ptb.hpp

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//    OpenVPN -- An application to securely tunnel IP networks
//               over a single port, with support for SSL/TLS-based
//               session authentication and key exchange,
//               packet encryption, packet authentication, and
//               packet compression.
//
//    Copyright (C) 2012- OpenVPN Inc.
//
//    SPDX-License-Identifier: MPL-2.0 OR AGPL-3.0-only WITH openvpn3-openssl-exception
//
 
// Generates ICMP "packet too big" response
 
#pragma once
 
#include <openvpn/common/socktypes.hpp>
#include <openvpn/ip/csum.hpp>
#include <openvpn/ip/ip4.hpp>
#include <openvpn/ip/ip6.hpp>
#include <openvpn/ip/icmp4.hpp>
#include <openvpn/ip/icmp6.hpp>
#include <openvpn/ip/ping6.hpp>
#include <openvpn/ip/ipcommon.hpp>
#include <openvpn/buffer/buffer.hpp>
 
namespace openvpn {
class Ptb
{
  public:
    static void generate_icmp_ptb(BufferAllocated &buf, std::uint16_t nexthop_mtu)
    {
        if (buf.empty())
            return;
 
        switch (IPCommon::version(buf[0]))
        {
        case IPCommon::IPv4:
            if (buf.length() <= sizeof(struct IPv4Header))
                break;
 
            generate_icmp4_ptb(buf, nexthop_mtu);
            break;
 
        case IPCommon::IPv6:
            if (buf.length() <= sizeof(struct IPv6Header))
                break;
 
            generate_icmp6_ptb(buf, nexthop_mtu);
            break;
        }
    }
 
  private:
    static void generate_icmp6_ptb(BufferAllocated &buf, std::uint16_t nexthop_mtu)
    {
        // ICMPv6 data includes original IPv6 header and as many bytes of payload as possible
        auto data_size = std::min(buf.length(), std::max(sizeof(ICMPv6), static_cast<size_t>(nexthop_mtu) - sizeof(ICMPv6)));
 
        // sanity check
        // we use headroom for adding IPv6 + ICMPv6 headers
        if ((buf.offset() < sizeof(ICMPv6)) || (buf.capacity() < (sizeof(ICMPv6) + data_size)))
            return;
 
        IPv6Header *ipv6 = (IPv6Header *)buf.c_data();
 
        uint8_t *b = buf.prepend_alloc(sizeof(ICMPv6));
        ICMPv6 *icmp = (ICMPv6 *)b;
 
        // IPv6 header
        icmp->head.version_prio = (6 << 4);
        icmp->head.flow_lbl[0] = 0;
        icmp->head.flow_lbl[1] = 0;
        icmp->head.flow_lbl[2] = 0;
        icmp->head.payload_len = htons(static_cast<uint16_t>(sizeof(ICMPv6) - sizeof(IPv6Header) + data_size));
        icmp->head.nexthdr = IPCommon::ICMPv6;
        icmp->head.hop_limit = 64;
        icmp->head.saddr = ipv6->daddr;
        icmp->head.daddr = ipv6->saddr;
 
        // ICMP header
        icmp->type = ICMPv6::PACKET_TOO_BIG;
        icmp->code = 0;
        icmp->mtu = htonl(nexthop_mtu);
        icmp->checksum = 0;
        icmp->checksum = Ping6::csum_icmp(icmp, sizeof(ICMPv6) + data_size);
 
        buf.set_size(sizeof(ICMPv6) + data_size);
    }
 
    static void generate_icmp4_ptb(BufferAllocated &buf, std::uint16_t nexthop_mtu)
    {
        // ICMP data includes original IP header and first 8 bytes of payload
        auto data_size = sizeof(IPv4Header) + ICMPv4::MIN_DATA_SIZE;
 
        // sanity check
        // we use headroom for adding IPv4 + ICMPv4 headers
        if ((buf.offset() < sizeof(ICMPv4)) || (buf.capacity() < (sizeof(ICMPv4) + data_size)))
            return;
 
        IPv4Header *ipv4 = (IPv4Header *)buf.c_data();
 
        uint8_t *b = buf.prepend_alloc(sizeof(ICMPv4));
        ICMPv4 *icmp = (ICMPv4 *)b;
 
        icmp->head.saddr = ipv4->daddr;
        icmp->head.daddr = ipv4->saddr;
        icmp->head.version_len = IPv4Header::ver_len(IPCommon::IPv4, sizeof(IPv4Header));
        icmp->head.tos = 0;
        icmp->head.tot_len = htons(static_cast<uint16_t>(sizeof(ICMPv4) + data_size));
        icmp->head.id = 0;
        icmp->head.frag_off = 0;
        icmp->head.ttl = 64;
        icmp->head.protocol = IPCommon::ICMPv4;
        icmp->head.check = 0;
        icmp->head.check = IPChecksum::checksum(b, sizeof(IPv4Header));
 
        icmp->type = ICMPv4::DEST_UNREACH;
        icmp->code = ICMPv4::FRAG_NEEDED;
        icmp->unused = 0;
        icmp->nexthop_mtu = htons(nexthop_mtu);
        icmp->checksum = 0;
        icmp->checksum = IPChecksum::checksum(b + sizeof(IPv4Header), sizeof(ICMPv4) - sizeof(IPv4Header) + data_size);
 
        buf.set_size(sizeof(ICMPv4) + data_size);
    }
};
} // namespace openvpn