This document discusses network tunneling protocols and tools. It describes how protocols like SSH, GRE, and ICMP can be used to encapsulate other protocols and bypass network restrictions. Examples of network tunneling tools that operate over HTTP, DNS, and ICMP are provided. The document notes both legitimate and malicious uses of tunneling, and outlines challenges in detecting tunneling traffic and payloads.
3. A delivery network protocol
encapsulates a payload
network protocol
The delivery protocol
usually operates at the
same or higher level (e.g.
in the TCP/IP stack) than
the payload protocol
4. Protocol Tunneling
◦ Order in Protocol Encapsulation
◦ Obfuscation rather than hiding
◦ Practical use cases & misuse
Covert Channels
◦ Channels not intended for information
transfer
◦ Hiding in unused protocol fields, utilizing
fields such as IP ID, TCP Sequence number
etc.
◦ Network Steganography
5. Carry data over incompatible
delivery-networks
Provide a (encrypted) path
through a public network
◦ Monitoring vs Anti-Censorship
Allowing “some kind” of
traffic may lead to “any kind”
6. Pre-existing network-based security
tools (firewalls, IDS) may not be able to
apply the controls to the tunneled
traffic
◦ Evading traffic regulation
Lack of host-based security controls
◦ Defense in depth
Inability for ingress and egress filtering
‘Open-ended’ tunnel may forward
traffic to other internal hosts
7. Advanced Persistent Threats
(APTs) - Remote Control & Data
exfiltration
◦ Backdoors with OS commands, file
transfer capabilities are installed in
target systems.
◦ Upload collected files using
common ports such as HTTP (80),
HTTPS (443) and DNS (53)
bypassing detection.
Covert channels for malware
◦ e.g. C&C communications over DNS
(i.e. Feederbot, W32.Morto )
9. The original IP packet is encrypted
The ESP header indicates that the entire
packet is the payload (IP-in-IP)
Inserts a new IP header (next header is ESP)
Image taken from http://www.free-it.de/archiv/talks_2005/paper-11156/paper-11156.html
10. Security services from gateway to gateway or
from host to gateway over an insecure
network
The entire original packet is encrypted
◦ Internal traffic behind the gateways is not protected
Often used to implement Virtual Private
Networks (IPsec VPNs)
◦ Site-to-site
◦ Client-to-site
11. “GRE (Generic Routing Encapsulation)
specifies a protocol for encapsulation of an
arbitrary protocol over another arbitrary
network layer protocol” – RFC 2784 and 2890
Point-to-point links
Image taken from http://netwild.ru/pptp/
12. Ethernet over IPv4/IPv6
(e.g. Openstack Neutron)
Support for tunneling
broadcasting/multicasting
◦ e.g. Delivering routing updates to multiple sites
IPv4/IPv6 over IPv4/IPv6
No default encryption/security services
◦ IPSec Tunnel/Transport over GRE
13. Tunnel Brokers provide a network tunneling
service
6in4 – IPv6 over IPv4
4in6 – IPv4 over IPv6
ISATAP
Teredo – IPv6 over UDP over IPv4
…and others
14. Secure channel over an insecure
network between an SSH client
and an SSH server (e.g. OpenSSH)
typically listening at TCP port 22
Public-key cryptography for server
(and client) authentication
Remote command execution, file
transfer (SCP, SFTP), TCP port and
X forwarding, tunneling
15. Local-port forwarding when traffic coming to
a local port is forwarded to a specified
remote host/port
Destination is relative to the SSH server’s
location and mostly unrestricted
SSH client can be configured to act either as a
local-only service or public to other hosts
16. Remote-port forwarding when traffic coming
to a remote port is forwarded to a specified
local host/port
Destination is relative to the SSH client’s
location and mostly unrestricted
SSH server can be configured to act either as
a local-only service or public to other hosts
17. Performs successfully for single-
host/port communications
◦ Simple Web (HTTP)
◦ Mail (SMTP, POP3, IMAP)
◦ SSH
Fails for more complex network
services
◦ Web with External References / Surfing
Solution: Chain to a Web Proxy
◦ FTP
◦ Peer-to-Peer
18. The SOCKS protocol proxies TCP
connections/forwards UDP packets from
client to server through a proxy
A local SOCKS proxy is created on the SSH
client’s side and can forward traffic to
arbitrary remote hosts and ports
Firewall Traversal / Content-filtering
circumvention
19. Run remote X Window System based
applications but displayed locally
Need for X server for Windows
Secure the X protocol by tunneling it over SSH
ssh –X user@host <application>
◦ Run a remote browser visiting a blocked website
20. “An ICMP ECHO_REQUEST packet contains an
additional 8 bytes worth of ICMP header
followed by an arbitrary-amount of data” –
ping(8) man page
LOKI (Phrack Issue 49) utilized it to establish
a covert channel between client/server
IP over ICMP
TCP over ICMP
21.
22. Various network protocols are encapsulated
using the HTTP protocol
HTTP is rarely blocked
Bypass restrictions
◦ Firewalls
◦ Proxy server / Content-filtering
23.
24. Transport arbitrary data by encoding them into DNS
messages
Wide support and availability of the global DNS infrastructure
Few organizations block DNS traffic from individual clients to
the Internet (e.g. captive portals in public Wi-Fi)
Effective for bypassing security measures such as firewalls or
ACLs
Used for two-way communication or data exfiltration
25. Around since 1998
NSTX (Nameserver Transfer Protocol)
OzymanDNS (Dan Kaminsky) – “Tunneling
Audio, Video and SSH over DNS”
Used mostly for bypassing paywalls
26. Mapping domain names and IP addresses
Record types
◦ A, AAAA, CNAME, MX, NS, PTR, TXT, NULL
EDNS for UDP payloads larger than 512 bytes
◦ Increased bandwidth
Internal users can contact arbitrary external
domains through the organization’s DNS
servers/resolvers
27. Image taken from http://nirlog.com/2006/03/28/dns-amplification-attack/
28.
29. Maximum 253 characters in domain
Maximum 63 characters per subdomain
Case-insensitive (Base32 encoding)
TXT requests allow for maximum characters
in response + Base64 encoding
Bandwidth up to 110KB/s, 150ms latency
(Van Leijenhorst, 2008)
33. Combined with NetCat
◦ Establish a local/remote port forward over SSH with
an SSH server
◦ Create a FIFO special file (a named pipe) on both
sides
◦ Listen for UDP requests / Relay through the SSH
tunnel
◦ Forward UDP requests / Relay through the SSH
tunnel
tcp_to_udp & udp_to_tcp
socat Relay & UDPTunnel (UDP over TCP)
34.
35. In the case of HTTP browsing, DNS requests are
still submitted by the client
Monitoring can reveal DNS requests for common
websites along SSH traffic.
Solution: forward DNS requests also to the SSH
server.
◦ (e.g. Firefox network.proxy.socks_remote_dns)
Multi-hop setups
◦ Client (SSH lpf) -> Host 1 (SSH dpf) -> Host 2 -> Web
36. SSH Traffic Volume & SSH Tunnel Endpoints
Tunnel Hunter (Dusi et al., 2008)
◦ Naïve Bayes Classifier
◦ Packet size & Packet inter-arrival time
◦ Detect Tunneling & Classify the actual protocol
(BitTorrent, POP, SMTP, HTTP) with high accuracy
◦ Limitations with respect to multiple SSH
authentication types, data compression, login
failures, network protocols
SSH server in non-standard ports (e.g. 443)
◦ EmergingThreats Snort Rules, Cisco IDS
Degrade SSH performance (TCP over TCP )
37. Image taken from http://www.sectechno.com/2010/10/31/bypassing-firewalls-using-icmp-tunnel/
38.
39. ICMPTX (IP over ICMP)
ICMP Tunnel (IP over ICMP)
Hans (IP over ICMP)
itun (IP over ICMP)
Ptunnel (TCP over ICMP)
Droid-VPN , Troid-VPN (Android Apps,
need root)
PD-Proxy, Wi-Free, Tunnel Guru
40. Detection Signatures
◦ ICMP_PingTunnel_Detected
◦ LOKI ICMP tunneling back door
◦ ICMP Raw Sockets
Non-standard average packet size
High ICMP traffic volume between tunnel
endpoints
Disallow ICMP traffic
41. The Tunnel Client initiates an HTTP
connection to the Tunnel Server
The application encapsulates the application
requests in HTTP requests destined to the
Tunnel Server
The Tunnel Server unwraps and forwards
46. Increased DNS traffic (network traffic profiling)
Maximum DNS request packet size
Large number of DNS TXT requests
Number of DNS requests, unique hostnames to a single
domain
Composition of hostnames
◦ Length, unique characters, character frequency analysis
Split DNS
◦ Web proxies (but not clients) can resolve external domains
47. Determining which tunneling messages are
malicious
◦ Real-time Blackhole Lists (DNSBL lookups)
23.42.168.192.dnsbl.example.net
example.net.dnslist.example.com
◦ NIST National Software Reference Library
84C0C5914FF0B825141BA2C6A9E3D6F4.md5.dshield.or
g
Mail server performs DNS TXT requests (SPF)
51. Using existing core network protocols in
innovative ways
Ability to bypass filtering controls and make
monitoring difficult (SSH encrypted tunnels)
Need for improved tunneling detection (both
delivery and payload protocols) methods and
even forensic capabilities
