implementation of local address-bound dialer

go.mod

@@ -5,6 +5,7 @@ go 1.13
 require (
 	github.com/GehirnInc/crypt v0.0.0-20230320061759-8cc1b52080c5 // indirect
 	github.com/coreos/go-systemd/v22 v22.5.0 // indirect
+	github.com/hashicorp/go-multierror v1.1.1 // indirect
 	github.com/kr/pretty v0.3.1 // indirect
 	github.com/tg123/go-htpasswd v1.2.1
 	golang.org/x/crypto v0.7.0

go.sum

@@ -8,6 +8,10 @@ github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSs
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/godbus/dbus/v5 v5.0.4/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
+github.com/hashicorp/errwrap v1.0.0 h1:hLrqtEDnRye3+sgx6z4qVLNuviH3MR5aQ0ykNJa/UYA=
+github.com/hashicorp/errwrap v1.0.0/go.mod h1:YH+1FKiLXxHSkmPseP+kNlulaMuP3n2brvKWEqk/Jc4=
+github.com/hashicorp/go-multierror v1.1.1 h1:H5DkEtf6CXdFp0N0Em5UCwQpXMWke8IA0+lD48awMYo=
+github.com/hashicorp/go-multierror v1.1.1/go.mod h1:iw975J/qwKPdAO1clOe2L8331t/9/fmwbPZ6JB6eMoM=
 github.com/kr/pretty v0.2.1/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=

hintdialer.go

@@ -0,0 +1,138 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+
+	"github.com/hashicorp/go-multierror"
+)
+
+var (
+	ErrNoSuitableAddress  = errors.New("no suitable address")
+	ErrBadIPAddressLength = errors.New("bad IP address length")
+	ErrUnknownNetwork     = errors.New("unknown network")
+)
+
+type BoundDialerContextKey struct{}
+
+type BoundDialerDefaultSink interface {
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+type BoundDialer struct {
+	defaultDialer BoundDialerDefaultSink
+	defaultHints  []net.IP
+}
+
+func NewBoundDialer(defaultDialer BoundDialerDefaultSink, defaultHints []net.IP) *BoundDialer {
+	if defaultDialer == nil {
+		defaultDialer = &net.Dialer{}
+	}
+	return &BoundDialer{
+		defaultDialer: defaultDialer,
+		defaultHints:  defaultHints,
+	}
+}
+
+func (d *BoundDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	hints := d.defaultHints
+	if hintsOverride := ctx.Value(BoundDialerContextKey{}); hintsOverride != nil {
+		if hintsOverrideValue, ok := hintsOverride.([]net.IP); ok {
+			hints = hintsOverrideValue
+		}
+	}
+
+	if len(hints) == 0 {
+		return d.defaultDialer.DialContext(ctx, network, address)
+	}
+
+	var netBase string
+	switch network {
+	case "tcp", "tcp4", "tcp6":
+		netBase = "tcp"
+	case "udp", "udp4", "udp6":
+		netBase = "udp"
+	case "ip", "ip4", "ip6":
+		netBase = "ip"
+	default:
+		return d.defaultDialer.DialContext(ctx, network, address)
+	}
+
+	var resErr error
+	for _, lIP := range hints {
+		lAddr, restrictedNetwork, err := ipToLAddr(netBase, lIP)
+		if err != nil {
+			resErr = multierror.Append(resErr, fmt.Errorf("ipToLAddr(%q) failed: %w", lIP.String(), err))
+			continue
+		}
+		if network != netBase && network != restrictedNetwork {
+			continue
+		}
+
+		conn, err := (&net.Dialer{
+			LocalAddr: lAddr,
+		}).DialContext(ctx, restrictedNetwork, address)
+		if err != nil {
+			resErr = multierror.Append(resErr, fmt.Errorf("dial failed: %w", err))
+		} else {
+			return conn, nil
+		}
+	}
+
+	if resErr == nil {
+		resErr = ErrNoSuitableAddress
+	}
+	return nil, resErr
+}
+
+func (d *BoundDialer) Dial(network, address string) (net.Conn, error) {
+	return d.DialContext(context.Background(), network, address)
+}
+
+func ipToLAddr(network string, ip net.IP) (net.Addr, string, error) {
+	v6 := true
+	if ip4 := ip.To4(); len(ip4) == net.IPv4len {
+		ip = ip4
+		v6 = false
+	} else if len(ip) != net.IPv6len {
+		return nil, "", ErrBadIPAddressLength
+	}
+
+	var lAddr net.Addr
+	var lNetwork string
+	switch network {
+	case "tcp", "tcp4", "tcp6":
+		lAddr = &net.TCPAddr{
+			IP: ip,
+		}
+		if v6 {
+			lNetwork = "tcp6"
+		} else {
+			lNetwork = "tcp4"
+		}
+	case "udp", "udp4", "udp6":
+		lAddr = &net.UDPAddr{
+			IP: ip,
+		}
+		if v6 {
+			lNetwork = "udp6"
+		} else {
+			lNetwork = "udp4"
+		}
+	case "ip", "ip4", "ip6":
+		lAddr = &net.IPAddr{
+			IP: ip,
+		}
+		if v6 {
+			lNetwork = "ip6"
+		} else {
+			lNetwork = "ip4"
+		}
+	default:
+		return nil, "", ErrUnknownNetwork
+	}
+
+	return lAddr, lNetwork, nil
+}