Finding IP Addresses of Other Network Interfaces on Linux

In environments where an attacker gains access to a local LAN, such as in a DMZ or multi-tiered architecture, the primary goal shifts from exploiting immediate exposed services to mapping out the full scope of reachable systems. Modern network design often means hosts possess multiple interfaces—management, backup, and operational—meaning visible services on one interface may not reflect the capabilities of others. The article outlines methods for discovering these hidden IP addresses. If traditional discovery methods like SNMP or specialized queries fail, brute-forcing through ARP queries is presented as an alternative tactic. A critical finding detailed is that Linux hosts can respond to ARP requests containing multiple IP addresses across all their interfaces, a behavior distinct from Solaris, Windows, and AIX.
The practical application of this technique involves using tools like arp-scan against defined network ranges (e.g., 10.0.0.0/8). Scan speeds are noted as potentially fast, allowing coverage of large subnets within several hours under specific conditions. Furthermore, the text addresses limitations regarding broad broadcast requests by offering a method to target hosts via unicast ARP requests using specific destination MAC addresses if authorization restricts scanning an entire LAN segment. A technical note is provided on how host kernel settings can affect this discovery process.

Local access scenarios for pentesting involve multi-tiered architectures and DMZs.
Pentest objectives may include determining the number of multihomed systems.
The article describes methods to find IP addresses of remote network interfaces.
SNMP (generic) or a "special NetBOIS query" can list system IP addresses if available.
ARP queries can be used for brute-forcing unknown IP addresses.
Linux hosts respond to ARP requests for all their IP addresses across all interfaces.
Tools like arp-scan allow scanning of Class B networks; one example scan covered 172.16.1.0/16.
Broadcasting ARP requests over specific ranges (e.g., 192.168.0.0/16) can cover local subnets.
Unicasting requests is possible by specifying a destination MAC address in arp-scan.
Modifying the kernel setting `arpignore` from 0 to 1 via `/proc/sys/net/ipv4/conf/all/arpignore` remedies system behavior.

The methodology described—leveraging Layer 2 discovery mechanisms like ARP to map out latent network presence—reveals a critical tension between security through obscurity and the imperative of comprehensive risk assessment. The initial assumption driving this reconnaissance is that external or segment-specific scanning provides a complete picture, an assumption frequently invalidated by the reality of multi-homed enterprise systems. This scenario forces analysts away from merely auditing the visible ingress points toward uncovering the entire system topology, effectively expanding the attack surface map under plausible deniability (if not explicitly authorized).
The technical detail regarding Linux ARP responses is a prime example of how subtle implementation differences between operating systems dictate reconnaissance paths; the counter-intuitive nature of responding to cross-LAN ARP probes forces practitioners to question foundational assumptions about network segmentation efficacy. While SNMP and NetBOIS represent high-level, formalized service discovery, brute-forcing via ARP operates at a much lower, more intrusive level of the OSI model. The mention of kernel overrides suggests that system hardening choices (or lack thereof) directly influence discoverability, turning configuration drift into an exploitable intelligence vector.
If this information were deployed in a coordinated influence campaign—for instance, by a sophisticated threat actor seeking to maximize persistence against an organization resistant to conventional intrusion detection—the playbook would involve low-and-slow network mapping to avoid volume-based alerts, using ARP scanning across multiple MAC addresses or interfaces as a "living off the land" technique that blends into background network noise. The article itself does not exhibit overt persuasive manipulation, but rather presents a technically sound toolkit for expanding an initial foothold; however, one must question whether this technical capacity is framed purely as defensive knowledge or as an enablement script for lateral movement exploitation.
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