Network Protocols Reference

A comprehensive reference for network protocols commonly encountered in security testing, network analysis, and system administration.

When to Use This Skill

Use this skill when you need to:

• Understand how a specific network protocol works
• Identify which protocols are running on a network during reconnaissance
• Analyze service discovery mechanisms (mDNS, DNS-SD, SSDP, WSD)
• Understand authentication and directory protocols (OAuth 2.0, RADIUS, LDAP, AD)
• Explain protocol behavior for security assessments
• Troubleshoot network connectivity issues related to specific protocols

Service Discovery Protocols

Multicast DNS (mDNS)

Purpose: IP address resolution within small, local networks without a dedicated name server.

How it works:

• Multicasts a query within the subnet
• Host with the specified name responds with its IP address
• All devices in the subnet update their mDNS caches

Key characteristics:

• Domain suffix:

  .local

  only
• Port: UDP 5353
• IPv4 multicast:

  224.0.0.251

• IPv6 multicast:

  ff02::fb

• Ethernet MAC (IPv4):

  01:00:5E:00:00:FB

• Ethernet MAC (IPv6):

  33:33:00:00:00:FB

• Scope: Confined to local network, does not cross routers

Special behavior:

• Domain Name Relinquishment: Host releases domain name by sending packet with TTL of zero
• Conflict handling: Conflicts with non-mDNS hosts in

  .local

  domain require network configuration adjustments

Security testing considerations:

• mDNS traffic is visible to all devices on the subnet
• Can reveal hostnames and IP addresses of devices
• Useful for network reconnaissance in local environments

DNS-SD (DNS Service Discovery)

Purpose: Discovering services on a network by querying specific domain names.

How it works:

• Queries specific service type domains (e.g.,

  _printers._tcp.local

  )
• Response includes all related domains and available services

Common service types:

• _printers._tcp.local

  - Printers

• _http._tcp.local

  - Web services

• _ssh._tcp.local

  - SSH services

• _smb._tcp.local

  - SMB file sharing

Security testing considerations:

• Reveals available services on the network
• Can enumerate service types without direct probing
• Often used in conjunction with mDNS

SSDP (Simple Service Discovery Protocol)

Purpose: Discovery of network services, primarily used by UPnP.

How it works:

• Text-based protocol using UDP
• Uses multicast addressing
• Built on HTTPU (HTTP over UDP)

Key characteristics:

• Port: UDP 1900
• IPv4 multicast:

  239.255.255.250

• Protocol foundation: HTTPU (HTTP over UDP)

Security testing considerations:

• UPnP devices often have security vulnerabilities
• SSDP can reveal device types and capabilities
• Common target for network reconnaissance
• Can be used to discover IoT devices

Web Service for Devices (WSD)

Purpose: Identify available services on a network (e.g., printers).

How it works:

• Devices broadcast UDP packets
• Service seekers send requests
• Service providers announce their offerings

Security testing considerations:

• Can enumerate network services
• Often used in enterprise environments
• May reveal sensitive service information

Authentication and Authorization Protocols

OAuth 2.0

Purpose: Secure, selective sharing of user information between services.

How it works:

1. User authenticates with authorization server
2. User grants authorization to the client service
3. Authorization server generates access token
4. Client service uses token to access specified user data

Common use cases:

• "Sign in with Google" / "Sign in with Facebook"
• API access delegation
• Third-party service integration

Security testing considerations:

• Token handling and storage
• Scope validation
• Authorization code flow security
• Token expiration and refresh mechanisms

RADIUS (Remote Authentication Dial-In User Service)

Purpose: Network access protocol for authentication, authorization, and accounting.

How it works:

• User credentials verified by RADIUS server
• May include network address verification
• Post-authentication: network access granted
• Session details tracked for billing and statistics

Key characteristics:

• Primary users: ISPs, enterprise networks
• Functions: AAA (Authentication, Authorization, Accounting)
• Common ports: UDP 1812 (authentication), UDP 1813 (accounting)

Security testing considerations:

• Credential handling
• Shared secret security
• Accounting data integrity
• Replay attack prevention

File Sharing and Session Protocols

SMB (Server Message Block)

Purpose: Sharing files, printers, and ports.

How it works:

• Operates directly over TCP or via NetBIOS over TCP
• Dual compatibility enhances connectivity with various devices

Key characteristics:

• Direct TCP: Port 445
• NetBIOS over TCP: Ports 137, 138
• Common in: Windows environments, file servers

Security testing considerations:

• SMBv1 is deprecated and vulnerable
• Common target for lateral movement
• Check for SMB signing enforcement
• Enumerate shares and permissions

NetBIOS (Network Basic Input/Output System)

Purpose: Manages network sessions and connections for resource sharing.

How it works:

• Supports unique names for devices
• Supports group names for multiple devices
• Enables targeted or broadcast messaging
• Communication modes: connectionless (no acknowledgment) or connection-oriented (session-based)

Key characteristics:

• Traditional protocols: IPC/IPX
• Common usage: TCP/IP
• Associated protocol: NetBEUI (fast but verbose due to broadcasting)

Security testing considerations:

• NetBIOS name enumeration
• Can reveal system names and user information
• Often disabled in modern security configurations

Directory and Identity Protocols

LDAP (Lightweight Directory Access Protocol)

Purpose: Management and access of directory information over TCP/IP.

How it works:

• Supports various operations for querying and modifying directory information
• Interacts with databases designed for LDAP communication

Key characteristics:

• Primary use: Distributed directory information services
• Common ports: TCP 389 (unencrypted), TCP 636 (LDAPS)
• Operations: Search, modify, add, delete directory entries

Security testing considerations:

• Anonymous bind vulnerabilities
• LDAP injection attacks
• Information disclosure through enumeration
• Certificate validation for LDAPS

Active Directory (AD)

Purpose: Network-accessible database for centralized management of network entities.

How it works:

• Contains objects: users, groups, privileges, resources
• Hierarchical structure of domains
• Subdomains allow further segmentation
• Each subdomain may maintain its own server and user base

Key characteristics:

• Centralized management: User access to network resources
• Query capabilities: Retrieve contact details, locate resources
• Structure: Domains, subdomains, organizational units

Security testing considerations:

• Kerberos authentication security
• Group Policy Object (GPO) analysis
• Privilege escalation paths
• Lateral movement opportunities
• Password policy enforcement

Protocol Quick Reference Table

Protocol	Primary Port(s)	Transport	Purpose
mDNS	UDP 5353	UDP	Local name resolution
DNS-SD	UDP 5353	UDP	Service discovery
SSDP	UDP 1900	UDP	UPnP service discovery
WSD	UDP (varies)	UDP	Device service discovery
RADIUS	UDP 1812/1813	UDP	AAA for network access
SMB	TCP 445	TCP	File/print sharing
NetBIOS	TCP 137-139	TCP	Session management
LDAP	TCP 389/636	TCP	Directory services
OAuth 2.0	TCP 443	TCP/HTTPS	Authorization

Common Security Testing Scenarios

Network Reconnaissance

1. mDNS enumeration: Scan for

   .local

   hosts on the subnet
2. SSDP discovery: Query

   239.255.255.250:1900

   for UPnP devices
3. SMB enumeration: Check ports 445, 137-139 for file shares
4. LDAP enumeration: Query port 389 for directory information

Service Discovery Analysis

1. DNS-SD queries: Request specific service types to enumerate services
2. WSD monitoring: Capture broadcast packets for service announcements
3. Protocol fingerprinting: Identify services by their protocol behavior

Authentication Testing

1. RADIUS testing: Verify authentication flow and accounting
2. OAuth 2.0 validation: Test token handling and scope enforcement
3. AD security: Assess Kerberos and LDAP security configurations

Troubleshooting Tips

• mDNS not working: Check firewall rules for UDP 5353, verify

  .local

  domain configuration
• SSDP discovery failing: Ensure UDP 1900 is open, check multicast routing
• SMB connectivity issues: Verify ports 445 or 137-139, check NetBIOS configuration
• LDAP queries failing: Check port 389/636, verify bind credentials and permissions
• AD authentication problems: Verify Kerberos configuration, check time synchronization

Best Practices

1. Disable unnecessary protocols: Turn off NetBIOS, SMBv1, and other legacy protocols when not needed
2. Use encrypted variants: Prefer LDAPS over LDAP, SMB signing enabled
3. Segment networks: Isolate service discovery traffic to appropriate VLANs
4. Monitor protocol traffic: Watch for unusual mDNS, SSDP, or LDAP activity
5. Keep services updated: Regularly patch SMB, LDAP, and other protocol implementations