a1db99adea
and harmonize them as follows - s/outgoing/automatic outbound/ - s/Incoming/Inbound and manual/ (are not affected by this option.) - s/only through network/only to network/ - s/this option. This option/this option. It/ - s/network types/networks/ and also pick up a few nits in doc/p2p-bad-ports.md
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When Bitcoin Core automatically opens outgoing P2P connections, it chooses a peer (address and port) from its list of potential peers. This list is populated with unchecked data gossiped over the P2P network by other peers.
A malicious actor may gossip an address:port where no Bitcoin node is listening, or one where a service is listening that is not related to the Bitcoin network. As a result, this service may occasionally get connection attempts from Bitcoin nodes.
"Bad" ports are ones used by services which are usually not open to the public and usually require authentication. A connection attempt (by Bitcoin Core, trying to connect because it thinks there is a Bitcoin node on that address:port) to such service may be considered a malicious action by an ultra-paranoid administrator. An example for such a port is 22 (ssh). On the other hand, connection attempts to public services that usually do not require authentication are unlikely to be considered a malicious action, e.g. port 80 (http).
Below is a list of "bad" ports which Bitcoin Core avoids when choosing a peer to connect to. If a node is listening on such a port, it will likely receive fewer incoming connections.
1: tcpmux
7: echo
9: discard
11: systat
13: daytime
15: netstat
17: qotd
19: chargen
20: ftp data
21: ftp access
22: ssh
23: telnet
25: smtp
37: time
42: name
43: nicname
53: domain
69: tftp
77: priv-rjs
79: finger
87: ttylink
95: supdup
101: hostname
102: iso-tsap
103: gppitnp
104: acr-nema
109: pop2
110: pop3
111: sunrpc
113: auth
115: sftp
117: uucp-path
119: nntp
123: NTP
135: loc-srv /epmap
137: netbios
139: netbios
143: imap2
161: snmp
179: BGP
389: ldap
427: SLP (Also used by Apple Filing Protocol)
465: smtp+ssl
512: print / exec
513: login
514: shell
515: printer
526: tempo
530: courier
531: chat
532: netnews
540: uucp
548: AFP (Apple Filing Protocol)
554: rtsp
556: remotefs
563: nntp+ssl
587: smtp (rfc6409)
601: syslog-conn (rfc3195)
636: ldap+ssl
989: ftps-data
990: ftps
993: ldap+ssl
995: pop3+ssl
1719: h323gatestat
1720: h323hostcall
1723: pptp
2049: nfs
3659: apple-sasl / PasswordServer
4045: lockd
5060: sip
5061: sips
6000: X11
6566: sane-port
6665: Alternate IRC
6666: Alternate IRC
6667: Standard IRC
6668: Alternate IRC
6669: Alternate IRC
6697: IRC + TLS
10080: Amanda
For further information see: