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use crate::dnscrypt_certs::*;
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use crate::errors::*;
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use byteorder::{BigEndian, ByteOrder, WriteBytesExt};
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pub const DNS_MAX_HOSTNAME_SIZE: usize = 256;
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pub const DNS_HEADER_SIZE: usize = 12;
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pub const DNS_OFFSET_FLAGS: usize = 2;
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pub const DNS_MAX_PACKET_SIZE: usize = 8192;
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const DNS_MAX_INDIRECTIONS: usize = 16;
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const DNS_FLAGS_TC: u16 = 2u16 << 8;
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const DNS_FLAGS_QR: u16 = 128u16 << 8;
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const DNS_FLAGS_RA: u16 = 128;
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const DNS_OFFSET_QUESTION: usize = DNS_HEADER_SIZE;
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const DNS_TYPE_OPT: u16 = 41;
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const DNS_TYPE_TXT: u16 = 16;
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const DNS_CLASS_INET: u16 = 1;
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#[inline]
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pub fn qdcount(packet: &[u8]) -> u16 {
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BigEndian::read_u16(&packet[4..])
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}
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#[inline]
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pub fn ancount(packet: &[u8]) -> u16 {
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BigEndian::read_u16(&packet[6..])
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}
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fn ancount_inc(packet: &mut [u8]) -> Result<(), Error> {
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let mut ancount = ancount(packet);
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ensure!(ancount < 0xffff, "Too many answer records");
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ancount += 1;
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BigEndian::write_u16(&mut packet[6..], ancount);
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Ok(())
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}
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#[inline]
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fn nscount(packet: &[u8]) -> u16 {
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BigEndian::read_u16(&packet[8..])
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}
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#[inline]
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pub fn arcount(packet: &[u8]) -> u16 {
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BigEndian::read_u16(&packet[10..])
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}
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fn arcount_inc(packet: &mut [u8]) -> Result<(), Error> {
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let mut arcount = arcount(packet);
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ensure!(arcount < 0xffff, "Too many additional records");
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arcount += 1;
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BigEndian::write_u16(&mut packet[10..], arcount);
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Ok(())
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}
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#[inline]
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pub fn tid(packet: &[u8]) -> u16 {
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BigEndian::read_u16(&packet[0..])
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}
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#[inline]
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pub fn set_tid(packet: &mut [u8], tid: u16) {
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BigEndian::write_u16(&mut packet[0..], tid);
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}
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#[inline]
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pub fn authoritative_response(packet: &mut [u8]) {
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let current_flags = BigEndian::read_u16(&packet[DNS_OFFSET_FLAGS..]);
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BigEndian::write_u16(
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&mut packet[DNS_OFFSET_FLAGS..],
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current_flags | DNS_FLAGS_QR | DNS_FLAGS_RA,
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);
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}
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#[inline]
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pub fn truncate(packet: &mut [u8]) {
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let current_flags = BigEndian::read_u16(&packet[DNS_OFFSET_FLAGS..]);
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BigEndian::write_u16(
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&mut packet[DNS_OFFSET_FLAGS..],
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current_flags | DNS_FLAGS_TC | DNS_FLAGS_QR | DNS_FLAGS_RA,
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);
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}
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#[inline]
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pub fn is_response(packet: &[u8]) -> bool {
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BigEndian::read_u16(&packet[DNS_OFFSET_FLAGS..]) & DNS_FLAGS_QR == DNS_FLAGS_QR
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}
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#[inline]
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pub fn is_truncated(packet: &[u8]) -> bool {
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BigEndian::read_u16(&packet[DNS_OFFSET_FLAGS..]) & DNS_FLAGS_TC == DNS_FLAGS_TC
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}
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pub fn qname(packet: &[u8]) -> Result<Vec<u8>, Error> {
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assert!(std::usize::MAX > 0xffff);
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ensure!(qdcount(packet) == 1, "Unexpected query count");
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let packet_len = packet.len();
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let mut offset = DNS_HEADER_SIZE;
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let mut qname = Vec::with_capacity(DNS_MAX_HOSTNAME_SIZE);
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let mut indirections = 0;
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loop {
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ensure!(offset < packet_len, "Short packet");
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match packet[offset] as usize {
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label_len if label_len & 0xc0 == 0xc0 => {
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ensure!(packet_len - offset > 1, "Short packet");
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let new_offset = (BigEndian::read_u16(&packet[offset..]) & 0x3fff) as usize;
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indirections += 1;
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ensure!(
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new_offset >= DNS_HEADER_SIZE
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&& new_offset != offset
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&& indirections < DNS_MAX_INDIRECTIONS,
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"Too many indirections"
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);
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offset = new_offset;
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}
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0 => {
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if qname.is_empty() {
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qname.push(b'.')
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}
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break;
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}
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label_len => {
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ensure!(packet_len - offset > 1, "Short packet");
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offset += 1;
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ensure!(packet_len - offset > label_len, "Short packet");
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if !qname.is_empty() {
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qname.push(b'.')
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}
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ensure!(
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qname.len() < DNS_MAX_HOSTNAME_SIZE - label_len,
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"Name too long"
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);
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qname.extend_from_slice(&packet[offset..offset + label_len]);
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offset += label_len;
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}
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}
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}
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Ok(qname)
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}
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fn skip_name(packet: &[u8], offset: usize) -> Result<usize, Error> {
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let packet_len = packet.len();
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ensure!(offset < packet_len - 1, "Short packet");
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let mut qname_len: usize = 0;
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let mut offset = offset;
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loop {
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let label_len = match packet[offset] as usize {
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label_len if label_len & 0xc0 == 0xc0 => {
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ensure!(packet_len - offset >= 2, "Incomplete offset");
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offset += 2;
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break;
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}
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label_len => label_len,
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} as usize;
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ensure!(
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packet_len - offset - 1 > label_len,
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"Malformed packet with an out-of-bounds name"
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);
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qname_len += label_len + 1;
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ensure!(qname_len <= DNS_MAX_HOSTNAME_SIZE, "Name too long");
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offset += label_len + 1;
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if label_len == 0 {
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break;
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}
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}
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Ok(offset)
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}
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fn traverse_rrs<F: FnMut(usize) -> Result<(), Error>>(
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packet: &[u8],
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mut offset: usize,
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rrcount: u16,
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mut cb: F,
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) -> Result<usize, Error> {
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let packet_len = packet.len();
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for _ in 0..rrcount {
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offset = skip_name(packet, offset)?;
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ensure!(packet_len - offset >= 10, "Short packet");
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cb(offset)?;
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let rdlen = BigEndian::read_u16(&packet[offset + 8..]) as usize;
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offset += 10;
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ensure!(
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packet_len - offset <= rdlen,
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"Record length would exceed packet length"
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);
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offset += rdlen;
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}
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Ok(offset)
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}
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fn traverse_rrs_mut<F: FnMut(&mut [u8], usize) -> Result<(), Error>>(
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packet: &mut [u8],
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mut offset: usize,
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rrcount: u16,
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mut cb: F,
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) -> Result<usize, Error> {
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let packet_len = packet.len();
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for _ in 0..rrcount {
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offset = skip_name(packet, offset)?;
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ensure!(packet_len - offset >= 10, "Short packet");
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cb(packet, offset)?;
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let rdlen = BigEndian::read_u16(&packet[offset + 8..]) as usize;
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offset += 10;
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ensure!(
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packet_len - offset <= rdlen,
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"Record length would exceed packet length"
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);
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offset += rdlen;
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}
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Ok(offset)
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}
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pub fn min_ttl(packet: &[u8], min_ttl: u32, max_ttl: u32, failure_ttl: u32) -> Result<u32, Error> {
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ensure!(qdcount(packet) == 1, "Unsupported number of questions");
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let packet_len = packet.len();
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ensure!(packet_len > DNS_OFFSET_QUESTION, "Short packet");
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ensure!(packet_len <= DNS_MAX_PACKET_SIZE, "Large packet");
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let mut offset = skip_name(packet, DNS_OFFSET_QUESTION)?;
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assert!(offset > DNS_OFFSET_QUESTION);
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ensure!(packet_len - offset > 4, "Short packet");
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offset += 4;
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let (ancount, nscount, arcount) = (ancount(packet), nscount(packet), arcount(packet));
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let rrcount = ancount + nscount + arcount;
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let mut found_min_ttl = if rrcount > 0 { max_ttl } else { failure_ttl };
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offset = traverse_rrs(packet, offset, rrcount, |offset| {
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let qtype = BigEndian::read_u16(&packet[offset..]);
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let ttl = BigEndian::read_u32(&packet[offset + 4..]);
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if qtype != DNS_TYPE_OPT && ttl < found_min_ttl {
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found_min_ttl = ttl;
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}
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Ok(())
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})?;
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if found_min_ttl < min_ttl {
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found_min_ttl = min_ttl;
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}
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ensure!(packet_len == offset, "Garbage after packet");
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Ok(found_min_ttl)
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}
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fn add_edns_section(packet: &mut Vec<u8>, max_payload_size: u16) -> Result<(), Error> {
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let opt_rr: [u8; 11] = [
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0,
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(DNS_TYPE_OPT >> 8) as u8,
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DNS_TYPE_OPT as u8,
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(max_payload_size >> 8) as u8,
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max_payload_size as u8,
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0,
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0,
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0,
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0,
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0,
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0,
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];
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ensure!(
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DNS_MAX_PACKET_SIZE - packet.len() >= opt_rr.len(),
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"Packet would be too large to add a new record"
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);
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arcount_inc(packet)?;
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packet.extend(&opt_rr);
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Ok(())
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}
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pub fn set_edns_max_payload_size(packet: &mut Vec<u8>, max_payload_size: u16) -> Result<(), Error> {
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ensure!(qdcount(packet) == 1, "Unsupported number of questions");
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let packet_len = packet.len();
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ensure!(packet_len > DNS_OFFSET_QUESTION, "Short packet");
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ensure!(packet_len <= DNS_MAX_PACKET_SIZE, "Large packet");
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let mut offset = skip_name(packet, DNS_OFFSET_QUESTION)?;
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assert!(offset > DNS_OFFSET_QUESTION);
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ensure!(packet_len - offset >= 4, "Short packet");
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offset += 4;
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let (ancount, nscount, arcount) = (ancount(packet), nscount(packet), arcount(packet));
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offset = traverse_rrs(packet, offset, ancount + nscount, |_offset| Ok(()))?;
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let mut edns_payload_set = false;
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traverse_rrs_mut(packet, offset, arcount, |packet, offset| {
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let qtype = BigEndian::read_u16(&packet[offset..]);
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if qtype == DNS_TYPE_OPT {
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ensure!(!edns_payload_set, "Duplicate OPT RR found");
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BigEndian::write_u16(&mut packet[offset + 2..], max_payload_size);
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edns_payload_set = true;
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}
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Ok(())
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})?;
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if edns_payload_set {
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return Ok(());
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}
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add_edns_section(packet, max_payload_size)?;
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Ok(())
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}
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pub fn serve_certificates<'t>(
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client_packet: &[u8],
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expected_qname: &str,
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dnscrypt_encryption_params_set: impl IntoIterator<Item = &'t DNSCryptEncryptionParams>,
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) -> Result<Option<Vec<u8>>, Error> {
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ensure!(client_packet.len() >= DNS_HEADER_SIZE, "Short packet");
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ensure!(qdcount(&client_packet) == 1, "No question");
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ensure!(
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!is_response(&client_packet),
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"Question expected, but got a response instead"
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);
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let offset = skip_name(client_packet, DNS_HEADER_SIZE)?;
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ensure!(client_packet.len() - offset >= 4, "Short packet");
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let qtype = BigEndian::read_u16(&client_packet[offset..]);
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let qclass = BigEndian::read_u16(&client_packet[offset + 2..]);
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if qtype != DNS_TYPE_TXT || qclass != DNS_CLASS_INET {
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return Ok(None);
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}
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let qname_v = qname(&client_packet)?;
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let qname = std::str::from_utf8(&qname_v)?;
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if !qname.eq_ignore_ascii_case(expected_qname) {
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return Ok(None);
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}
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let mut packet = (&client_packet[..offset + 4]).to_vec();
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authoritative_response(&mut packet);
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for dnscrypt_encryption_params in dnscrypt_encryption_params_set {
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let cert_bin = dnscrypt_encryption_params.dnscrypt_cert().as_bytes();
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ensure!(cert_bin.len() <= 0xff, "Certificate too long");
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ancount_inc(&mut packet)?;
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packet.write_u16::<BigEndian>(0xc000 + DNS_HEADER_SIZE as u16)?;
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packet.write_u16::<BigEndian>(DNS_TYPE_TXT)?;
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packet.write_u16::<BigEndian>(DNS_CLASS_INET)?;
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packet.write_u32::<BigEndian>(28800)?;
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packet.write_u16::<BigEndian>(1 + cert_bin.len() as u16)?;
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packet.write_u8(cert_bin.len() as u8)?;
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packet.extend_from_slice(&cert_bin[..]);
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ensure!(packet.len() < DNS_MAX_PACKET_SIZE, "Packet too large");
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}
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Ok(Some(packet))
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}
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