Libipq: payload modification

[MiniComa]

Hello,

I use iptables to put UDP packets into QUEUE. Then I take them,modify payload and want to put them back in QUEUE. Do I need to recalculate UDP checksum? If yes, could any of you give me a link of C code with UDP checksum?

Also should I use NF_QUEUE as a verdict?

status = ipq_set_verdict(h, m->packet_id,NF_QUEUE,m->data_len,BUFSIZE);

MC

[nini09]

uint16_t udp_checksum ( const void * buff,
size_t len,
in_addr_t src_addr,
in_addr_t dest_addr
)


Calculate the UDP checksum (calculated with the whole packet).

Parameters:
buff The UDP packet.
len The UDP packet length.
src_addr The IP source address (in network format).
dest_addr The IP destination address (in network format).

Returns:
The result of the checksum.

Definition at line 29 of file udp.c.

00030 {
00031 const uint16_t *buf=buff;
00032 uint16_t *ip_src=(void *)&src_addr, *ip_dst=(void *)&dest_addr;
00033 uint32_t sum;
00034 size_t length=len;
00035
00036 // Calculate the sum //
00037 sum = 0;
00038 while (len > 1)
00039 {
00040 sum += *buf++;
00041 if (sum & 0x80000000)
00042 sum = (sum & 0xFFFF) + (sum >> 16);
00043 len -= 2;
00044 }
00045
00046 if ( len & 1 )
00047 // Add the padding if the packet lenght is odd //
00048 sum += *((uint8_t *)buf);
00049
00050 // Add the pseudo-header //
00051 sum += *(ip_src++);
00052 sum += *ip_src;
00053
00054 sum += *(ip_dst++);
00055 sum += *ip_dst;
00056
00057 sum += htons(IPPROTO_UDP);
00058 sum += htons(length);
00059
00060 // Add the carries //
00061 while (sum >> 16)
00062 sum = (sum & 0xFFFF) + (sum >> 16);
00063
00064 // Return the one's complement of sum //
00065 return ( (uint16_t)(~sum) );
00066 }