NG_PATCH(4) FreeBSD Kernel Interfaces Manual NG_PATCH(4)


ng_patchtrivial mbuf data modifying netgraph node type


#include < netgraph/ng_patch.h>


The patch node performs data modification of packets passing through it. Modifications are restricted to a subset of C language operations on unsigned integers of 8, 16, 32 or 64 bit size. These are: set to new value (=), addition (+=), subtraction (-=), multiplication (*=), division (/=), negation (= -), bitwise AND (&=), bitwise OR (|=), bitwise eXclusive OR (^=), shift left (<<=), shift right (>>=). A negation operation is the one exception: integer is treated as signed and second operand (the value) is not used. There may be several modification operations, they are all applied to a packet sequentially in order they were specified by user. Data payload of packet is viewed as array of bytes, with zero offset corresponding to the very first byte of packet headers, and length bytes beginning from offset are taken as a single integer in network byte order.


This node type has two hooks:
Packets received on this hook are modified according to rules specified in config and then forwarded to out hook, if it exists and connected. Otherwise they are reflected back to the in hook.
Packets received on this hook are forwarded to in hook without any changes.


This node type supports the generic control messages, plus the following:
This command sets the sequence of modify operations that will be applied to incoming data on a hook. The following struct ng_patch_config must be supplied as an argument:

struct ng_patch_op { 
 uint64_t value; 
 uint32_t offset; 
 uint16_t length; /* 1,2,4 or 8 bytes */ 
 uint16_t mode; 
/* Patching modes */ 
#define NG_PATCH_MODE_SET 1 
#define NG_PATCH_MODE_ADD 2 
#define NG_PATCH_MODE_SUB 3 
#define NG_PATCH_MODE_MUL 4 
#define NG_PATCH_MODE_DIV 5 
#define NG_PATCH_MODE_NEG 6 
#define NG_PATCH_MODE_AND 7 
#define NG_PATCH_MODE_OR 8 
#define NG_PATCH_MODE_XOR 9 
#define NG_PATCH_MODE_SHL 10 
#define NG_PATCH_MODE_SHR 11 
struct ng_patch_config { 
 uint32_t count; 
 uint32_t csum_flags; 
 struct ng_patch_op ops[]; 

The csum_flags can be set to any combination of CSUM_IP, CSUM_TCP, CSUM_SCTP and CSUM_UDP (other values are ignored) for instructing the IP stack to recalculate the corresponding checksum before transmitting packet on output interface. The ng_patch node does not do any checksum correction by itself.

This control message obtains current set of modify operations, returned as struct ng_patch_config.
Returns node statistics as a struct ng_patch_stats.
Clear node statistics.
This command is identical to NGM_PATCH_GET_STATS, except that the statistics are also atomically cleared.


This node shuts down upon receipt of a NGM_SHUTDOWN control message, or when all hooks have been disconnected.


The ng_patch node allows to modify TTL and TOS/DSCP fields in IP packets. Suppose you have two adjacent simplex links to remote network (e.g. satellite), so that the packets expiring in between will generate unwanted ICMP-replies which have to go forth, not back. Thus you need to raise TTL of every packet entering link by 2 to ensure the TTL will not reach zero there. So you setup ipfw(8) rule with netgraph action to inject packets going to other end of simplex link by the following ngctl(8) script:

/usr/sbin/ngctl -f- <<-SEQ 
 mkpeer ipfw: patch 200 in 
 name ipfw:200 ttl_add 
 msg ttl_add: setconfig { count=1 csum_flags=1 ops=[ \ 
  { mode=2 value=3 length=1 offset=8 } ] } 
/sbin/ipfw add 150 netgraph 200 ip from any to simplex.remote.net

Here “ ttl_add” node of type ng_patch configured to add (mode NG_PATCH_MODE_ADD) a value of 3 to a one-byte TTL field, which is 9th byte of IP packet header.

Another example would be two consecutive modifications of packet TOS field: say, you need to clear the IPTOS_THROUGHPUT bit and set the IPTOS_MINCOST bit. So you do:

/usr/sbin/ngctl -f- <<-SEQ 
 mkpeer ipfw: patch 300 in 
 name ipfw:300 tos_chg 
 msg tos_chg: setconfig { count=2 csum_flags=1 ops=[ \ 
  { mode=7 value=0xf7 length=1 offset=1 }  \ 
  { mode=8 value=0x02 length=1 offset=1 } ] } 
/sbin/ipfw add 160 netgraph 300 ip from any to any not dst-port 80

This first does NG_PATCH_MODE_AND clearing the fourth bit and then NG_PATCH_MODE_OR setting the third bit.

In both examples the csum_flags field indicates that IP checksum (but not TCP or UDP checksum) should be recalculated before transmit.

Note: one should ensure that packets are returned to ipfw after processing inside netgraph(4), by setting appropriate sysctl(8) variable:

sysctl net.inet.ip.fw.one_pass=0


The ng_patch node type was implemented in FreeBSD 8.1.


Maxim Ignatenko <gelraen.ua@gmail.com>. This manual page was written by Vadim Goncharov <vadimnuclight@tpu.ru>.


Node blindly tries to apply every patching operation to each packet (except those which offset if greater than length of the packet), so be sure that you supply only the right packets to it (e.g. changing bytes in the ARP packets meant to be in IP header could corrupt them and make your machine unreachable from the network).

!!! WARNING !!!

Output path of the IP stack assumes correct fields and lengths in the packets - changing them by mistake to incorrect values can cause unpredictable results including kernel panics.

March 5, 2012 FreeBSD