RE: TCP checksum?

[Shipman, Jeffrey E]

Thanks John, that function works great. However, I'm still having problems
where I'm not getting any responses from the destination. I've been setting
the destination address to the source address (this is just to play around
and learn things), and it's not working properly. If I connect straight to
the address (192.168.1.20), it works great. However, if I connect to
192.168.1.1, it looks like in the packet it's using 192.168.1.20, but all
I get no SYN/ACK back...my machine just keeps on sending out SYNs. Does
anyone know of things I need to also check in order to make sure this will
work. When I finish my module, it will be modifying all sorts of fields
inside of the TCP packet and I'd like to be sure that I can get responses
from remote machines.

Thanks.

Jeff Shipman - CCD 
Sandia National Laboratories 
[phone] / MS-1372 

-----Original Message-----
From: John McBride [mailto:[e-mail]]
Sent: Thursday, June 13, 2002 11:19 AM
To: Shipman, Jeffrey E
Subject: RE: TCP checksum?

Jeffrey, 
find snippet I used to update checksum in my netfilter module - 
partly nicked from nat core code 
/***************************************************************************
*/ 
static void update_checksum(struct sk_buff *skb) 
{ 
        struct tcphdr *th; 
        struct iphdr * iph; 
        int len; 
        iph = skb->nh.iph; 
        th = (struct tcphdr *)((u_int32_t *)iph + iph->ihl); 
        len = skb->len; 
        /* IP checksum */ 
        iph->check = 0; 
        iph->check = ip_fast_csum((u8 *)iph, iph->ihl); 
        
        if (iph->protocol != IPPROTO_TCP) 
                return; 
        
        th->check = 0; 
        th->check = tcp_v4_check(th, 
                                 len - 4*iph->ihl, 
                                 iph->saddr, iph->daddr, 
                                 csum_partial((char *)th, len-4*iph->ihl,
0)); 
} 
/***************************************************************************
*/ 

-----Original Message----- 
From: Shipman, Jeffrey E [mailto:[e-mail]] 
Sent: Thursday, June 13, 2002 4:11 PM 
To: '[e-mail]' 
Subject: TCP checksum? 

I'm looking for a function similar to skb_checksum(), but 
for the tcphdr->check field. I'm playing around with a module 
I've written for netfilter and I would like to modify options of 
the IP and TCP headers. For example, right now I'm trying 
to set the destination IP to the source IP, but the TCP checksum 
is coming out incorrectly. How can I calculate this checksum? 
Thanks a lot in advance. Also, if anyone knows where some 
documentation about the TCP/IP stack in the kernel are, please 
let me know. 
Jeff Shipman - CCD 
Sandia National Laboratories 
[phone] / MS-1372 

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