If you want to debug why your C/C++ application segfaults once in a while (daemon that segfaults once in few days) under Linux the obvious way is to tell your program to create core dump files on segfault. It took me few hours to make my program coredump - by default under Linux setuid programs do not dump core. To force a program to do so, you should execute this after setuid:
prctl(PR_SET_DUMPABLE, 1);
and include:
#include <sys/prctl.h>
Of course you should also enable core dump with setrlimit:
setrlimit(RLIMIT_CORE, &limit);
To tune where your core dumps are stored, search google for core_pattern or /proc/sys/kernel/core_pattern, I don’t feel like copy-pasting.
Hope this helps somebody.
Ever wanted to be able to create site screenshots, just like alexa? Want to do it on your server with a script? Than read this article.
The idea is simple: install xvnc on your server, so you’ll have a virtual X display, than install Opera browser (Firefox does not support -geometry parameter) and finally install ImageMagick (the chances are that you already have it) so you can screenshot your virtual display. On Gentoo Linux I’ve done this by typing:
USE='X qt server png jpeg -kde' emerge vnc opera imagemagick
Create a user, under which xvnc/Opera will run and switch to it. Start vncserver with:
vncserver :1 -depth 24 -geometry 1024x768
it will ask for a password, you will need it if you want to connect to this display from a desktop computer. Of course you can change resolution, which is 1024×768 in this case (do not raise -depth, at least without reading “man vncserver”). Now start opera browser with:
/usr/bin/opera -display :1
Of course you will se nothing. Connect from your computer to the remote vnc server with:
vncviewer xx.xx.xx.xx:1
where xx.xx.xx.xx of course is the IP address of your server. You should see Opera window. Remove all the tollbars, so you have only meny on top. SECURE YOUR BROWSER - you may want to completely disable JavaScript or at least disable all popups. Close the browser. Now is a good time to decide if you want to install Macromedia Flash (installation is trivial).
So now you should be able to make screenshot from the console - run opera with URL as a parameter, also specifying -geometry and screenshot the site with import (part of ImageMagick package). This simple shell script runs Opera, waits 25 seconds for the site to load (unfortunately I didn’t find a good way to check if site is loaded) and screenshots it:
#!/bin/bash
export DISPLAY=":1"
/usr/bin/opera -display :1 -geometry 1024x768+0+0 -nomail -nosession "$1" > /dev/null 2> /dev/null &
/usr/bin/sleep 25
/usr/bin/import -window root -display :1 -crop 1024x768+7+50 "$2"
/usr/bin/killall opera
Usage is:
./screenshot http://www.google.com/ google.png
where screenshot is the name of the script. After executing it you will have google.png with screenshot of google’s website. If you see browser’s menus or frames, play with -crop parameter of import. Do not pass unchecked/unescaped parameters to the script, somebody will be able to execute a program on your server! Also have in mind that running browser on your server is not the most secure thing in the world. Actually it is as secure as browsing with Opera browser, so the choice is yours. Just don’t blame me if something wrong happens.
You can also use this sample Perl webserver. It will site screenshot when called as:
http://xx.xx.xx.xx/shot?http://www.google.com/
The server is single process, so you are limited to a single user at a time, also have in mind that you will have to wait 25+ sec for your screenshot. Before starting it, make sure you have installed: HTTP::Daemon and HTTP::Status. Also have in mind that this is only a TEST server that IS VULNERABLE, but it’s OK for testing and use in trusted environment.
#!/usr/bin/perl
$SIG{PIPE} = 'IGNORE';
use HTTP::Daemon;
use HTTP::Status;
my $d = HTTP::Daemon->new(
'LocalPort' => 8899
) || die;
print "Please contact me at: url, “>\n”;
while (my $c = $d->accept) {
if (my $r = $c->get_request) {
if ($r->method eq ‘GET’ and $r->url->path eq “/shot”) {
$site=urlDecode($r->url->query);
`/path/to/screenshot $site /path/to/writable/area/site.png`;
$c->send_file_response(”/path/to/writable/area/site.png”);
}
else {
$c->send_error(RC_FORBIDDEN)
}
}
$c->close;
undef($c);
}
sub urlDecode {
my ($string) = @_;
$string =~ tr/+/ /;
$string =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack(”C”, hex($1))/eg;
return $string;
}
This webserver should be run like:
nohup perl ws.pl &
as the same user as which vncserver is running (root is not a good choice, really).
Have fun (and screenshots). This guide is intentionally not a total copy-paste solution, if you have hard time with missing steps - better outsource this task to somebody capable.
This works for me, you can click the counter button at the bottom of the page and you will see screenshot of my site on counter’s page (this is a free Bulgarian web counter written by me, only the high performance engine actually).