Documentation/op-guide: document TLS changes in 3.2

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

Documentation/op-guide/security.md

@@ -193,6 +193,80 @@ The proxy communicates with etcd members through both the `--advertise-client-ur
 
 When client authentication is enabled for an etcd member, the administrator must ensure that the peer certificate specified in the proxy's `--peer-cert-file` option is valid for that authentication. The proxy's peer certificate must also be valid for peer authentication if peer authentication is enabled.
 
+## Notes for TLS authentication
+
+Since [v3.2.0](https://github.com/coreos/etcd/blob/master/CHANGELOG.md#v320-2017-06-09), [TLS certificates get reloaded on every client connection](https://github.com/coreos/etcd/pull/7829). This is useful when replacing expiry certs without stopping etcd servers; it can be done by overwriting old certs with new ones. Refreshing certs for every connection should not have too much overhead, but can be improved in the future, with caching layer. Example tests can be found [here](https://github.com/coreos/etcd/blob/b041ce5d514a4b4aaeefbffb008f0c7570a18986/integration/v3_grpc_test.go#L1601-L1757).
+
+Since [v3.2.0](https://github.com/coreos/etcd/blob/master/CHANGELOG.md#v320-2017-06-09), [server denies incoming peer certs with wrong IP `SAN`](https://github.com/coreos/etcd/pull/7687). For instance, if peer cert contains IP addresses in Subject Alternative Name (SAN) field, server authenticates a peer only when the remote IP address matches one of those IP addresses. This is to prevent unauthorized endpoints from joining the cluster. For example, peer B's CSR (with `cfssl`) is:
+
+```json
+{
+  "CN": "etcd peer",
+  "hosts": [
+    "*.example.default.svc",
+    "*.example.default.svc.cluster.local",
+    "10.138.0.27"
+  ],
+  "key": {
+    "algo": "rsa",
+    "size": 2048
+  },
+  "names": [
+    {
+      "C": "US",
+      "L": "CA",
+      "ST": "San Francisco"
+    }
+  ]
+}
+```
+
+when peer B's actual IP address is `10.138.0.2`, not `10.138.0.27`. When peer B tries to join the cluster, peer A will reject B with the error `x509: certificate is valid for 10.138.0.27, not 10.138.0.2`, because B's remote IP address does not match the one in Subject Alternative Name (SAN) field.
+
+Since [v3.2.0](https://github.com/coreos/etcd/blob/master/CHANGELOG.md#v320-2017-06-09), [server resolves TLS `DNSNames` when checking `SAN`](https://github.com/coreos/etcd/pull/7767). For instance, if peer cert contains any DNS names in Subject Alternative Name (SAN) field, server authenticates a peer only when forward-lookups on those DNS names have matching IP with the remote IP address. For example, peer B's CSR (with `cfssl`) is:
+
+```json
+{
+  ...
+  "hosts": [
+    "b.com"
+  ],
+  ...
+}
+```
+
+when peer B's remote IP address is `10.138.0.2`. When peer B tries to join the cluster, peer A looks up the incoming host `b.com` to get the list of IP addresses (e.g. `dig b.com`). And rejects B if the list does not contain the IP `10.138.0.2`, with the error `tls: 10.138.0.2 does not match any of DNSNames ["b.com"]`.
+
+Since [v3.2.2](https://github.com/coreos/etcd/blob/master/CHANGELOG.md#v322-2017-07-07), [server accepts connections if IP matches, without checking DNS entries](https://github.com/coreos/etcd/pull/8223). For instance, if peer cert contains IP addresses and DNS names in Subject Alternative Name (SAN) field, and the remote IP address matches one of those IP addresses, server just accepts connection without further checking the DNS names. For example, peer B's CSR (with `cfssl`) is:
+
+```json
+{
+  ...
+  "hosts": [
+    "invalid.domain",
+    "10.138.0.2"
+  ],
+  ...
+}
+```
+
+when peer B's remote IP address is `10.138.0.2` and `invalid.domain` is a invalid host. When peer B tries to join the cluster, peer A successfully authenticates B, since Subject Alternative Name (SAN) field has a valid matching IP address. See [issue#8206](https://github.com/coreos/etcd/issues/8206) for more detail.
+
+Since [v3.2.5](https://github.com/coreos/etcd/blob/master/CHANGELOG.md#v325-2017-08-04), [server supports reverse-lookup on wildcard DNS `SAN`](https://github.com/coreos/etcd/pull/8281). For instance, if peer cert contains only DNS names (no IP addresses) in Subject Alternative Name (SAN) field, server first reverse-lookups the remote IP address to get a list of names mapping to that address (e.g. `nslookup IPADDR`). Then accepts the connection if those names have a matching name with peer cert's DNS names (either by exact or wildcard match). If none is matched, server forward-lookups each DNS entry in peer cert (e.g. look up `example.default.svc` when the entry is `*.example.default.svc`), and accepts connection only when the host's resolved addresses have the matching IP address with the peer's remote IP address. For example, peer B's CSR (with `cfssl`) is:
+
+```json
+{
+  ...
+  "hosts": [
+    "*.example.default.svc",
+    "*.example.default.svc.cluster.local"
+  ],
+  ...
+}
+```
+
+when peer B's remote IP address is `10.138.0.2`. When peer B tries to join the cluster, peer A reverse-lookup the IP `10.138.0.2` to get the list of host names. And either exact or wildcard match the host names with peer B's cert DNS names in Subject Alternative Name (SAN) field. If none of reverse/forward lookups worked, it returns an error `"tls: "10.138.0.2" does not match any of DNSNames ["*.example.default.svc","*.example.default.svc.cluster.local"]`. See [issue#8268](https://github.com/coreos/etcd/issues/8268) for more detail.
+
 ## Frequently asked questions
 
 ### I'm seeing a SSLv3 alert handshake failure when using TLS client authentication?