How to Trace an Access Path Across Multiple Firewalls
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<p>The post <a href="https://www.firemon.com/blog/trace-network-traffic-multiple-firewalls/">How to Trace an Access Path Across Multiple Firewalls</a> appeared first on <a href="https://www.firemon.com">www.firemon.com</a>.</p><p>When a connection fails or succeeds unexpectedly, the first question is simple: <em>Why?</em></p><p>But answering that question is not simple in modern environments.</p><p>A single connection between two systems may traverse:</p><ul> <li>Multiple firewalls</li> <li>Different rule sets and priorities</li> <li>Network zones and routing paths</li> </ul><p>Reviewing one device at a time does not provide a complete answer.</p><p>This post explains how to trace an access path across multiple firewalls so you can determine exactly why traffic is allowed or blocked.</p><h2><strong>The Core Problem: Access Is Determined Across the Environment</strong></h2><p>Native firewall management tools are typically scoped to a single device. They show:</p><ul> <li>Rules</li> <li>Logs</li> <li>Configuration state</li> </ul><p>They do not show how traffic is evaluated across multiple enforcement points.</p><p>A connection is only successful if it is allowed at every step along the path.</p><h2><strong>What Is an Access Path?</strong></h2><p>An access path is the sequence of evaluations that determine whether traffic can move from a source to a destination. It includes:</p><ul> <li>Source and destination systems</li> <li>Ports and protocols</li> <li>Every firewall, control point, and layer 3 device along the route</li> <li>The rules that allow or deny traffic at each step</li> </ul><p>Tracing an access path means identifying each of these elements and how they interact.</p><h2><strong>Inputs and Outputs</strong></h2><p><strong>Inputs</strong></p><ul> <li>Source system</li> <li>Destination system</li> <li>Port and protocol</li> <li>Firewall configurations across the environment</li> <li>Network topology and routing paths</li> <li>Object groups and address mappings</li> </ul><p><strong>Outputs</strong></p><ul> <li>Whether the connection is allowed or blocked</li> <li>The sequence of enforcement points involved</li> <li>The rules that permit or deny traffic</li> <li>Alternate paths that may allow connectivity</li> </ul><h2><strong>Step 1: Define the Connection</strong></h2><p><strong>Start with a specific question:</strong></p><p>Can App_Server communicate with DB_Server on port 1433?</p><p>Without a clearly defined connection, tracing becomes ambiguous.</p><h2><strong>Step 2: Identify the Expected Path</strong></h2><p>Determine how traffic should flow through the network. This includes:</p><ul> <li>Source zone or network</li> <li>Intermediate segments</li> <li>Destination zone or network</li> </ul><p>In many environments, there are multiple possible paths.</p><p>Understanding topology is required before evaluating rules.</p><h2><strong>Step 3: Evaluate Each Enforcement Point</strong></h2><p><strong>At each firewall or control point:</strong></p><p>1. Identify relevant rules</p><p>2. Evaluate rule order and precedence</p><p>3. Determine whether traffic is allowed or denied</p><h4><strong>Example</strong></h4><p><strong>Firewall A:</strong></p><p>Allow App → DB (1433)</p><p><strong>Firewall B:</strong></p><p>Deny App → DB (1433)</p><p>The connection is blocked because all enforcement points must allow the traffic.</p><h2><strong>Step 4: Expand Object Groups</strong></h2><p>Rules often reference object groups rather than individual systems.</p><p>These groups may include multiple addresses.</p><h4>Example</h4><p>Allow App → DB_Group (1433)</p><p><strong>DB_Group may resolve to:</strong></p><p>DB_Server</p><p>Backup_DB</p><p>Reporting_DB</p><p>Tracing requires evaluating all expanded members.</p><h2><strong>Step 5: Evaluate Rule Interactions</strong></h2><p>Rules do not operate independently.</p><p><strong>Key factors include:</strong></p><ul> <li>Rule order</li> <li>Overlapping conditions</li> <li>Broad rules that override specific ones</li> </ul><h4><strong>Example</strong></h4><p>Rule 1: Allow App → Any (Any Port)</p><p>Rule 2: Deny App → DB (1433)</p><p>The deny rule exists but is never reached.</p><h2><strong>Step 6: Account for Alternate Paths</strong></h2><p>Even if one path blocks traffic, another path may allow it.</p><h4>Example</h4><p><strong>Path 1:</strong></p><p>Firewall A denies App → DB</p><p><strong>Path 2:</strong></p><p>Firewall B allows App → DB</p><p>If routing allows Path 2, the connection succeeds.</p><p>Tracing must include all possible paths.</p><h2><strong>Step 7: Determine Final Outcome</strong></h2><p><strong>After evaluating:</strong></p><ul> <li>All enforcement points</li> <li>Rule interactions</li> <li>Object expansions</li> <li>Possible paths</li> </ul><p><strong>You can determine:</strong></p><ul> <li>Whether the connection is allowed or blocked</li> <li>Which rule is responsible</li> <li>Where control should be adjusted</li> </ul><h2><strong>Example: Why a Connection Is Allowed</strong></h2><p><strong>Question</strong></p><p>Can App_Server reach DB_Server on port 1433?</p><p><strong>Configuration Findings</strong></p><p>Firewall A: Allow App → DB_Group (1433)</p><p>Firewall B: No explicit deny</p><p>Rule order: Broad allow takes precedence</p><p><strong>Effective Outcome</strong></p><p>App → DB (1433)</p><p>App → Backup_DB (1433)</p><p>The connection is allowed due to object group expansion and rule precedence.</p><h2><strong>Why Manual Tracing Breaks Down</strong></h2><p><strong>Manual tracing becomes difficult when:</strong></p><ul> <li>Multiple devices are involved</li> <li>Rule sets are large and complex</li> <li>Object groups expand significantly</li> </ul><p><strong>This leads to:</strong></p><ul> <li>Incomplete analysis</li> <li>Incorrect assumptions</li> <li>Slow troubleshooting</li> </ul><h2><strong>Evaluating Access Paths Using a Policy Model</strong></h2><p>A <a href="https://www.firemon.com/products/policy-manager/">policy model</a> provides a structured way to evaluate access.</p><p><strong>It combines:</strong></p><ul> <li>Firewall configurations</li> <li>Network topology</li> <li>Object resolution</li> <li>Rule evaluation logic</li> </ul><p><strong>This allows teams to:</strong></p><ul> <li>Evaluate connectivity across the environment</li> <li>Identify all applicable rules</li> <li>Understand why access is allowed or blocked</li> </ul><h2><strong>The Role of FireMon</strong></h2><p>FireMon enables access path evaluation by:</p><ul> <li>Building a normalized policy model across firewalls</li> <li>Incorporating topology and routing</li> <li>Evaluating connectivity between systems</li> <li>Identifying the rules that control access</li> </ul><p>This provides a consistent way to answer: Why is this connection allowed or blocked?</p><div class="cta-banner" style="--background: url('https://www.firemon.com/wp-content/uploads/2024/10/cta-bg.webp');--mobile-background: url('https://www.firemon.com/wp-content/uploads/2024/10/cta-bg-small.webp');"> <p class="cta-banner--title h3">Schedule a FireMon Demo</p> <p> <a href="https://www.firemon.com/request-a-demo/" class="btn btn--primary btn--s">Book Now</a></p></div><h2><strong>Key Takeaways</strong></h2><ul> <li>Access is determined across multiple enforcement points</li> <li>A connection must be allowed at every step</li> <li>Rule order and object expansion affect outcomes</li> <li>Alternate paths can enable unexpected connectivity</li> <li>Evaluating access requires both policy and topology</li> </ul><h2><strong>Wrapping it Up</strong></h2><p>When troubleshooting access, the goal is not to find a rule.</p><p>The goal is to leverage both control point and network topology to provide an end-to-end view of the connection, allowing for a clear explanation as to why traffic is or is not allowed.</p><div class="spu-placeholder" style="display:none"></div><div class="addtoany_share_save_container addtoany_content addtoany_content_bottom"><div class="a2a_kit a2a_kit_size_20 addtoany_list" 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