pkt_filter_lpf.cc

Go to the documentation of this file.

// Copyright (C) 2013-2020 Internet Systems Consortium, Inc. ("ISC")
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

#include <config.h>
#include <dhcp/dhcp4.h>
#include <dhcp/iface_mgr.h>
#include <dhcp/pkt4.h>
#include <dhcp/pkt_filter_lpf.h>
#include <dhcp/protocol_util.h>
#include <exceptions/exceptions.h>
#include <fcntl.h>
#include <net/ethernet.h>
#include <linux/filter.h>
#include <linux/if_ether.h>
#include <linux/if_packet.h>

namespace {

using namespace isc::dhcp;

struct sock_filter dhcp_sock_filter [] = {
    // Make sure this is an IP packet: check the half-word (two bytes)
    // at offset 12 in the packet (the Ethernet packet type).  If it
    // is, advance to the next instruction.  If not, advance 11
    // instructions (which takes execution to the last instruction in
    // the sequence: "drop it").
    // #0
    BPF_STMT(BPF_LD + BPF_H + BPF_ABS, ETHERNET_PACKET_TYPE_OFFSET),
    // #1
    BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, ETHERTYPE_IP, 0, 11),

    // Make sure it's a UDP packet.  The IP protocol is at offset
    // 9 in the IP header so, adding the Ethernet packet header size
    // of 14 bytes gives an absolute byte offset in the packet of 23.
    // #2
    BPF_STMT(BPF_LD + BPF_B + BPF_ABS,
             ETHERNET_HEADER_LEN + IP_PROTO_TYPE_OFFSET),
    // #3
    BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 9),

    // Make sure this isn't a fragment by checking that the fragment
    // offset field in the IP header is zero.  This field is the
    // least-significant 13 bits in the bytes at offsets 6 and 7 in
    // the IP header, so the half-word at offset 20 (6 + size of
    // Ethernet header) is loaded and an appropriate mask applied.
    // #4
    BPF_STMT(BPF_LD + BPF_H + BPF_ABS, ETHERNET_HEADER_LEN + IP_FLAGS_OFFSET),
    // #5
    BPF_JUMP(BPF_JMP + BPF_JSET + BPF_K, 0x1fff, 7, 0),

    // Check the packet's destination address. The program will only
    // allow the packets sent to the broadcast address or unicast
    // to the specific address on the interface. By default, this
    // address is set to 0 and must be set to the specific value
    // when the raw socket is created and the program is attached
    // to it. The caller must assign the address to the
    // prog.bf_insns[8].k in the network byte order.
    // #6
    BPF_STMT(BPF_LD + BPF_W + BPF_ABS,
             ETHERNET_HEADER_LEN + IP_DEST_ADDR_OFFSET),
    // If this is a broadcast address, skip the next check.
    // #7
    BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0xffffffff, 1, 0),
    // If this is not broadcast address, compare it with the unicast
    // address specified for the interface.
    // #8
    BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, 0x00000000, 0, 4),

    // Get the IP header length.  This is achieved by the following
    // (special) instruction that, given the offset of the start
    // of the IP header (offset 14) loads the IP header length.
    // #9
    BPF_STMT(BPF_LDX + BPF_B + BPF_MSH, ETHERNET_HEADER_LEN),

    // Make sure it's to the right port.  The following instruction
    // adds the previously extracted IP header length to the given
    // offset to locate the correct byte.  The given offset of 16
    // comprises the length of the Ethernet header (14) plus the offset
    // of the UDP destination port (2) within the UDP header.
    // #10
    BPF_STMT(BPF_LD + BPF_H + BPF_IND, ETHERNET_HEADER_LEN + UDP_DEST_PORT),
    // The following instruction tests against the default DHCP server port,
    // but the action port is actually set in PktFilterBPF::openSocket().
    // N.B. The code in that method assumes that this instruction is at
    // offset 11 in the program.  If this is changed, openSocket() must be
    // updated.
    // #11
    BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, DHCP4_SERVER_PORT, 0, 1),

    // If we passed all the tests, ask for the whole packet.
    // #12
    BPF_STMT(BPF_RET + BPF_K, (u_int)-1),

    // Otherwise, drop it.
    // #13
    BPF_STMT(BPF_RET + BPF_K, 0),
};

}

using namespace isc::util;

namespace isc {
namespace dhcp {

SocketInfo
PktFilterLPF::openSocket(Iface& iface,
                         const isc::asiolink::IOAddress& addr,
                         const uint16_t port, const bool,
                         const bool) {

    // Open fallback socket first. If it fails, it will give us an indication
    // that there is another service (perhaps DHCP server) running.
    // The function will throw an exception and effectively cease opening
    // raw socket below.
    int fallback = openFallbackSocket(addr, port);

    // The fallback is open, so we are good to open primary socket.
    int sock = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
    if (sock < 0) {
        close(fallback);
        isc_throw(SocketConfigError, "Failed to create raw LPF socket");
    }

    // Set the close-on-exec flag.
    if (fcntl(sock, F_SETFD, FD_CLOEXEC) < 0) {
        close(sock);
        close(fallback);
        isc_throw(SocketConfigError, "Failed to set close-on-exec flag"
                  << " on the socket " << sock);
    }

    // Create socket filter program. This program will only allow incoming UDP
    // traffic which arrives on the specific (DHCP) port). It will also filter
    // out all fragmented packets.
    struct sock_fprog filter_program;
    memset(&filter_program, 0, sizeof(filter_program));

    filter_program.filter = dhcp_sock_filter;
    filter_program.len = sizeof(dhcp_sock_filter) / sizeof(struct sock_filter);

    // Configure the filter program to receive unicast packets sent to the
    // specified address. The program will also allow packets sent to the
    // 255.255.255.255 broadcast address.
    dhcp_sock_filter[8].k = addr.toUint32();

    // Override the default port value.
    dhcp_sock_filter[11].k = port;
    // Apply the filter.
    if (setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &filter_program,
                   sizeof(filter_program)) < 0) {
        close(sock);
        close(fallback);
        isc_throw(SocketConfigError, "Failed to install packet filtering program"
                  << " on the socket " << sock);
    }

    struct sockaddr_ll sa;
    memset(&sa, 0, sizeof(sockaddr_ll));
    sa.sll_family = AF_PACKET;
    sa.sll_ifindex = iface.getIndex();

    // For raw sockets we construct IP headers on our own, so we don't bind
    // socket to IP address but to the interface. We will later use the
    // Linux Packet Filtering to filter out these packets that we are
    // interested in.
    if (bind(sock, reinterpret_cast<const struct sockaddr*>(&sa),
             sizeof(sa)) < 0) {
        close(sock);
        close(fallback);
        isc_throw(SocketConfigError, "Failed to bind LPF socket '" << sock
                  << "' to interface '" << iface.getName() << "'");
    }

    // Set socket to non-blocking mode.
    if (fcntl(sock, F_SETFL, O_NONBLOCK) != 0) {
        // Get the error message immediately after the bind because the
        // invocation to close() below would override the errno.
        char* errmsg = strerror(errno);
        close(sock);
        close(fallback);
        isc_throw(SocketConfigError, "failed to set SO_NONBLOCK option on the"
                  " LPF socket '" << sock << "' to interface '"
                  << iface.getName() << "'" << ", reason: " << errmsg);
    }

    return (SocketInfo(addr, port, sock, fallback));

}

Pkt4Ptr
PktFilterLPF::receive(Iface& iface, const SocketInfo& socket_info) {
    uint8_t raw_buf[IfaceMgr::RCVBUFSIZE];
    // First let's get some data from the fallback socket. The data will be
    // discarded but we don't want the socket buffer to bloat. We get the
    // packets from the socket in loop but most of the time the loop will
    // end after receiving one packet. The call to recv returns immediately
    // when there is no data left on the socket because the socket is
    // non-blocking.
    // @todo In the normal conditions, both the primary socket and the fallback
    // socket are in sync as they are set to receive packets on the same
    // address and port. The reception of packets on the fallback socket
    // shouldn't cause significant lags in packet reception. If we find in the
    // future that it does, the sort of threshold could be set for the maximum
    // bytes received on the fallback socket in a single round. Further
    // optimizations would include an asynchronous read from the fallback socket
    // when the DHCP server is idle.
    int datalen;
    do {
        datalen = recv(socket_info.fallbackfd_, raw_buf, sizeof(raw_buf), 0);
    } while (datalen > 0);

    // Now that we finished getting data from the fallback socket, we
    // have to get the data from the raw socket too.
    int data_len = read(socket_info.sockfd_, raw_buf, sizeof(raw_buf));
    // If negative value is returned by read(), it indicates that an
    // error occurred. If returned value is 0, no data was read from the
    // socket. In both cases something has gone wrong, because we expect
    // that a chunk of data is there. We signal the lack of data by
    // returning an empty packet.
    if (data_len <= 0) {
        return Pkt4Ptr();
    }

    InputBuffer buf(raw_buf, data_len);

    // @todo: This is awkward way to solve the chicken and egg problem
    // whereby we don't know the offset where DHCP data start in the
    // received buffer when we create the packet object. In general case,
    // the IP header has variable length. The information about its length
    // is stored in one of its fields. Therefore, we have to decode the
    // packet to get the offset of the DHCP data. The dummy object is
    // created so as we can pass it to the functions which decode IP stack
    // and find actual offset of the DHCP data.
    // Once we find the offset we can create another Pkt4 object from
    // the reminder of the input buffer and set the IP addresses and
    // ports from the dummy packet. We should consider doing it
    // in some more elegant way.
    Pkt4Ptr dummy_pkt = Pkt4Ptr(new Pkt4(DHCPDISCOVER, 0));

    // Decode ethernet, ip and udp headers.
    decodeEthernetHeader(buf, dummy_pkt);
    decodeIpUdpHeader(buf, dummy_pkt);

    // Read the DHCP data.
    std::vector<uint8_t> dhcp_buf;
    buf.readVector(dhcp_buf, buf.getLength() - buf.getPosition());

    // Decode DHCP data into the Pkt4 object.
    Pkt4Ptr pkt = Pkt4Ptr(new Pkt4(&dhcp_buf[0], dhcp_buf.size()));

    // Set the appropriate packet members using data collected from
    // the decoded headers.
    pkt->setIndex(iface.getIndex());
    pkt->setIface(iface.getName());
    pkt->setLocalAddr(dummy_pkt->getLocalAddr());
    pkt->setRemoteAddr(dummy_pkt->getRemoteAddr());
    pkt->setLocalPort(dummy_pkt->getLocalPort());
    pkt->setRemotePort(dummy_pkt->getRemotePort());
    pkt->setLocalHWAddr(dummy_pkt->getLocalHWAddr());
    pkt->setRemoteHWAddr(dummy_pkt->getRemoteHWAddr());

    return (pkt);
}

int
PktFilterLPF::send(const Iface& iface, uint16_t sockfd, const Pkt4Ptr& pkt) {

    OutputBuffer buf(14);

    // Some interfaces may have no HW address - e.g. loopback interface.
    // For these interfaces the HW address length is 0. If this is the case,
    // then we will rely on the functions which construct the IP/UDP headers
    // to provide a default HW addres. Otherwise, create the HW address
    // object using the HW address of the interface.
    if (iface.getMacLen() > 0) {
        HWAddrPtr hwaddr(new HWAddr(iface.getMac(), iface.getMacLen(),
                                    iface.getHWType()));
        pkt->setLocalHWAddr(hwaddr);
    }


    // Ethernet frame header.
    // Note that we don't validate whether HW addresses in 'pkt'
    // are valid because they are checked by the function called.
    writeEthernetHeader(pkt, buf);

    // IP and UDP header
    writeIpUdpHeader(pkt, buf);

    // DHCPv4 message
    buf.writeData(pkt->getBuffer().getData(), pkt->getBuffer().getLength());

    sockaddr_ll sa;
    memset(&sa, 0x0, sizeof(sa));
    sa.sll_family = AF_PACKET;
    sa.sll_ifindex = iface.getIndex();
    sa.sll_protocol = htons(ETH_P_IP);
    sa.sll_halen = 6;

    int result = sendto(sockfd, buf.getData(), buf.getLength(), 0,
                        reinterpret_cast<const struct sockaddr*>(&sa),
                        sizeof(sockaddr_ll));
    if (result < 0) {
        isc_throw(SocketWriteError, "failed to send DHCPv4 packet, errno="
                  << errno << " (check errno.h)");
    }

    return (0);

}


} // end of isc::dhcp namespace
} // end of isc namespace