iptables/extensions/libxt_u32.man

U32 tests whether quantities of up to 4 bytes extracted from a packet have
specified values. The specification of what to extract is general enough to
find data at given offsets from tcp headers or payloads.
.TP
[\fB!\fP] \fB\-\-u32\fP \fItests\fP
The argument amounts to a program in a small language described below.
.IP
tests := location "=" value | tests "&&" location "=" value
.IP
value := range | value "," range
.IP
range := number | number ":" number
.PP
a single number, \fIn\fP, is interpreted the same as \fIn:n\fP. \fIn:m\fP is
interpreted as the range of numbers \fB>=n\fP and \fB<=m\fP.
.IP "" 4
location := number | location operator number
.IP "" 4
operator := "&" | "<<" | ">>" | "@"
.PP
The operators \fB&\fP, \fB<<\fP, \fB>>\fP and \fB&&\fP mean the same as in C.
The \fB=\fP is really a set membership operator and the value syntax describes
a set. The \fB@\fP operator is what allows moving to the next header and is
described further below.
.PP
There are currently some artificial implementation limits on the size of the
tests:
.IP "    *"
no more than 10 of "\fB=\fP" (and 9 "\fB&&\fP"s) in the u32 argument
.IP "    *"
no more than 10 ranges (and 9 commas) per value
.IP "    *"
no more than 10 numbers (and 9 operators) per location
.PP
To describe the meaning of location, imagine the following machine that
interprets it. There are three registers:
.IP
A is of type \fBchar *\fP, initially the address of the IP header
.IP
B and C are unsigned 32 bit integers, initially zero
.PP
The instructions are:
.IP
number B = number;
.IP
C = (*(A+B)<<24) + (*(A+B+1)<<16) + (*(A+B+2)<<8) + *(A+B+3)
.IP
&number C = C & number
.IP
<< number C = C << number
.IP
>> number C = C >> number
.IP
@number A = A + C; then do the instruction number
.PP
Any access of memory outside [skb\->data,skb\->end] causes the match to fail.
Otherwise the result of the computation is the final value of C.
.PP
Whitespace is allowed but not required in the tests. However, the characters
that do occur there are likely to require shell quoting, so it is a good idea
to enclose the arguments in quotes.
.PP
Example:
.IP
match IP packets with total length >= 256
.IP
The IP header contains a total length field in bytes 2-3.
.IP
\-\-u32 "\fB0 & 0xFFFF = 0x100:0xFFFF\fP"
.IP
read bytes 0-3
.IP
AND that with 0xFFFF (giving bytes 2-3), and test whether that is in the range
[0x100:0xFFFF]
.PP
Example: (more realistic, hence more complicated)
.IP
match ICMP packets with icmp type 0
.IP
First test that it is an ICMP packet, true iff byte 9 (protocol) = 1
.IP
\-\-u32 "\fB6 & 0xFF = 1 &&\fP ...
.IP
read bytes 6-9, use \fB&\fP to throw away bytes 6-8 and compare the result to
1. Next test that it is not a fragment. (If so, it might be part of such a
packet but we cannot always tell.) N.B.: This test is generally needed if you
want to match anything beyond the IP header. The last 6 bits of byte 6 and all
of byte 7 are 0 iff this is a complete packet (not a fragment). Alternatively,
you can allow first fragments by only testing the last 5 bits of byte 6.
.IP
 ... \fB4 & 0x3FFF = 0 &&\fP ...
.IP
Last test: the first byte past the IP header (the type) is 0. This is where we
have to use the @syntax. The length of the IP header (IHL) in 32 bit words is
stored in the right half of byte 0 of the IP header itself.
.IP
 ... \fB0 >> 22 & 0x3C @ 0 >> 24 = 0\fP"
.IP
The first 0 means read bytes 0-3, \fB>>22\fP means shift that 22 bits to the
right. Shifting 24 bits would give the first byte, so only 22 bits is four
times that plus a few more bits. \fB&3C\fP then eliminates the two extra bits
on the right and the first four bits of the first byte. For instance, if IHL=5,
then the IP header is 20 (4 x 5) bytes long. In this case, bytes 0-1 are (in
binary) xxxx0101 yyzzzzzz, \fB>>22\fP gives the 10 bit value xxxx0101yy and
\fB&3C\fP gives 010100. \fB@\fP means to use this number as a new offset into
the packet, and read four bytes starting from there. This is the first 4 bytes
of the ICMP payload, of which byte 0 is the ICMP type. Therefore, we simply
shift the value 24 to the right to throw out all but the first byte and compare
the result with 0.
.PP
Example:
.IP
TCP payload bytes 8-12 is any of 1, 2, 5 or 8
.IP
First we test that the packet is a tcp packet (similar to ICMP).
.IP
\-\-u32 "\fB6 & 0xFF = 6 &&\fP ...
.IP
Next, test that it is not a fragment (same as above).
.IP
 ... \fB0 >> 22 & 0x3C @ 12 >> 26 & 0x3C @ 8 = 1,2,5,8\fP"
.IP
\fB0>>22&3C\fP as above computes the number of bytes in the IP header. \fB@\fP
makes this the new offset into the packet, which is the start of the TCP
header. The length of the TCP header (again in 32 bit words) is the left half
of byte 12 of the TCP header. The \fB12>>26&3C\fP computes this length in bytes
(similar to the IP header before). "@" makes this the new offset, which is the
start of the TCP payload. Finally, 8 reads bytes 8-12 of the payload and
\fB=\fP checks whether the result is any of 1, 2, 5 or 8.