#3868 avoid KeyError when canonicaldomains is missing

paramiko assumes that `canonicaldomains` is always present when `CanonicalizeHostname` is set, and it errors out when it is not, preventing the whole host config from being parsed.

Xpra-org/xpra#3868 (comment)

paramiko/config.py

@@ -233,7 +233,7 @@ def lookup(self, hostname):
         if canon and hostname.count(".") <= maxdots:
             # NOTE: OpenSSH manpage does not explicitly state this, but its
             # implementation for CanonicalDomains is 'split on any whitespace'.
-            domains = options["canonicaldomains"].split()
+            domains = options.get("canonicaldomains", "").split()
             hostname = self.canonicalize(hostname, options, domains)
             # Overwrite HostName again here (this is also what OpenSSH does)
             options["hostname"] = hostname

tests/test_config.py

@@ -510,6 +510,20 @@ def test_hostname_tokenization(self):
         result = load_config("hostname-tokenized").lookup("whatever")
         assert result["hostname"] == "prefix.whatever"
 
+    def test_with_canonicaldomains_missing(self):
+        # this config is valid,
+        # don't assume that `CanonicalizeHostname` implies that
+        # `canonicaldomains` is specified.
+        SSHConfig.from_text(
+            """
+CanonicalizeHostname always
+
+Host foo
+    Port 22
+"""
+        ).lookup("foo")
+
+
 
 class TestSSHConfigDict:
     def test_SSHConfigDict_construct_empty(self):