TheTAZZone - Internet Chaos

Ettercap Part II

TUTORIAL BY JAYMILL230 FOR THETAZZONE/TAZFORUM

TAZ Forum :: A Computer, Gaming, and Social Network Community of Friends :: TAZForum :: View topic – Tutorial- Ettercap Part II

In the first part of this tutorial, we looked at the basic features of ettercap, from general network sniffing to Man in the Middle attacks. However, we only scratched the surface. In part two we will look at the more advanced features, including ettercap’s built in plugins, an example of a few filters, along with one or two others, and how to defend yourself against ettercap and other similar programs.

Plug-ins

Ettercap comes bundled with several very useful plugins. I’m not going to go over them all, but I will show you several examples, and you should be able to apply these to the others. The ones we will cover are;

* arp_cop
* chk_poison
* find_ettercap
* finger
* search_promic

first lets look at the plug-in selection screen, we can get there by going to Plugins–> Manage the Plugins, or by pressing ctrl-p

Screen-Shot #1

Now, as you can see, we have the name of each of each of the plug-ins, their version, and a brief description of what each one of them do. For a complete list, and a better description of each, check here (warning: PDF file). The first of the plugins that we are going to cover is “arp_cop.” So we will follow these steps, we are just going to select out device (for me it would be ath0 or eth0 (or the name of your device on windows). Now I am going to press ctrl-p and bring up the “manage plugins” menu. Find arp_cop and double click it. In the info box on the bottom you should see that it will say

arp_cop: plugin running…

Now we can go to Start–>Sniffing. Now ettercap will begin to monitor the arp packets and the arp tables that are going through the network, however without any kind of poisoning. If ettercap finds that all the sudden one NIC is sending out arp packets consistent with arp poisoning, then it will put an alert into the info box giving you that NIC’s IP address, and MAC address. It will not necessarily mean that someone is doing something malicious, however it will point out suspicious activity.

The next plugin is the one that I use the most, chk_poison. This plugin doesn’t require much explanation, you go through the steps in part 1 to begin arp poisoning. Start your sniffing. Now go to the plugins menu, and open up chk_poison. It will tell you if the arp poisoning was successful and you are now in the middle sniffing all the traffic. Again, it will be telling you all of this in the info box (sensing a pattern?).

Screen-Shot #2

The number 3 plugin we are are going to look at is find ettercap plugin. Treat it the same way as arp_cop, and it will go about doing exactly what it sounds like, finding etterap on a network. It does this by a couple of methods, 1st, acting like arp_cop (arp_cop is more general though), and second is sniffing the types of packets, and the headers on those packets, to find out if they are the same that ettercap would use.

Finger is not always reliable, and kind of a pain in the ass, if you need to fingerprint a host, I would suggest using nmap. If you are unfamiliar with this program, I would HIGHLY recommend becoming familiar with it. Nokia writes an excellent tutorial (in 5 parts, covers alot) that you can find here; http://tazforum.thetazzone.com/viewtopic.php?t=684.

GW_discover will scan the hosts that you picked up when you scanned, and try to determine which IP address is the gateway if it has been hidden in one way or another. This is great if you are looking for the router to attack.

Search_promisc is a great plugin for finding any computer sniffing the network in promiscuous mode, i.e., it will find wireshark/tcpdump/ethereal/almost any other while they are not running in silent mode. Usuaully this is not the default mode, so you should be all set.

This should pretty much sum up which each of the plugins that I have found most useful, but many others are useful as well, read the documentation, play around with it, and if you can find a good use for it, post it, and I’ll enter it into the tutorial (giving you credit of course). Next we get to look at the part of the tutorial that allows us to do just about anything Smile

Filters

Filters in ettercap are, to put it simply, fairly amazing. They allow you to do the simple, just look at TCP/UDP connections, to changing what somebody sees on the webpage. This can be used in a humorous or malicious way. In the example I will show you, we will be able to change all the images on a web page into something else, Lets go and take a look shall we?

First lets take a look at a simple ettercap filter;

if (ip.proto == TCP && tcp.dst == 80) {
if (search(DATA.data, “Accept-Encoding”)) {
replace(“Accept-Encoding”, “Accept-Rubbish!”);
# note: replacement string is same length as original string
msg(“zapped Accept-Encoding!\n”);
}
}
if (ip.proto == TCP && tcp.src == 80) {
replace(“img src=”, “img src=\”http://www.irongeek.com/images/jollypwn.png\” “);
replace(“IMG src=”, “img src=\”http://www.irongeek.com/images/jollypwn.png\” “);
msg(“Filter Ran.\n”);
}

**credit to IronGeek for the example!**

If you have coded before, you will be able to make pretty short work of the code, and if you haven’t coded before, this may not be the place to start. But lets quickly walk through it;

* line-1 ) if its a TCP transmission on port 80 do the following;
* line 2 ) Look inside of the transmission, and accept the encoding (go figure)
* line 3 ) replace accept encoding to accept-rubbish (to be honest not quite sure about this line, perhaps someone could clarify it for me?
* line 4 ) a note by the programmer
* line 5) put a message in the info box on ettercap that says “zapped accept-encoding”
* line 7 ) same as line 1
* line 8 ) replace string one with string 2
* line 9 ) same thing
* line 10 ) place another message in the info box

This code basically changes all the <img> tags in a webpage to a string of the script authors choice in this case, its a jolly roger, but really it could be anything.

Now lets take a look at all of the commands that we have to our disposal while writing a filter;

SCRIPTS SYNTAX
A script is a compound of instructions. It is executed sequentially and you can make branches with the ‘if’ statements. ‘if’ and ‘if/else’ statements are the only supported. No loops are implemented. The syntax is almost like C code except that you have to put ‘if’ blocks into graph parentheses ‘{‘ ‘}’, even if they contain only one instruction. NOTE: you have to put a space between the ‘if’ and the ‘(‘. You must not put the space between the function name and the ‘(‘. Example:
if (conditions) { }
func(args…);
The conditions for an ‘if’ statement can be either functions or comparisons. Two or more conditions can be linked together with logical operators like OR ‘||’ and AND ‘&&’. Example:
if (tcp.src == 21 && search(DATA.data, “ettercap”)) {
} Pay attention to the operator precedence. You cannot use parentheses to group conditions, so be careful with the order. An AND at the beginning of a conditions block will exclude all the other tests if it is evaluated as false. The parsing is left-to-right, when an operator is found: if it is an AND and the previous condition is false, all the statement is evaluated as false; if it is an OR the parsing goes on even if the condition is false. Example:
if (ip.proto == UDP || ip.proto == TCP && tcp.src == 80) {
} if (ip.proto == TCP && tcp.src == 80 || ip.proto == UDP) {
} the former condition will match all udp or http traffic. The latter is wrong, because if the packet is not tcp, the whole condition block will be evaluated as false. If you want to make complex conditions, the best way is to split them into nested ‘if’ blocks.
Every instruction in a block must end with a semicolon ‘;’. Comparisons are implemented with the ‘==’ operator and can be used to compare numbers, strings or ip addresses. An ip address MUST be enclosed within two single quotes (eg. ‘192.168.0.7’). You can also use the ‘less than’ (‘<‘), ‘greater than’ (‘>’), ‘less or equal’ (‘<=’) and ‘greater or equal’ (‘>=’) operators. The lvalue of a comparison must be an offset (see later) Example:
if (DATA.data + 20 == “ettercap” && ip.ttl > 16) {
} Assignments are implemented with the ‘=’ operator and the lvalue can be an offset (see later). The rvalue can be a string, an integer or a hexadecimal value. Example:
ip.ttl = 0xff;
DATA.data + 7 = “ettercap NG”; You can also use the ‘inc’ and ‘dec’ operations on the packet fields. The operators used are ‘+=’ and ‘-=’. The rvalue can be an integer or a hexadecimal value. Example:
ip.ttl += 5;

OFFSET DEFINITION
An offset is identified by a virtual pointer. In short words, an offset is a pointer to the packet buffer. The virtual pointer is a tuple <L, O, S>, where L is the iso/osi level, O is the offset in that level and S is the size of the virtual pointer. You can make algebraic operations on a virtual pointer and the result is still an offset. Specifying ‘vp + n’ will result in a new virtual pointer <L, O+n, S>. And this is perfectly legal, we have changed the internal offset of that level. Virtual pointers are in the form ‘name.field.subfield’. For example ‘ip.ttl’ is the virtual pointer for the Time To Live field in the IP header of a packet. It will be translated as <L=3, O=9, S=1>. Indeed it is the 9th byte of level 3 and its size is 1 byte. ‘ip.ttl + 1’ is the same as ‘ip.proto’ since the 10th byte of the IP header is the protocol encapsulated in the IP packet. The list of all supported virtual pointers is in the file etterfilter.tbl. You can add your own virtual pointers by adding a new table or modifying the existing ones. Refer to the comments at the beginning of the file for the syntax of etterfilter.tbl file.

SCRIPTS FUNCTIONS

search(where, what)
this function searches the string ‘what’ in the buffer ‘where’. The buffer can be either DATA.data or DECODED.data. The former is the payload at layer DATA (ontop TCP or UDP) as it is transmitted on the wire, the latter is the payload decoded/decrypted by dissectors.
So, if you want to search in an SSH connection, it is better to use ‘DECODED.data’ since ‘data’ will be encrypted.
The string ‘what’ can be binary. You have to escape it. example:
search(DATA.data, “\x41\x42\x43”)

regex(where, regex)
this function will return true if the ‘regex’ has matched the buffer ‘where’. The considerations about ‘DECODED.data’ and ‘DATA.data’ mentioned for the function ‘search’ are the same for the regex function. NOTE: regex can be used only against a string buffer. example:
regex(DECODED.data, “.*login.*”)

pcre_regex(where, pcre_regex … )
this function will evaluate a perl compatible regular expression. You can match against both DATA and DECODED, but if your expression modifies the buffer, it makes sense to operate only on DATA. The function accepts 2 or 3 parameters depending on the operation you want. The two parameter form is used only to match a pattern. The three parameter form means that you want to make a substitution. In both cases, the second parameter is the search string.
You can use $n in the replacement string. These placeholders are referred to the groups created in the search string. (e.g. pcre_regex(DATA.data, “^var1=([:digit:]*)&var2=([:digit:]*)”, “var1=$2&var2=$1”) will swap the value of var1 and var2).
NOTE: The pcre support is optional in ettercap and will be enabled only if you have the libpcre installed. The compiler will warn you if you try to compile a filter that contains pcre expressions but you don’t have libpcre. Use the -w option to suppress the warning. example:
pcre_regex(DATA.data, “.*foo$”)
pcre_regex(DATA.data, “([^ ]*) bar ([^ ]*)”, “foo $1 $2”)

replace(what, with)
this function replaces the string ‘what’ with the string ‘with’. They can be binary string and must be escaped. The replacement is always performed in DATA.data since is the only payload which gets forwarded. The ‘DECODED.data’ buffer is used only internally and never reaches the wire. example:
replace(“ethercap”, “ettercap”)

inject(what)
this function injects the content of the file ‘what’ after the packet being processed. It always injects in DATA.data. You can use it to replace the entire packet with a fake one using the drop() function right before the inject() command. In that case the filtering engine will drop the current packet and inject the fake one. example:
inject(“./fake_packet”)

log(what, where)
this function dumps in the file ‘where’ the buffer ‘what’. No information is stored about the packet, only the payload is dumped. So you will see the stream in the file. If you want to log packets in a more enhanced mode, you need to use the ettercap -L option and analyze it with etterlog ( 8 ) .
The file ‘where’ must be writable to the user EC_UID (see etter.conf(5) ). example:
log(DECODED.data, “/tmp/interesting.log”)

msg(message)
this function displays a message to the user in the User Messages window. It is useful to let the user know whether a particular filter has been successful or not. example:
msg(“Packet filtered successfully”)

drop()
this function marks the packet “to be dropped”. The packet will not be forwarded to the real destination. example:
drop()

kill()
this function kills the connection that owns the matched packet. If it is a TCP connection, a RST is sent to both sides of the connection. If it is an UDP connection, an ICMP PORT UNREACHABLE is sent to the source of the packet. example:
kill()

exec(command)
this function executes a shell command. You have to provide the full path to the command since it is executed without any environment. There is no way to determine if the command was successful or not. Furthermore, it is executed asynchronously since it is forked by the main process. example:
exec(“/bin/cat /tmp/foo >> /tmp/bar”)

exit()
this function causes the filter engine to stop executing the code. It is useful to stop the execution of the script on some circumstance checked by an ‘if’ statement. example:
exit()

Using these commands, we can manipulate just about any packet that comes through us destined for just about anywhere. If you write some interesting scrips, let me know, I’ll make sure to give you credit and post them with this tutorial!

**screenshot links not up yet, more examples of filters to come, stay tuned**

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