Sniffing and Evasion

Basic Knowledge

• Sniffing is capturing packets as they pass on the wire to review for interesting information
• MAC (Media Access Control) - physical or burned-in address - assigned to NIC for communications at the Data Link layer
  • 48 bits long
  • Displayed as 12 hex characters separated by colons
  • First half of address is the organizationally unique identifier - identifies manufacturer
  • Second half ensures no two cards on a subnet will have the same address
• NICs normally only process signals meant for it
• Promiscuous mode - NIC must be in this setting to look at all frames passing on the wire
• CSMA/CD - Carrier Sense Multiple Access/Collision Detection - used over Ethernet to decide who can talk
• Collision Domains
  • Traffic from your NIC (regardless of mode) can only be seen within the same collision domain
  • Hubs by default have one collision domain
  • Switches have a collision domain for each port

Protocols Susceptible

• SMTP is sent in plain text and is viewable over the wire. SMTP v3 limits the information you can get, but you can still see it.
• FTP sends user ID and password in clear text
• TFTP passes everything in clear text
• IMAP, POP3, NNTP and HTTP all send over clear text data
• TCP shows sequence numbers (usable in session hijacking)
• TCP and UCP show open ports
• IP shows source and destination addresses

ARP

• Stands for Address Resolution Protocol
• Resolves IP address to a MAC address
• Packets are ARP_REQUEST and ARP_REPLY
• Each computer maintains it’s own ARP cache, which can be poisoned
• Commands
  • arp -a - displays current ARP cache
  • arp -d * - clears ARP cache
• Works on a broadcast basis - both requests and replies are broadcast to everyone
• Gratuitous ARP - special packet to update ARP cache even without a request
  • This is used to poison cache on other machines

IPv6

• Uses 128-bit address
• Has eight groups of four hexadecimal digits
• Sections with all 0s can be shorted to nothing (just has start and end colons)
• Double colon can only be used once
• Loopback address is ::1

• Scope applies for multicast and anycast
• Traditional network scanning is computationally less feasible

Wiretapping

• Lawful interception - legally intercepting communications between two parties
• Active - interjecting something into the communication
• Passive - only monitors and records the data
• PRISM - system used by NSA to wiretap external data coming into US

Active and Passive Sniffing

• Passive sniffing - watching network traffic without interaction; only works for same collision domain
• Active sniffing - uses methods to make a switch send traffic to you even though it isn’t destined for your machine
• Span port - switch configuration that makes the switch send a copy of all frames from other ports to a specific port
  • Not all switches have the ability to do this
  • Modern switches sometimes don’t allow span ports to send data - you can only listen
• Network tap - special port on a switch that allows the connected device to see all traffic
• Port mirroring - another word for span port

MAC Flooding

• Switches either flood or forward data
• If a switch doesn’t know what MAC address is on a port, it will flood the data until it finds out
• CAM Table - the table on a switch that stores which MAC address is on which port
  • If table is empty or full, everything is sent to all ports
• This works by sending so many MAC addresses to the CAM table that it can’t keep up
• Tools
  • Etherflood
  • Macof
• Switch port stealing - tries to update information regarding a specific port in a race condition
• MAC Flooding will often destroy the switch before you get anything useful, doesn’t last long and it will get you noticed. Also, most modern switches protect against this.

ARP Poisoning

• Also called ARP spoofing or gratuitous ARP
• This can trigger alerts because of the constant need to keep updating the ARP cache of machines
• Changes the cache of machines so that packets are sent to you instead of the intended target
• Countermeasures
  • Dynamic ARP Inspection using DHCP snooping
  • XArp can also watch for this
  • Default gateway MAC can also be added permanently into each machine’s cache
• Tools
  • Cain and Abel
  • WinArpAttacker
  • Ufasoft
  • dsniff

DHCP Starvation

• Attempt to exhaust all available addresses from the server
• Attacker sends so many requests that the address space allocated is exhausted
• DHCPv4 packets - DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, DHCPACK
• DHCPv6 packets - Solicit, Advertise, Request (Confirm/Renew), Reply
• DHCP Steps
  1. Client sends DHCPDISCOVER
  2. Server responds with DHCPOFFER
  3. Client sends request for IP with DHCPREQUEST
  4. Server sends address and config via DHCPACK
• Tools
  • Yersinia
  • DHCPstarv
• Mitigation is to configure DHCP snooping
• Rogue DHCP Server - setup to offer addresses instead of real server. Can be combined with starvation to real server.

Spoofing

• MAC Spoofing - changes your MAC address. Benefit is CAM table uses most recent address.
• Port security can slow this down, but doesn’t always stop it
• MAC Spoofing makes the switch send all packets to your address instead of the intended one until the CAM table is updated with the real address again
• IRDP Spoofing - hacker sends ICMP Router Discovery Protocol messages advertising a malicious gateway
• DNS Poisoning - changes where machines get their DNS info from, allowing attacker to redirect to malicious websites

Sniffing Tools

• Wireshark
  • Previously known as Ethereal
  • Can be used to follow streams of data
  • Can also filter the packets so you can find a specific type or specific source address
  • Example filters
    • ! (arp or icmp or dns) - filters out the “noise” from ARP, DNS and ICMP requests
    • http.request - displays HTTP GET requests
    • tcp contains string - displays TCP segments that contain the word “string”
    • ip.addr==172.17.15.12 && tcp.port==23 - displays telnet packets containing that IP
    • tcp.flags==0x16 - filters TCP requests with ACK flag set
• tcpdump
  • Recent version is WinDump (for Windows)
  • Syntax
    • tcpdump flag(s) interface
    • tcpdump -i eth1 - puts the interface in listening mode
• tcptrace
  • Analyzes files produced by packet capture programs such as Wireshark, tcpdump and Etherpeek
• Other Tools
  • Ettercap - also can be used for MITM attacks, ARP poisoning. Has active and passive sniffing.
  • Capsa Network Analyzer
  • Snort - usually discussed as an Intrusion Detection application
  • Sniff-O-Matic
  • EtherPeek
  • WinDump
  • WinSniffer

Devices To Evade

• Intrusion Detection System (IDS) - hardware or software devices that examine streams of packets for malicious behavior
  • Signature based - compares packets against a list of known traffic patterns
  • Anomaly based - makes decisions on alerts based on learned behavior and “normal” patterns
  • False negative - case where traffic was malicious, but the IDS did not pick it up
  • HIDS (Host-based intrusion detection system) - IDS that is host-based
  • NIDS (Network-based intrusion detection system) - IDS that scans network traffic
• Snort - a widely deployed IDS that is open source
  • Includes a sniffer, traffic logger and a protocol analyzer
  • Runs in three different modes
    • Sniffer - watches packets in real time
    • Packet logger - saves packets to disk for review at a later time
    • NIDS - analyzes network traffic against various rule sets
  • Configuration is in /etc/snort on Linux and c:\snort\etc in Windows
  • Rule syntax
    • alert tcp !HOME_NET any -> $HOME_NET 31337 (msg : “BACKDOOR ATTEMPT-Backorifice”)
      • This alerts about traffic coming not from an external network to the internal one on port 31337
  • Example output
    • 10/19-14:48:38.543734 0:48:542:2A:67 -> 0:10:B5:3C:34:C4 type:0x800 len:0x5EA xxx -> xxx TCP TTL:64 TOS:0x0 ID:18112 IpLen:20 DgmLen:1500 DF
    • Important info is bolded
• Firewall
  • An appliance within a network that protects internal resources from unauthorized access
  • Only uses rules that implicitly denies traffic unless it is allowed
  • Oftentimes uses network address translation (NAT) which can apply a one-to-one or one-to-many relationship between external and internal IP addresses
  • Screened subnet - hosts all public-facing servers and services
  • Bastion hosts - hosts on the screened subnet designed to protect internal resources
  • Private zone - hosts internal hosts that only respond to requests from within that zone
  • Multi-homed - firewall that has two or more interfaces
  • Packet-filtering - firewalls that only looked at headers
  • Stateful inspection - firewalls that track the entire status of a connection
  • Circuit-level gateway - firewall that works on Layer 5 (Session layer)
  • Application-level gateway - firewall that works like a proxy, allowing specific services in and out

Evasion Techniques

• Slow down - faster scanning such as using nmap’s -T5 switch will get you caught. Pros use -T1 switch to get better results
• Flood the network - trigger alerts that aren’t your intended attack so that you confuse firewalls/IDS and network admins
• Fragmentation - splits up packets so that the IDS can’t detect the real intent
• Unicode encoding - works with web requests - using Unicode characters instead of ascii can sometimes get past
• Tools
  • Nessus - also a vulnerability scanner
  • ADMmutate - creates scripts not recognizable by signature files
  • NIDSbench - older tool for fragmenting bits
  • Inundator - flooding tool

Firewall Evasion

• ICMP Type 3 Code 13 will show that traffic is being blocked by firewall
• ICMP Type 3 Code 3 tells you the client itself has the port closed
• Firewall type can be discerned by banner grabbing
• Firewalking - going through every port on a firewall to determine what is open
• Tools
  • CovertTCP
  • ICMP Shell
  • 007 Shell
• The best way around a firewall will always be a compromised internal machine

Honeypots

• A system setup as a decoy to entice attackers
• Should not include too many open services or look too easy to attack
• High interaction - simulates all services and applications and is designed to be completely compromised
• Low interaction - simulates a number of services and cannot be completely compromised
• Examples
  • Specter
  • Honeyd
  • KFSensor