shadowsocks/shadowsocks-rust · error

failed to encode DNS response

Error message

failed to encode DNS response

What it means

In the fake-DNS UDP server's run loop, converting the constructed response Message to wire bytes via rsp_message.to_vec() fails; the error is logged as "failed to encode DNS response" and returned as an io::Error. The UDP client receives no reply.

Source

Thrown at crates/shadowsocks-service/src/local/fake_dns/udp_server.rs:80

                    error!("failed to parse DNS request, error: {}", err);
                    continue;
                }
            };

            let rsp_message = match handle_dns_request(&req_message, &self.manager).await {
                Ok(m) => m,
                Err(err) => {
                    error!("failed to handle DNS request, error: {}", err);

                    Message::error_msg(req_message.id, req_message.op_code, ResponseCode::ServFail)
                }
            };

            let rsp_buffer = match rsp_message.to_vec() {
                Ok(b) => b,
                Err(err) => {
                    error!("failed to encode DNS response, error: {}", err);
                    return Err(io::Error::new(io::ErrorKind::Other, err));
                }
            };
            let _ = self.listener.send_to(&rsp_buffer, peer_addr).await;
        }
    }
}

View on GitHub (pinned to 8eb0f0a65b)

Solutions

1. Upgrade shadowsocks-service/hickory-proto to matching versions
2. Log and inspect the underlying encoder error and the offending query
3. Verify the fake-dns address mapping state isn't producing invalid record data
4. Work around by using real DNS resolution for the affected client

// client sees no UDP reply; guard with timeout and fallback:
match timeout(UDP_DNS_TIMEOUT, udp_dns_query(q)).await {
    Ok(Err(_)) | Err(_) => fallback_to_system_resolver(q).await,
    Ok(Ok(m)) => m,
}

When it happens

Trigger: run() handles an inbound UDP DNS packet, builds rsp_message (typically an address-mapping answer), and Message::to_vec() returns a hickory encoding error.

Common situations: Oversized or unusual response failing to serialize; hickory-proto version incompatibility; corrupted/edge-case query triggering an invalid response construction.

Related errors

• failed to encode DNS response
• unexpected response from 8.8.8.8:53
• redir destination must not be an domain name address
• tun destination must not be an domain name address
• missing `local_dns_addr` or `remote_dns_addr` in configurati

AI-assisted analysis of shadowsocks/shadowsocks-rust@8eb0f0a65b (2026-09-09). Data as JSON: /api/errors/3a341db97c4f1403. Report an issue: GitHub.