The key detail is that the server logs show no record of the attempts, indicating the SYN packets are not reaching the server’s application or TCP stack. A host-based firewall (like iptables or Windows Firewall) operating at the kernel level would drop these packets before they are processed by the TCP listener, explaining both the lack of SYN-ACKs in the capture (if the capture is on the server interface after the firewall drop) and the absence of log entries.

DNS tunneling often encodes data into subdomains, resulting in DNS queries with fully qualified domain names (FQDNs) that exceed the typical length. The `dsize` option checks the UDP packet size. A threshold like >100 bytes is a practical, common signature for detecting abnormally large DNS request packets that may contain tunneled data, making it the most effective choice from the options provided.

While other options are suspicious, they are not definitive. A legitimate outbound SSH connection would use the server’s own private key to authenticate the session. If the packet capture shows the server presenting a host key that does not match its verified, legitimate key, it is conclusive evidence that the SSH traffic is not originating from the legitimate SSH service but from a rogue process or attacker’s tool, confirming a compromise.

The pattern describes a horizontal port scan. The key indicators are the high number of distinct external IP addresses (horizontal scan) on a specific, common port (443), with minimal data transfer per connection (just enough to determine if the port is open/closed). DDoS bots typically send high volumes to fewer targets. C2 beaconing is usually to one or a few IPs at regular intervals. Browsing a site would involve more data transfer and connections to a smaller set of domains.

A common feature of rootkits is to hide processes and their network connections from user-space tools like `ps` and `netstat`. However, when `netstat -tunap` is run (which shows PID/Program name), a rootkit that only hides from `ps` may fail to hide the socket association, resulting in connections listed with a PID but a blank or missing program name. This discrepancy is a classic red flag for a rootkit’s presence.

This Suricata/Snort rule detects the use of the common command-line tool ‘curl’, which is a dual-use tool. It is frequently used legitimately by developers and administrators for API calls, debugging, and data transfer. It is also abused by attackers for downloading payloads. The most efficient and appropriate triage step is a simple inquiry to establish context before taking any disruptive action, as the activity is suspicious but not conclusively malicious on its own.

The payload uses directory traversal sequences (`../../../`) to attempt to escape the web root directory and access a known sensitive system file (`/etc/passwd`). This is the hallmark of a Local File Inclusion (LFI) attack. The immediate goal is to read local files on the server’s filesystem. While a successful LFI can sometimes lead to RCE (e.g., by including log files), the displayed payload’s direct goal is file disclosure.

A near-zero latency between a SYN and its SYN-ACK is physically impossible for traffic crossing any network distance. This artifact typically occurs when the sensor is connected to a SPAN/mirror port on a switch. The switch sends both the original SYN (sent to the server) and a copy of the server’s response (the SYN-ACK) out the mirror port to the sensor in rapid succession, making them appear simultaneous in the capture file.

While ping scans are common, alerts for specific, large ICMP packet sizes are often tuned to detect data tunneling. Attackers can use the optional data field in ICMP Echo Request packets to encapsulate other types of traffic (like shell commands or exfiltrated data), creating a covert channel that may bypass firewalls if ICMP is permitted. The alert is for the unusual *size*, not the mere presence of ICMP Type 8.

The key detail is that the server logs show no record of the attempts, indicating the SYN packets are not reaching the server’s application or TCP stack. A host-based firewall (like iptables or Windows Firewall) operating at the kernel level would drop these packets before they are processed by the TCP listener, explaining both the lack of SYN-ACKs in the capture (if the capture is on the server interface after the firewall drop) and the absence of log entries.0