Paulo Coghi's questions

On Ubuntu Server 20.04, Redis (5.0.7) doesn't start after setting supervised systemd on /etc/redis/redis.conf, but no error is printed when running it manually with /usr/bin/redis-server /etc/redis/redis.conf.

Also, when running manually, its possible to find it listening on port 6379 with netstat -tulpn.

To reproduce

As root:

• Update packages and install Redis with apt update && apt install redis-server
• On /etc/redis/redis.conf, change the line with supervised no to supervised systemd
• Try to restart Redis with service redis-server restart or systemctl restart redis

The output of the above command is:

Job for redis-server.service failed because the control process exited with error code.
See "systemctl status redis-server.service" and "journalctl -xe" for details.

And the output of systemctl status redis-server.service is:

● redis-server.service - Advanced key-value store
     Loaded: loaded (/lib/systemd/system/redis-server.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Wed 2021-02-03 12:15:56 -03; 28s ago
       Docs: http://redis.io/documentation,
             man:redis-server(1)
    Process: 3851 ExecStart=/usr/bin/redis-server /etc/redis/redis.conf (code=exited, status=1/FAILURE)

Feb 03 12:15:56 mywebsite.com systemd[1]: redis-server.service: Scheduled restart job, restart counter is at 5.
Feb 03 12:15:56 mywebsite.com systemd[1]: Stopped Advanced key-value store.
Feb 03 12:15:56 mywebsite.com systemd[1]: redis-server.service: Start request repeated too quickly.
Feb 03 12:15:56 mywebsite.com systemd[1]: redis-server.service: Failed with result 'exit-code'.
Feb 03 12:15:56 mywebsite.com systemd[1]: Failed to start Advanced key-value store.

No log is printed on /var/log/redis/redis-server.log when trying to restart Redis service, but the text below is printed when running it manually:

4231:C 03 Feb 2021 12:27:52.053 # oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo
4231:C 03 Feb 2021 12:27:52.053 # Redis version=5.0.7, bits=64, commit=00000000, modified=0, pid=4231, just started
4231:C 03 Feb 2021 12:27:52.053 # Configuration loaded
4232:M 03 Feb 2021 12:27:52.063 * Increased maximum number of open files to 10032 (it was originally set to 1024).
                _._
           _.-``__ ''-._
      _.-``    `.  `_.  ''-._           Redis 5.0.7 (00000000/0) 64 bit
  .-`` .-```.  ```\/    _.,_ ''-._
 (    '      ,       .-`  | `,    )     Running in standalone mode
 |`-._`-...-` __...-.``-._|'` _.-'|     Port: 6379
 |    `-._   `._    /     _.-'    |     PID: 4232
  `-._    `-._  `-./  _.-'    _.-'
 |`-._`-._    `-.__.-'    _.-'_.-'|
 |    `-._`-._        _.-'_.-'    |           http://redis.io
  `-._    `-._`-.__.-'_.-'    _.-'
 |`-._`-._    `-.__.-'    _.-'_.-'|
 |    `-._`-._        _.-'_.-'    |
  `-._    `-._`-.__.-'_.-'    _.-'
      `-._    `-.__.-'    _.-'
          `-._        _.-'
              `-.__.-'

4232:M 03 Feb 2021 12:27:52.065 # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128.
4232:M 03 Feb 2021 12:27:52.065 # Server initialized
4232:M 03 Feb 2021 12:27:52.066 * Ready to accept connections

Software versions:

Redis server v=5.0.7 sha=00000000:0 malloc=jemalloc-5.2.1 bits=64 build=636cde3b5c7a3923

Ubuntu 20.04.2 LTS - 64 bit

systemd 245 (245.4-4ubuntu3.4)

When the number of blocked processes grow indefinitely, how to identify exactly what are these processes.

Since I'm inside a system container (LXC/OpenVZ), I can't modify the hosts' kernel modules to use tools like perf-tools or bcc-tools/bpfcc-tools

The question How to track newly created processes in Linux? doesn't cover the scenario of system containers, and most of the answers rely on tools like perf-tools and bcc-tools.

I am using Proxmox 3.4 with OpenVZ and venet. In various servers, everything is working fine.

But in a new dedicated server, I cant ping my only container.

The public IP of the HN is 158.69.241.xxx and for the container 149.56.179.xxx.

Routing table

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
149.56.179.xxx  0.0.0.0         255.255.255.255 UH    0      0        0 venet0
158.69.241.0    0.0.0.0         255.255.255.0   U     0      0        0 vmbr0
0.0.0.0         158.69.241.254  0.0.0.0         UG    0      0        0 vmbr0

/etc/network/interfaces

auto lo
iface lo inet loopback

auto vmbr0
iface vmbr0 inet static
        address 158.69.241.xxx
        netmask 255.255.255.0
        broadcast 158.69.241.255
        gateway 158.69.241.254
        bridge_ports eth0
        bridge_stp off
        bridge_fd 0

Netfilter configuration

$ iptables -t nat -L && iptables -t filter -L && iptables -t mangle -L

Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination        

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination        

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination        
Chain INPUT (policy ACCEPT)
target     prot opt source               destination        

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination        

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination        
Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination        

Chain INPUT (policy ACCEPT)
target     prot opt source               destination        

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination        

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination        

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination

In Monit, how can I check the health of my Nginx server (if it is returning 502, for example) and how to restart it when this occurs.

The environment is an Ubuntu 14.04 64bit

I am developing a free DNS service and I can't totally solve the situation when two or more customers try to add the same domain (nearly at the same time).

Here are a few solutions to handle this situation, but none of them seems to be good or viable.

Solution 1:

Use different pairs of name server for each customer

Creating multiple and distinct ns's (like ns1, ns2.. ns49, ns50, ns51..) allows me to use different pairs of servers with different customers trying to add same domain.

The real owner will use only his pair in domain registrar (ie: ns8 and ns9) so only his records will be accepted and propagated to the whole internet.

The problem with Solution 1:

It is vulnerable to a mass attack

If a malicious person create a really big number of accounts, it would be impossible to have an equivalent number of pairs, if all of these accounts try to add the same domain.

Solution 2:

Only allow one user account to use a domain

If one user already added a specific domain and correctly configured it in his domain registrar, no other account will be able to add the same domain.

The problem with Solution 2:

Grace time

It can take hours to validate an added domain (please, correct me If I am wrong). It would require me to give a "grace time" to allow each recently added domain to stay "unverified" until we can validate it through the respective domain registrar.

Also, during this grace time, no other account would be able to add the same domain unless we use solution 1 ( but remember #1 has a vulnerability ).

How free (or paid) DSN services solve this problem? What are their approaches (since any user apparently can add whatever domain it wants without restrictions) ?

Edit: about the duplicate

There is a slight difference between my question and Era's one. I am the provider and not the customer. And Era's question only points to the problem from the customer's eyes.

Also, even @Jacob from DigitalOcean stated that they do a "first come, first serve" approach which I am trying to avoid and is the reason for my question to exist.

Is it possible to enable PHP APC in Virtualmin? If yes, how?

(almost no information in foruns, official documentation, web, google..)

Important: (this is the reason for this question)

Virtualmin, by default, only works and integrates well with 3 PHP modes:

1. Apache mod_php

2. CGI wrapper

3. FCGId

Is APC compatible with any of these modes?

What is the max distance for cat5e cables in 1000Mbps (1 Gigabit) Full Duplex mode?

I intend to configure a server as a gateway.

I'll use it to manage network users, as well as share the internet connection for this network.

I will not share the Internet with squid, but I still wanted to be able to establish rules of access, such as sites that can not be accessed.

How can I do this without using squid?

Important note:

I consider squid an excellent software. My problem is not with the squid in particular, but with any kind of proxy, because I have clients that use a series of programs that simply can not access the Internet via proxy.

Rather to score my question negatively, you could put your points of view, and you might understand that this is a case study.

I'm searching for open source software to create my own cloud computing environment, like Ubuntu UEC or Eucalyptus.

What I need is that each virtual machine can use resources from multiple nodes (CPU, RAM). And apparently, neither UEC nor Eucalyptus offer this feature.

An example:

I have 5 hardware nodes with each 2x Quadcore. I want that one VM can use up to 20 cores (in this case).

My question

Which open source software can meet this requirement?

#Edit#

At first, I think this is possible with current technology, with computers connected using gigabit network.

Companies like the VPS.net offer services that make a single virtual machine to be formed from the resources of multiple nodes. (At least it seems to be so)

Is this just an abstraction made ​​by VPS.net? Is this approach recommended?

I need to transfer a few gigs of files from server A to B. I own SSH access to server A, but I have only FTP access to B.

Since I can not send one .tar file from A to B, for the reason that I can not untar it later, I need to perform an FTP transfer.

How can I transfer (via FTP) all files including the directories and all sub directories?

I tried using mput * and mput *.*, but they did not work.

About solutions like LFTP or others

The lftp is a great solution, but I can not install it on the server, because it is not a dedicated server or a VPS. It is a common shared webhosting with SSH access.