159 votes
Accepted

What causes the 'Connection Refused' message?

Note: This message is a symptom of the problem you are trying to solve. Understanding the cause of the message will ultimately lead you to solving your problem. The message 'Connection Refused' has ...
  • 115k
111 votes

How to scan local network for SSH-able computers?

From the command line you could use: sudo nmap -sS -p 22 192.168.10.0/24 Substitute for the local address space on your network. I sometimes use this when I plug in a headless rasberry pi and want ...
  • 1,211
110 votes

A previous IT worker probably left some backdoors. How can I eliminate them?

The only way to be absolutely certain is to wipe every system clean and to reinstall from scratch. You will also need to audit all of the locally generated software and configurations to ensure that ...
  • 115k
98 votes

Testing UDP port connectivity

To test if udp port is responding, use netcat. An example from the man page: nc -v -u -z -w 3 example.host 20-30 Send UDP packets to ports 20-30 of example.host, and report which ones did ...
  • 1,661
91 votes

How do I delete a route from Linux routing table

with the route -n command you'll obtain Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 0.0.0.0 192.168.178.1 0.0.0.0 UG 0 ...
76 votes
Accepted

What is the ipv6 equivalent of 0.0.0.0/0

The IPv6 equivalent of IPv4's 0.0.0.0 is ::/0.
  • 1,086
74 votes

Testing UDP port connectivity

both on client ans server install nc: yum install nc (for centos) on server listen UDP port: nc -ul 6111 (add the -6 option if you're testing an ipv6 connection) on client nc -u <server> 6111 ...
  • 841
73 votes
Accepted

How to manage DNS in NetworkManager via console (nmcli)?

Here is the command to modify an existing connection. nmcli con mod $connectionName ipv4.dns "8.8.8.8 8.8.4.4" connectionName can be found by command: nmcli con. In the question case, it ...
68 votes
Accepted

Does Wi-Fi traffic from one client to another travel via the access point?

Yes, the communication is traveling through the access point. In this case the AP is functioning exactly like a switch does in a wired network. It is possible to have two devices communicate directly,...
  • 1,471
66 votes

Domain Controller thinks its on a Public Network

Whether the network of a domain controller is classified as domain network doesn't depend on the gateway configuration. The behaviour of a false network classification can be caused the NLA (network ...
  • 2,085
64 votes

Some ISPs don't charge for bandwidth used on Youtube/Netflix/Facebook. How do they monitor that?

HTTPS obscures the content of the traffic, but not the endpoints. So, for instance, my ISP does not know that I'm responding to this particular question, because I'm using HTTPS, but they do still ...
62 votes
Accepted

unable to connect to public postgresql rds instance

The issue was that the inbound rule in the Security Group specified a security group as the source. Changing it to a CIDR that included my IP address fixed the issue. Open the database security ...
59 votes
Accepted

Users with Google's IP address. How is it possible?

What you're seeing is the Google proxy address. Mobile users with a chrome browser (either Android or iOS) that have the bandwidth management features turned on will often be seen as using one of ...
  • 5,577
57 votes

How do I make a connection private on Windows Server 2012 R2

A GUI way of making this change: Hit Winkey + R to open Run prompt and type gpedit.msc Navigate to: Computer Configuration/Windows Settings/Security Setting /Network List Manager Policies. Choose ...
  • 721
56 votes
Accepted

Why should I use an FQDN instead of the server's IP address?

Using an IP address ensures that you are not relying on a DNS server. It also has the benefit of preventing attacks through DNS spoofing. Using a FQDN instead of an IP address means that, if you were ...
55 votes
Accepted

Why does removing an unused IP address from an interface kill connections unrelated to that address

In linux, IP addresses have a notion of 'primary' and 'secondary' addresses. The primary is typically the first address you add to the system. Removing the primary address has the implicit operation ...
  • 23.1k
52 votes

How to scan local network for SSH-able computers?

nmap -p 22 --open -sV 192.168.178.0/24
  • 648
52 votes

Rogue DHCP Server Can't be found

On one of the affected Windows clients start a packet capture (Wireshark, Microsoft Network Monitor, Microsoft Message Analyzer, etc.), then from an elevated command prompt run ipconfig /release. The ...
  • 109k
47 votes

How to set the preferred network interface in Linux

Here's the complete answer, in case it helps others: To make packets with destinations 192.168.10.* use eth0, and all other packets use eth1: 1) View your current routing table ip route list One ...
  • 641
47 votes
Accepted

What does "Connection: close" mean when used in the response message?

Yes, this is correct. The server can just say "I don't support your keep-alive request and will just close the connection when I am finished". From RFC 2616, Section 14.10: HTTP/1.1 defines ...
  • 97.9k
46 votes
Accepted

Some ISPs don't charge for bandwidth used on Youtube/Netflix/Facebook. How do they monitor that?

Firstly they know the YouTube IP address. ISP's have an IP database. For example YouTube's ASN is AS15169. On the server side they would make a grouping for each service. One of them is the default ...
45 votes
Accepted

What happens if two local systems download the same resource on same port?

A TCP connection (which underlies HTTP and many other protocols) is uniquely (at a given point in time) defined by 4 parameters: The local IP The local port The remote IP The remote port Even if you ...
  • 1,030
42 votes

Why should I use an FQDN instead of the server's IP address?

DNS is not just FQDN = IP The important thing about DNS is that it provides more than just A records (hostname = IP). DNS provides different types of records such as MX, CNAME, TXT, etc... that may ...
42 votes
Accepted

What makes a private IP address not routable?

Private IP addresses are routable, albeit they are not publicly routed. Basically, a router will route a private address to private/internal LAN, rather than to the internet. To expand my answer: a ...
  • 45.1k
41 votes

A previous IT worker probably left some backdoors. How can I eliminate them?

First, the most important goal of a fired sysadmin is to clean up his past, particularly if it was a departure in bad standing. If an attack on his previous system were to happen he wouldn't stand to ...
  • 4,924
40 votes
Accepted

Network printer exploited (read: hacked) to print antisemitic documents. How to fix?

This attack disproportionately affected universities because, for historical reasons, many universities use public IPv4 addresses for most or all of their network, and for academic reasons have little ...
40 votes
Accepted

How do I use Linux to find unused IP addresses on my network?

Any well-behaved device on an Ethernet LAN is free to ignore nearly any traffic, so PINGs, port scans, and the like are all unreliable. Devices are not, however, free to ignore ARP requests, afaik. ...
  • 79k
38 votes

How do I make a connection private on Windows Server 2012 R2

Using Powershell. Here is an example of changing the network profile of a network interface called Ethernet to "Private." This is a one line command. Set-NetConnectionProfile -InterfaceAlias Ethernet ...
  • 481
38 votes
Accepted

What's the Wireshark packet receiving and processing procedure on a Windows machine?

The I/O model in Windows is based on a stack of components. Data must flow through the various components of that stack that exists between the physical network card, and the application that will ...
  • 55.2k
38 votes

Rogue DHCP Server Can't be found

Found it!! It was my DCS-5030L D-Link Network Camera! I have NO idea why this happened. This is how I found it. I changed my laptop IP Address to 10.255.255.150/255.255.255.0/10.255.255.1 and the DNS ...

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