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validate high order two bits of first dns label octet #494

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merged 5 commits into from
Jan 3, 2023
Merged

validate high order two bits of first dns label octet #494

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483b041
validate high order two bits of first dns label octet
bwillcoxpubfork Dec 13, 2022
45df48a
fixup! validate high order two bits of first dns label octet
bwillcoxpubfork Dec 13, 2022
fe503ff
Update src/dns.cpp
bwillcox Jan 3, 2023
b8c7c40
fixup! Update src/dns.cpp
bwillcoxpubfork Jan 3, 2023
cac18c6
fixup! Update src/dns.cpp
bwillcoxpubfork Jan 3, 2023
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13 changes: 11 additions & 2 deletions src/dns.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -86,14 +86,23 @@ void DNS::skip_to_dname_end(InputMemoryStream& stream) const {
break;
}
else {
if ((value & 0xc0)) {
const uint8_t offset_discriminator = value & 0xc0;
if (offset_discriminator == 0xc0) {
// This is an offset label, skip the second byte and we're done
stream.skip(1);
break;
}
else {
else if (offset_discriminator == 0) {
// This is an offset label, skip the second byte and we're done
stream.skip(1);
break;
}
else if ((value & 0x3f) == value) {
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Uh I think the diff was bad, this is duplicated now. Sorry!

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no worries; all looks ok now I think...

// This is an actual label, skip its contents
stream.skip(value);
} else {
// high order two bits of the first octet of a label must be either 11 or 00
throw malformed_packet();
}
}
}
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16 changes: 11 additions & 5 deletions tests/src/dns_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -571,6 +571,7 @@ TEST_F(DNSTest, BadLabelSize) {

// add bad length
const size_t bad_label_len{0x80};
const size_t label_offset = payload_sz;
payload[payload_sz++] = bad_label_len;

// fill label for incorrect length and terminate
Expand All @@ -590,13 +591,18 @@ TEST_F(DNSTest, BadLabelSize) {
payload + payload_sz);
payload_sz += sizeof(type_class);

// SUCCEED moves from dns_decompression_pointer_out_of_bounds to malformed_packet after fix
const DNS packet(payload, payload_sz);
EXPECT_EQ(packet.questions_count(), 1);
// invalid high two bits of label first octest is detected early now
try {
const auto queries{packet.queries()};
const DNS packet(payload, payload_sz);
FAIL();
} catch (dns_decompression_pointer_out_of_bounds& oob) {
} catch (malformed_packet& mp) {
SUCCEED();
}

// check the other invalid value of high two bits in label size
payload[label_offset] = 0x10;
try {
const DNS packet(payload, payload_sz);
FAIL();
} catch (malformed_packet& mp) {
SUCCEED();
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