Provided by: ettercap-common_0.8.3.1-12_amd64 
NAME
etterfilter - Filter compiler for ettercap content filtering engine
SYNOPSIS
etterfilter [OPTIONS] FILE
DESCRIPTION
The etterfilter utility is used to compile source filter files into binary filter files
that can be interpreted by the JIT interpreter in the ettercap(8) filter engine. You have
to compile your filter scripts in order to use them in ettercap. All syntax/parse errors
will be checked at compile time, so you will be sure to produce a correct binary filter
for ettercap.
GENERAL OPTIONS
-o, --output <FILE>
you can specify the output file for a source filter file. By default the output is
filter.ef.
-t, --test <FILE>
you can analyze a compiled filter file with this option. etterfilter will print in
a human readable form all the instructions contained in it. It is a sort of
"disassembler" for binary filter files.
-d, --debug
prints some debug messages during the compilation. Use it more than once to
increase the debug level ( etterfilter -ddd ... ).
-w, --suppress-warnings
Don't exit on warnings. With this option the compiler will compile the script even
if it contains warnings.
STANDARD OPTIONS
-v, --version
Print the version and exit.
-h, --help
prints the help screen with a short summary of the available options.
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.
Since etterfilter support both IP address families, you should care whether you use
'ip.proto' which is specific for the IPv4 address family or it's IPv6 couterpart
'ipv6.nh'. Especially for the L4 protocol matching using 'ip.proto' and/or
'ipv6.nh', you should be careful if you're really acting on the right protocol.
This should be enforced using the L3 protocol identifier 'eth.proto'.
Example:
if (eth.proto == IP && ip.proto == TCP && tcp.dst == 80 || tcp.src == 80) {
}
if (eth.proto == IP6 && ipv6.nh == TCP && tcp.dst == 80 || tcp.src == 80) {
}
if (tcp.dst == 80 || tcp.src == 80) {
}
The first example correctly matches http traffic only on IPv4 while the second
would match http traffic only on IPv6. The thrid example matches http regardless
it's IP address familiy.
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' or '2001:db8::2'). 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;
More examples can be found in the etter.filter.examples file.
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. Note that since etterfilter also
supports processing of IPv6, the above mentioned only applies for IPv4 packets
while counterpart in IPv6 would be 'ipv6.nh'.
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")
execinject(command)
this function operates similar to the inject function except that it uses the
output of a shell command to inject data rather than the contents of a file. 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 execinject() command. In that
case the filtering engine will drop the current packet and inject the fake one.
example:
execinject("/bin/cat /tmp/foo")
execreplace(command)
this function operates similar to the replace function except that it uses the
output of a shell command to replace entire data rather than the contents of a
file. An other difference, is that orinal packet content is pass to the shell
command in stdin. It always injects in DATA.data. You can use it to replace the
entire packet with a fake one depending on the original one.
example:
execreplace("tr A-Z a-z")
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()
EXAMPLES
Here are some examples of using etterfilter.
etterfilter filter.ecf -o filter.ef
Compiles the source filter.ecf into a binary filter.ef
ORIGINAL AUTHORS
Alberto Ornaghi (ALoR) <alor@users.sf.net>
Marco Valleri (NaGA) <naga@antifork.org>
PROJECT STEWARDS
Emilio Escobar (exfil) <eescobar@gmail.com>
Eric Milam (Brav0Hax) <jbrav.hax@gmail.com>
OFFICIAL DEVELOPERS
Mike Ryan (justfalter) <falter@gmail.com>
Gianfranco Costamagna (LocutusOfBorg) <costamagnagianfranco@yahoo.it>
Antonio Collarino (sniper) <anto.collarino@gmail.com>
Ryan Linn <sussuro@happypacket.net>
Jacob Baines <baines.jacob@gmail.com>
CONTRIBUTORS
Dhiru Kholia (kholia) <dhiru@openwall.com>
Alexander Koeppe (koeppea) <format_c@online.de>
Martin Bos (PureHate) <purehate@backtrack.com>
Enrique Sanchez
Gisle Vanem <giva@bgnett.no>
Johannes Bauer <JohannesBauer@gmx.de>
Daten (Bryan Schneiders) <daten@dnetc.org>
SEE ALSO
etter.filter.examples
ettercap(8) etterlog(8) etter.conf(5) ettercap_curses(8) ettercap_plugins(8)
ettercap-pkexec(8)