support nat on icmp sockets

Author: Arian8j2

forwarder/src/socket/icmp.rs

@@ -13,6 +13,11 @@ use std::{
     net::{SocketAddr, SocketAddrV6},
 };
 
+/// magic bytes that are injected to end of icmp echo reply packets that
+/// we craft and it get discarded later when parsing, it's purpose is to
+/// detect automatic echo reply packets of kernel and ignore them
+const ECHO_REPLY_MAGIC: [u8; 3] = [0x24, 0x74, 0x33];
+
 /// `IcmpSocket` that is very similiar to `UdpSocket`
 #[derive(Debug)]
 pub struct IcmpSocket {
@@ -50,7 +55,7 @@ impl IcmpSocket {
 
 impl SocketTrait for IcmpSocket {
     fn send_to(&self, buffer: &[u8], to: &SocketAddr) -> io::Result<usize> {
-        let packet = craft_icmp_packet(buffer, &self.udp_socket_addr, to)?;
+        let packet = craft_icmp_packet(buffer, &self.udp_socket_addr, to, false)?;
         let mut to_addr = *to;
         // in linux `send_to` on icmpv6 socket requires destination port to be zero
         to_addr.set_port(0);
@@ -115,7 +120,7 @@ impl NonBlockingSocketTrait for NonBlockingIcmpSocket {
         let dst_addr = self
             .connected_addr
             .ok_or_else(|| Into::<io::Error>::into(io::ErrorKind::NotConnected))?;
-        let packet = craft_icmp_packet(buffer, &self.icmp_socket.udp_socket_addr, &dst_addr)?;
+        let packet = craft_icmp_packet(buffer, &self.icmp_socket.udp_socket_addr, &dst_addr, true)?;
         self.icmp_socket.socket.send(&packet)
     }
 
@@ -137,30 +142,64 @@ fn craft_icmp_packet(
     payload: &[u8],
     source_addr: &SocketAddr,
     dst_addr: &SocketAddr,
+    is_echo_request: bool,
 ) -> io::Result<Vec<u8>> {
-    let echo_header = IcmpEchoHeader {
-        id: dst_addr.port(),
-        seq: source_addr.port(),
+    // when we are sending echo reply we inject few magic bytes to the
+    // end of payload so when receiving reply packets we can determine
+    // if the echo reply packet is automatically sent from kernel
+    // (in case /proc/sys/net/ipv4/icmp_echo_ignore_all is not turned off)
+    // or we actually sent it
+    let payload = if !is_echo_request {
+        let payload_with_magic_len = payload.len() + ECHO_REPLY_MAGIC.len();
+        // TODO: this allocation is really bad, find another way for it
+        let mut buffer = vec![0u8; payload_with_magic_len];
+        buffer[..payload.len()].copy_from_slice(payload);
+        buffer[payload.len()..].copy_from_slice(&ECHO_REPLY_MAGIC);
+        buffer
+    } else {
+        payload.to_vec()
+    };
+
+    // read comments on `receiver::parse_icmp_packet` on why the
+    // source and destination place changes based on echo reply or request
+    let echo_header = if is_echo_request {
+        IcmpEchoHeader {
+            id: source_addr.port(),
+            seq: dst_addr.port(),
+        }
+    } else {
+        IcmpEchoHeader {
+            id: dst_addr.port(),
+            seq: source_addr.port(),
+        }
     };
 
     let icmp_header = if source_addr.is_ipv4() {
-        let icmp_type = Icmpv4Type::EchoRequest(echo_header);
-        Icmpv4Header::with_checksum(icmp_type, payload)
+        let icmp_type = if is_echo_request {
+            Icmpv4Type::EchoRequest(echo_header)
+        } else {
+            Icmpv4Type::EchoReply(echo_header)
+        };
+        Icmpv4Header::with_checksum(icmp_type, &payload)
             .to_bytes()
             .to_vec()
     } else {
-        let icmp_type = Icmpv6Type::EchoRequest(echo_header);
+        let icmp_type = if is_echo_request {
+            Icmpv6Type::EchoRequest(echo_header)
+        } else {
+            Icmpv6Type::EchoReply(echo_header)
+        };
         let source_ip = as_socket_addr_v6(*source_addr).ip().octets();
         let destination_ip = as_socket_addr_v6(*dst_addr).ip().octets();
-        Icmpv6Header::with_checksum(icmp_type, source_ip, destination_ip, payload)
+        Icmpv6Header::with_checksum(icmp_type, source_ip, destination_ip, &payload)
             .map_err(|_| Into::<io::Error>::into(io::ErrorKind::InvalidInput))?
             .to_bytes()
             .to_vec()
     };
 
     let mut header_and_payload = Vec::with_capacity(icmp_header.len() + payload.len());
     header_and_payload.extend_from_slice(&icmp_header);
-    header_and_payload.extend_from_slice(payload);
+    header_and_payload.extend_from_slice(&payload);
     Ok(header_and_payload)
 }
 
@@ -187,29 +226,64 @@ pub fn parse_icmp_packet(packet: &mut [u8], is_ipv6: bool) -> Option<IcmpPacket<
     };
 
     let icmp = IcmpSlice::from_slice(is_ipv6, &packet[payload_start_index..])?;
-    // we only work with icmp echo requests so if any other type of icmp
+    // we only work with icmp echo requests and replies so if any other type of icmp
     // packet we receive we just ignore it
-    let correct_icmp_type = if is_ipv6 {
+    let echo_request = if is_ipv6 {
         etherparse::icmpv6::TYPE_ECHO_REQUEST
     } else {
         etherparse::icmpv4::TYPE_ECHO_REQUEST
     };
-    if icmp.type_u8() != correct_icmp_type || icmp.code_u8() != 0 {
+    let echo_reply = if is_ipv6 {
+        etherparse::icmpv6::TYPE_ECHO_REPLY
+    } else {
+        etherparse::icmpv4::TYPE_ECHO_REPLY
+    };
+
+    let is_echo_request = icmp.type_u8() == echo_request;
+    let is_echo_reply = icmp.type_u8() == echo_reply;
+
+    let is_correct_icmp_type = is_echo_request || is_echo_reply;
+    if !is_correct_icmp_type || icmp.code_u8() != 0 {
         return None;
     }
 
     let bytes5to8 = icmp.bytes5to8();
-    // icmp is on layer 3 so it has no idea about ports
-    // we use identification part of icmp packet as destination port
-    // to identify packets that are really meant for us
-    let dst_port = u16::from_be_bytes([bytes5to8[0], bytes5to8[1]]);
-
-    // we also use sequence part of icmp packet as source port
-    let src_port = u16::from_be_bytes([bytes5to8[2], bytes5to8[3]]);
+    let id = u16::from_be_bytes([bytes5to8[0], bytes5to8[1]]);
+    let seq = u16::from_be_bytes([bytes5to8[2], bytes5to8[3]]);
 
     let payload_len = icmp.payload().len();
-    let total_len = packet.len();
-    let payload = &mut packet[total_len - payload_len..];
+    let packet_len = packet.len();
+
+    let payload = if is_echo_request {
+        &mut packet[packet_len - payload_len..]
+    } else {
+        // filter the reply packets that doesn't have the magic bytes
+        let payload = &mut packet[packet_len - payload_len..];
+        let magic_len = ECHO_REPLY_MAGIC.len();
+        if payload_len < magic_len {
+            return None;
+        }
+        if payload[payload_len - magic_len..] != ECHO_REPLY_MAGIC {
+            return None;
+        }
+        // striping magic bytes off the payload
+        &mut payload[..payload_len - magic_len]
+    };
+
+    // icmp is on layer 3 so it has no idea about ports so we use
+    // identification and sequence part of icmp packet as src and dst port
+    // but the problem is that if for example port 1010 sends a packet to 8000
+    // the id and seq is like this:
+    //      | ID: 1010 | SEQ: 8000 |
+    // now if the server wants to send echo reply from 8000 to 1010 the packet
+    // will be like this:
+    //      | ID: 8000 | SEQ: 1010 |
+    // because now the sender is 8000 and the receiver is 1010
+    // the important part is the ID, if the ID of echo reply is different than
+    // the echo request then NAT has no clue how to forward this packet, so we
+    // swap the id and seq position based on the packet being reply or request
+    // so the ID field will not get changed
+    let (src_port, dst_port) = if is_echo_reply { (seq, id) } else { (id, seq) };
 
     Some(IcmpPacket {
         payload,