fix(dumpcap): Handle dumpcap invocation for unix transports

Currently there is a flaw in the plugin interface. Plugins cannot
specify how dumpcap should be invoked to capture traffic. The default
is ethernet traffic. There are Unix transports available which are
neither CAN traffic nor ethernet traffic. In this case the wrong
method `_eth_cmd()` is called which cannot create a valid CLI for
dumpcap.

src/gallia/dumpcap.py

@@ -18,7 +18,13 @@
 from urllib.parse import urlparse
 
 from gallia.log import get_logger
-from gallia.transports import ISOTPTransport, RawCANTransport, TargetURI
+from gallia.transports import (
+    ISOTPTransport,
+    RawCANTransport,
+    TargetURI,
+    UnixLinesTransport,
+    UnixTransport,
+)
 from gallia.utils import auto_int, split_host_port
 
 logger = get_logger("gallia.dumpcap")
@@ -48,25 +54,39 @@ async def start(
         artifacts_dir: Path,
     ) -> Dumpcap | None:
         ts = int(datetime.now().timestamp())
-        if target.scheme in [ISOTPTransport.SCHEME, RawCANTransport.SCHEME]:
-            outfile = artifacts_dir.joinpath(f"candump-{ts}.pcap.gz")
-            src_addr = (
-                auto_int(target.qs["src_addr"][0]) if "src_addr" in target.qs else None
-            )
-            dst_addr = (
-                auto_int(target.qs["dst_addr"][0]) if "dst_addr" in target.qs else None
-            )
-            cmd = cls._can_cmd(
-                target.netloc,
-                src_addr,
-                dst_addr,
-            )
-        else:
-            outfile = artifacts_dir.joinpath(f"eth-{ts}.pcap.gz")
-            cmd = await cls._eth_cmd(target.netloc)
+
+        match target.scheme:
+            case ISOTPTransport.SCHEME | RawCANTransport.SCHEME:
+                outfile = artifacts_dir.joinpath(f"candump-{ts}.pcap.gz")
+                src_addr = (
+                    auto_int(target.qs["src_addr"][0])
+                    if "src_addr" in target.qs
+                    else None
+                )
+                dst_addr = (
+                    auto_int(target.qs["dst_addr"][0])
+                    if "dst_addr" in target.qs
+                    else None
+                )
+                cmd = cls._can_cmd(
+                    target.netloc,
+                    src_addr,
+                    dst_addr,
+                )
+            # Unix domain sockets are not supported by dumpcap.
+            case UnixTransport.SCHEME | UnixLinesTransport.SCHEME:
+                return None
+            # There is currently no API for transport plugins to
+            # register a scheme and a corresponding invocation
+            # for dumpcap. So this match…case is best effort,
+            # since it defaults to ethernet.
+            case _:
+                outfile = artifacts_dir.joinpath(f"eth-{ts}.pcap.gz")
+                cmd = await cls._eth_cmd(target.netloc)
 
         if cmd is None:
             return None
+
         cmd_str = shlex.join(cmd)
         try:
             proc = await asyncio.create_subprocess_exec(