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Auto-escalate max-qname-len when MTU is too small for the domain #22

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Auto-escalate max-qname-len when MTU is too small for the domain #22

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33 changes: 31 additions & 2 deletions client/client.go
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Original file line number Diff line number Diff line change
Expand Up @@ -172,6 +172,27 @@ func (ts *TunnelServer) effectiveMaxQnameLen() int {
return 101
}

// autoEscalateMaxQnameLen attempts to find a working max QNAME length when
// the current setting produces an MTU that is too small. It tries progressively
// larger values up to the RFC 1035 maximum of 253 bytes. If a working value is
// found, it updates MaxQnameLen and returns the new MTU. Otherwise it returns 0.
func (t *Tunnel) autoEscalateMaxQnameLen() int {
original := t.TunnelServer.effectiveMaxQnameLen()
for _, candidate := range []int{150, 200, 253} {
if candidate <= original {
continue
}
mtu := DNSNameCapacity(t.TunnelServer.Addr, candidate, t.TunnelServer.MaxNumLabels) - t.wireConfig.DataOverhead()
if mtu >= 50 {
log.Warnf("max-qname-len %d is too small for domain %s (MTU < 50); auto-raised to %d (MTU %d)",
original, t.TunnelServer.Addr, candidate, mtu)
t.TunnelServer.MaxQnameLen = candidate
return mtu
}
}
return 0
}

// Tunnel represents a DNS tunnel connection. Create with NewTunnel, then
// either call the step-by-step Initiate* methods (for embedding in frameworks
// like xray-core) or call ListenAndServe for a fully managed session.
Expand Down Expand Up @@ -337,9 +358,13 @@ func (t *Tunnel) InitiateKCPConn(mtu int) error {
if mtu <= 0 {
maxQnameLen := t.TunnelServer.effectiveMaxQnameLen()
mtu = DNSNameCapacity(t.TunnelServer.Addr, maxQnameLen, t.TunnelServer.MaxNumLabels) - t.wireConfig.DataOverhead()
if mtu < 50 {
mtu = t.autoEscalateMaxQnameLen()
}
}
if mtu < 50 {
return fmt.Errorf("MTU %d is too small (minimum 50)", mtu)
return fmt.Errorf("MTU %d is too small (minimum 50); try increasing -max-qname-len (current: %d, max: 253) or use a shorter -domain",
mtu, t.TunnelServer.effectiveMaxQnameLen())
}
t.TunnelServer.MTU = mtu
log.Infof("effective MTU %d", mtu)
Expand Down Expand Up @@ -496,7 +521,11 @@ func (t *Tunnel) ListenAndServe(listenAddr string) error {
maxQnameLen := t.TunnelServer.effectiveMaxQnameLen()
mtu := DNSNameCapacity(t.TunnelServer.Addr, maxQnameLen, t.TunnelServer.MaxNumLabels) - t.wireConfig.DataOverhead()
if mtu < 50 {
return fmt.Errorf("MTU %d is too small (minimum 50)", mtu)
mtu = t.autoEscalateMaxQnameLen()
}
if mtu < 50 {
return fmt.Errorf("MTU %d is too small (minimum 50); try increasing -max-qname-len (current: %d, max: 253) or use a shorter -domain",
mtu, t.TunnelServer.effectiveMaxQnameLen())
}
log.Infof("effective MTU %d", mtu)

Expand Down