feat: socks5 proxy server (#3336)

- `SOCKS5_ENABLED=off`
- `SOCKS5_LISTENING_ADDRESS=":1080"`
- `SOCKS5_USER=`
- `SOCKS5_PASSWORD=`

internal/socks5/constants.go

@@ -0,0 +1,86 @@
+package socks5
+
+import "fmt"
+
+// See https://datatracker.ietf.org/doc/html/rfc1928#section-3
+type authMethod byte
+
+const (
+	authNotRequired      authMethod = 0
+	authGssapi           authMethod = 1
+	authUsernamePassword authMethod = 2
+	authNotAcceptable    authMethod = 255
+)
+
+func (a authMethod) String() string {
+	switch a {
+	case authNotRequired:
+		return "no authentication required"
+	case authGssapi:
+		return "GSSAPI"
+	case authUsernamePassword:
+		return "username/password"
+	case authNotAcceptable:
+		return "no acceptable methods"
+	default:
+		return fmt.Sprintf("unknown method (%d)", a)
+	}
+}
+
+// Subnegotiation version
+// See https://datatracker.ietf.org/doc/html/rfc1929#section-2
+const (
+	authUsernamePasswordSubNegotiation1 byte = 1
+)
+
+// SOCKS versions.
+const (
+	socks5Version byte = 5
+)
+
+// See https://datatracker.ietf.org/doc/html/rfc1928#section-4
+type cmdType byte
+
+const (
+	connect      cmdType = 1
+	bind         cmdType = 2
+	udpAssociate cmdType = 3
+)
+
+func (c cmdType) String() string {
+	switch c {
+	case connect:
+		return "connect"
+	case bind:
+		return "bind"
+	case udpAssociate:
+		return "UDP associate"
+	default:
+		return fmt.Sprintf("unknown command (%d)", c)
+	}
+}
+
+// See https://datatracker.ietf.org/doc/html/rfc1928#section-4 and
+// https://datatracker.ietf.org/doc/html/rfc1928#section-5
+type addrType byte
+
+const (
+	ipv4       addrType = 1
+	domainName addrType = 3
+	ipv6       addrType = 4
+)
+
+// See https://datatracker.ietf.org/doc/html/rfc1928#section-6
+type replyCode byte
+
+const (
+	succeeded replyCode = iota
+	generalServerFailure
+	connectionNotAllowedByRuleset
+	networkUnreachable
+	hostUnreachable
+	connectionRefused
+	ttlExpired
+	commandNotSupported
+	addressTypeNotSupported
+)