contrib: recognize CJDNS seeds as such

An IPv6 address from fc00::/8 could be either from the CJDNS network or
from a private-unroutable-reserved segment of IPv6. A seed node with
such an address must be from the CJDNS network, otherwise other peers
will not be able to connect to it.

Author: vasild

contrib/seeds/generate-seeds.py

@@ -61,7 +61,7 @@ def name_to_bip155(addr):
             raise ValueError(f'Invalid I2P {vchAddr}')
     elif '.' in addr: # IPv4
         return (BIP155Network.IPV4, bytes((int(x) for x in addr.split('.'))))
-    elif ':' in addr: # IPv6
+    elif ':' in addr: # IPv6 or CJDNS
         sub = [[], []] # prefix, suffix
         x = 0
         addr = addr.split(':')
@@ -77,7 +77,14 @@ def name_to_bip155(addr):
                 sub[x].append(val & 0xff)
         nullbytes = 16 - len(sub[0]) - len(sub[1])
         assert((x == 0 and nullbytes == 0) or (x == 1 and nullbytes > 0))
-        return (BIP155Network.IPV6, bytes(sub[0] + ([0] * nullbytes) + sub[1]))
+        addr_bytes = bytes(sub[0] + ([0] * nullbytes) + sub[1])
+        if addr_bytes[0] == 0xfc:
+            # Assume that seeds with fc00::/8 addresses belong to CJDNS,
+            # not to the publicly unroutable "Unique Local Unicast" network, see
+            # RFC4193: https://datatracker.ietf.org/doc/html/rfc4193#section-8
+            return (BIP155Network.CJDNS, addr_bytes)
+        else:
+            return (BIP155Network.IPV6, addr_bytes)
     else:
         raise ValueError('Could not parse address %s' % addr)