Transcript Transparent Caching

Transparent Caching
The art of caching network traffic
without requiring user / browser side
configuration.
Who am I?
Overview
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What is transparent caching, and why use it?
Tools available
How to set it up
Common problems
Alternatives
What is transparent caching?
• Transparently proxying / caching network
traffic without requiring user configuration or
knowledge.
• A way to simplify caching for the end user
• Forces all users to use the cache.
Why use transparent caching?
• Ease of use. No configuration required by the
end user.
• Catching all users. No users can bypass the
cache.
Reasons to not use it
• It is not a magical tool without problems.
• Technical issues
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Networking issues. “Woodo magic”
Stability / Reliability
Only port 80
FTP not supported
To be efficient modern browsers are required
Reasons to not use it (cont.)
• Political reasons
– What is an internet connection?
– Privacy
• No user control. Users can’t bypass the cache.
What is involved?
• TCP level routing
• Reverse NAT or related technology to hijack
port 80 traffic.
• A proxy with some knowledge of transparent
proxying
• A cache
Tools available
• TCP level Routing
– Policy routing / route maps
– TCP / layer 4 switches
• with or without NAT
– Cisco WCCP
• Host level NAT
– Linux firewall code
– FreeBSD firewall code
– IP-Filter
Using policy routing to redirect
traffic
• A standard router configured to route TCP port
80 to the cache server.
Router
Internet
Cache Server
User 1
User 2
User 3
User ..
Policy routing (cont.)
• Benefits
– Can usually be deployed without extra hardware
• Drawbacks
– Only static routing
– No fault tolerance. Port 80 traffic disrupted if cache
server fails.
– More CPU load on the router
Running the cache on a router
• Small network / firewall. Host used as router.
Router /
Cache Server
Internet
User 1
User 2
User 3
User ..
Caching router / firewall
• Benefits
– Less hardware required
– Well suited for small to medium sized firewalls.
• Drawbacks
– Stability / reliability. Can disrupt all communication.
– If running on a firewall: make sure the firewall
protects the cache software.
Cisco Web Cache Control
Protocol (WCCP)
• Developed by Cisco for Cisco
• Automated configuration. Proxy servers
announce their presence to the router.
• Load balancing
• Fault recovery
• Commercial Licensing required. Not currently
an option for free software.
TCP level / layer 4 switching
• The use of a smart and efficient network device
to redirect traffic.
Router
User 1
TCP Switch
User 2
User 3
User ..
Internet
...
Cache
Server
TCP switch benefits
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Can bypass the cache if it malfunctions
Good reliability
Can distribute the load on multiple cache servers
Can do the required NAT, allowing the use of any
OS on the cache server.
– Some do HTTP proxy translation, allowing the use
of any proxy software.
TCP switch drawbacks
– One more expensive box to purchase
– Using NAT requires switch vendor support in the
proxy software to support old browsers.
Request formats
• Proxy request
TCP connection from client to proxy
GET http://www.example.com/file HTTP/1.0
...
• Server request
TCP connection from client to server IP
GET /path/to/file HTTP/1.0
Host: www.example.com (if supported)
...
Problems related to request formats
• A transparent proxy must reconstruct the URL
of the request.
• Host: headers not always available. HTTP/1.1
feature or 1.0 add-on.
• IP address from NAT translation.
What happens at the TCP level?
• Normal communication / proxying
– IP based routing
– TCP is end-to-end
– One IP address, one Host
• Transparent proxying
– TCP based routing
– TCP is no longer end-to-end
– One IP address, “multiple hosts”
Problems at the TCP level
• TCP normally relies on two IP protocols. TCP
and ICMP. Of these only TCP can be reliably
redirected.
• ICMP is required for Path MTU discovery.
• TCP resets if a single packet travels another
path bypassing the redirection.
Things to consider when
configuring OS level NAT
• Try not to disturb traffic to/from the cache
server host.
• Make sure that the proxy traffic is not redirected
back to the proxy.
• Be prepared to do packet level traces, preferably
from a separate box.
Recommended steps when building
a transparent proxy
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Think it over. Is it really required?
Build and test the proxy server
Configure NAT on the proxy server
Test it using a local LAN client
Set up TCP level routing.
Common problems
• Communication hangs for some users
– Most likely caused by MTU related problems.
• Connection reset errors
– Usually misconfigured NAT or TCP routing.
• Bad performance
– Possibly CPU bottleneck in the router.
TCP Reset errors (cont.)
• Error messages seen from the proxy.
– TCP routing or NAT affects traffic generated by the
proxy.
• Error messages seen by the browser (popup)
– TCP routing or NAT failure, causing some client
traffic to bypass the redirection.
Alternatives
• PAC files
• Blocking port 80
– Selectively or everything
– Possibly with an automated message
Selectively blocking port 80 with a
message
• A good alternative to transparent proxying
• Uses the same techniques as transparent
proxying for hijacking port 80, but only to
deliver the instructions.
Blocking port 80, benefits
• Forces the users to configure their proxy
settings
• Users are automatically provided with
configuration instructions when needed. Less
calls to support line.
• Users get the information on why caching is
good for them.
• PAC file allows easy configuration of
exceptions
Blocking port 80, drawbacks
• Not all browsers supports proxy settings
• Users are required to be capable of following
instructions.
Summary
• Transparent caching is a good tool in most
configurations to ease user side configuration.
• It has some important limitations. Not a full
replacement for standard proxying.
• For many automatic instructions on how to
configure proxy settings achieves the same
goals.
Sources for more information
– Squid FAQ
http://squid.nlanr.net/Squid/FAQ/
– Router manuals on policy routing
– IP-Filter home page
http://cheops.anu.edu.au/~avalon/ip-filter.html
– Linux 2.0 ipfadm
http://www.xos.nl/linux/ipfwadm/
– Linux 2.2 ipchains
http://www.rustcorp.com/linux/ipchains/
Questions
Example Cisco IP policy route map
• Policy route map, routing port 80 (www) to
server 10.11.12.13
! Enable policy routing
interface Ethernet0
ip policy route-map proxy-redirect
! Route to proxy server
route-map proxy-redirect permit 10
match ip address 110
set ip next-hop 10.11.12.13
! Only policy route client www traffic
access-list 110 deny
tcp any any neq www
access-list 110 deny
tcp host 10.11.12.13 any
access-list 110 permit tcp any any
Example Linux ipfwadm NAT
• Linux 2.0 redirecting eth0 TCP port 80 to Squid
on port 3128
– Kernel options:
CONFIG_IP_FIREWALL=y
CONFIG_IP_ALWAYS_DEFRAG=y
– ipfwadm ruleset
# Accept local traffic
ipfwadm -I -a accept -W eth0 -D this.host
# Redirect port 80 to Squid on 3128
ipfwadm -I -a accept -W eth0 -P tcp -D 0.0.0.0/0 80 -r 3128
Example Linux ipchains NAT
• Linux 2.2 redirecting eth0 TCP port 80 to Squid
on port 3128
– Kernel options:
CONFIG_IP_FIREWALL=y
CONFIG_IP_ALWAYS_DEFRAG=y
– ipchains ruleset
# Accept local traffic
ipchains -A input -j ACCEPT -i eth0 -d 10.11.12.13/32
# Redirect port 80 to Squid on port 3128
ipchains -A input -j REDIRECT 3128 -i eth0 -p tcp -d 0.0.0.0/0 80
Example IP-Filter NAT
• ipnat ruleset redirecting TCP port 80 to Squid
on port 3128
# Redirect direct web traffic to local web server.
rdr de0 10.11.13.13/32 port 80 -> 127.0.0.1 port 80 tcp
# Redirect everything else to squid on port 3128
rdr de0 0.0.0.0/0 port 80 -> 127.0.0.1 port 3128 tcp
Running Squid on Linux
What is Linux
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Linux is like any other UNIX
POSIX standards
GNU tools
Best of SysV and BSD families
Filesystem performance
• To few performance counters for I/O to make
any good measurements
• Asynchronous writes by default (like fastfs on
Solaris)
• noatime mount option
Kernel performance / tuning
• Memory freelist tuning on smaller systems
– /proc/sys/vm/freepages
• Filedescriptor limits
– Default 256
– Later revisions of 2.2 may allow 1024
– Patches available for higher limits
Hands on transparent caching
• Linux configuration
– Kernel configuration
• Firewalling & Transparent proxy support
– ipfwadm configuration
ipfwadm -I accept -D thishost
ipfwadm -I accept -P tcp -D 0.0.0.0/0
80 -r 3128
Hands on transparent caching (cont.)
• Squid configuration
httpd_accel_host virtual
httpd_accel_uses_host_header on