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+%include "default.mgp"
+%default 1 bgrad
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+%nodefault
+%back "blue"
+
+%center
+%size 7
+
+
+netfilter/iptables training
+%size 4
+
+Nov 05/06/07, 2007
+Day 1
+
+%center
+%size 4
+by
+
+Harald Welte <laforge@netfilter.org>
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Contents
+
+ Day 1
+
+ Introduction
+ Highly Scalable Linux Network Stack
+ Netfilter Hooks
+ Packet selection based on IP Tables
+ The Connection Tracking Subsystem
+ The NAT Subsystem
+ Packet Mangling
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Introduction
+
+
+Who is speaking to you?
+ an independent Free Software developer
+ who earns his living off Free Software since 1997
+ who is one of the authors of the Linux kernel firewall system called netfilter/iptables
+ [who can claim to be the first to have enforced the GNU GPL in court]
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Introduction
+
+Linux and Networking
+ Linux is a true child of the Internet
+ Early adopters: ISP's, Universities
+ Lots of work went into a highly scalable network stack
+ Not only for client/server, but also for routers
+ Features unheared of in other OS's
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Introduction
+
+Did you know, that a stock 2.6.x linux kernel can provide
+
+ a stateful packet filter ?
+ fully symmetric NA(P)T ?
+ policy routing ?
+ QoS / traffic shaping ?
+ IPv6 firewalling ?
+ packet filtering, NA(P)T on a bridge ?
+ layer 2 (mac) address translation ?
+ packet forwarding rates of up to 2.1Mpps ?
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Introduction
+
+Why did we need netfilter/iptables?
+Because ipchains...
+
+ has no infrastructure for passing packets to userspace
+ makes transparent proxying extremely difficult
+ has interface address dependent Packet filter rules
+ has Masquerading implemented as part of packet filtering
+ code is too complex and intermixed with core ipv4 stack
+ is neither modular nor extensible
+ only barely supports one special case of NAT (masquerading)
+ has only stateless packet filtering
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Introduction
+
+Who's behind netfilter/iptables
+
+ The core team
+ Paul 'Rusty' Russel
+ co-author of iptables in Linux 2.2
+ James Morris
+ Marc Boucher
+ Harald Welte
+ Jozsef Kadlecsik
+ Martin Josefsson
+ Patrick McHardy
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Netfilter Hooks
+
+ What is netfilter?
+
+ System of callback functions within network stack
+ Callback function to be called for every packet traversing certain point (hook) within network stack
+ Protocol independent framework
+ Hooks in layer 3 stacks (IPv4, IPv6, DECnet, ARP)
+ Multiple kernel modules can register with each of the hooks
+
+Traditional packet filtering, NAT, ... is implemented on top of this framework
+
+Can be used for other stuff interfacing with the core network stack, like DECnet routing daemon.
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Netfilter Hooks
+
+Netfilter architecture in IPv4
+%font "typewriter"
+%size 4
+in --->[1]--->[ROUTE]--->[3]--->[4]---> out
+ | ^
+ | |
+ | [ROUTE]
+ v |
+ [2] [5]
+ | ^
+ | |
+ v |
+%font "standard"
+1=NF_IP_PRE_ROUTING
+2=NF_IP_LOCAL_IN
+3=NF_IP_FORWARD
+4=NF_IP_POST_ROUTING
+5=NF_IP_LOCAL_OUT
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Netfilter Hooks
+
+Netfilter Hooks
+
+ Any kernel module may register a callback function at any of the hooks
+
+ The module has to return one of the following constants
+
+ NF_ACCEPT continue traversal as normal
+ NF_DROP drop the packet, do not continue
+ NF_STOLEN I've taken over the packet do not continue
+ NF_QUEUE enqueue packet to userspace
+ NF_REPEAT call this hook again
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP tables
+
+ Packet selection using IP tables
+
+ The kernel provides generic IP tables support
+
+ Each kernel module may create it's own IP table
+
+ The four major parts of the firewalling subsystem are implemented using IP tables
+ Packet filtering table 'filter'
+ NAT table 'nat'
+ Packet mangling table 'mangle'
+ The 'raw' table for conntrack exemptions
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP Tables
+
+ Managing chains and tables
+
+ An IP table consists out of multiple chains
+ A chain consists out of a list of rules
+ Every single rule in a chain consists out of
+ match[es] (rule executed if all matches true)
+ target (what to do if the rule is matched)
+ implicit packet and byte counter
+
+%size 4
+matches and targets can either be builtin or implemented as kernel modules
+
+%size 5
+ The userspace tool iptables is used to control IP tables
+ handles all different kinds of IP tables
+ supports a plugin/shlib interface for target/match specific options
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP Tables
+
+Basic iptables commands
+
+ To build a complete iptables command, we must specify
+ which table to work with
+ which chain in this table to use
+ an operation (insert, add, delete, modify)
+ one or more matches (optional)
+ a target
+
+The syntax is
+%font "typewriter"
+%size 3
+iptables -t table -Operation chain -j target match(es)
+%font "standard"
+%size 5
+
+Example:
+%font "typewriter"
+%size 3
+iptables -t filter -A INPUT -j ACCEPT -p tcp --dport smtp
+%font "standard"
+%size 5
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP Tables
+
+Matches
+ Basic matches
+ -p protocol (tcp/udp/icmp/...)
+ -s source address (ip/mask)
+ -d destination address (ip/mask)
+ -i incoming interface
+ -o outgoing interface
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP Tables
+
+ addrtype match
+ matches source/destionation address type
+ types are UNICAST/LOCAL/BROADCAST/ANYCAST/MULTICAST/...
+ ah match
+ matches IPSEC AH SPI (range)
+ comment match
+ always matches, allows user to place comment in rule
+ connmark match
+ connection marking, see later
+ conntrack match
+ more extended version of 'state'
+ match on timeout, fine-grained state, original tuples
+ dscp match
+ matches DSCP codepoint (formerly-known as TOS bits)
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP Tables
+
+ ecn match
+ matches ECN bits of tcp and ip header
+ esp match
+ matches IPSEC ESP SPI (range)
+ hashlimit match
+ dynamic limiting
+ helper match
+ allows matching of conntrack helper name
+ iprange match
+ match on arbitrary IP address ranges (not a mask)
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP Tables
+
+ length match
+ match on packet length
+ limit
+ static rate limiting
+ mac
+ match on source mac address
+ mark
+ match on nfmark (fwmark)
+ multiport
+ match on multiple ports
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP Tables
+
+ owner
+ match on socket owner (uid, gid, pid, sid, command name)
+ physdev
+ match underlying device in case of bridge
+ pkttype
+ match link-layer packet type (unicast,broadcast,multicast)
+ realm
+ match routing realm
+ recent
+ see special section below
+ tcpmss
+ match on TCP maximum segment size
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+IP Tables
+
+Targets
+ very dependent on the particular table
+
+ Table specific targets will be discussed later
+
+ Generic Targets, always available
+ ACCEPT accept packet within chain
+ DROP silently drop packet
+ QUEUE enqueue packet to userspace
+ LOG log packet via syslog
+ ULOG log packet via ulogd
+ RETURN return to previous (calling) chain
+ foobar jump to user defined chain
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Packet Filtering
+
+Overview
+
+ Implemented as 'filter' table
+ Registers with three netfilter hooks
+
+ NF_IP_LOCAL_IN (packets destined for the local host)
+ NF_IP_FORWARD (packets forwarded by local host)
+ NF_IP_LOCAL_OUT (packets from the local host)
+
+Each of the three hooks has attached one chain (INPUT, FORWARD, OUTPUT)
+
+Every packet passes exactly one of the three chains. Note that this is very different compared to the old 2.2.x ipchains behaviour.
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Packet Filtering
+
+Targets available within 'filter' table
+
+ Builtin Targets to be used in filter table
+ ACCEPT accept the packet
+ DROP silently drop the packet
+ QUEUE enqueue packet to userspace
+ RETURN return to previous (calling) chain
+ foobar user defined chain
+
+ Targets implemented as loadable modules
+ REJECT drop the packet but inform sender
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ Connection tracking...
+
+ implemented seperately from NAT
+ enables stateful filtering
+ implementation
+ hooks into NF_IP_PRE_ROUTING to track packets
+ hooks into NF_IP_POST_ROUTING and NF_IP_LOCAL_IN to see if packet passed filtering rules
+ protocol modules (currently TCP/UDP/ICMP/SCTP)
+ application helpers currently (FTP,IRC,H.323,talk,SNMP)
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ Connection tracking...
+
+ divides packets in the following four categories
+ NEW - would establish new connection
+ ESTABLISHED - part of already established connection
+ RELATED - is related to established connection
+ INVALID - (multicast, errors...)
+ does _NOT_ filter packets itself
+ can be utilized by iptables using the 'state' match
+ is used by NAT Subsystem
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ State tracking for TCP is obvious
+ TCP inherently stateful
+ Two TCP state machines on each end have well-defined behaviour
+ Passive tracking of state machines
+ In more recent 2.6.x kernels, tracking of TCP window (seq/ack)
+ Max idle timeout of fully-established session: 5 days
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ State tracking for UDP: How is this possible?
+ UDP itself not stateful at all
+ However, higher-level protocols mostly match request-reply
+ First packet (request) is assumed to be NEW
+ First matching reply packet is assumed to confirm connection
+ Further packets in either direction refresh timeout
+ Timeouts: 30sec unreplied, 180sec confirmed
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ State tracking on ICMP: What's that?
+ ICMP Errors (e.g. host/net unreachable, ttl exceeded)
+ They can always be categorized as RELATED to other connections
+ ICMP request/reply (ECHO REQUEST, INFO REQUEST)
+ can be treated like UDP request/reply case
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ State tracking on SCTP: What's SCTP?
+ Streaming Control Transfer Protocol
+ Linux has SCTP in the network stack, so why should the packet filter not support it?
+ Pretty much like TCP in most cases
+ Doesn't support more advanced features such as failover of an endpoint
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ State tracking on other protocols
+ 'generic' protocol: no layer-4 tuple information
+ 'gre' helper in patch-o-matic
+
+ State tracking of higher-layer protocols
+ implemented as 'connection tracking helpers'
+ currently in-kernel: amanda, ftp, irc, tftp
+ currently in patch-o-matic: pptp, h.323, sip, quake, ...
+ have to be explicitly loaded (ip_conntrack_*.[k]o)
+ work by issuing so-called "expectations"
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ Exemptions to connection tracking
+ Usually connection tracking is called first in PRE_ROUTING
+ Sometimes, filtering is preferred before this conntrack lookup
+ Therefore, the "raw" table was introduced
+ In some rare cases, one might want to not track certain packets
+ The NOTRACK can be used in the "raw" table
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ Configuration / Tuning
+ module parameter "hashsize"
+ number of hash table buckets
+ /proc/sys/net/ipv4/ip_conntrack_max
+ maximum number of tracked connections
+ /proc/sys/net/ipv4/ip_conntrack_buckets (read-only)
+ number of hash table buckets
+ /proc/net/ip_conntrack
+ list of connections
+ /proc/net/ip_conntrack_expect
+ list of pending expectations
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Connection Tracking Subsystem
+
+ Configuration / Tuning
+ /proc/sys/net/ip_conntrack_log_invalid
+ log invalid packets?
+ /proc/sys/net/ip_conntrack_tcp_be_liberal
+ basically disables window tracking, if "1"
+ /proc/sys/net/ip_conntrack_tcp_loose
+ how many packets required until sync in case of pickup
+ if set to zero, disables pickup
+ /proc/sys/net/ip_conntrack_tcp_max_retrans
+ maximum number of retransmitted packets without seeing a n ACK
+ /proc/sys/net/ip_conntrack_*timeout*
+ timeout values of respective protocol states
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Network Address Translation
+
+ Network Address Translation
+
+ Previous Linux Kernels only implemented one special case of NAT: Masquerading
+ Linux 2.4.x / 2.6.x can do any kind of NAT.
+ NAT subsystem implemented on top of netfilter, iptables and conntrack
+ Following targets available within 'nat' Table
+ SNAT changes the packet's source whille passing NF_IP_POST_ROUTING
+ DNAT changes the packet's destination while passing NF_IP_PRE_ROUTING
+ MASQUERADE is a special case of SNAT
+ REDIRECT is a special case of DNAT
+ SAME
+ NETMAP
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Network Address Translation
+
+ Source NAT
+ SNAT Example:
+%font "typewriter"
+%size 3
+iptables -t nat -A POSTROUTING -j SNAT --to-source 1.2.3.4 -s 10.0.0.0/8
+%font "standard"
+%size 4
+
+ MASQUERADE Example:
+%font "typewriter"
+%size 3
+iptables -t nat -A POSTROUTING -j MASQUERADE -o ppp0
+%font "standard"
+%size 5
+
+ Destination NAT
+ DNAT example
+%font "typewriter"
+%size 3
+iptables -t nat -A PREROUTING -j DNAT --to-destination 1.2.3.4:8080 -p tcp --dport 80 -i eth1
+%font "standard"
+%size 4
+
+ REDIRECT example
+%font "typewriter"
+%size 3
+iptables -t nat -A PREROUTING -j REDIRECT --to-port 3128 -i eth1 -p tcp --dport 80
+%font "standard"
+%size 5
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Packet Mangling
+
+ Purpose of 'mangle' table
+ packet manipulation except address manipulation
+
+ Integration with netfilter
+ 'mangle' table hooks in all five netfilter hooks
+ priority: after conntrack
+
+Simple example:
+%font "typewriter"
+%size 3
+iptables -t mangle -A PREROUTING -j MARK --set-mark 10 -p tcp --dport 80
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Packet Mangling
+
+ Targets specific to the 'mangle' table:
+ DSCP
+ manipulate DSCP field
+ ECN
+ manipulate ECN bits
+ IPV4OPTSSTRIP
+ strip IPv4 options
+ MARK
+ change the nfmark field of the skb
+ TCPMSS
+ set TCP MSS option
+ TOS
+ manipulate the TOS bits
+ TTL
+ set / increase / decrease TTL field
+ CLASSIFY
+ classify packet (for tc/iproute)
+ CONNMARK
+ set mark of connection
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+The raw Table
+
+ Purpose of 'raw' table
+ to allow for filtering rules _before_ conntrack
+ Targets specific to the 'raw' table:
+ NOTRACK
+ don't do connection tracking
+
+ The table can also be useful for flood protection rules that happen before traversing the (computational) expensive connection tracking subsystem.
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Advanced Netfilter concepts
+
+%size 4
+ Userspace logging
+ flexible replacement for old syslog-based logging
+ packets to userspace via multicast netlink sockets
+ easy-to-use library (libipulog)
+ plugin-extensible userspace logging daemon (ulogd)
+ Can even be used to directly log into MySQL
+
+ Queuing
+ reliable asynchronous packet handling
+ packets to userspace via unicast netlink socket
+ easy-to-use library (libipq)
+ provides Perl bindings
+ experimental queue multiplex daemon (ipqmpd)
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Advanced Netfilter concepts
+
+ Firewalling on a Bridge (ebtables + iptables)
+ totally transparent to layer 2 and above
+ no attack vector since firewall has no IP address
+ even possible to do NAT on the bridge
+ or even NAT of MAC addresses
+
+ ipset - Faster matching
+ iptables are a linear list of rules
+ ipset represents a 'group' scheme
+ Implements different data types for different applications
+ hash table (for random addresses)
+ bitmask (for let's say a /24 network)
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Advanced Netfilter concepts
+
+ ipv6 packet filtering
+ ip6tables almost identical to iptables
+ no connection tracking in mainline yet, but patches exist
+ ip6_conntrack
+ initial copy+paste 'port' by USAGI
+ was not accepted because of code duplication
+ nf_conntrack
+ generalized connection tracking, supports ipv4 and ipv6
+ mutually exclusive with ip_conntrack
+ as of now, no ipv4 nat on to of nf_conntrack
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%page
+netfilter/iptables tutorial
+Thanks
+
+ Thanks to
+ the BBS scene, Z-Netz, FIDO, ...
+ for heavily increasing my computer usage in 1992
+ KNF (http://www.franken.de/)
+ for bringing me in touch with the internet as early as 1994
+ for providing a playground for technical people
+ for telling me about the existance of Linux!
+ Alan Cox, Alexey Kuznetsov, David Miller, Andi Kleen
+ for implementing (one of?) the world's best TCP/IP stacks
+ Paul 'Rusty' Russell
+ for starting the netfilter/iptables project
+ for trusting me to maintain it today
+ Astaro AG
+ for sponsoring parts of my netfilter work
personal git repositories of Harald Welte. Your mileage may vary