domaininfo: New package to track domain (security) information

This patch introduces a new "domaininfo" package, which implements a
database with information about domains.  In particular, it tracks
incoming and outgoing security levels.

That information is used in incoming and outgoing SMTP to prevent
downgrades.

internal/domaininfo/domaininfo.pb.go

@@ -0,0 +1,96 @@
+// Code generated by protoc-gen-go.
+// source: domaininfo.proto
+// DO NOT EDIT!
+
+/*
+Package domaininfo is a generated protocol buffer package.
+
+It is generated from these files:
+	domaininfo.proto
+
+It has these top-level messages:
+	Domain
+*/
+package domaininfo
+
+import proto "github.com/golang/protobuf/proto"
+import fmt "fmt"
+import math "math"
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion2 // please upgrade the proto package
+
+type SecLevel int32
+
+const (
+	// Does not do TLS.
+	SecLevel_PLAIN SecLevel = 0
+	// TLS client connection (no certificate validation).
+	SecLevel_TLS_CLIENT SecLevel = 1
+	// TLS, but with invalid certificates.
+	SecLevel_TLS_INSECURE SecLevel = 2
+	// TLS, with valid certificates.
+	SecLevel_TLS_SECURE SecLevel = 3
+)
+
+var SecLevel_name = map[int32]string{
+	0: "PLAIN",
+	1: "TLS_CLIENT",
+	2: "TLS_INSECURE",
+	3: "TLS_SECURE",
+}
+var SecLevel_value = map[string]int32{
+	"PLAIN":        0,
+	"TLS_CLIENT":   1,
+	"TLS_INSECURE": 2,
+	"TLS_SECURE":   3,
+}
+
+func (x SecLevel) String() string {
+	return proto.EnumName(SecLevel_name, int32(x))
+}
+func (SecLevel) EnumDescriptor() ([]byte, []int) { return fileDescriptor0, []int{0} }
+
+type Domain struct {
+	Name string `protobuf:"bytes,1,opt,name=name" json:"name,omitempty"`
+	// Security level for mail coming from this domain (they send to us).
+	IncomingSecLevel SecLevel `protobuf:"varint,2,opt,name=incoming_sec_level,json=incomingSecLevel,enum=domaininfo.SecLevel" json:"incoming_sec_level,omitempty"`
+	// Security level for mail going to this domain (we send to them).
+	OutgoingSecLevel SecLevel `protobuf:"varint,3,opt,name=outgoing_sec_level,json=outgoingSecLevel,enum=domaininfo.SecLevel" json:"outgoing_sec_level,omitempty"`
+}
+
+func (m *Domain) Reset()                    { *m = Domain{} }
+func (m *Domain) String() string            { return proto.CompactTextString(m) }
+func (*Domain) ProtoMessage()               {}
+func (*Domain) Descriptor() ([]byte, []int) { return fileDescriptor0, []int{0} }
+
+func init() {
+	proto.RegisterType((*Domain)(nil), "domaininfo.Domain")
+	proto.RegisterEnum("domaininfo.SecLevel", SecLevel_name, SecLevel_value)
+}
+
+func init() { proto.RegisterFile("domaininfo.proto", fileDescriptor0) }
+
+var fileDescriptor0 = []byte{
+	// 189 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x09, 0x6e, 0x88, 0x02, 0xff, 0xe2, 0x12, 0x48, 0xc9, 0xcf, 0x4d,
+	0xcc, 0xcc, 0xcb, 0xcc, 0x4b, 0xcb, 0xd7, 0x2b, 0x28, 0xca, 0x2f, 0xc9, 0x17, 0xe2, 0x42, 0x88,
+	0x28, 0x2d, 0x61, 0xe4, 0x62, 0x73, 0x01, 0x73, 0x85, 0x84, 0xb8, 0x58, 0xf2, 0x12, 0x73, 0x53,
+	0x25, 0x18, 0x15, 0x18, 0x35, 0x38, 0x83, 0xc0, 0x6c, 0x21, 0x27, 0x2e, 0xa1, 0xcc, 0xbc, 0xe4,
+	0xfc, 0xdc, 0xcc, 0xbc, 0xf4, 0xf8, 0xe2, 0xd4, 0xe4, 0xf8, 0x9c, 0xd4, 0xb2, 0xd4, 0x1c, 0x09,
+	0x26, 0xa0, 0x0a, 0x3e, 0x23, 0x11, 0x3d, 0x24, 0x93, 0x83, 0x53, 0x93, 0x7d, 0x40, 0x72, 0x41,
+	0x02, 0x30, 0xf5, 0x30, 0x11, 0x90, 0x19, 0xf9, 0xa5, 0x25, 0xe9, 0xf9, 0xa8, 0x66, 0x30, 0xe3,
+	0x33, 0x03, 0xa6, 0x1e, 0x26, 0xa2, 0xe5, 0xce, 0xc5, 0x01, 0x37, 0x8f, 0x93, 0x8b, 0x35, 0xc0,
+	0xc7, 0xd1, 0xd3, 0x4f, 0x80, 0x41, 0x88, 0x8f, 0x8b, 0x2b, 0xc4, 0x27, 0x38, 0xde, 0xd9, 0xc7,
+	0xd3, 0xd5, 0x2f, 0x44, 0x80, 0x51, 0x48, 0x80, 0x8b, 0x07, 0xc4, 0xf7, 0xf4, 0x0b, 0x76, 0x75,
+	0x0e, 0x0d, 0x72, 0x15, 0x60, 0x82, 0xa9, 0x80, 0xf2, 0x99, 0x93, 0xd8, 0xc0, 0x41, 0x60, 0x0c,
+	0x08, 0x00, 0x00, 0xff, 0xff, 0x2c, 0x78, 0x65, 0x5b, 0x16, 0x01, 0x00, 0x00,
+}