UDP Proxy

UdpProxy receives UDP datagrams from clients and forwards each one to a backend server, then relays the backend’s reply back to the original sender. This request-reply model covers protocols such as DNS, NTP, RADIUS, and syslog.

Basic usage

use rust_web_server::udp_proxy::UdpProxy;

// Forward DNS queries round-robin across two resolvers
UdpProxy::new(["8.8.8.8:53", "8.8.4.4:53"])
    .reply_timeout_ms(2_000)
    .bind("0.0.0.0:53")
    .unwrap();

.bind() blocks the calling thread indefinitely. Run it in a dedicated thread or process.

Per-datagram thread model

Each received datagram is handled entirely in its own OS thread so the main receive loop is never blocked waiting for a backend reply:

Main loop calls recv_from() on the bound socket.
A new thread is spawned for each datagram.
The thread creates an ephemeral UdpSocket bound to 0.0.0.0:0.
The thread sends the datagram to the selected backend.
The thread waits up to reply_timeout for a reply.
The reply is sent back to the original client address via the shared socket.

No session state is kept between datagrams. Each datagram is independent.

Round-robin backend selection

An AtomicUsize counter selects backends without locking, cycling through the list evenly across datagrams:

UdpProxy::new([
    "resolver-1:53",
    "resolver-2:53",
    "resolver-3:53",
])

Timeouts and buffer size

UdpProxy::new(backends)
    .reply_timeout_ms(5_000)    // wait for backend reply (default: 5 000 ms)
    .buffer_size(65_536)        // per-datagram buffer bytes (default: 65 536)

If the backend does not reply within reply_timeout_ms, the datagram is silently dropped — the client receives no response for that query. This matches the behaviour of most UDP protocols, which implement their own retransmission logic.

Config-file usage

Activate UdpProxy from rws.config.toml with a [[udp_proxy]] section:

[[udp_proxy]]
bind = "0.0.0.0:53"
backends = ["dns1.internal:53", "dns2.internal:53"]
read_timeout_ms = 500

Multiple [[udp_proxy]] sections are supported — each starts an independent listener.

Use cases

Protocol	Typical port
DNS forwarding	53
NTP relay	123
syslog relay	514
SNMP proxying	161
RADIUS	1812 / 1813
DTLS passthrough	any