King Street Wharf, Sydney) for a chance to have a few drinks with guys (and gals) from your friendly hosting company. In addition to this, we’re lucky to have special guests Tom and Scott of Github fame dropping in and you’ll have the opportunity to have some beers, tell stories of 14′ kangaroos, and discuss all things Github.

Anchor will be putting on drinks from 6pm.

Details:
6pm, Monday, Jan 18th
James Squire Brewhouse
22 Promenade King St Wharf, Sydney

Unfortunately, SSH doesn’t produce this error, although it darn well should…

I just had a Github customer report that they couldn’t access their repos via SSH, despite it all working properly yesterday, and “not having changed anything”. A bit of debug logging and an inspired leap of intuition on the part of another sysadmin in the office, and the answer was quickly found.

First off, the symptoms:

• Debug logging showed that the user was connecting successfully, presenting six SSH keys (none of which were the key of interest) before disconnecting;
• The SSH key was in the user’s SSH agent (you can verify this with a quick ssh-add -l);
• There were more than six keys in the SSH agent

This last symptom is the key point. As an anti-brute-force measure (I assume), SSH won’t allow a user to connect and present more than MaxAuthTries credentials (whether they be passwords or keys) before being forcibly disconnected. The default value for this parameter (if you haven’t realised already) is six.

Whilst this makes a lot of sense for passwords (and a lesser, but still valid, measure for keys) it does mean that you effectively have a hard limit of six keys in your agent simultaneously (at least without using SSH configs to specify a single key to present to the server). Any more than six keys, and you run the very real risk that the key you need to give to a particular server will be number seven in your agent, and all your authentications will fail miserably.

Bumping the value of MaxAuthTries to a much larger value works fine for Github — password auth is disabled, and if you can manage to brute force a key you’re welcome to what you can get — but you certainly can’t rely on inflating MaxAuthTries everywhere to get you out of trouble, so: keep those SSH agents lean, or at least specify IdentityFile for all your servers.

Whilst I’m a fan of using percentages for my disk space checks, sometimes an explicit size is more appropriate. So, you’d expect the following to work nicely:

$USER1$/check_disk -w 5G -c 1G -p /data/foo

If you don’t actually test that this works (by artificially filling your disk and seeing what happens), you may be dismayed to find that you only get alerted when the disk has 5MB of free disk space. Why is this?

Because Nagios, despite the fact that nobody has sweated the megabytes for about a gazillion years, doesn’t support ‘G’ as a suffix for thresholds. Oh, it’ll make a good show of pretending — after all, the output formatting options have ‘GB’ as an option — but nope, for your thresholds it’s “5000M” all the way.

ROCK ON!

I’ve never been a real fan of the output of big “industry analysis” firms, since their reports never seemed to really tell the whole story, and didn’t match up with my experiences “in the trenches”. Now I know why. A representative sample:

“I see. So, the companies in your magic quadrant, are they all paying clients of yours?”

“Well, yes they are,” He said, proudly.

“Well, if they are all paying clients, then what’s so ‘magic’ about being in the quadrant?”

“The companies are not all rated at the same level, some are rated much higher than others.”

“And should I be surprised to hear that the companies that pay you more so you can afford to have entire teams cover them full-time; you tend to know a lot about, and they tend to get better ratings?”

No answer.

“Maybe you should stop calling it the ‘Magic Quadrant’ and call it what it really is; perhaps ‘The Quadrant of Companies That Can Afford To Be In It’.

Go read the whole article, though, it’s pure gold.

Remember the good old days, when Melissa and ILOVEYOU were the major virus threats, spreading via e-mail and causing all sorts of embarrassing conversations at work? Or maybe even earlier than that, when the only way you could get a virus was by engaging in risky sex? (I mean Software EXchange, of course… get your mind out of the gutter)

These days, anti-virus protection for e-mail is fairly thorough, and nobody’s really swapping floppies full of 16 colour games at recess. Malware authors have moved on to new and more fertile ground — embedding their junk in web pages, and relying on browser exploits to gain access to computers. Of course, with this method, you can only get infected if you actually visit a page that has an infestation, so the malware authors have two options: either entice you to visit their sites, or modify existing websites that users will visit in the course of their day — legitimate sites that people know and trust, but with a little added infection.

Enticing people to a whole dodgy site is usually just a matter of providing something people love to look at and sticking it in search engines. Since the attacker has to have a stable, identifiable presence for the search engines to direct users to, that can also be used by anti-malware lists like stopbadware.org to protect web users, so this isn’t a particularly effective means of attack, and is waning somewhat in popularity. Far more effective is infecting a legitimate website with some form of malware. How does it happen, though? In our experience, there are four vectors for infection:

1. Brute-force password guessing, where the attacker has a botnet they control to just repeatedly try lots and lots of usernames and passwords. They’re bound to get lucky sooner or later.
2. Some sort of web-based exploit, typically a vulnerability in the web application that allows the attacker to run code of their choosing; this is then either used directly to edit files, or bootstrapped into sufficient access to edit files via another method.
3. Password “scraping”, where the attacker gets direct access to the FTP password for your site. This can either be some sort of malware on the workstation of the web developer (or someone else related to the management of the website) that gets the password off the local machine (in a saved password file, or via a keylogger), or else via the “lost password” functionality provided by the hosting provider. Once the attacker has the FTP password for the site, they are free to login to the live site and make whatever changes they like.
4. Direct modification of the website code on the client-side computer, relying on the developer not to notice it and then upload the compromised content to the live site. We recently had our first “confirmed” case of this (where the web developer found the malicious modifications in their local copy), and they swear blind they didn’t download the HTML from the live site (which would bring the “infection” onto the local machine from the infected live site — which we’ve seen before, and categorise under vectors 1 and/or 2).

The countermeasures required to combat all these vectors boil down to a few simple precautions.

• Use strong passwords. (Protects against vector 1) Yes, they’re a pain to manage, but a weak password is just an open invitation to getting repeatedly and painfully owned. Of course, the strongest password is a keypair, which leads us to…
• Don’t use FTP. (Protects against vectors 1 and 3) The list of reasons for this is long, but for securing your website, FTP is a pain because you can only use passwords[1]. Switch to using SFTP (the file transfer component of the SSH protocol) and you can use public keys, which are, for all practical purposes, unguessable. You should also encrypt your keys with a passphrase, which means that even if the attacker does get access to your workstation and copies the key, it’ll be useless to them — unless they keylog your passphrase, which brings us to…
• Keep your workstation secure. (Vectors 3 and 4) It seems that attackers have realised that the weakest link in the website security chain is still the Windows desktop, and they’re increasingly hitting it as the first step in taking over websites (if you get the right workstation, you can get the credentials to hundreds or thousands of websites, because one web developer often works on many different sites). So, on any machine you connect to webservers from, you need to be doubly, triply sure that it’s rock solid — and that’s just a matter of following all the good advice out on the web. Antivirus, antispyware and firewall software, constantly running, well-configured, and kept up to date; keep up to date with your application patches, especially for your web browser, e-mail client, and core OS; don’t visit dodgy web sites; and so on.
• Protect your e-mail. (Vector 3) If someone can get access to your e-mail, they can also get access to your website, by using the password recovery feature (or impersonating you to your hosting company). If they delete the e-mails that are coming in before you notice them, you’ll never know what’s going on, and all the password changes and workstation security in the world won’t help you.
• Don’t use shared hosting. (Vectors 1 and 3). This might seem an odd thing to say, given that we sell shared hosting, but it is a legitimate way to reduce your vulnerability. If you use a dedicated server (including a VPS), you (or we) can configure it to only allow logins from certain IP addresses, rather than the entire Internet. This means that even if an attacker does get your password (or SSH key), via brute force or sniffing it off your workstation, they can’t login from their own machine because it won’t have an authorized IP address. On shared hosting, this configuration is impractical, because hundreds of people have legitimate access to the server, from a great many different IP addresses.
• Code responsibly. (Vector 2) It is said that “PHP is great, because its ease of use means that any idiot can produce a security hole — and most of them do”. Whilst this is a little derogatory to the many (several? few? one? PLEASE?) good PHP programmers out there, it is certainly fair to say that the capabilities of many people who write dynamic code aren’t up to the challenge of writing code that is exposed to the extremely hostile conditions that are the public Internet.

  Thus, if you are not familiar with the common security practices and problems with the language or environment that you are developing for, stop right now and go learn a little. There’s plenty of good information out there on the Internet from people who have learnt the lessons the hard way. Celebrate the benefits of literacy by learning from their mistakes rather than having to educate yourself by cleaning up an infected website. If you feel that isn’t something you can commit to, then please, for the sake of the Internet, find someone else to write the code.

• Keep your web applications patched. (Vector 2) Whilst some sites do use custom-built web applications, many sites choose to use a standard CMS or other application to manage their website. This is great, because hopefully someone else is taking a bit of responsibility for the security of the software, but that doesn’t do you any good if you don’t keep it up-to-date. Far, far too many people install a CMS once, then forget about it. Almost all of these applications have a vulnerability at some point, and not keeping them up to date is absolutely fatal, because once a vulnerability is found in a piece of software, an attacker can typically use Google to find all of the publicly-available instances of the vulnerable software, and quickly attack them all.

  This means that you need to keep yourself well-informed of any security updates for your off-the-shelf web applications. Subscribe to a relevant security announcements mailing list, or ensure that your vendor sends them to you. (If your commercial CMS vendor doesn’t have this ability, find a new CMS vendor).

Websites get compromised all the time, by a variety of methods. You should reinforce your defences, lest you’re the next target.

1. The first person to mention Kerberos or other unused-in-practice authentication schemes in a comment gets a free laughing at. If you think SFTP and SCP aren’t supported in widely used web development programs, try finding something that supports GSSAPI…

This is the output of iptables -L on a webmin-managed box I just saw:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
ACCEPT     tcp  --  anywhere             anywhere            tcp flags:ACK/ACK
ACCEPT     all  --  anywhere             anywhere            state ESTABLISHED
ACCEPT     all  --  anywhere             anywhere            state RELATED
ACCEPT     udp  --  anywhere             anywhere            udp spt:domain dpts:1024:65535
ACCEPT     icmp --  anywhere             anywhere            icmp any
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ftp
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ssh
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:smtp
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:domain
ACCEPT     udp  --  anywhere             anywhere            udp dpt:domain
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:http
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:pop3
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:imap
ACCEPT     udp  --  anywhere             anywhere            udp dpt:imap
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:https
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:mysql
ACCEPT     tcp  --  anywhere             anywhere            tcp spt:mysql
ACCEPT     tcp  --  anywhere             anywhere            tcp dpts:terabase:samsung-unidex
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ndmp
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:dnp
LOG        all  --  anywhere             anywhere            LOG level debug prefix `DROPPED = '
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:ftp-data 

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             anywhere            tcp spt:ftp-data dpt:ftp-data
ACCEPT     tcp  --  anywhere             anywhere            tcp spt:ftp dpt:ftp

Lovely that it has all those ports and whatnot opened up, but what’s with the ACCEPT policies?

Webmin: Now with FAILWALL management!

I should have been in marketing.

Recently, I read an article from a fairly prominent “cloud computing” vendor, which contained a line that basically said “Let the cloud worry about your scalability and performance problems”. I nearly snorted my late-mid-morning can of mother out my nose when I read it. Here’s why.

“Let the cloud worry about your scalability” is nothing more than a thinly disguised version of “just throw more hardware at it”. This is a “solution” beloved of salespeople everywhere, because it’s plausible, real easy to say, and makes a whole pile more money for the company providing the hardware. However, while it can be an appropriate solution in the right circumstances, and with appropriate evidence of its effectiveness in those particular circumstances, it usually isn’t the only option, it often isn’t the best option, and sometimes it isn’t an effective option at all.

The dirty little secret of hosting is that your scaling ability is solely determined by your application — the technologies it uses and its internal architecture. Yes, you can probably get more performance or concurrent users out of throwing more hardware at it this time, but sooner or later more memory or faster CPUs isn’t going to do anything useful.

I suppose, in some perverse way, just telling developers that “the cloud will provide” could be construed as a kindness. In the same way that we give high school kids the simplified approximation of motion that is Newton’s Laws, rather than the complicated and fiddly reality of relativity, saying “let the cloud scale you to being the next Facebook” might be a useful approximation to let developers ignore extraneous details and focus on getting things “right enough”.

The vast majority of sites, even those who aspire to be the next Twitter, will never get to anywhere near that scale. Even if it is the goal (and plenty of sites manage to occupy a satisfying — and dare I say it, profitable — niche without needing a second datacentre full of equipment), a new site is only going to get that big by focusing on satisfying users and creating compelling applications.

Spending your time writing Yet Another Key-Value Store is an awesome way to spend a lazy weekend, but when you’re burning your rent cheque and credit rating trying to get your “next big thing” site off the ground, every minute spent not awesomising your user experience is putting you 35c closer to having to go back to working for The Man.

For whatever reason, though, it makes me uncomfortable to lie to people about things like this, even if I might think it’s in their interest. I know, first hand, the shock and pain that comes from finding out that your site, beloved by millions, is suddenly overloaded and unreliable — and, even worse, that throwing hardware at the problem won’t do a damned thing. It’s an awful feeling.

While you can’t be worrying about scaling to a million users when your site has a grand total of three users (one of which is your mum), you have to prepare for it when things starts to take off, and have a plan in place to deal with it.

Sooner or later, you’re going to have to sit down, find the pain points in your current architecture, and work out how to solve them. If you’re not comfortable doing that yourself, then you need smart systems people who know how. I can guarantee you that “the cloud” isn’t going to advise you on how to restructure your file storage so it will horizontally scale to a petabyte of data. Don’t rely on it to scale you out of trouble.