Introduction
HTTP tunneling is a method in which non-legitimate data is sent using HTTP protocol. This is acheived using a client-server HTTP tunneling application, and is usually done to bypass the security provided by common firewalls, as firewalls usually allow HTTP traffic. A properly formatted HTTP packet is used which passes the standard HTTP checks and L4 firewalls. HTTP tunneling works with a host running the server side applciation. The outside user runs the client side applciation, which is used to send HTTP encapsulated communication to the server application. The server then interprets this information and opens the connection to the desired host, and this bypasses the security.
Real Servers and Server Farms
To provide services to clients, you configure real servers (the actual physical servers) on the ACE. Real servers provide client services such as HTTP or XML content, hosting websites, FTP file uploads or downloads, redirection for web pages that have moved to another location, and so on. The ACE also allows you to configure backup servers in case a server is taken out of service for any reason.
Servers are organized into related groups called server farms. Servers within server farms often contain identical content (referred to as mirrored content) so that if one server becomes inoperative, another server can take its place immediately. Also, mirrored content allows several servers to share the load of increased demand.
Blocking HTTP Tunnels
Most standard firewalls cant block HTTP tunnels which are cleaverly disguised. Although firewalls do have HTTP inspection; it only checks for proper HTTP packet and header, which is there in HTTP tunnels. Several HTTP tunneling methods exist, like HTTP CONNECT, POST, GET, PUT, DELETE tunneling. To block this traffic requires a deeper packet inspection using custom regex. Custom regex is not possible in case of firewalls like PIX and FWSM
pixfirewall(config-cmap)# match request header ?
mpf-class-map mode commands/options:
accept Accept field
accept-charset Accept-Charset field
accept-encoding Accept-Encoding field
accept-language Accept-Language field
allow Allow field
authorization Authorization field
…
pixfirewall(config-cmap)# match request header X-?
ERROR: % Unrecognized command
ACE/PIX/ASA come with a built-in “port-misuse” directive in HTTP inspection engine. But it doesn’t really help here. The engine looks for these fixed regexes:
FIRETHRO: "[/\\\\]cgi[-]bin[/\\\\]proxy"
HTTP_PORT: "[ \t]+photo[.]exectech[-]va[.]com"
UAGENT_GNUTELLA: "[Gg][Nn][Uu][Tt][Ee][Ll][Ll][Aa]"
YAHOO: "YMSG.*"
KAZAA: "[Xx][-][Kk][Aa][Zz][Aa][Aa].*“
Configure ACE to block HTTP Tunneling
Follow these steps to block HTTP tunneling using ACE:
1) Define a class-map that matches port 80:
switch/lab(config)# class-map match-any http
switch/lab(config-cmap)# match port tcp eq www
2) Build header regexes: if we see both headers, match!
switch/lab(config)# class-map type http inspect match-all headers
switch/lab(config-cmap-http-insp)# match header X-Counter header-value .*
switch/lab(config-cmap-http-insp)# match header X-Session header-value .*
3) Instruct ACE to drop packets that match the regex
switch/lab(config)# policy-map type inspect http all-match htpolicy
switch/lab(config-pmap-ins-http)# class headers
switch/lab(config-pmap-ins-http-c)# reset
4) Apply the HTTP policy to port-80 traffic:
switch/lab(config)# policy-map multi-match nofirepass
switch/lab(config-pmap)# class http
switch/lab(config-pmap-c)# inspect http policy htpolicy url-logging
5) Map the policy to the client-side interface:
switch/lab(config)# int vlan 20
switch/lab(config-if)# service-policy input nofirepass
SSL Tunneling
Following is a sample configuration for SSL tunneling
parameter-map type http CASE_PARAM
case-insensitive
persistence-rebalance
set header-maxparse-length 65535
set content-maxparse-length 65535
class-map match-all CLEAR_TEXT_VIP
2 match virtual-address 172.20.120.19 tcp eq www
policy-map multi-match JORGE-MULTIMATCH
class CLEAR_TEXT_VIP
loadbalance vip inservice
loadbalance policy POLICY_TO_ENCRYPT_TRAFFIC
loadbalance vip icmp-reply active
appl-parameter http advanced-options CASE_PARAM
policy-map type loadbalance first-match POLICY_TO_ENCRYPT_TRAFFIC
class class-default
serverfarm ENCRYPTED-SERVERFARM
ssl-proxy client SSL-PROXY-JORGE
ssl-proxy service SSL-PROXY-JORGE
key TAC-key
cert TAC-cert
serverfarm host ENCRYPTED-SERVERFARM
rserver JORGE-SERVER 443
inservice
Verify
Check for the following ACE logs to verify if blocking is working fine
%ACE-6-302022: Built TCP connection 0x76 for vlan20:10.20.10.100/52431 (10.20.10.100/52431) to vlan100:10.10.10.101/80 (10.10.10.101/80)
%ACE-5-304001: User:10.20.10.100 Accessed URL 10.10.10.101:/cgi-bin/firepass-1.1.2a/fpserver/fpserver.cgi
%ACE-6-302023: Teardown TCP connection 0x76 for vlan20:10.20.10.100/52431 to vlan100:10.10.10.101/80 duration 0:00:00 bytes 508 Policy Close
