The Advisory Boar

A fast, easy-to-maintain, nuisance-free sharpening technique that works well for kitchen knives and woodworking tools alike.

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An article about Vodafone injecting javascript into web pages reminded me of a problem I investigated when Hassath couldn't send mail when connected through her phone's hotspot.

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I have now lost count of the number of forwarded copies of the “A scientist found a bird that hadn’t been seen in half a century, then killed it” mail that people have sent me. The collection of a Bougainville Moustached Kingfisher specimen by an AMNH team in Guadalcanal has drawn intense criticism and reignited the debate about whether scientific collection is justified or even necessary.

[Illustration: J G Keulemans (1842–1912), Novitates Zoologicae]

I don't have anything new or insightful to add to this debate.

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I've written about the various SSH improvements in Ansible 2, including a rewrite of the connection plugin. Unfortunately, the problem that originally motivated the rewrite currently remains unsolved.

Competing prompts

If you ssh to a host for which your known_hosts file has no entry, you are shown the host's key fingerprint and are prompted with Are you sure you want to continue connecting (yes/no)?. If you run ansible against multiple unknown hosts, however, the host key prompts will just stack up:

The authenticity of host 'magpie (a.b.c.d)' can't be established.
ECDSA key fingerprint is 2a:5a:4c:4b:e0:40:de:8b:9b:e6:0f:90:45:68:89:fc.
Are you sure you want to continue connecting (yes/no)? The authenticity of host 'hobby (e.f.g.h)' can't be established.
RSA key fingerprint is 61:84:90:47:f7:0f:7b:a2:d5:09:98:6f:bb:3c:50:d9.
Are you sure you want to continue connecting (yes/no)? The authenticity of host 'raven (i.j.k.l)' can't be established.
RSA key fingerprint is ab:97:c2:7d:b6:8e:c3:ab:78:a2:20:04:af:9c:6f:2b.
Are you sure you want to continue connecting (yes/no)?

The processes compete for input, so typing “yes” may or may not work:

Please type 'yes' or 'no': yes
Please type 'yes' or 'no': yes
Please type 'yes' or 'no': 

Worse still, if some of the targeted hosts are known, output from their tasks may cause the prompts to scroll off the screen, and ansible will appear to hang.

Inter-ssh locking

The solution is to acquire a lock before executing ssh and releasing it once the host key prompt (if any) is negotiated. Ansible 2 had some code copied from 1.9 to implement this, but it was agonisingly broken. It wouldn't have always acquired the lock or released it correctly, but the actual locking was commented out anyway because of lower-level changes, so it just scanned known_hosts twice for every connection. Even if the locking had worked, the lock would have been held until ssh exited.

I submitted patches (12195, 12212, 12236, 12276) to add a connection locking infrastructure and use it to hold a lock only until ssh had verified the host key (not until it finished). Although most of the changes were merged, the actual ssh locking was rejected because it would (unavoidably) wait for ssh to timeout while trying to connect to unreachable hosts.

One of the maintainers recently said they may reconsider this (because it's painful to deal with any number of newly provisioned hosts otherwise), so I have opened a new PR, but it has not yet been merged.

Update: The maintainers went with a different approach to solve the problem. Instead of using locking inside the connection plugin, this checks the host key as a separate step at the strategy level, at the expense of having to parse the known_hosts file to check if a host's key is already known. I think that's a fragile solution, but it does eliminate the locking concerns and improve upon the status quo.

Another update: The commit referenced above was reverted later the same day, for some reason the maintainers did not see fit to record in the commit message. So we're right back to the broken starting point.

Lars Svensson's classic “Identification Guide to European Passerines” was first published a few decades ago. It is no longer available from Amazon, but I have been keeping an eye on copies from other sellers on the Amazon marketplace, and I am increasingly puzzled by their proposed prices.

The absurdly high price isn't because the book is new, because there's a used copy for sale at $1847.20. Even the cheapest used copy right now is priced at $458.60, and that's still far more than I can imagine anyone wanting to pay.

The sellers don't look shady at first glance, and many are highly rated over a significant period. Maybe they didn't notice that the book is available elsewhere for twenty-odd euro? But no, it's probably an “algorithm” (note: those are scare quotes) at work.

I was at the UP Bird Festival in Chambal, tempted mostly by the memory of seeing many crocodiles. There were very few crocodiles this time, but we found a medium-sized Mugger Crocodylus palustris sunning itself on the bank towards the end of our trip.

I had promised Hassath that I would take a crocodile selfie but alas, I managed to omit the actual crocodile. The crocodile-shaped object up on the bank is (what else?) a log.

The crocodile was there, though, just beyond the edge of the frame.

Every morning, children stream past our house in both directions on their way to school. There are the nearly grown-up, very self-conscious young ladies on the way to the inter-college, dressed in blue and white with neatly plaited and be-ribboned hair. There are groups of brown-and-white children, always squabbling over some snack. There are tiny red-and-blue primary school kids who drift past like tumbleweed—so easily distracted that it's a marvel that they ever make it to school.

And then there are the troublemakers, the wretched blue-and-brown boys who derive entertainment from pinching the valve-caps off our car tyres, or snapping off the occasional windshield wiper. We stuck a webcam in the window overlooking the car to keep an eye on these miscreants. It worked pretty well. A few of the smaller children still write their names on the windows when the car is dusty, but we haven't lost any more valve caps.

But now the webcam has become a local attraction, and we hear children of every colour walking past talking about the “CCTV”, bringing their friends around to point it out, and waving or posing (or dancing!) for the camera. A blue-and-white pair—not yet as serious as their elder sisters–recently made faces at it and ran away horrified but giggling when I replied with a cheerful “Hi”.

Ubiquitous surveillance? What fun!

Two years ago, I bought a pair of Alpen Wings 8x42 ED binoculars. These are one of the least expensive mid-range birding binoculars, but a big step up from my earlier Nikon Trailblazer 8x42 at three times the price. Even so, I didn't expect them to be so much better than anything I had used before.

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While writing about ansible_ssh_pipelining earlier, it occurred to me that pipelining could be made to work with requiretty, thus saving having to edit /etc/sudoers, and even making it possible to use su (which always requires a tty). This would mean pipelining could be enabled by default, for a noticeable performance boost.

Here's a working implementation (see the commit message for gory details) that I've submitted as a PR for Ansible 2. Let's hope it's merged soon.

SSH pipelining is an Ansible feature to reduce the number of connections to a host.

Ansible will normally create a temporary directory under ~/.ansible (via ssh), then for each task, copy the module source to the directory (using sftp or scp) and execute the module (ssh again).

With pipelining enabled, Ansible will connect only once per task using ssh to execute python, and write the module source to its stdin. Even with persistent ssh connections enabled, it's a noticeable improvement to make only one ssh connection per task.

Unfortunately, pipelining is disabled by default because it is incompatible with sudo's requiretty setting (or su, which always requires a tty). This is because of a quirk of the Python interpreter, which enters interactive mode automatically when you pipe in data from a (pseudo) tty.

Update 2015-11-18: I've submitted a pull request to make pipelining work with requiretty. The rest of this post still remains true, but if the PR is merged, the underlying problem will just go away.

Pipelining can be enabled globally by setting “pipelining=True” in the ssh section of ansible.cfg, or setting “ANSIBLE_SSH_PIPELINING=1” in the environment.

With Ansible 2 (not yet released), you can also set ansible_ssh_pipelining in the inventory or in a playbook. You can leave it enabled in ansible.cfg, but turn it off for some hosts (where requiretty must remain enabled), or even write a play with pipelining disabled in order to remove requiretty from /etc/sudoers.

- lineinfile:
    dest: /etc/sudoers
    line: 'Defaults requiretty'
    state: absent
  sudo_user: root
  vars:
      ansible_ssh_pipelining: no

The above lineinfile recipe is simplistic, but it shows that it's now possible to disable requiretty, even if it's by replacing /etc/sudoers altogether.

Note the use of another Ansible 2 feature above: vars can also be set for individual tasks (and blocks), not only plays.