获取MTU值

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SINT32 rutWan_getInterfaceMTU(char *ifname)
{
struct ifreq ifr;
int sockfd, err;

   if( (sockfd = socket(PF_INET, SOCK_DGRAM, 0)) <= 0 )
   {
      return -1;
   }

   strncpy(ifr.ifr_name, ifname, IFNAMSIZ);
   err = ioctl(sockfd, SIOCGIFMTU, (void*)&ifr);
   close(sockfd);

   if (err == -1)
   {
      return -1;
   }
   return ifr.ifr_mtu;
}

// 如何设置TCP MSS

void SetTCPMSSRules(const char *ifName)
{
   char cmd[BUFLEN_128];
   int mtu = 0;

   if( (mtu = rutWan_getInterfaceMTU(ifName)) == -1 ) // 可以为MTU值
   {
      cmsLog_error("cannot get MTU information");
      return;
   }
   /* setup TCP MSS option manipulation */
   snprintf(cmd, sizeof(cmd),
      "iptables -I FORWARD -o %s -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss %d", ifName, (mtu-40));
   rut_doSystemAction("rut", cmd);
   snprintf(cmd, sizeof(cmd),
      "iptables -I FORWARD -i %s -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss %d", ifName, (mtu-40));
   rut_doSystemAction("rut", cmd);

}