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PostPosted: Thu Mar 31, 2016 1:31 pm 
Total Newbie

Joined: Thu Mar 31, 2016 1:22 pm
Posts: 1
I installed an iptables modules to match Modbus protocol, compilation runed well, I integreat the .ko module with commande insmod, and now when I type iptables -m modbus -h, I have the help for the protocol, so evry thing is good until now, but when I tried to execute an iptables filtre, it don't work:

aa@ubuntu:~$ sudo iptables -A INPUT -p tcp -m modbus --unitid  11
iptables: Invalid argument. Run `dmesg' for more information.
aa@ubuntu:~$ dmesg
[ 3692.909462] ip_tables: modbus match: invalid size 0 != 40

I think it's a problem in the match function of my code, I checked, but I couldn't figure the problem, I just verify the tcp header of my packet in the MATCH:

Triggers when a packet comes in matching the registeres

static int match(const struct sk_buff *skb,
      const struct net_device *in,
      const struct net_device *out,
      const void *matchinfo,int offset,
      const void *hdr, u_int16_t datalen,
      int *hotdrop)

  const struct iphdr *iph;
  const struct tcphdr *tcph;
  u_int8_t tcplen;

  /* Examine the TCP header, which is 32 bytes after the IP
     header.  "hdr" points to just after IP header */
  const struct modbus_tcp *modbus;
  const struct ipt_modbus *modbusinfo = matchinfo;
  const struct modbus_data *data;

  iph = ip_hdr(skb);

  tcph = (void *)iph + iph->ihl*4;

  /* TCP header length caluculation*/
  tcplen = tcph->doff*4;

  /* Match our structure to the data part */
  modbus = hdr+tcplen;

  /* If length is less then the total of IP and TCP header, that
     should be part of three way handshake .. allow it */
  if (ntohs(iph->tot_len) == 20+tcplen) {
    if(modbusinfo->allow_tcp == 1)
      return 1;
      return 0;


  /* Return the "OR"s of all the parameters given.  If any
     of the given parameters is true, the whole thing is true */       

      return (func_code_check(modbusinfo->funccode_flags,(modbus->modbus_d).func_code, modbusinfo->func_code[0],modbusinfo->func_code[1], modbusinfo->invflags_funccode) || unitid_check(modbusinfo->unitid_flags,(modbus->modbus_d).unit_id, modbusinfo->unit_id,modbusinfo->invflags_unitid) || refnum_check(modbusinfo->refnum_flags,(modbus->modbus_d).ref_num, modbusinfo->ref_num,modbusinfo->invflags_refnum) || length_check(modbusinfo->length_flags,(modbus->modbus_h).length, modbusinfo->length[0],modbusinfo->length[1], modbusinfo->invflags_length));


Any idea what may be the problems, i use iptables version v1.4.7 , and kernel 2.6.32-21-generic
this is a link to a version of the code on wich i have start modifing in case if you want make a look at the original code : https://github.com/stevenylai/ralink_sdk/blob/5d3729a93e1f722c658d29606347c35816cd0ec8/source/linux-2.4.x/net/ipv4/netfilter/ipt_modbus.c

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