Restore deleted PANEL Ubuntu/Debian(Top or Bottom)

1. gconftool-2 – -shutdown

2. rm -rf ~/.gconf/apps/panel

3. pkill gnome-panel

Reboot

USBmon, HOW TO

* Introduction

The name “usbmon” in lowercase refers to a facility in kernel which is
used to collect traces of I/O on the USB bus. This function is analogous
to a packet socket used by network monitoring tools such as tcpdump(1)
or Ethereal. Similarly, it is expected that a tool such as usbdump or
USBMon (with uppercase letters) is used to examine raw traces produced
by usbmon.

The usbmon reports requests made by peripheral-specific drivers to Host
Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
usbmon may not correspond to bus transactions precisely. This is the same
situation as with tcpdump.

* How to use usbmon to collect raw text traces

Unlike the packet socket, usbmon has an interface which provides traces
in a text format. This is used for two purposes. First, it serves as a
common trace exchange format for tools while more sophisticated formats
are finalized. Second, humans can read it in case tools are not available.

To collect a raw text trace, execute following steps.

1. Prepare

Mount debugfs (it has to be enabled in your kernel configuration), and
load the usbmon module (if built as module). The second step is skipped
if usbmon is built into the kernel.

# mount -t debugfs none_debugs /sys/kernel/debug
# modprobe usbmon
#

Verify that bus sockets are present.

# ls /sys/kernel/debug/usb/usbmon
0s 0u 1s 1t 1u 2s 2t 2u 3s 3t 3u 4s 4t 4u
#

Now you can choose to either use the socket ‘0u’ (to capture packets on all
buses), and skip to step #3, or find the bus used by your device with step #2.
This allows to filter away annoying devices that talk continuously.

2. Find which bus connects to the desired device

Run “cat /proc/bus/usb/devices”, and find the T-line which corresponds to
the device. Usually you do it by looking for the vendor string. If you have
many similar devices, unplug one and compare two /proc/bus/usb/devices outputs.
The T-line will have a bus number. Example:

T: Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 0
D: Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
P: Vendor=0557 ProdID=2004 Rev= 1.00
S: Manufacturer=ATEN
S: Product=UC100KM V2.00

Bus=03 means it’s bus 3.

3. Start ‘cat’

# cat /sys/kernel/debug/usb/usbmon/3u > /tmp/1.mon.out

to listen on a single bus, otherwise, to listen on all buses, type:

# cat /sys/kernel/debug/usb/usbmon/0u > /tmp/1.mon.out

This process will be reading until killed. Naturally, the output can be
redirected to a desirable location. This is preferred, because it is going
to be quite long.

4. Perform the desired operation on the USB bus

This is where you do something that creates the traffic: plug in a flash key,
copy files, control a webcam, etc.

5. Kill cat

Usually it’s done with a keyboard interrupt (Control-C).

At this point the output file (/tmp/1.mon.out in this example) can be saved,
sent by e-mail, or inspected with a text editor. In the last case make sure
that the file size is not excessive for your favourite editor.

* Raw text data format

Two formats are supported currently: the original, or ‘1t’ format, and
the ‘1u’ format. The ‘1t’ format is deprecated in kernel 2.6.21. The ‘1u’
format adds a few fields, such as ISO frame descriptors, interval, etc.
It produces slightly longer lines, but otherwise is a perfect superset
of ‘1t’ format.

If it is desired to recognize one from the other in a program, look at the
“address” word (see below), where ‘1u’ format adds a bus number. If 2 colons
are present, it’s the ‘1t’ format, otherwise ‘1u’.

Any text format data consists of a stream of events, such as URB submission,
URB callback, submission error. Every event is a text line, which consists
of whitespace separated words. The number or position of words may depend
on the event type, but there is a set of words, common for all types.

Here is the list of words, from left to right:

– URB Tag. This is used to identify URBs, and is normally an in-kernel address
of the URB structure in hexadecimal, but can be a sequence number or any
other unique string, within reason.

– Timestamp in microseconds, a decimal number. The timestamp’s resolution
depends on available clock, and so it can be much worse than a microsecond
(if the implementation uses jiffies, for example).

– Event Type. This type refers to the format of the event, not URB type.
Available types are: S – submission, C – callback, E – submission error.

– “Address” word (formerly a “pipe”). It consists of four fields, separated by
colons: URB type and direction, Bus number, Device address, Endpoint number.
Type and direction are encoded with two bytes in the following manner:
Ci Co Control input and output
Zi Zo Isochronous input and output
Ii Io Interrupt input and output
Bi Bo Bulk input and output
Bus number, Device address, and Endpoint are decimal numbers, but they may
have leading zeros, for the sake of human readers.

– URB Status word. This is either a letter, or several numbers separated
by colons: URB status, interval, start frame, and error count. Unlike the
“address” word, all fields save the status are optional. Interval is printed
only for interrupt and isochronous URBs. Start frame is printed only for
isochronous URBs. Error count is printed only for isochronous callback
events.

The status field is a decimal number, sometimes negative, which represents
a “status” field of the URB. This field makes no sense for submissions, but
is present anyway to help scripts with parsing. When an error occurs, the
field contains the error code.

In case of a submission of a Control packet, this field contains a Setup Tag
instead of an group of numbers. It is easy to tell whether the Setup Tag is
present because it is never a number. Thus if scripts find a set of numbers
in this word, they proceed to read Data Length (except for isochronous URBs).
If they find something else, like a letter, they read the setup packet before
reading the Data Length or isochronous descriptors.

– Setup packet, if present, consists of 5 words: one of each for bmRequestType,
bRequest, wValue, wIndex, wLength, as specified by the USB Specification 2.0.
These words are safe to decode if Setup Tag was ‘s’. Otherwise, the setup
packet was present, but not captured, and the fields contain filler.

– Number of isochronous frame descriptors and descriptors themselves.
If an Isochronous transfer event has a set of descriptors, a total number
of them in an URB is printed first, then a word per descriptor, up to a
total of 5. The word consists of 3 colon-separated decimal numbers for
status, offset, and length respectively. For submissions, initial length
is reported. For callbacks, actual length is reported.

– Data Length. For submissions, this is the requested length. For callbacks,
this is the actual length.

– Data tag. The usbmon may not always capture data, even if length is nonzero.
The data words are present only if this tag is ‘=’.

– Data words follow, in big endian hexadecimal format. Notice that they are
not machine words, but really just a byte stream split into words to make
it easier to read. Thus, the last word may contain from one to four bytes.
The length of collected data is limited and can be less than the data length
report in Data Length word.

Here is an example of code to read the data stream in a well known programming
language:

class ParsedLine {
int data_len; /* Available length of data */
byte data[];

void parseData(StringTokenizer st) {
int availwords = st.countTokens();
data = new byte[availwords * 4];
data_len = 0;
while (st.hasMoreTokens()) {
String data_str = st.nextToken();
int len = data_str.length() / 2;
int i;
int b; // byte is signed, apparently?! XXX
for (i = 0; i < len; i++) {
// data[data_len] = Byte.parseByte(
// data_str.substring(i*2, i*2 + 2),
// 16);
b = Integer.parseInt(
data_str.substring(i*2, i*2 + 2),
16);
if (b >= 128)
b *= -1;
data[data_len] = (byte) b;
data_len++;
}
}
}
}

Examples:

An input control transfer to get a port status.

d5ea89a0 3575914555 S Ci:1:001:0 s a3 00 0000 0003 0004 4 <
d5ea89a0 3575914560 C Ci:1:001:0 0 4 = 01050000

An output bulk transfer to send a SCSI command 0x5E in a 31-byte Bulk wrapper
to a storage device at address 5:

dd65f0e8 4128379752 S Bo:1:005:2 -115 31 = 55534243 5e000000 00000000 00000600 00000000 00000000 00000000 000000
dd65f0e8 4128379808 C Bo:1:005:2 0 31 >

* Raw binary format and API

The overall architecture of the API is about the same as the one above,
only the events are delivered in binary format. Each event is sent in
the following structure (its name is made up, so that we can refer to it):

struct usbmon_packet {
u64 id; /* 0: URB ID – from submission to callback */
unsigned char type; /* 8: Same as text; extensible. */
unsigned char xfer_type; /* ISO (0), Intr, Control, Bulk (3) */
unsigned char epnum; /* Endpoint number and transfer direction */
unsigned char devnum; /* Device address */
u16 busnum; /* 12: Bus number */
char flag_setup; /* 14: Same as text */
char flag_data; /* 15: Same as text; Binary zero is OK. */
s64 ts_sec; /* 16: gettimeofday */
s32 ts_usec; /* 24: gettimeofday */
int status; /* 28: */
unsigned int length; /* 32: Length of data (submitted or actual) */
unsigned int len_cap; /* 36: Delivered length */
union { /* 40: */
unsigned char setup[SETUP_LEN]; /* Only for Control S-type */
struct iso_rec { /* Only for ISO */
int error_count;
int numdesc;
} iso;
} s;
int interval; /* 48: Only for Interrupt and ISO */
int start_frame; /* 52: For ISO */
unsigned int xfer_flags; /* 56: copy of URB’s transfer_flags */
unsigned int ndesc; /* 60: Actual number of ISO descriptors */
}; /* 64 total length */

These events can be received from a character device by reading with read(2),
with an ioctl(2), or by accessing the buffer with mmap. However, read(2)
only returns first 48 bytes for compatibility reasons.

The character device is usually called /dev/usbmonN, where N is the USB bus
number. Number zero (/dev/usbmon0) is special and means “all buses”.
Note that specific naming policy is set by your Linux distribution.

If you create /dev/usbmon0 by hand, make sure that it is owned by root
and has mode 0600. Otherwise, unpriviledged users will be able to snoop
keyboard traffic.

The following ioctl calls are available, with MON_IOC_MAGIC 0x92:

MON_IOCQ_URB_LEN, defined as _IO(MON_IOC_MAGIC, 1)

This call returns the length of data in the next event. Note that majority of
events contain no data, so if this call returns zero, it does not mean that
no events are available.

MON_IOCG_STATS, defined as _IOR(MON_IOC_MAGIC, 3, struct mon_bin_stats)

The argument is a pointer to the following structure:

struct mon_bin_stats {
u32 queued;
u32 dropped;
};

The member “queued” refers to the number of events currently queued in the
buffer (and not to the number of events processed since the last reset).

The member “dropped” is the number of events lost since the last call
to MON_IOCG_STATS.

MON_IOCT_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 4)

This call sets the buffer size. The argument is the size in bytes.
The size may be rounded down to the next chunk (or page). If the requested
size is out of [unspecified] bounds for this kernel, the call fails with
-EINVAL.

MON_IOCQ_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 5)

This call returns the current size of the buffer in bytes.

MON_IOCX_GET, defined as _IOW(MON_IOC_MAGIC, 6, struct mon_get_arg)
MON_IOCX_GETX, defined as _IOW(MON_IOC_MAGIC, 10, struct mon_get_arg)

These calls wait for events to arrive if none were in the kernel buffer,
then return the first event. The argument is a pointer to the following
structure:

struct mon_get_arg {
struct usbmon_packet *hdr;
void *data;
size_t alloc; /* Length of data (can be zero) */
};

Before the call, hdr, data, and alloc should be filled. Upon return, the area
pointed by hdr contains the next event structure, and the data buffer contains
the data, if any. The event is removed from the kernel buffer.

The MON_IOCX_GET copies 48 bytes to hdr area, MON_IOCX_GETX copies 64 bytes.

MON_IOCX_MFETCH, defined as _IOWR(MON_IOC_MAGIC, 7, struct mon_mfetch_arg)

This ioctl is primarily used when the application accesses the buffer
with mmap(2). Its argument is a pointer to the following structure:

struct mon_mfetch_arg {
uint32_t *offvec; /* Vector of events fetched */
uint32_t nfetch; /* Number of events to fetch (out: fetched) */
uint32_t nflush; /* Number of events to flush */
};

The ioctl operates in 3 stages.

First, it removes and discards up to nflush events from the kernel buffer.
The actual number of events discarded is returned in nflush.

Second, it waits for an event to be present in the buffer, unless the pseudo-
device is open with O_NONBLOCK.

Third, it extracts up to nfetch offsets into the mmap buffer, and stores
them into the offvec. The actual number of event offsets is stored into
the nfetch.

MON_IOCH_MFLUSH, defined as _IO(MON_IOC_MAGIC, 8)

This call removes a number of events from the kernel buffer. Its argument
is the number of events to remove. If the buffer contains fewer events
than requested, all events present are removed, and no error is reported.
This works when no events are available too.

FIONBIO

The ioctl FIONBIO may be implemented in the future, if there’s a need.
In addition to ioctl(2) and read(2), the special file of binary API can
be polled with select(2) and poll(2). But lseek(2) does not work.

* Memory-mapped access of the kernel buffer for the binary API

The basic idea is simple:

To prepare, map the buffer by getting the current size, then using mmap(2).
Then, execute a loop similar to the one written in pseudo-code below:

struct mon_mfetch_arg fetch;
struct usbmon_packet *hdr;
int nflush = 0;
for (;;) {
fetch.offvec = vec; // Has N 32-bit words
fetch.nfetch = N; // Or less than N
fetch.nflush = nflush;
ioctl(fd, MON_IOCX_MFETCH, &fetch); // Process errors, too
nflush = fetch.nfetch; // This many packets to flush when done
for (i = 0; i < nflush; i++) {
hdr = (struct ubsmon_packet *) &mmap_area[vec[i]];
if (hdr->type == ‘@’) // Filler packet
continue;
caddr_t data = &mmap_area[vec[i]] + 64;
process_packet(hdr, data);
}
}

Thus, the main idea is to execute only one ioctl per N events.

Although the buffer is circular, the returned headers and data do not cross
the end of the buffer, so the above pseudo-code does not need any gathering.

Refrence : http://www.kernel.org/

Music On Console

MP3 player favorit sejak dl. Untuk menjalankan :

[root@kopipait~#]mocp

What is MOC?

MOC (music on console) is a console audio player for LINUX/UNIX designed to be powerful and easy to use.

You just need to select a file from some directory using the menu similar to Midnight Commander, and MOC will start playing all files in this directory beginning from the chosen file. There is no need to create play lists like in other players.

If you want to combine some files from one or few directories on one play list, you can do this. The play list will be remembered between runs or you can save it as an m3u file to load it whenever you want.

Need the console where MOC is running for more important things? Need to close the X terminal emulator? You don’t have to stop playing – just press q and the interface will be detached leaving the server running. You can attach it later, or you can attach one interface in the console, and another in the X terminal emulator, no need to switch just to play another file.

MOC plays smoothly, regardless of system or I/O load because it uses the output buffer in a separate thread. It doesn’t cause gaps between files, because the next file to be played is precached while playing the current file.

Internet stream (Icecast, Shoutcast) are supported.

Key mapping can be fully customized.

Supported file formats are: mp3, Ogg Vorbis, FLAC, Musepack, Speex, WAVE, AIFF, AU (and other less popular formats supported by libsndfile. New formats support is under development.

Other features:

• Simple mixer.
• Color themes.
• Searching the menu (the play list or a directory) like M-s in Midnight Commander.
• The way MOC creates titles from tags is configurable.
• Optional character set conversion for file tags using iconv().
• OSS, JACK, and ALSA output.

http://moc.daper.net

Modem Smart EVDO AC2726

Pertama kali modem dicolok, ternyata dia dikenali degan USB-CD bukan modem. Berikut cara singkat agar modem tipe ini dapat bekerja :

– hasil lsusb :

Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 009: ID 19d2:fff1
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub

– Update kernel versi terbaru, versi 2.6.28-13 sudah bisa melakukan sudo modprobe usbserial

– install USB Modeswitch

– edit /etc/usbmodeswitch.conf

– tambahkan :

########################################################
# ZTE AC2726 (EVDO)
#
# Contributor: Wasim Baig
DefaultVendor=  0x19d2
DefaultProduct= 0xfff5
TargetVendor=   0x19d2
TargetProduct=  0xfff1
MessageContent="5553424312345678c00000008000069f010000000000000000000000000000"

Buat rule baru di /etc/udev/rules.d :

########################################################
# /etc/udev/rules.d/99-zte-ac2726.rules
#
# Smart ZTE AC2726 (EVDO)
#
SUBSYSTEM=="usb", SYSFS{idVendor}=="19d2", SYSFS{idProduct}=="fff5", RUN+="/usr/sbin/usb_modeswitch --default-vendor 0x19d2 --default-product 0xfff1 --message-content 5553424312345678c00000008000069f030000000000000000000000000000"

– cabut colok kembali USB,  lihat hasil lsusb apakah sudah berubah dari 19d2:fff5 ke 19d2:fff1

– selanjutnya #sudo modprobe usbserial vendor=0x19d2 product=0xfff1

Tinggal melakukan wvdial, pada wfdial.conf :

[Dialer smart]
Init1 = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
Stupid Mode = 1
Modem = /dev/ttyUSB0
Phone = #777
Idle Seconds = 300
Password = smart
Modem Type = USB Modem
Compuserve = 0
Baud = 921600
Auto DNS = 1
Dial Command = ATDT
Ask Password = 0
ISDN = 0
Username = smart

Thanks to : Heriman, yg sudah berkenan menjawab pertanyaan saya di  forum.linux.or.id

19d2:fff5 sudah berubah ke 19d2:fff1

IP Forwarding & (Source) NAT

Suatu Desa dan Seorang Anak

Ilustrasi Kampungnya

Inilah peta kecamatan OtotKwat yang terletak di salah satu distrik di Negeri Kayangan.

Pada Suatu Hari…

Hiduplah keluarga bahagia di Desa Suka Nyapu. Keluarga itu memiliki seorang anak yang masih balita. Mereka menjalani kehidupan seperti biasa, hingga pada suatu hari sang ibu sakit flu. Karena pileknya tak tertahankan dan bapak sedang nun jauh di sana bekerja, maka ibu terpaksa minta tolong pada anaknya untuk membeli obat di toko obat Bu Jamilah yang terletak di Desa Suka Ngelap, walaupun sang anak sebenarnya tidak tahu jalan. Yang diketahui oleh anaknya hanyalah kalau mau keluar dari Desa Suka Nyapu, jalannya ya hanya itu saja.

Ibu berpesan kepada anaknya: “Nak, tolong belikan obat UsirPhileg di toko Bu Jamilah. Naik sepeda saja, nanti kalau sudah ketemu Pak Hasan tanyakan jalan ke Desa Suka Ngelap.” Dan berangkatlah sang anak naik sepeda kecilnya. Klutak klutek klutik, akhirnya sang anak bertemu dengan Pak Hasan. Berkatalah sang anak kepada Pak Hasan: “Pak, saya mau ke toko obat Bu Jamilah di Desa Suka Ngelap. Saya lewat mana ya pak?” Dan Pak Hasan pun menjawab: “Belok kanan nak, nanti kamu akan sampai ke Desa Suka Ngelap.”

Sesampainya di Desa Suka Ngelap di toko Bu Jamilah, anak itu kemudian membeli obat, dan menuju kembali pulang. Bertanyalah sang anak kepada Bu Jamilah: “Bu, kembali ke Desa Suka Nyapu lewat mana?” Dan dia memperoleh jawaban: “Lewat jalan yang tadi saja Nak, kemudian nanti kalau sudah ketemu Pak Hasan tanyakan saja jalan menuju Desa Suka Nyapu.”

Klutak klutek klutik, bertemulah anak itu dengan Pak Hasan lagi. Kembali dia bertanya: “Pak, saya mau ke Desa Suka Nyapu lewat mana ya?” Jawaban yang diperoleh: “Belok kiri nak, nanti kau akan sampai kembali ke desamu.” Akhirnya, dengan arahan Pak Hasan, sampailah sang anak ke desanya dengan membawa obat flu untuk ibunya yang sedang sakit.

Pada Hari Yang Lain…

“Ibu, aku ingin mandi, tapi sabunnya habis.. hiks.. hiks..” Sang ibu pun berkata kepada anaknya: “Ya beli dong nak, di Desa Suka Mandi pasti ada yang jual sabun.” Tanpa basa-basi, sang anak langsung berangkat menuju Desa Suka Mandi. Dia tak merasa perlu bertanya kepada ibunya, karena toh jalan keluar dari desanya ya hanya itu, dan nanti dia bisa bertanya kepada Pak Hasan.

Sesampainya di tempat Pak Hasan, dia bertanya: “Pak, ke Desa Suka Mandi lewat mana ya?” Dan Pak Hasan pun menjawab: “Bapak tidak tahu, Nak, jadi kamu lurus saja dan nanti tanyakan ke Pak Amir.” Berangkatlah dia mengikuti petunjuk Pak Hasan. Tak lama kemudian, sampailah dia ke tempat Pak Amir. Lagi-lagi dia menanyakan pertanyaan yang sebelumnya telah dia tanyakan ke Pak Hasan.Tapi di luar dugaan, Pak Amir berkata: “Jangan ke sana, Nak, pulanglah. Di sana sedang ada kerusuhan!” Tanpa patah semangat, sang anak terus-menerus mendesak agar boleh meneruskan perjalanan ke Desa Suka Mandi. Karena ternyata Pak Amir tak bergeming, maka pulanglah sang anak dengan kecewa. Dasar balita ndak tau jalan, ketika bertemu Pak Hasan, bertanya lagi dia tentang jalan pulangnya. Berkat petunjuk Pak Hasan, akhirnya dia sampai ke desanya dan kembali ke rumah.

Selang beberapa hari, kembali sang anak melanjutkan usahanya menuju Desa Suka Mandi. Dengan melalui jalan yang sama seperti beberapa hari sebelumnya, sampailah anak itu ke tempat Pak Amir. Karena keadaan sudah aman, berkatalah Pak Amir kepada anak itu: “Jalan terus saja nak, nanti bertanyalah kepada Pak Badri.” Dengan riang gembira, berjalanlah dia sesuai dengan arahan Pak Amir. Sesampainya di tempat Pak Badri, kembali dia menanyakan jalan ke Desa Suka Mandi. Dijawab oleh Pak Badri: “Belok Kanan, ikuti jalan!” Cihuyyy, dia berjalan dan akhirnya sampailah ke toko sabun itu.

Karena mendapati bahwa pembelinya balita, bertanyalah si penjual: “Rumahmu di mana, Nak?” Sang anak menjawab: “Rumah saya ada di Desa Suka Nyapu.” Penjualnya kemudian berkata: “Aku tak tahu di mana itu, Nak. Berangkatlah ke Pak Badri, kemudian tanyakan jalan menuju arah pulangmu.” Sambil tersenyum, berangkatlah anak itu. Sesampainya di tempat Pak Badri, dia menanyakan jalan yang mana yang menuju ke arah desanya. Dia memberitahukan bahwa desanya adalah Desa Suka Nyapu. Di luar dugaan, Pak Badri menjawab: “Aduh Nak, aku tak tahu ke mana arah pulangmu. Aku cuma tahu jalan ke Desa Suka Mandi, Desa Suka Cuci, Desa Suka Ngepel, dan Desa Suka Setrika. Ke 2 desa yang terakhir melewati Pos Pak Amir. Selebihnya aku tak tahu.” Menangislah sang anak, dan karena dia tak bisa pulang, maka dia menjalani sisa hidupnya sampai mati di Pos Pak Badri.

Sayang sekali hidupnya harus berakhir seperti itu. Kejadian itu tidak perlu terjadi andaikata sebelumnya Pak Amir berkata: “Nak, sepulangmu nanti katakan kepada Pak Badri bahwa kau berasal dari Pos Pak Amir,” karena Pak Badri memang tahu ke mana jalan yang harus ditempuh kalau anak itu akan kembali menuju Pos Pak Amir, tapi selebihnya tidak. Setelah sampai di tempat Pak Amir, anak itu bisa diteruskan menuju tujuan yang sesungguhnya, yaitu rumahnya di Desa Suka Nyapu.

Computer Network Version of Previous Story

Dalam dunia jaringan, padanannya kira-kira begini:

• Anak balita yang berjalan-jalan adalah IP Packet.
• Pak Hasan, Pak Amir, dan Pak Badri pada perangkat jaringan berperan sebagai ROUTER.
• Desa-desa di kecamatan OtotKwat pada jaringan merupakan suatu Network Segment.
• Warna biru menunjukkan proses routing. Tentu saja, pada proses ini IP Forwarding aktif.
• Warna hijau menunjukkan kondisi di mana router tidak melakukan IP Forwarding. Dalam dunia Linux, kondisi ini berlaku ketika /proc/sys/net/ipv4/ip_forward bernilai 0.
• Warna merah merupakan ilustrasi peristiwa timeout.
• Warna oranye merupakan ilustrasi peristiwa destination unreachable.
• Warna hijau aneh merupakan ilustrasi peristiwa source NAT.

Skema Jaringan

Pada skema di bawah ini, kotak kuning adalah internet, sedangkan warna hijau (atau biru? terserah) muda adalah jaringan suatu gedung.

Cara Kerja

Menurut cerita sebelumnya, dari Desa Suka Nyapu sang anak menuju ke Desa Suka Ngelap. Pada versi ini, berarti ada paket dari network 10.1.21.0/24 ke network 222.111.212.248/29. Kalau memperhatikan cerita sebelumnya, tidak ada operasi Source NAT di situ. Kok bisa sampai? Padahal asalnya adalah IP Privat, dan tujuannya adalah IP Publik! Tak masalah. Berapa pun IPnya, asal router punya info tentang itu ya ndak masalah. Yang penting adalah router punya info untuk paket itu untuk kembali. Kasus yang persis sama berlaku untuk cerita kalau ada paket dari 172.16.92.16/28 ke 222.111.212.248/29, atau untuk cerita 10.1.21.0/24 ke 202.212.222.128/28.

Pada cerita sebelumnya pun tergambarkan bahwa paket dari 10.1.21.0/24 ke 9.10.11.0/23 tanpa operasi NAT ya bisa sampai kok. Perkaranya adalah, siapa pun yang dihubungi di 9.10.11.0/23 tidak dapat mengirim jawaban kembali ke 10.1.21.0/24. Kenapa? Karena Pak Badri tidak tahu di mana itu desa 10.1.21.0/24. Kenapa ndak tau? Karena tidak ada dalam routing table-nya. Alasan lain yang sedikit lebih masuk akal: Silakan baca RFC1918 yang menyatakan bahwa 10.0.0.0/8, 172.16.0.0/12, dan 192.168.0.0/16 tidak boleh routeable di internet. Kenapa angkanya itu? Ya suka-suka yang bikin keputusan. Tugas kita adalah menikmati keputusan, bukan ikut memikirkan keputusannya.

Sebentar, bukannya paket ndak bisa nyampe karena IP privat diblok? Pada dasarnya tidak. Ngapain diblok? Tidak diblok pun tetap tidak bisa balik lha ndak ada info route-nya. Kalau mau diblok ya boleh-boleh saja, itung-itung ngurangi traffic yang ndak perlu.

Nah setelah ada operasi S-NAT pada paket yang menuju ke 9.10.11.0/23, maka harusnya paket bisa balik ke tujuan, karena Pak Badri akan melihat bahwa paket itu berasal dari jaringan Pak Amir. Setelah dikembalikan ke Pak Amir, maka Pak Amir bisa mengembalikan paket data tersebut ke jaringan asalnya. Kalau tidak di-NAT bagaimana? Ya sesuai cerita di atas: Tewas.

Kesimpulan

IP Forwarding merupakan peristiwa di mana router meneruskan paket yang diterima pada suatu interface ke interface yang lain sesuai tujuan paket.

Source NAT merupakan peristiwa di mana router mengubah paket, menggantikan SourceAddress paket tersebut menjadi suatu alamat tertentu.

Bisakah IP Forwarding bekerja tanpa S-NAT? BISA. Bisakah S-NAT bekerja tanpa IP Forwarding? Bisa aja sih, tinggal set iptables kalo anda pake Linux, atau perintah NAT lain yang sesuai untuk router anda. Tapi ya ngapain???

Credit goes to : KAMAS MUHAMMAD <kamas@lc.vlsm.org>