Shallow Thoughts : : Jun

How to un-deny a host blocked by denyhosts

We had a little crisis Friday when our server suddenly stopped accepting ssh connections.

The problem turned out to be denyhosts, a program that looks for things like failed login attempts and blacklists IP addresses.

But why was our own IP blacklisted? It was apparently because I'd been experimenting with a program called mailsync, which used to be a useful program for synchronizing IMAP folders with local mail folders. But at least on Debian, it has broken in a fairly serious way, so that it makes three or four tries with the wrong password before it actually uses the right one that you've configured in .mailsync. These failed logins are a good way to get yourself blacklisted, and there doesn't seem to be any way to fix mailsync or the c-client library it uses under the covers.

Okay, so first, stop using mailsync. But then how to get our IP off the server's blacklist? Just editing /etc/hosts.deny didn't do it -- the IP reappeared there a few minutes later.

A web search found lots of solutions -- you have to edit a long list of files, but no two articles had the same file list. It appears that it's safest to remove the IP from every file in /var/lib/denyhosts.

So here are the step by step instructions.

First, shut off the denyhosts service:

service denyhosts stop

Go to /var/lib/denyhosts/ and grep for any file that includes your IP:

grep aa.bb.cc.dd *

(If you aren't sure what your IP is as far as the outside world is concerned, Googling what's my IP will helpfully tell you, as well as giving you a list of other sites that will also tell you.)

Then edit each of these files in turn, removing your IP from them (it will probably be at the end of the file).

When you're done with that, you have one more file to edit: remove your IP from the end of /etc/hosts.deny

You may also want to add your IP to /etc/hosts.allow, but it may not make much difference, and if you're on a dynamic IP it might be a bad idea since that IP will eventually be used by someone else.

Finally, you're ready to re-start denyhosts:

service denyhosts start

Whew, un-blocked. And stay away from mailsync. I wish I knew of a program that actually worked to keep IMAP and mbox mailboxes in sync.

Tags: linux
[ 12:59 Jun 26, 2016    More linux | permalink to this entry | ]

Cave 6" as a Quick-Look Scope

I haven't had a chance to do much astronomy since moving to New Mexico, despite the stunning dark skies. For one thing, those stunning dark skies are often covered with clouds -- New Mexico's dramatic skyscapes can go from clear to windy to cloudy to hail or thunderstorms and back to clear and hot over the course of a few hours. Gorgeous to watch, but distracting for astronomy, and particularly bad if you want to plan ahead and observe on a particular night. The Pajarito Astronomers' monthly star parties are often clouded or rained out, as was the PEEC Nature Center's moon-and-planets star party last week.

That sort of uncertainty means that the best bet is a so-called "quick-look scope": one that sits by the door, ready to be hauled out if the sky is clear and you have the urge. Usually that means some kind of tiny refractor; but it can also mean leaving a heavy mount permanently set up (with a cover to protect it from those thunderstorms) so it's easy to carry out a telescope tube and plunk it on the mount.

I have just that sort of scope sitting in our shed: an old, dusty Cave Astrola 6" Newtonian on an equatorian mount. My father got it for me on my 12th birthday. Where he got the money for such a princely gift -- we didn't have much in those days -- I never knew, but I cherished that telescope, and for years spent most of my nights in the backyard peering through the Los Angeles smog.

Eventually I hooked up with older astronomers (alas, my father had passed away) and cadged rides to star parties out in the Mojave desert. Fortunately for me, parenting standards back then allowed a lot more freedom, and my mother was a good judge of character and let me go. I wonder if there are any parents today who would let their daughter go off to the desert with a bunch of strange men? Even back then, she told me later, some of her friends ribbed her -- "Oh, 'astronomy'. Suuuuuure. They're probably all off doing drugs in the desert." I'm so lucky that my mom trusted me (and her own sense of the guys in the local astronomy club) more than her friends.

The Cave has followed me through quite a few moves, heavy, bulky and old fashioned as it is; even when I had scopes that were bigger, or more portable, I kept it for the sentimental value. But I hadn't actually set it up in years. Last week, I assembled the heavy mount and set it up on a clear spot in the yard. I dusted off the scope, cleaned the primary mirror and collimated everything, replaced the finder which had fallen out somewhere along the way, set it up ... and waited for a break in the clouds.

I'm happy to say that the optics are still excellent. As I write this (to be posted later), I just came in from beautiful views of Hyginus Rille and the Alpine Valley on the moon. On Jupiter the Great Red Spot was just rotating out. Mars, a couple of weeks before opposition, is still behind a cloud (yes, there are plenty of clouds). And now the clouds have covered the moon and Jupiter as well. Meanwhile, while I wait for a clear view of Mars, a bat makes frenetic passes overhead, and something in the junipers next to my observing spot is making rhythmic crunch, crunch, crunch sounds. A rabbit chewing something tough? Or just something rustling in the bushes?

I just went out again, and now the clouds have briefly uncovered Mars. It's the first good look I've had at the Red Planet in years. (Tiny achromatic refractors really don't do justice to tiny, bright objects.) Mars is the most difficult planet to observe: Dave liks to talk about needing to get your "Mars eyes" trained for each Mars opposition, since they only come every two years. But even without my "Mars eyes", I had no trouble seeing the North pole with dark Acidalia enveloping it, and, in the south, the sinuous chain of Sini Sabaeus, Meridiani, Margaritifer, and Mare Erythraeum. (I didn't identify any of these at the time; instead, I dusted off my sketch pad and sketched what I saw, then compared it with XEphem's Mars view afterward.)

I'm liking this new quick-look telescope -- not to mention the childhood memories it brings back.

Tags: astronomy, science
[ 08:53 Jun 18, 2016    More science/astro | permalink to this entry | ]

Visual diffs and file merges with vimdiff

I needed to merge some changes from a development file into the file on the real website, and discovered that the program I most often use for that, meld, is in one of its all too frequent periods where its developers break it in ways that make it unusable for a few months. (Some of this is related to GTK, which is a whole separate rant.)

That led me to explore some other diff/merge alternatives. I've used tkdiff quite a bit for viewing diffs, but when I tried to use it to merge one file into another I found its merge just too hard to use. Likewise for emacs: it's a wonderful editor but I never did figure out how to get ediff to show diffs reliably, let alone merge from one file to another.

But vimdiff looked a lot easier and had a lot more documentation available, and actually works pretty well.

I normally run vim in an xterm window, but for a diff/merge tool, I want a very wide window which will show the diffs side by side. So I used gvimdiff instead of regular vimdiff: gvimdiff docs.dev/filename docs.production/filename

Configuring gvimdiff to see diffs

gvimdiff initially pops up a tiny little window, and it ignores Xdefaults. Of course you can resize it, but who wants to do that every time? You can control the initial size by setting the lines and columns variables in .vimrc. About 180 columns by 60 lines worked pretty well for my fonts on my monitor, showing two 80-column files side by side. But clearly I don't want to set that in .vimrc so that it runs every time I run vim; I only want that super-wide size when I'm running a side-by-side diff.

You can control that by checking the &diff variable in .vimrc:

if &diff
    set lines=58
    set columns=180
endif

If you do decide to resize the window, you'll notice that the separator between the two files doesn't stay in the center: it gives you lots of space for the right file and hardly any for the left. Inside that same &diff clause, this somewhat arcane incantation tells vim to keep the separator centered:

    autocmd VimResized * exec "normal \<C-w>="

I also found that the colors, in the vim scheme I was using, made it impossible to see highlighted text. You can go in and edit the color scheme and make your own, of course, but an easy way quick fix is to set all highlighting to one color, like yellow, inside the if $diff section:

    highlight DiffAdd    cterm=bold gui=none guibg=Yellow
    highlight DiffDelete cterm=bold gui=none guibg=Yellow
    highlight DiffChange cterm=bold gui=none guibg=Yellow
    highlight DiffText   cterm=bold gui=none guibg=Yellow

Merging changes

Okay, once you can view the differences between the two files, how do you merge from one to the other? Most online sources are quite vague on that, but it's actually fairly easy:

]c	jumps to the next difference
[c	jumps to the previous difference
dp	makes them both look like the left side (apparently stands for diff put
do	makes them both look like the right side (apparently stands for diff obtain

The only difficult part is that it's not really undoable. u (the normal vim undo keystroke) works inconsistently after dp: the focus is generally in the left window, so u applies to that window, while dp modified the right window and the undo doesn't apply there. If you put this in your .vimrc

nmap du :wincmd w<cr>:normal u<cr>:wincmd w<cr>

then you can use du to undo changes in the right window, while u still undoes in the left window. So you still have to keep track of which direction your changes are going.

Worse, neither undo nor this du command restores the highlighting showing there's a difference between the two files. So, really, undoing should be reserved for emergencies; if you try to rely on it much you'll end up being unsure what has and hasn't changed.

In the end, vimdiff probably works best for straightforward diffs, and it's probably best get in the habit of always merging from right to left, using do. In other words, run vimdiff file-to-merge-to file-to-merge-from, and think about each change before doing it to make it less likely that you'll need to undo.

And hope that whatever silly transient bug in meld drove you to use vimdiff gets fixed quickly.

Tags: linux, editors, vim
[ 20:10 Jun 09, 2016    More linux/editors | permalink to this entry | ]

Walking your Goat at the Summer Concert

I love this place. We just got back from this week's free Friday concert at Ashley Pond. Not a great band this time (the previous two were both excellent). But that's okay -- it's still fun to sit on the grass on a summer evening and watch the swallows wheeling over the pond and the old folks dancing up near the stage and the little kids and dogs dashing pell-mell through the crowd, while Dave, dredging up his rock-star past, explains why this band's sound is so muddy (too many stacked effects pedals).

And then on the way out, I'm watching appreciatively as the teen group, who were earlier walking a slack line strung between two trees, has now switched to juggling clubs. (I know old people are supposed to complain about "kids today", but honestly, the kids here seem smart and fit and into all kinds of cool activities.) One of the jugglers has just thrown three clubs and a ball, and is mostly keeping them all in the air, when I hear a bleat to my right -- it's a girl walking by with a goat on a leash.

Just another ordinary Friday evening in Los Alamos.

Tags: los alamos
[ 20:45 Jun 03, 2016    More misc | permalink to this entry | ]