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tun_recv: removed mssfix limit for IPv4 traffic if DF is not set #329

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tun_recv: removed mssfix limit for IPv4 traffic if DF is not set #329

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11 changes: 10 additions & 1 deletion openvpn/client/cliproto.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -443,12 +443,21 @@ class Session : ProtoContextCallbackInterface,
if (buf.size())
{
const ProtoContext::ProtoConfig &c = proto_context.conf();

bool df = true;

if (IPCommon::version(buf[0]) == IPCommon::IPv4 && buf.size() >= sizeof(struct IPv4Header))
{
df = IPv4Header::is_df_set(buf.c_data());
}

// when calculating mss, we take IPv4 and TCP headers into account
// here we need to add it back since we check the whole IP packet size, not just TCP payload
constexpr size_t MinTcpHeader = 20;
constexpr size_t MinIpHeader = 20;
size_t mss_no_tcp_ip_encap = c.mss_fix + (MinTcpHeader + MinIpHeader);
if (c.mss_fix > 0 && buf.size() > mss_no_tcp_ip_encap)

if (df && c.mss_fix > 0 && buf.size() > mss_no_tcp_ip_encap)
{
Ptb::generate_icmp_ptb(buf, clamp_to_typerange<unsigned short>(mss_no_tcp_ip_encap));
tun->tun_send(buf);
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7 changes: 7 additions & 0 deletions openvpn/ip/ip4.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -43,6 +43,12 @@ struct IPv4Header
return static_cast<uint8_t>(((len >> 2) & 0x0F) | (version & 0x0F) << 4);
}

static bool is_df_set(const unsigned char *data)
{
auto *hdr = reinterpret_cast<const IPv4Header *>(data);
return ntohs(hdr->frag_off) & IPv4Header::DF;
}

std::uint8_t version_len;

std::uint8_t tos;
Expand All @@ -52,6 +58,7 @@ struct IPv4Header
enum
{
OFFMASK = 0x1fff,
DF = 0x4000,
};
std::uint16_t frag_off;

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