Month: November 2012

IT

X11 forwarding request failed on channel 0

Since upgrading to Fedora 17, I've been getting this message "X11 forwarding request failed on channel 0". I haven't done anything to fix it for a while, but today I got so annoyed with this message, that I decided to fix it.

First I google'd around a bit. What does this message mean? I read a lot of reactions that suggest to fix something on ssh_config on the client side or in sshd_config on the server side. These suggestions did not work for me.

So, how do we debug this?

On the server side, open the firewall on a non-standard port. I used port 222:
iptables -I INPUT -s [client-ip] -p tcp --dport 222 -j ACCEPT
Then I ran sshd in non-forking debug mode on this port:
/usr/sbin/sshd -d -p 222
Then we login from the client, using verbose mode:
ssh -vvv [server]

This generates a lot of logs on both sides. The log on the client side contains:
debug3: Ignored env GTK_IM_MODULE
debug3: Ignored env XAUTHORITY
debug3: Ignored env CCACHE_HASHDIR
debug3: Ignored env _
debug2: channel 0: request shell confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug1: Remote: No xauth program; cannot forward with spoofing.
debug2: channel_input_status_confirm: type 100 id 0
X11 forwarding request failed on channel 0

So, what's the problem? The server has no xauth program. The old versions of the ssh client silently failed when the server had no xauth, this new version is just a little more verbose.

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IT

MySQL – How to get Top N rows for each group

We have the following table and data:

CREATE TEMPORARY TABLE girls(
name text,
haircolor text,
score INT
);
 
INSERT INTO girls VALUES ('Megan','brunette',9);
INSERT INTO girls VALUES ('Tiffany','brunette',5);
INSERT INTO girls VALUES ('Kimberly','brunette',7);
INSERT INTO girls VALUES ('Hester','blonde',10);
INSERT INTO girls VALUES ('Caroline','blonde',5);
 
SELECT * from girls;
+----------+-----------+-------+
| name     | haircolor | score |
+----------+-----------+-------+
| Megan    | brunette  |     9 | 
| Tiffany  | brunette  |     5 | 
| Kimberly | brunette  |     7 | 
| Hester   | blonde    |    10 | 
| Caroline | blonde    |     5 | 
+----------+-----------+-------+
5 rows in set (0.00 sec)

Session Variables

MySQL, at least on the versions I've checked, does not support ROW_NUMBER() function that can assign a sequence number within a group, the MySQL session variables can be used to build a workaround. Session variables do not need to be declared first and can be used to do calculations and perform actions based on them. They appear to require initialization. For instance:

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© GeekLabInfo MySQL - How to get Top N rows for each group is a post from GeekLab.info. You are free to copy materials from GeekLab.info, but you are required to link back to http://www.geeklab.info

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