mdns: add workaround for https://github.com/avahi/avahi/issues/379

Avahi's mDNS-reflection feature does not understand NSEC, so
it corrupts mDNS packets by not rewriting compressed labels

copyparty/__main__.py

@@ -1125,6 +1125,7 @@ def add_zc_mdns(ap):
     ap2.add_argument("--zmv", action="store_true", help="verbose mdns")
     ap2.add_argument("--zmvv", action="store_true", help="verboser mdns")
     ap2.add_argument("--zm-no-pe", action="store_true", help="mute parser errors (invalid incoming MDNS packets)")
+    ap2.add_argument("--zm-nwa-1", action="store_true", help="disable workaround for avahi-bug #379 (corruption in Avahi's mDNS reflection feature)")
     ap2.add_argument("--zms", metavar="dhf", type=u, default="", help="list of services to announce -- d=webdav h=http f=ftp s=smb -- lowercase=plaintext uppercase=TLS -- default: all enabled services except http/https (\033[32mDdfs\033[0m if \033[33m--ftp\033[0m and \033[33m--smb\033[0m is set, \033[32mDd\033[0m otherwise)")
     ap2.add_argument("--zm-ld", metavar="PATH", type=u, default="", help="link a specific folder for webdav shares")
     ap2.add_argument("--zm-lh", metavar="PATH", type=u, default="", help="link a specific folder for http shares")

copyparty/mdns.py

@@ -25,6 +25,7 @@ from .stolen.dnslib import (
     DNSHeader,
     DNSQuestion,
     DNSRecord,
+    set_avahi_379,
 )
 from .util import CachedSet, Daemon, Netdev, list_ips, min_ex
 
@@ -72,6 +73,9 @@ class MDNS(MCast):
         self.ngen = ngen
         self.ttl = 300
 
+        if not self.args.zm_nwa_1:
+            set_avahi_379()
+
         zs = self.args.name + ".local."
         zs = zs.encode("ascii", "replace").decode("ascii", "replace")
         self.hn = "-".join(x for x in zs.split("?") if x) or (

copyparty/stolen/dnslib/dns.py

@@ -8,7 +8,7 @@ from itertools import chain
 from .bimap import Bimap, BimapError
 from .bit import get_bits, set_bits
 from .buffer import BufferError
-from .label import DNSBuffer, DNSLabel
+from .label import DNSBuffer, DNSLabel, set_avahi_379
 from .ranges import IP4, IP6, H, I, check_bytes
 
 
@@ -426,7 +426,7 @@ class RR(object):
             if rdlength:
                 rdata = RDMAP.get(QTYPE.get(rtype), RD).parse(buffer, rdlength)
             else:
-                rdata = ""
+                rdata = RD(b"a")
             return cls(rname, rtype, rclass, ttl, rdata)
         except (BufferError, BimapError) as e:
             raise DNSError("Error unpacking RR [offset=%d]: %s" % (buffer.offset, e))

copyparty/stolen/dnslib/label.py

@@ -11,6 +11,25 @@ LDH = set(range(33, 127))
 ESCAPE = re.compile(r"\\([0-9][0-9][0-9])")
 
 
+avahi_379 = 0
+
+
+def set_avahi_379():
+    global avahi_379
+    avahi_379 = 1
+
+
+def log_avahi_379(args):
+    global avahi_379
+    n = avahi_379
+    if n == 2:
+        return
+    if n == 1:
+        avahi_379 = 2
+    t = "Invalid pointer in DNSLabel [offset=%d,pointer=%d,length=%d]; this is probably avahi-bug #379, packet corruption in Avahi's mDNS reflection feature. Copyparty has a workaround and is OK, but other devices need either --zm4 or --zm6"
+    raise BufferError(t % args)
+
+
 class DNSLabelError(Exception):
     pass
 
@@ -96,8 +115,9 @@ class DNSBuffer(Buffer):
                     )
                 if pointer < self.offset:
                     self.offset = pointer
+                elif avahi_379:
+                    log_avahi_379((self.offset, pointer, len(self.data)))
                 else:
-
                     raise BufferError(
                         "Invalid pointer in DNSLabel [offset=%d,pointer=%d,length=%d]"
                         % (self.offset, pointer, len(self.data))