Netfilter Hooks

Netfilter is a subsystem in the Linux 2.4 kernel. Netfilter provides an generic and abstract interface to the standard routing code. This is currently used in Linux kernel for packet filtering, mangling, NAT(network address translation) and queuing packets to the userspace.I have been using netfilters in my final year project(in VoIP security) as a packet capture mechanism.Netfilter makes connection tracking possible through the use of various hooks in the kernel’s network code.

These hooks are places that kernel code, either statically built or in the form of a loadable module, can register functions to be called for specific network events. An example of such an event is the reception of a packet.

Linux , since 2.4,  supports hooks for IPv4 and IPv6. Netfilter defines five hooks for IPv4. The declaration of the symbols for these can be found in “linux/netfilter_ipv4.h”. These hooks are displayed in the table below:

Hook	Called at
NF_IP_PRE_ROUTING	After sanity checks, before routing decisions.
NF_IP_LOCAL_IN	After routing decisions if packet is for this host.
NF_IP_FORWARD	If the packet is destined for another interface.
NF_IP_LOCAL_OUT	Packets coming from local processes on their way out.
NF_IP_POST_ROUTING	Just before outbound packets “hit the wire”.

The NF_IP_PRE_ROUTING hook, called as the first hook after a packet has been received, has been used for packet capture in this project. The following diagrams diagrammatically represents where exactly does each of these hooks operate at.

At any of these hooks we can define Callback functions which act as handler routines. These functions are invoked when the corresponding network event occurs. Now the fate of the packet is determined by the return code of the hook function. Various return codes available are:

Return Code	Meaning
NF_DROP	Discard the packet.
NF_ACCEPT	Keep the packet.
NF_STOLEN	Forget about the packet.
NF_QUEUE	Queue packet for userspace.
NF_REPEAT	Call this hook function again.

The NF_DROP return code means that this packet should be dropped completely and any resources allocated for it should be released. NF_ACCEPT tells Netfilter that so far the packet is still acceptable and that it should move to the next stage of the network stack. NF_STOLEN is an interesting one because it tells Netfilter to “forget” about the packet. What this tells Netfilter is that the hook function will take processing of this packet from here and that Netfilter should drop all processing of it. This does not mean, however, that resources for the packet are released. The packet and it’s respective sk_buff structure are still valid, it’s just that the hook function has taken ownership of the packet away from Netfilter. Unfortunately I’m not exactly clear on what NF_QUEUE really does so for now I won’t discuss it. The last return value, NF_REPEAT requests that Netfilter calls the hook function again. Obviously one must be careful using NF_REPEAT so as to avoid an endless loop.

I will try to add more details on the same with sample codes in my future posts.

PS: The post contains adopted contents. Content was actually prepared for the project documentation purpose.

ENOMEM error

ENOMEM is an OS error code , as defined in kern/include/kern/errno.h ,which is returned due to insufficient

memory. The name ENOMEM stands for Error NO MEMory. Its one of the error codes returned by the fork() call

which means no more storage space available.In connection with sockets they are raised when there isn’t enough

resources available to create a socket. The value of the error code is 12.

And also I have read that from the point of view of an application, ENOMEM is pretty diffiult to be handled constructively. One way to deal with it is to fail instantly and to release all the allocated resources as soon as possible,
avoiding operations requiring allocating new resources [Refer.].

Right now I am having this error when I try to send a large number of UDP packets to a receiver. It happens at the receiver end at a point and then there is a steep increase in the packet loss. Now I am trying to fix it and once its done I will update this post. And if someone can help me with the same, please do it by leaving a comment. And thanx in advance..

Linux network buffers

1.Introduction

I borrowed this document from Mr.Harald Welte (laforge@gnumonks.org) and since I found it very useful I
just thought of sharing this with you all..and more importantly i can always keep it as a note for myself.
So please dont sue me for copyright issues..ok..

2.skbuff

skbuffs are the buffers in which the linux kernel handles network packets. The packet is received by the network card, put into a skbuff and then passed to the network stack, which uses the skbuff all the time.

2.1 struct sk_buff

The struct sk_buff is defined in <linux/skbuff.h> as follows:

next

next buffer in list
prev

previous buffer in list
list

list we are on
sk

socket we belong to
stamp

timeval we arrived at
dev

device we are leaving by
rx_dev

device we arrived at
h

transport layer header (tcp,udp,icmp,igmp,spx,raw)
nh

network layer header (ip,ipv6,arp,ipx,raw)
mac

link layer header
dst

FIXME:
cb

control buffer, used internally
len

length of actual data
csum

checksum
used

FIXME: data moved to user and not MSG_PEEK
is_clone

we are a clone
cloned

head may be cloned
pkt_type

packet class
ip_summed

driver fed us ip checksum
priority

packet queuing priority
users

user count
protocol

packet protocol from driver
security

security level of packet
truesize

real size of the buffer
head

pointer to head of buffer
data

data head pointer
tail

tail pointer
end

end pointer
destructor

destructor function

nfmark

netfilter mark
nfcache

netfilter internal caching info
nfct

associated connection, if any

tc_index

traffic control index

2.2 skb support functions

There are a bunch of skb support functions provided by the sk_buff layer. I briefly describe the most important ones in this section.

allocation / free / copy / clone and expansion functions

struct sk_buff *alloc_skb(unsigned int size, int gfp_mask)

This function allocates a new skb. This is provided by the skb layer to initialize some privat data and do memory statistics. The returned buffer has no headroom and a tailroom of /size/ bytes.

void kfree_skb(struct sk_buff *skb)

Decrement the skb’s usage count by one and free the skb if no references left.

struct sk_buff *skb_get(struct sk_buff *skb)

Increments the skb’s usage count by one and returns a pointer to it.

struct sk_buff *skb_clone(struct sk_buff *skb, int gfp_mask)

This function clones a skb. Both copies share the packet data but have their own struct sk_buff. The new copy is not owned by any socket, reference count is 1.

struct sk_buff *skb_copy(const struct sk_buff *skb, int gfp_mask)

Makes a real copy of the skb, including packet data. This is needed, if You wish to modify the packet data. Reference count of the new skb is 1.

struct skb_copy_expand(const struct sk_buff *skb, int new_headroom, int new_tailroom, int gfp_mask)

Make a copy of the skb, including packet data. Additionally the new skb has a haedroom of /new_headroom/ bytes size and a tailroom of /new_tailroom/ bytes.

anciliary functions

int skb_cloned(struct sk_buff *skb)

Is the skb a clone?

int skb_shared(struct sk_Buff *skb)

Is this skb shared? (is the reference count > 1)?

operations on lists of skb’s

struct sk_buff *skb_peek(struct sk_buff_head *list_)

peek a skb from front of the list; does not remove skb from the list

struct sk_buff *skb_peek_tail(struct sk_buff_head *list_)

peek a skb from tail of the list; does not remove sk from the list

__u32 skb_queue_len(sk_buff_head *list_)

return the length of the given skb list

void skb_queue_head(struct sk_buff_head *list_, struct sk_buff *newsk)

enqueue a skb at the head of a given list

void skb_queue_tail(struct sk_buff_head *list_, struct sk_buff *newsk)

enqueue a skb at the end of a given list.

struct sk_buff *skb_dequeue(struct sk_buff_head *list_)

dequeue a skb from the head of the given list.

struct sk_buff *sbk_dequeue_tail(struct sk_buff_head *list_)

dequeue a skb from the tail of the given list

operations on skb data

unsigned char *skb_put(struct sk_buff *sbk, int len)

extends the data area of the skb. if the total size exceeds the size of the skb, the kernel will panic. A pointer to the first byte of new data is returned.

unsigned char *skb_push(struct sk_buff *skb, int len)

extends the data area of the skb. if the total size exceeds the size of the skb, the kernel will panic. A pointer to the first byte of new data is returned.

unsigned char *skb_pull(struct sk_buff *skb, int len)

remove data from the start of a buffer, returning the bytes to headroom. A pointr to the next data in the buffer is returned.

int skb_headroom(struct sk_buff *skb)

return the amount of bytes of free space at the head of skb

int skb_tailroom(struct sk_buff *skb)

return the amount of bytes of free space at the end of skb

struct sk_buff *skb_cow(struct sk_buff *skb, int headroom)

if the buffer passed lacks sufficient headroom or is a clone it is copied and additional headroom made available.