标签：#network

The botnet was reportedly tied to a Russia-based residential proxy network.

That "authenticity of host can't be established" message isn't just noise. Here's what's actually happening — and why blindly typing "yes" is a security mistake. Every developer has seen this: The authenticity of host 'example.com (203.0.113.1)' can't be established. ED25519 key fingerprint is SHA256:abc123xyz... Are you sure you want to continue connecting (yes/no/[fingerprint])? Almost everyone types yes without reading it. Then they move on. This message is SSH trying to protect you from one of the most dangerous attacks in network security: the man-in-the-middle attack. Understanding what's happening here — and what the ~/.ssh/known_hosts file actually does — will change how you think about every SSH connection you make. The Problem SSH Is Solving When you connect to ssh user@example.com , how do you know you're actually talking to example.com ? You can't rely on the IP address — IP addresses can be spoofed or rerouted. You can't rely on DNS — DNS can be poisoned. You can't rely on the network path — traffic can be intercepted at any point between you and the server. Without verification, an attacker positioned between you and the server could intercept the connection, pose as the server, decrypt everything you send, re-encrypt it, and forward it along. You'd type your password or authenticate with your key and never know the attacker saw every keystroke. This is a man-in-the-middle (MITM) attack . It's not theoretical. It happens on compromised networks, corporate proxies, malicious Wi-Fi hotspots, and misconfigured infrastructure. SSH's defense is host key verification . Every SSH server has a unique cryptographic identity — its host key. Before you exchange any sensitive data, the server proves it holds the private key corresponding to a public key you've previously verified. If the keys don't match, SSH warns you — loudly. What a Host Key Actually Is When OpenSSH is installed on a server, it automatically generates a set of host key pairs. These live in /etc

⚠️ For defensive/educational purposes only. Sniff only networks you own or are explicitly authorized to test. Unauthorized network monitoring is illegal in most jurisdictions. The uncomfortable truth about your factory floor If your plant uses Modbus TCP — and statistically, it probably does — every register read, every coil write, every sensor value is flying across your network in plaintext . No encryption. No authentication. No signature. Nothing. Modbus was designed in 1979 by Modicon for serial communication between a PLC and a few field devices on a dedicated cable. The threat model was "someone might physically tap the wire." The solution was "don't let strangers into the control room." Forty-five years later, that same protocol is running over your corporate VLAN, talking to cloud historians, and occasionally — if your IT/OT segmentation has gaps — reachable from the internet. Let me show you what that looks like from the wire. The 5-line sniffer This is a defensive monitoring tool. Same code your blue team would use to baseline normal traffic and detect anomalies. Requires scapy : pip install scapy from scapy.all import sniff , TCP , Raw def show_modbus ( pkt ): if TCP in pkt and pkt [ TCP ]. dport == 502 and Raw in pkt : payload = pkt [ Raw ]. load print ( f " { pkt [ ' IP ' ]. src } → { pkt [ ' IP ' ]. dst } : { payload . hex () } " ) sniff ( filter = " tcp port 502 " , prn = show_modbus , store = False ) Run it on a span port, a TAP, or a mirror VLAN, and within seconds you'll see something like this: 192.168.1.50 → 192.168.1.10: 0001000000060103006400 02 192.168.1.10 → 192.168.1.50: 00010000000701030441f00000 192.168.1.50 → 192.168.1.10: 00020000000601100065000102 Every byte tells a story. Let's decode the first packet. Decoding what you just captured The Modbus TCP frame format is documented in the spec (it's public — that's part of the problem): Bytes 0-1: Transaction ID Bytes 2-3: Protocol ID (always 0x0000 for Modbus) Bytes 4-5: Length Byte 6: Unit