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bitcoin/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:

pull/23306

pull/23542

fetch.spec.whatwg.org

chromium.googlesource.com

hg.mozilla.org