diff --git a/Cargo.lock b/Cargo.lock
index 290e4fe..07b10cf 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -273,6 +273,7 @@ dependencies = [
  "dirs",
  "futures",
  "hex",
+ "libc",
  "rand",
  "rtnetlink",
  "secp256k1",
diff --git a/Cargo.toml b/Cargo.toml
index 1b985ef..a25b225 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -16,6 +16,7 @@ hex = "0.4"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 tun = { version = "0.7", features = ["async"] }
+libc = "0.2"
 rtnetlink = "0.14"
 tokio = { version = "1", features = ["rt", "macros", "signal", "sync"] }
 futures = "0.3"
diff --git a/src/bin/fips.rs b/src/bin/fips.rs
index ae561ce..0a34c48 100644
--- a/src/bin/fips.rs
+++ b/src/bin/fips.rs
@@ -2,21 +2,46 @@
 //!
 //! Loads configuration and creates the top-level node instance.
 
-use fips::{log_ipv6_packet, shutdown_tun_interface, Config, Node, TunDevice};
-use tracing::{error, info, warn, Level};
+use fips::{
+    build_dest_unreachable, log_ipv6_packet, should_send_icmp_error, shutdown_tun_interface,
+    Config, DestUnreachableCode, FipsAddress, Node, TunDevice, TunTx,
+};
+use tracing::{debug, error, info, warn, Level};
 use tracing_subscriber::{fmt, EnvFilter};
 
 /// TUN packet reader loop.
 ///
-/// Reads packets from the TUN device and logs them at DEBUG level.
+/// Reads packets from the TUN device, logs them, and sends ICMPv6
+/// Destination Unreachable responses for packets we can't route.
+///
 /// This runs in a separate thread since TUN reads are blocking.
-fn run_tun_reader(mut device: TunDevice, mtu: u16) {
+fn run_tun_reader(mut device: TunDevice, mtu: u16, our_addr: FipsAddress, tun_tx: TunTx) {
     let mut buf = vec![0u8; mtu as usize + 100]; // Extra space for headers
 
     loop {
         match device.read_packet(&mut buf) {
             Ok(n) if n > 0 => {
-                log_ipv6_packet(&buf[..n]);
+                let packet = &buf[..n];
+                log_ipv6_packet(packet);
+
+                // Currently no routing capability - send ICMPv6 Destination Unreachable
+                // for all packets that qualify for an error response
+                if should_send_icmp_error(packet) {
+                    if let Some(response) = build_dest_unreachable(
+                        packet,
+                        DestUnreachableCode::NoRoute,
+                        our_addr.to_ipv6(),
+                    ) {
+                        debug!(
+                            len = response.len(),
+                            "Sending ICMPv6 Destination Unreachable"
+                        );
+                        if tun_tx.send(response).is_err() {
+                            info!("TUN writer channel closed, reader stopping");
+                            break;
+                        }
+                    }
+                }
             }
             Ok(_) => {
                 // Zero-length read, continue
@@ -124,8 +149,11 @@ async fn main() {
                 match output {
                     Ok(out) => {
                         if out.status.success() {
-                            info!("ip link show {}:\n{}", device.name(),
-                                String::from_utf8_lossy(&out.stdout));
+                            info!(
+                                "ip link show {}:\n{}",
+                                device.name(),
+                                String::from_utf8_lossy(&out.stdout)
+                            );
                         }
                     }
                     Err(e) => {
@@ -145,14 +173,31 @@ async fn main() {
 
     info!("FIPS initialized successfully");
 
-    // Spawn TUN reader task if TUN is active
+    // Spawn TUN reader and writer threads if TUN is active
     let tun_name = if let Some(tun_device) = node.take_tun_device() {
         let mtu = tun_device.mtu();
         let name = tun_device.name().to_string();
-        info!(mtu, name = %name, "Starting TUN packet reader");
+        let our_addr = *tun_device.address();
 
+        // Create writer (dups the fd for independent write access)
+        let (writer, tun_tx) = match tun_device.create_writer() {
+            Ok(w) => w,
+            Err(e) => {
+                error!("Failed to create TUN writer: {}", e);
+                std::process::exit(1);
+            }
+        };
+
+        info!(mtu, name = %name, "Starting TUN reader and writer");
+
+        // Spawn writer thread
         std::thread::spawn(move || {
-            run_tun_reader(tun_device, mtu);
+            writer.run();
+        });
+
+        // Spawn reader thread
+        std::thread::spawn(move || {
+            run_tun_reader(tun_device, mtu, our_addr, tun_tx);
         });
 
         Some(name)
diff --git a/src/icmp.rs b/src/icmp.rs
new file mode 100644
index 0000000..4e3c944
--- /dev/null
+++ b/src/icmp.rs
@@ -0,0 +1,377 @@
+//! ICMPv6 message handling for FIPS.
+//!
+//! Implements ICMPv6 error message generation per RFC 4443.
+//! Currently supports Destination Unreachable (Type 1) for
+//! packets that cannot be routed.
+
+use std::net::Ipv6Addr;
+
+/// ICMPv6 message types.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum Icmpv6Type {
+    /// Destination Unreachable (error).
+    DestinationUnreachable = 1,
+    /// Packet Too Big (error).
+    PacketTooBig = 2,
+    /// Time Exceeded (error).
+    TimeExceeded = 3,
+    /// Parameter Problem (error).
+    ParameterProblem = 4,
+    /// Echo Request.
+    EchoRequest = 128,
+    /// Echo Reply.
+    EchoReply = 129,
+}
+
+/// ICMPv6 Destination Unreachable codes.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+pub enum DestUnreachableCode {
+    /// No route to destination.
+    NoRoute = 0,
+    /// Communication administratively prohibited.
+    AdminProhibited = 1,
+    /// Beyond scope of source address.
+    BeyondScope = 2,
+    /// Address unreachable.
+    AddressUnreachable = 3,
+    /// Port unreachable.
+    PortUnreachable = 4,
+    /// Source address failed policy.
+    SourcePolicy = 5,
+    /// Reject route to destination.
+    RejectRoute = 6,
+}
+
+/// IPv6 header next-header value for ICMPv6.
+pub const IPPROTO_ICMPV6: u8 = 58;
+
+/// Minimum IPv6 MTU - ICMPv6 responses must not exceed this.
+const MIN_IPV6_MTU: usize = 1280;
+
+/// IPv6 header length.
+const IPV6_HEADER_LEN: usize = 40;
+
+/// ICMPv6 header length (type + code + checksum + unused/data).
+const ICMPV6_HEADER_LEN: usize = 8;
+
+/// Maximum original packet bytes to include in ICMPv6 error.
+const MAX_ORIGINAL_PACKET: usize = MIN_IPV6_MTU - IPV6_HEADER_LEN - ICMPV6_HEADER_LEN;
+
+/// Check if we should send an ICMPv6 error for this packet.
+///
+/// Returns false if the packet is:
+/// - Too short to be valid IPv6
+/// - Not IPv6
+/// - An ICMPv6 error message itself
+/// - Has a multicast source address
+/// - Has an unspecified source address (::)
+pub fn should_send_icmp_error(packet: &[u8]) -> bool {
+    // Must have at least an IPv6 header
+    if packet.len() < IPV6_HEADER_LEN {
+        return false;
+    }
+
+    // Must be IPv6
+    let version = packet[0] >> 4;
+    if version != 6 {
+        return false;
+    }
+
+    // Extract source address
+    let src = Ipv6Addr::from(<[u8; 16]>::try_from(&packet[8..24]).unwrap());
+
+    // Don't send errors for unspecified source
+    if src.is_unspecified() {
+        return false;
+    }
+
+    // Don't send errors for multicast source (first byte 0xff)
+    if src.octets()[0] == 0xff {
+        return false;
+    }
+
+    // Don't send errors for ICMPv6 error messages (types 0-127)
+    let next_header = packet[6];
+    if next_header == IPPROTO_ICMPV6 && packet.len() > IPV6_HEADER_LEN {
+        let icmp_type = packet[IPV6_HEADER_LEN];
+        // ICMPv6 error messages are types 0-127
+        if icmp_type < 128 {
+            return false;
+        }
+    }
+
+    true
+}
+
+/// Build an ICMPv6 Destination Unreachable response.
+///
+/// Takes the original packet that couldn't be delivered and returns
+/// a complete IPv6 packet containing the ICMPv6 error response.
+///
+/// Arguments:
+/// - `original_packet`: The packet that couldn't be routed
+/// - `code`: The specific unreachable reason
+/// - `our_addr`: Our FIPS address (source of the error)
+///
+/// Returns None if the original packet is invalid.
+pub fn build_dest_unreachable(
+    original_packet: &[u8],
+    code: DestUnreachableCode,
+    our_addr: Ipv6Addr,
+) -> Option<Vec<u8>> {
+    // Validate original packet
+    if original_packet.len() < IPV6_HEADER_LEN {
+        return None;
+    }
+
+    // Extract destination from original packet (becomes our destination)
+    let dest_addr = Ipv6Addr::from(<[u8; 16]>::try_from(&original_packet[8..24]).unwrap());
+
+    // Calculate how much of the original packet to include
+    let original_len = original_packet.len().min(MAX_ORIGINAL_PACKET);
+    let icmpv6_len = ICMPV6_HEADER_LEN + original_len;
+    let total_len = IPV6_HEADER_LEN + icmpv6_len;
+
+    let mut response = vec![0u8; total_len];
+
+    // === IPv6 Header ===
+    // Version (4) + Traffic Class (8) + Flow Label (20)
+    response[0] = 0x60; // Version 6, TC high bits = 0
+    // response[1..4] = 0 (TC low bits + flow label)
+
+    // Payload length (ICMPv6 header + body)
+    let payload_len = icmpv6_len as u16;
+    response[4..6].copy_from_slice(&payload_len.to_be_bytes());
+
+    // Next header = ICMPv6
+    response[6] = IPPROTO_ICMPV6;
+
+    // Hop limit
+    response[7] = 64;
+
+    // Source = our address
+    response[8..24].copy_from_slice(&our_addr.octets());
+
+    // Destination = original source
+    response[24..40].copy_from_slice(&dest_addr.octets());
+
+    // === ICMPv6 Header ===
+    let icmp_start = IPV6_HEADER_LEN;
+
+    // Type = Destination Unreachable
+    response[icmp_start] = Icmpv6Type::DestinationUnreachable as u8;
+
+    // Code
+    response[icmp_start + 1] = code as u8;
+
+    // Checksum placeholder (calculated below)
+    // response[icmp_start + 2..icmp_start + 4] = 0
+
+    // Unused (4 bytes of zeros for Dest Unreachable)
+    // response[icmp_start + 4..icmp_start + 8] = 0
+
+    // === ICMPv6 Body ===
+    // As much of original packet as fits
+    response[icmp_start + ICMPV6_HEADER_LEN..].copy_from_slice(&original_packet[..original_len]);
+
+    // Calculate checksum
+    let checksum = icmpv6_checksum(&response[icmp_start..], &our_addr, &dest_addr);
+    response[icmp_start + 2..icmp_start + 4].copy_from_slice(&checksum.to_be_bytes());
+
+    Some(response)
+}
+
+/// Calculate ICMPv6 checksum per RFC 4443.
+///
+/// The checksum is calculated over a pseudo-header plus the ICMPv6 message.
+fn icmpv6_checksum(icmpv6_message: &[u8], src: &Ipv6Addr, dst: &Ipv6Addr) -> u16 {
+    let mut sum: u32 = 0;
+
+    // Pseudo-header: source address (16 bytes)
+    for chunk in src.octets().chunks(2) {
+        sum += u16::from_be_bytes([chunk[0], chunk[1]]) as u32;
+    }
+
+    // Pseudo-header: destination address (16 bytes)
+    for chunk in dst.octets().chunks(2) {
+        sum += u16::from_be_bytes([chunk[0], chunk[1]]) as u32;
+    }
+
+    // Pseudo-header: upper-layer packet length (4 bytes, as u32)
+    let len = icmpv6_message.len() as u32;
+    sum += (len >> 16) as u32;
+    sum += (len & 0xffff) as u32;
+
+    // Pseudo-header: next header (padded to 4 bytes)
+    sum += IPPROTO_ICMPV6 as u32;
+
+    // ICMPv6 message (with checksum field = 0)
+    let mut i = 0;
+    while i + 1 < icmpv6_message.len() {
+        // Skip the checksum field (bytes 2-3)
+        if i == 2 {
+            i += 2;
+            continue;
+        }
+        sum += u16::from_be_bytes([icmpv6_message[i], icmpv6_message[i + 1]]) as u32;
+        i += 2;
+    }
+
+    // Handle odd byte
+    if i < icmpv6_message.len() {
+        sum += (icmpv6_message[i] as u32) << 8;
+    }
+
+    // Fold 32-bit sum to 16 bits
+    while sum >> 16 != 0 {
+        sum = (sum & 0xffff) + (sum >> 16);
+    }
+
+    // One's complement
+    !(sum as u16)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn make_ipv6_packet(src: Ipv6Addr, dst: Ipv6Addr, next_header: u8, payload: &[u8]) -> Vec<u8> {
+        let mut packet = vec![0u8; IPV6_HEADER_LEN + payload.len()];
+
+        // Version + TC + Flow Label
+        packet[0] = 0x60;
+
+        // Payload length
+        let len = payload.len() as u16;
+        packet[4..6].copy_from_slice(&len.to_be_bytes());
+
+        // Next header
+        packet[6] = next_header;
+
+        // Hop limit
+        packet[7] = 64;
+
+        // Source
+        packet[8..24].copy_from_slice(&src.octets());
+
+        // Destination
+        packet[24..40].copy_from_slice(&dst.octets());
+
+        // Payload
+        packet[IPV6_HEADER_LEN..].copy_from_slice(payload);
+
+        packet
+    }
+
+    #[test]
+    fn test_should_send_error_valid_packet() {
+        let src = "fd00::1".parse().unwrap();
+        let dst = "fd00::2".parse().unwrap();
+        let packet = make_ipv6_packet(src, dst, 17, &[0u8; 8]); // UDP
+
+        assert!(should_send_icmp_error(&packet));
+    }
+
+    #[test]
+    fn test_should_not_send_error_unspecified_source() {
+        let src = Ipv6Addr::UNSPECIFIED;
+        let dst = "fd00::2".parse().unwrap();
+        let packet = make_ipv6_packet(src, dst, 17, &[0u8; 8]);
+
+        assert!(!should_send_icmp_error(&packet));
+    }
+
+    #[test]
+    fn test_should_not_send_error_multicast_source() {
+        let src = "ff02::1".parse().unwrap();
+        let dst = "fd00::2".parse().unwrap();
+        let packet = make_ipv6_packet(src, dst, 17, &[0u8; 8]);
+
+        assert!(!should_send_icmp_error(&packet));
+    }
+
+    #[test]
+    fn test_should_not_send_error_for_icmp_error() {
+        let src = "fd00::1".parse().unwrap();
+        let dst = "fd00::2".parse().unwrap();
+        // ICMPv6 Destination Unreachable (type 1)
+        let icmp_payload = [1u8, 0, 0, 0, 0, 0, 0, 0];
+        let packet = make_ipv6_packet(src, dst, IPPROTO_ICMPV6, &icmp_payload);
+
+        assert!(!should_send_icmp_error(&packet));
+    }
+
+    #[test]
+    fn test_should_send_error_for_icmp_echo() {
+        let src = "fd00::1".parse().unwrap();
+        let dst = "fd00::2".parse().unwrap();
+        // ICMPv6 Echo Request (type 128) - informational, not error
+        let icmp_payload = [128u8, 0, 0, 0, 0, 0, 0, 0];
+        let packet = make_ipv6_packet(src, dst, IPPROTO_ICMPV6, &icmp_payload);
+
+        assert!(should_send_icmp_error(&packet));
+    }
+
+    #[test]
+    fn test_should_not_send_error_short_packet() {
+        let packet = vec![0u8; 20]; // Too short for IPv6
+        assert!(!should_send_icmp_error(&packet));
+    }
+
+    #[test]
+    fn test_build_dest_unreachable() {
+        let src: Ipv6Addr = "fd00::1".parse().unwrap();
+        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
+        let original = make_ipv6_packet(src, dst, 17, &[0u8; 8]);
+
+        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
+        let response = build_dest_unreachable(&original, DestUnreachableCode::NoRoute, our_addr);
+
+        assert!(response.is_some());
+        let response = response.unwrap();
+
+        // Check IPv6 header
+        assert_eq!(response[0] >> 4, 6); // Version
+        assert_eq!(response[6], IPPROTO_ICMPV6); // Next header
+
+        // Check source is our address
+        let resp_src = Ipv6Addr::from(<[u8; 16]>::try_from(&response[8..24]).unwrap());
+        assert_eq!(resp_src, our_addr);
+
+        // Check destination is original source
+        let resp_dst = Ipv6Addr::from(<[u8; 16]>::try_from(&response[24..40]).unwrap());
+        assert_eq!(resp_dst, src);
+
+        // Check ICMPv6 type and code
+        assert_eq!(response[IPV6_HEADER_LEN], 1); // Type = Dest Unreachable
+        assert_eq!(response[IPV6_HEADER_LEN + 1], 0); // Code = No Route
+    }
+
+    #[test]
+    fn test_build_dest_unreachable_invalid_input() {
+        let short_packet = vec![0u8; 20];
+        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
+
+        let response = build_dest_unreachable(&short_packet, DestUnreachableCode::NoRoute, our_addr);
+        assert!(response.is_none());
+    }
+
+    #[test]
+    fn test_build_dest_unreachable_truncates_large_packet() {
+        let src: Ipv6Addr = "fd00::1".parse().unwrap();
+        let dst: Ipv6Addr = "fd00::2".parse().unwrap();
+        // Large payload
+        let original = make_ipv6_packet(src, dst, 17, &[0u8; 2000]);
+
+        let our_addr: Ipv6Addr = "fd00::ffff".parse().unwrap();
+        let response = build_dest_unreachable(&original, DestUnreachableCode::NoRoute, our_addr);
+
+        assert!(response.is_some());
+        let response = response.unwrap();
+
+        // Response must not exceed minimum MTU
+        assert!(response.len() <= MIN_IPV6_MTU);
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 4d021a9..fb7739b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -6,6 +6,7 @@
 pub mod bloom;
 pub mod cache;
 pub mod config;
+pub mod icmp;
 pub mod identity;
 pub mod node;
 pub mod peer;
@@ -52,4 +53,7 @@ pub use peer::{Peer, PeerError, PeerState, UpstreamPeer};
 pub use node::{Node, NodeError, NodeState};
 
 // Re-export TUN types
-pub use tun::{log_ipv6_packet, shutdown_tun_interface, TunDevice, TunError, TunState};
+pub use tun::{log_ipv6_packet, shutdown_tun_interface, TunDevice, TunError, TunState, TunTx, TunWriter};
+
+// Re-export ICMPv6 types
+pub use icmp::{build_dest_unreachable, should_send_icmp_error, DestUnreachableCode, Icmpv6Type};
diff --git a/src/tun.rs b/src/tun.rs
index 89ee0d1..bae7b4a 100644
--- a/src/tun.rs
+++ b/src/tun.rs
@@ -7,12 +7,18 @@
 use crate::{FipsAddress, TunConfig};
 use futures::TryStreamExt;
 use rtnetlink::{new_connection, Handle};
-use std::io::Read;
+use std::fs::File;
+use std::io::{Read, Write};
 use std::net::Ipv6Addr;
+use std::os::unix::io::{AsRawFd, FromRawFd};
+use std::sync::mpsc;
 use thiserror::Error;
-use tracing::{debug, info};
+use tracing::{debug, error, info};
 use tun::Layer;
 
+/// Channel sender for packets to be written to TUN.
+pub type TunTx = mpsc::Sender<Vec<u8>>;
+
 /// Errors that can occur with TUN operations.
 #[derive(Debug, Error)]
 pub enum TunError {
@@ -150,6 +156,72 @@ impl TunDevice {
         info!(name = %self.name, "Deleting TUN device");
         delete_interface(&self.name).await
     }
+
+    /// Create a TunWriter for this device.
+    ///
+    /// This duplicates the underlying file descriptor so that reads and writes
+    /// can happen independently on separate threads. Returns the writer and
+    /// a channel sender for submitting packets to be written.
+    pub fn create_writer(&self) -> Result<(TunWriter, TunTx), TunError> {
+        let fd = self.device.as_raw_fd();
+
+        // Duplicate the file descriptor for writing
+        let write_fd = unsafe { libc::dup(fd) };
+        if write_fd < 0 {
+            return Err(TunError::Configure(format!(
+                "failed to dup fd: {}",
+                std::io::Error::last_os_error()
+            )));
+        }
+
+        let write_file = unsafe { File::from_raw_fd(write_fd) };
+        let (tx, rx) = mpsc::channel();
+
+        Ok((
+            TunWriter {
+                file: write_file,
+                rx,
+                name: self.name.clone(),
+            },
+            tx,
+        ))
+    }
+}
+
+/// Writer thread for TUN device.
+///
+/// Services a queue of outbound packets and writes them to the TUN device.
+/// Multiple producers can send packets via the TunTx channel.
+pub struct TunWriter {
+    file: File,
+    rx: mpsc::Receiver<Vec<u8>>,
+    name: String,
+}
+
+impl TunWriter {
+    /// Run the writer loop.
+    ///
+    /// Blocks forever, reading packets from the channel and writing them
+    /// to the TUN device. Returns when the channel is closed (all senders dropped).
+    pub fn run(mut self) {
+        info!(name = %self.name, "TUN writer starting");
+
+        for packet in self.rx {
+            if let Err(e) = self.file.write_all(&packet) {
+                // "Bad address" is expected during shutdown when interface is deleted
+                let err_str = e.to_string();
+                if err_str.contains("Bad address") {
+                    info!(name = %self.name, "TUN interface deleted, writer stopping");
+                    break;
+                }
+                error!(name = %self.name, error = %e, "TUN write error");
+            } else {
+                debug!(name = %self.name, len = packet.len(), "TUN packet written");
+            }
+        }
+
+        info!(name = %self.name, "TUN writer stopped");
+    }
 }
 
 /// Log basic information about an IPv6 packet at DEBUG level.