Shadowsocks is a lightweight, efficient proxied tunnel often used to bypass network restrictions. However, many administrators and developers run into DNS resolution problems when routing traffic through Shadowsocks. This article provides a practical, technically detailed troubleshooting guide with concrete commands, configuration tips, and testing methods to restore correct DNS behavior while avoiding DNS leaks and performance pitfalls.
Understanding the root causes
Before changing configurations, it helps to know why DNS issues appear when using Shadowsocks. Common causes include:
- Local DNS still resolving directly — DNS queries bypass the tunnel and go to the ISP resolver, causing timeouts or leaks.
- UDP blocking or filtering — Many networks block DNS over UDP (port 53) or rate-limit it, causing queries to fail.
- Proxy client not handling DNS — Some Shadowsocks clients only tunnel TCP; DNS queries (UDP) are not forwarded without explicit UDP relay enabled.
- System resolver conflicts — systemd-resolved, NetworkManager, dnsmasq, and /etc/resolv.conf interactions can route queries incorrectly.
- MTU/fragmentation issues — Large EDNS0 responses or DNSSEC responses may be fragmented and dropped.
Diagnose the problem: systematic checks
Follow this checklist to isolate what’s failing. Run each step and note the results.
1) Confirm raw connectivity to the DNS server
Use dig to query a public resolver directly. If this fails, upstream network or firewall might be blocking UDP/53.
Example: dig +short @8.8.8.8 www.example.com
2) Check whether DNS queries are leaving the machine via the tunnel
Run tcpdump to inspect outbound traffic. On Linux:
sudo tcpdump -n -s 0 -A udp port 53
If you see DNS queries to the ISP’s IP rather than forwarded to the Shadowsocks local proxy (or not encapsulated into the tunnel), local resolution is leaking.
3) Verify Shadowsocks UDP support
Shadowsocks has both TCP and UDP modes. On the server, ensure the server process listens for UDP: for shadowsocks-libev use the -u flag (ss-server -u -p 8388 -k your_password -m aes-256-gcm).
On the client, ensure the client supports UDP relay (ss-local -u for shadowsocks-libev). If using a GUI client, enable “UDP Relay” or “Enable UDP forwarding”.
4) Inspect system resolver
Check /etc/resolv.conf, and if systemd-resolved is present, inspect resolvectl status. Mismatches between the stub resolver (127.0.0.53) and dnsmasq can cause queries to bypass local proxies.
5) Check for TCP fallback
Some resolvers accept TCP on port 53. Test: dig +tcp @8.8.8.8 www.example.com. If TCP works but UDP fails, UDP blocking is likely.
Quick practical fixes
Below are targeted solutions depending on the diagnosis above. Apply the one(s) that match your environment.
1) Route DNS through the Shadowsocks local proxy
The most reliable fix is to make the system resolver send DNS queries to a local DNS forwarder that itself is tunneled through Shadowsocks.
- Install a lightweight DNS forwarder such as dnsmasq or pdnsd.
- Configure dnsmasq to use a public resolver (8.8.8.8) and bind to 127.0.0.1:53.
- Point /etc/resolv.conf to 127.0.0.1.
- Start ss-local with UDP enabled (ss-local -u -s server_ip -p server_port -l 1080 -k password -m aes-256-gcm). dnsmasq queries will be sent to localhost and then forwarded through the Shadowsocks process (if the local proxy is set to forward DNS).
Note: If dnsmasq directly queries an external IP, it may bypass the tunnel. Combine this with iptables redirection described next to make dnsmasq send queries via the local SOCKS/HTTP proxy or redirect UDP to ss-local.
2) Redirect DNS UDP port 53 to local resolver (iptables)
On Linux with iptables you can transparently redirect outbound UDP port 53 to a local stub resolver which is handled inside the tunnel.
Example commands (run as root):
iptables -t nat -A OUTPUT -p udp –dport 53 -j DNAT –to-destination 127.0.0.1:53
Or to redirect to ss-local UDP relay (assume ss-local listens on 127.0.0.1:1080 with UDP handling):
iptables -t nat -A OUTPUT -p udp –dport 53 -j REDIRECT –to-ports 1080
Adjust based on your client’s UDP listening port. This ensures local applications sending UDP/53 get captured and handled by the local proxy. If using nftables, use equivalent nft rules.
3) Use TCP-based DNS or DoH/DoT
If UDP traffic is blocked, you can force DNS over TCP or encrypted DNS:
- Use dig +tcp or configure your resolver to prefer TCP. This is a workaround rather than a permanent fix.
- Install a DoH/DoT client (cloudflared, dnscrypt-proxy) configured to upstream via HTTPS/TLS to a public resolver. Run it locally on 127.0.0.1 and point /etc/resolv.conf to it. Encrypted DNS goes over port 443/853 which is often allowed and can be tunneled through Shadowsocks reliably.
4) Enable UDP relay on server and client
UDP DNS queries won’t traverse if the server or client lacks UDP relay support. For shadowsocks-libev:
Server: ss-server -u -s 0.0.0.0 -p 8388 -k password -m aes-256-gcm
Client: ss-local -u -s server_ip -p 8388 -l 1080 -k password -m aes-256-gcm
On GUI clients, look for the “UDP Relay” or “Enable UDP forwarding” option and enable it. After enabling, test again with dig while the tunnel is active.
5) Fix systemd-resolved and stub resolver conflicts
If systemd-resolved is enabled (common on modern distros), the stub resolver at 127.0.0.53 might be used by applications directly. To integrate it with the tunnel:
- Option A: Configure systemd-resolved to forward upstream to a local DoH or local dnsmasq instance. Edit /etc/systemd/resolved.conf and set DNS=127.0.0.1 and restart resolved.
- Option B: Disable the stub resolver symlink and replace /etc/resolv.conf with 127.0.0.1, or use resolvectl to set per-interface DNS servers that match the tunnel setup.
Performance and advanced considerations
EDNS0 and fragmentation
Large DNS responses with EDNS0 may be fragmented and then dropped by firewalls. If you suspect this, disable EDNS0 truncation or reduce the DNS query payload size. For dig: dig +noedns @resolver domain.
TTL and cache behavior
Local caches such as dnsmasq will respect TTLs. If a domain resolves intermittently, the cache may hide the issue. Clear caches when testing: systemd-resolve –flush-caches or restart dnsmasq.
DNS leak testing
To ensure DNS queries are actually going through the tunnel, use public leak-testing pages or run tcpdump on both the LAN interface and the tunnel interface. Confirm query destinations match the remote resolver (inside tunnel) rather than ISP’s servers.
Failover and redundancy
Configure a local forwarder with multiple upstream resolvers and short retry policies so that if one route fails, another is attempted quickly. For dnsmasq, list upstream servers in /etc/dnsmasq.conf:
server=127.0.0.1#5053 # Example local DoH client
server=8.8.8.8
Troubleshooting checklist and commands
- Check current resolvers: cat /etc/resolv.conf
- Check systemd-resolved: resolvectl status
- Capture DNS traffic: sudo tcpdump -n -s 0 -A udp port 53
- Test DNS over UDP: dig @8.8.8.8 www.example.com
- Test DNS over TCP: dig +tcp @8.8.8.8 www.example.com
- Check Shadowsocks listening: ss-local -l 1080 -u (or netstat -tunlp | grep ss-local)
- Flush caches: sudo systemd-resolve –flush-caches or sudo systemctl restart dnsmasq
- iptables redirect for UDP: iptables -t nat -A OUTPUT -p udp –dport 53 -j REDIRECT –to-ports 1080
Platform-specific tips
Windows
Windows applications may use the system resolver and bypass user-space proxies. Use a local DNS forwarder (e.g., Acrylic DNS Proxy) listening on 127.0.0.1 and ensure Shadowsocks Windows client has “Enable UDP” turned on. Alternatively, use system-wide routing tools such as Proxifier or ForceBindIP to force DNS through the proxy.
Android
Use the official Shadowsocks Android app with “Tun2socks” or “Global proxy” mode and enable “Remote DNS” or “Fake IP” options. If using Android 9+, configure Private DNS (DoT) to a local DoT client or remote DoH provider.
macOS
Use a local DNS proxy (dnscrypt-proxy or AdGuard Home) and set /etc/resolv.conf to 127.0.0.1. If using a system-level VPN profile, ensure it captures DNS queries or use the Shadowsocks client’s DNS settings.
When to escalate to server-side changes
If all client-side fixes fail, check the server:
- Confirm ss-server has UDP enabled and the server host firewall allows UDP/port.
- Inspect server CPU and network for packet drops—UDP can be more sensitive to load.
- Consider running a dedicated DNS resolver on the server (e.g., Unbound) configured to serve clients over TLS or DoH and expose it only to authenticated tunnels.
In summary, resolving Shadowsocks DNS issues requires identifying whether DNS queries are leaking, being blocked, or simply not forwarded due to client/server configuration. The most robust approaches are to run a local DNS forwarder bound to localhost, enable UDP relay on both client and server, and/or use encrypted TCP-based DNS (DoH/DoT) when UDP is unreliable. Use tcpdump, dig, and resolver configuration inspection to validate changes and prevent leaks.
For more practical guides and configuration examples related to private networking and proxy setups, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.