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LaForge I'm going to talk this evening about netfilter
LaForge netfilter is the packet filtering / packet mangling / NAT framework of the Linux 2.4 kernel series
LaForge Some slides and a script for this talk are avaliable at http://www.gnumonks.org/papers/netfilter-lk2000
LaForge I expect the autitorium to be familiar with TCP/IP basics, as well as being familiar with iptables and packet filtering in general
LaForge oops... ipchains of course :)
LaForge in other words: You should know how the 2.2 packet filtering (ipchains) works ;)
LaForge first a bit of an introduction:
LaForge What is netfilter?
LaForge Netfilter is a generalized framework of hooks in the network stack
LaForge any kernel module can plug into one or more of these hooks an will receive each packet traversing this hook
LaForge the netfilter hooks are currently implemented for IPv4, IPv6 and DECnet.
LaForge (i've heared recently, that somebody wants to implement them for IPX, too)
LaForge these hooks are placed in well-chosen points of the protocol stack.
LaForge The traditional packet filtering, as well as all kinds of network address translation and packet mangling are implemented on top of these hooks.
LaForge so netfilter is definitely more than a firewalling subsystem - it's a superset of that.
LaForge The next introductory question is:
LaForge Why did we need netfilter?
LaForge Because the old 2.2 code was way too complex
LaForge it was scattered around the whole IPv4 code
LaForge there were about 25 places in the IPv4 code, where we had a #ifdef CONFIG_IP_FIREWALL ... #else ... #endif
LaForge which is quite bad.
LaForge furthermore, all packet handling had to be done in kernel
LaForge masquerading was a hack to the packet filtering code
LaForge and the filtering rules are bound to interface addresses.
LaForge The 2.2 code was not very extensible, you could only write masquerading helper modules like ip_masq_irc / ip_masq_ftp / ...
LaForge ... now to the last part of the introduction:
LaForge Who did netfilter?
LaForge The main part of netfilter design and implementation was done by Rusty Russel
LaForge Rusty is also the co-author of ipchains and Linux Kernel Firewall Maintainer for the last years.
LaForge He got sponsored for one year to concentrate on the firewalling code - and the result was netfilter
LaForge Some other people joined him in different stages of the development: Marc Boucher, James Morris and the last one is myself.
LaForge The 'formal' core team consists out of us four people. Of course there are numoerous other contributors, you can see them at http://netfilter.kernelnotes.org/scoreboard.html
LaForge So let's begin the main part of this presentation:
LaForge PART 1 - netfilter basics
LaForge I was talking about these hooks at particular points in the network stack.
LaForge I'm going to concentrate on IPv4, as this seems to be the most important case :)
LaForge --->[1]--->[ROUTE]--->[3]--->[4]--->
LaForge | ^
LaForge | |
LaForge | [ROUTE]
LaForge v |
LaForge [2] [5]
LaForge | ^
LaForge | |
LaForge v |
LaForge
LaForge on the left hand, you have incoming packets, coming from the network
LaForge on the right hand, outgoing packets are leaving to the network
LaForge on the bottom of the picture is our local machine, the local userspace processes.
LaForge the 5 hooks are called:
LaForge 1 NF_IP_PRE_ROUTING
LaForge 2 NF_IP_LOCAL_IN
LaForge 3 NF_IP_FORWARD
LaForge 4 NF_IP_POST_ROUTING
LaForge 5 NF_IP_LOCAL_OUT
LaForge so let's view at the path a packet goes while being forwarded by our machine:
LaForge Firs it comes off the wire, it passes hook #1. The routing decision is made,
LaForge it passes hook #3 (forward), passes hook #4 (post_routing) and leaves off to the network again.
LaForge If we look on packets which have a local destionation (are locally terminated an are not routed), the following path:
LaForge packet comes off the wire
LaForge packet hits hook #1 (pre_routing)
LaForge routing decision decides that packet is local
LaForge packet hits hook #2 (local_in)
LaForge packet hits local process
LaForge
LaForge If we look at a locally-originated packet:
LaForge packet is generated by local process at the bottom
LaForge packet hits hook #5 (local_out)
LaForge routing code decides where to route the packet
LaForge packet passes hook #4 (post_routing)
LaForge packet hits the wire of the network
LaForge (btw: i want to concentrate on the talk and handle questions after the talk, this way i can concentrate on the talk...)
LaForge (anyway, you can collect the questions at #qc, if you want)
LaForge Now we know how packets traverse the netfilter hooks
LaForge As I said, any kernel module may register on one or more of these hooks, and a callback-function is called for each packet passing this particular hook
LaForge the module may then return a verdict about the packet's future:
LaForge NF_ACCEPT = continue traversal as normal
LaForge NF_DROP = drop the packet silently, do not continue
LaForge NF_STOLEN = I (as the hook-registered module) have taken over the packet, do not continue
LaForge NF_QUEUE = enqueue packet to userspace (i'm going to say more about this later)
LaForge NF_REPEAT = please call this hook again
LaForge packet filtering / NAT / packet mangling is implemented using IP tables on each of these netfilter hooks.
LaForge IP TABLES:
LaForge IP tables are tables of rules, which a packet traverses from top to bottom
LaForge each rule in an IP table consists out of matches, which specify how the packet must look like, if it is to match this rule
LaForge and one target, which tells us what to do if this particular rule matches.
LaForge IP tables are implemented as reusable component - in fact, netfilter it self uses currently three instances of IP tables.
LaForge But any other kernel module may also use IP tables (for example as an IPsec SPDB)
LaForge The three tables implemented in netfilter itself are: 'filter', 'nat' and 'mangle'
LaForge Connectiontracking:
LaForge Connection tracking is another part, which is implemented on top of the netfileter hooks.
LaForge conntrack enables us to do stateful firewalling. That is: Decide upon the fate of a packet not only by data from this packet, but also by information about the state of the connection the packet belongs to.
LaForge i'm going to say more about connection tracking later.
LaForge First I want to talk about the three IP tables:
LaForge PART II - packet filtering
LaForge Packet filtering is implemented using the three hooks NF_IP_LOCAL_IN
LaForge NF_IP_FORWAD and NF_IP_LOCAL_OUT
LaForge each packet passes only one of these three hooks:
LaForge locally originated packets traverse only NF_IP_LOCAL_OUT
LaForge locally terminated packets traverse only NF_IP_LOCAL_IN
LaForge and forwarded packets traverse only NF_IP_FORWARD
LaForge the 'filter' table connects one chain to each of these three hooks:
LaForge NF_IP_LOCAL_IN = INPUT chain
LaForge NF_IP_LOCAL_OUT = OUTPUT chian
LaForge NF_IP_FORWARD = FORWARD chain
LaForge (the names are the same as in 2.2 - only uppercase)
LaForge but BE AWARE: the behaviour which packet traverses which chain has changed from the 2.2 behaviour
LaForge i.e. a forwarded packet only hits the FORWARD chain, _not_ INPUT and OUTPUT also
LaForge to know how we insert filtering rules in the chains of the 'filter' table, we have to examine the IP tables a bit further
LaForge As I said, the IP tables are implemented very generic, so there's one userspace tool, which is able to configure/modify all kindes of tables/chains
LaForge each rule in a chain consists out of
LaForge - match(es) which specify things like source address, destination address, port numbers, ...
LaForge - target (what to do if this rule matches)
LaForge To configure these rules, we have the tool called 'iptables'
LaForge I'm going to explain some of the iptables commands:
LaForge To fully specify an iptables command, we need the following information:
LaForge - which table to work on
LaForge - which chain in this table to use
LaForge - the operation (append, insert , delete, modify, )
LaForge - at least one match
LaForge - and exactly one target
LaForge the syntax is something like:
LaForge iptables - t table -Operation chain -j target match(es)
LaForge to give a very basic example:
LaForge iptables -t filter -A INPUT -j ACCEPT -p tcp --dport smtp
LaForge which -A(ppend)s a rule to the INPUT chain of the 'filter' table
LaForge and the rule itself ACCEPTs all tcp packets which have a destination port of 25 (smtp)
LaForge now we have to know what matches and targets we have available
LaForge as targets, we have :
LaForge ACCEPT - accept the packet
LaForge DROP - silently drop the packet (this is the 2.2 DENY)
LaForge QUEUE - queue the packet to an userspace process
LaForge RETURN - return to previous (calling) chain
LaForge foobar - jump to an userdefined chain
LaForge REJECT - drop the packet and inform the sender about it
LaForge LOG - log the packet via syslog, continue traversal
LaForge ULOG - send the packet to an userspace logging process
LaForge MIRROR - change source/destination IP and resend the packet (for testing purpose)
LaForge now the available matches:
LaForge -p protocol (tcp/udp/icmp/...)
LaForge -s source address
LaForge -d destination address
LaForge -i incoming interface
LaForge -o outgoing interface
LaForge --dport destination port
LaForge --sport source port
LaForge --state (NEW,ESTABLISHED,RELATED,INVALID) (i'm comming back to that)
LaForge --mac-source source MAC address
LaForge --mark nfmark value
LaForge --tos TOS value of the packet
LaForge --ttl ttl value of the packet
LaForge --limit (limit the rate of this packet to a certain amount of pkts/timeframe)
LaForge
LaForge knowing about the matches and targets, you are now able to configure your packet filter.
LaForge I'm coming back to the connection tracking stuff
LaForge this is a real advantage of the new 2.4 code:
LaForge stateful firewalling
LaForge the connection tracking code keeps track of all current connections going through our router/firewall
LaForge each packet is assigned one of the state values:
LaForge NEW (packet would establish a new connection, if we let it pass)
LaForge ESTABLISHED (packet is part of an already established connection)
LaForge RELATED (packet is somehow related to an already established connection)
LaForge INVALID (packet is multicast or something else whe really don't know what it is
LaForge so now we could do something like:
LaForge iptables -A FORWARD -j ACCEPT -m state --state ESTABLISHED,RELATED
LaForge which lets only all packets belonging to an already established connection and the related ones pass.
LaForge if we now block all NEW packets from the 'outer' interface (internet)
LaForge and allow NEW packets from the inside interface, we'll have the basic config of most firewalls
LaForge so how does this differ from blocking packets which have the SYN flag set?
LaForge connection tracking is generic and currently handles TCP, UDP and ICMP
LaForge so for example we don't accept icmp echo replies, if we didn't send an icmp echo request before
LaForge the connection tracking is extensible in two ways:
LaForge - application helper modules (like ip_conntrack_ftp, ip_conntrack_irc) for specific protocols
LaForge - protocol helper modules (for tracking the state of other protocols than tcp/udp/icmp)
LaForge the ip_contrack_ftp for example marks all incoming ftp data connections as RELATED
LaForge now we can do active ftp through a firewall which doesn't have to accept all connections to internal ip's with ports > 1024 anymore!
LaForge ok... time for the next parT:
LaForge PART III - NAT
LaForge in 2.2 we only had the masquerading code, which deals with a special case of NAT (network address translation)
LaForge in 2.4 we have all kinds of differnet nat:
LaForge SNAT (source address NAT), and MASQUERADE as a special case of that
LaForge DNAT (destination address NAT), and REDIRECT as a special case
LaForge source nat is done at the POST_ROUTING hook
LaForge destination nat is done at the PRE_ROUTING hook
LaForge i'll begin with a small example of SNAT:
LaForge iptables -t nat -A POSTROUTING -j SNAT --to-source 1.2.3.4 -o eth0
LaForge this will NAT all packets to be sent out on eth0 to the new source address of 1.2.3.4
LaForge (it of course does the inverse mapping for the reply packets)
LaForge SNAT is useful for NAT cases, where you have a statically assigned IP address.
LaForge If your outgoing interfaces has a dynamically assigned IP address, you may use the MASQUERADE target.
LaForge iptables -t nat -A POSTROUTING -j MASQUERADE -o ppp0
LaForge is an example for masqing all traffic on interface ppp0.
LaForge the address to which the packets are nat'ed is the interface address of ppp0
LaForge it's always the current address of ppp0, so IP address changes don't need any special handling.
LaForge The next part is DNAT:
LaForge small example:
LaForge iptables -t nat -A PREROUTING -j DNAT --to-destination 1.2.3.4:8080 -t tcp --dport 80 -i eth0
LaForge which NAT's all tcp packets, coming through interface eth0 and going to a webserver to 1.2.3.4:808
LaForge 8080 of coruse
LaForge this is quite useful if you want to do transparent www proxying
LaForge REDIRECT is a special case of DNAT:
LaForge iptables -t nat -A PREROUTING -j REDIRECT --to-port 3128 -i eth1 -p tcp --dport 80
LaForge all tcp packets from eth1 going to any webserver on port 80 are redirected to a proxy running on the local machine
LaForge PART IV - Packet mangling
LaForge this is something really new, which 2.2.x code didn't have at all
LaForge the 'mangle' table lets you mangle any arbitrary information inside the packets while they pass our local machine
LaForge currently we have only three targets implemented:
LaForge TOS - change the TOS bit field in the header
LaForge TTL - change the TTL field in the header (increment/decrement/set)
LaForge MARK - set the packet's skb->nfmark fielt to a particular value
LaForge of course you can again use all the matches available for packet filtering and nat.
LaForge a simple example:
LaForge iptables -t mangle -A PREROUITING -j MARK --set-mark 10 -p tcp --dport 80
LaForge which set's the nfmark field of each packet's skb to 10, if it is tcp and has a destination port of 9-
LaForge 80
LaForge all matches and targets are implemented as separate modules, so you can at any time write new match and/or target modules
LaForge There are two more 'advanced concepts' of netfilter, I want to introduce:
LaForge - Queuing
LaForge if you have a rule, which has the target QUEUE, the packet is inserted into a special queue inside netfilter
LaForge the packets in this queue are transmitted over a netlink socket to a userspace process.
LaForge this userspace process can now do whatever it wans with the packet (including its data) and re-inject it at exactly the place it came from
LaForge the process can (of course) also set the verdict of this packet (like: DROP this packet, ACCEPT the other one)
LaForge this enables people to write some firewalling code in userspace, and (hopefully) keeps the kernel clean from too complex code.
LaForge - Userspace logging
LaForge Very similar to queuing, although it is unidirectional
LaForge if you insert a rule with the ULOG target, the packet is copied and sent through a netlink multicast socket
LaForge one or more userspace processes may listen to this netlink multicast socket and receive the copy of the packet
LaForge the userspace process may now gather all information it needs and log it to a logfile/database/whatever
LaForge we've already implemented ulogd, which is a plugin-extensible logging daemon attaching to the ULOG target
LaForge So.... we are heading the end of my talk.... last chapter:
LaForge Current development and future:
LaForge - full TCP sequence number tracking
LaForge - port more matches/targets to IPv6
LaForge - support for more application protocol helpers for NAT (RPC, SMB, SNMP, ...)
LaForge - more matches (like 'accept all packets as long as the number of connections to this port doesn't raise about N)
LaForge - multicast support
LaForge - infrastructure for having conntrack and nat helpers in userspace
LaForge
LaForge At the end some useful links:
LaForge This presentation:
LaForge http://www.gnumonks.org/papers/netfilter-lk2000
LaForge netfilter homepage: http://netfilter.kernelnotes.org
LaForge links to the mailinglist(s) and the archives, as well as the iptables userspace tool are on the netfilter homepage
LaForge we also have a bunch of documents you might be interested in: The 2.4 packet filtering howto, the 2.4 NAT howto, the netfilter hacking howto, and some more stuff
LaForge everything should be linked from the netfilter homepage
Blu3 Thank you, it was very informative :)
LaForge Thanks for your interest in this talk... I'll deal with questions now
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last edited 2009-02-28 15:15:04 by narendramind