"No such a file or directory" when setting NETGEAR router

I got this error when I was setting up a new NETGEAR X6 router after setting up the new password. I restored the factory settings multiple times, but it either showed "No such a file or directory" for the link "http://www.routerlogin.net/genie_index.html" or pop up a dialog for user name and password, but the default "admin" and "password" didn't work.

I finally realized that it was my fault, not NETGEAR's. My laptop connected to NETGEAR X6 using a cable, but to my old NETGEAR router using WiFi. When the new router redirected the page to "http://www.routerlogin.net/genie_index.html", my laptop messed up with www.routerlogin.net because it tried to the old NETGEAR router. Turning off WiFi of my laptop solved the problem, and X6 home page showed up correctly.

Allow the outside world access your server in a container

I run docker containers of Nexus and httpd on a CentOS 7 host. I added a nexus.service to start the containers using docker-compose. I could access the nexus server from any machine after I started the service. But the next day, I could not access that server from other machines, running curl -k -X GET https://<host-ip> always got time out. The containers were still running and they were still bound to all interfaces because I could run curl -k -X GET https://<host-ip> on that host. The below shows those three ports 80,443,15001 of all interfaces are listened. NOTE: Proto=tcp6 doesn’t means “not listening on ipv4”.

# netstat -l -t
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp6       0      0 [::]:http               [::]:*                  LISTEN     
tcp6       0      0 [::]:15001              [::]:*                  LISTEN     
tcp6       0      0 [::]:https              [::]:*                  LISTEN

If I restart the service docker using systemctl restart docker, and service nexus systemctl start nexus, everything works again. If I reboot the server, docker starts and the containers are running, but I cannot access the Nexus server from any machine except the host.

It turns out that the host runs chef-client in the early morning and after rebooting, and set net.ipv4.ip_forward = 0. I can run sysctl net.ipv4.ip_forward=1 to make remote access to the Nexus server, and sysctl net.ipv4.ip_forward=0 to deny any access.

If I restarts the service docker, docker set net.ipv4.ip_forward=1 automatically. Check the docker document Communcating to the outside world

IP packet forwarding is governed by the ip_forward system parameter. Packets can only pass between containers if this parameter is 1. Usually you will simply leave the Docker server at its default setting —ip-forward=true and Docker will go set ip_forward to 1 for you when the server starts up. If you set —ip-forward=false and your system’s kernel has it enabled, the —ip-forward=false option has no effect. To check the setting on your kernel or to turn it on manually:

​

VNC server won't start on my CentOS 7

I had to ask my boss to power down and power up the desktop in my office this morning. After the desktop came back, I could not access the vncserver because it failed to start. And I got this error when I run systemctl.

$ sudo systemctl start vncserver@\:2

Job for vncserver@:2.service failed because a configured resource limit was exceeded. See "systemctl status vncserver@:2.service" and "journalctl -xe" for details.

And I also found a Xvnc process started on :1 every time when I start the service.

I did find that `/usr/lib/systemd/system/vncserver@:2.service was missing and restored it like below:

[Unit]
Description=Remote desktop service (VNC)
After=syslog.target network.target

[Service]
Type=forking
User=bwang

# Clean any existing files in /tmp/.X11-unix environment
ExecStartPre=-/usr/bin/vncserver -kill %i
ExecStart=/usr/bin/vncserver %i -nolisten tcp -localhost -geometry 1920x1080 -de
pth 24 
PIDFile=/home/bwang/.vnc/%H%i.pid
ExecStop=-/usr/bin/vncserver -kill %i

[Install]

WantedBy=multi-user.target

And I ran systemctl daemon-reload, systemctl disable vncserver@\:2.service, systemctl enable vncserver@\:2.service multiple times, but got the same error again and again.

I also tried using vncserver\@\:2 or vncserver@:2 for service name, it won’t fix it.

Finally when I ran vncserver from the command line, it showed this message

$ vncserver :2 -nolisten tcp -localhost -geometry 1920x1080 -depth 24

Warning: bwang.corp.rhapsody.com:2 is taken because of /tmp/.X11-unix/X2
Remove this file if there is no X server bwang.corp.rhapsody.com:2
A VNC server is already running as :2

New 'bwang.corp.rhapsody.com:1 (bwang)' desktop is bwang.corp.rhapsody.com:1

Starting applications specified in /home/bwang/.vnc/xstartup
Log file is /home/bwang/.vnc/bwang.corp.rhapsody.com:1.log

Obviously, when the deskotp was powered down, /tmp/.X11-unix/X2 was left on the hard drive, and it blocked vncserver from starting on display 2 again.

Removing /tmp/.X11-unix/X2 fix the problem.

NOTE: using vncserver@:2 without escaping : is ok when running systemctl.

​

Spark SQL AnalysisException due to data type mismatch

If you run the following code, you will encounter AnalysisException with data type mismatch.

spark.sql("create table test_table(id int, data struct<name: string, age:int>)")
spark.sql("insert overwrite table test_table select 1, struct('joe', 15)")
val d = spark.read.table("test_table")

case class UserData(name: String, age: Int)
case class User(id: Int, data: UserData)
val u = Seq(User(1, UserData("joe", 15)), User(2, UserData("mary", 10))).toDF

val missing = {
  u.join(
    d,
    u("id") === d("id")
    && u("data")("name") === d("data")("name")
    && u("data")("age") === d("data")("age"),
    "outer"
  )
  .where( u("id").isNull || d("id").isNull)
}

// show this result
// +---+---------+----+----+                                                      
// |id |data     |id  |data|
// +---+---------+----+----+
// |2  |[mary,10]|null|null|
// +---+---------+----+----+
missing.show(false)

// Throws this error: org.apache.spark.sql.AnalysisException: cannot resolve '(`data` = test_table.`data`)'
// due to data type mismatch: differing types in '(`data` = test_table.`data`)'
// (struct<name:string,age:int> and struct<name:string,age:int>).;
val missing_2 = {
  u.join(d,
         u("id") === d("id") && u("data") === d("data"),
         "outer")
    .where(u("id").isNull || d("id").isNull)
}

Don’t be fooled by (struct<name:string,age:int> and struct<name:string,age:int>). The problem is caused by nullable, which is not shown in the error message. The DataFrame created from case classes has nullable=false for id and age because Scala Int cannot be null, while the SQL creates nullable fields. And if you compare a field with complex type (struct, array), Spark just thinks they are different as shown in missing_2. But if you compare field by field, there is no problem as shown in missing.

scala> u.schema.treeString
res4: String =
"root
 |-- id: integer (nullable = false)
 |-- data: struct (nullable = true)
 |    |-- name: string (nullable = true)
 |    |-- age: integer (nullable = false)
"

scala> d.schema.treeString
res5: String =
"root
 |-- id: integer (nullable = true)
 |-- data: struct (nullable = true)
 |    |-- name: string (nullable = true)
 |    |-- age: integer (nullable = true)
"

​

Mac OS SSH Kerberose Single-Sign-On

To use Kerberos Single-Sign-On on a MacBookPro, you can do as follows:

• Make a copy of /etc/krb5.conf from a server in your company network.
• Save it into /etc/krb5.confon your MacBookPro. You probably need sudo.
• Run kinit <your-login>@<realm_name>. If you don’t know what is the realm name is, take a look at /etc/krb5.conf

  [libdefaults]
  default_realm = MY.COMPANY.COM
  

• Add this line to ~/.ssh/config. Without this line, password method is preferred.

  GSSAPIAuthentication yes
  

• Then you can ssh to any host supporting kerberos without typing a password.
• If your kerberos ticket expires, just run kinit again. You don’t have to provide the principal <your-login>@<realm_name> at this time.
• Use klist to show when the ticket will expire.

​

Gnome Terminal Profile Export

I usually run screen on a remote server in PROD environment. Whenever I want to access the server, I SSH to that host, and run screen -D -R to resume the session I leave before. Using screen, you don’t have to worry about your SSH session suddenly goes down due to the Internet connection issue.

I also adjust the terminal running the SSH connection with a title and special color scheme so that I can quickly tell this is a PROD environment and need to be careful.

I create a Gnome terminal profile so that I only need to click “File” -> “Open Terminal” -> “prod-env”. “prod-env” is the profile name with all customized attributes:

• Title
• Terminal window size
• Automatically run a command ssh -t <host> screen -D -R
• Specified color-scheme

How can I share the profile among the multiple environments, e.g., a laptop and a desktop running Gnome environment? Of course, I can set it manually on both of machines. It would be better if I can export the profile and import it on other machines.

It is actually pretty simple. The following command works on CentOS 7.3 and Gnome 3.

• Export

  dconf dump /org/gnome/terminal/ > /tmp/terminal-profile.dconf
  

• Import

  cat /tmp/terminal-profile.dconf | dconf load
  

​

Screen in docker with error "Must be connected to a terminal"

I have a running docker container with command /bin/bash --login. When I run the command

$ docker exec -it devsh /bin/bash --login

I can access the container’s Bash, but when I run screen, I got this error:

$ screen
Must be connected to a terminal.

It turns out that I should use docker attach devsh. This command allows me to access the original shell in the container.

​