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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 less
+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](https://github.com/bitcoin/bitcoin/pull/23306#issuecomment-947516736)
+
+[pull/23542](https://github.com/bitcoin/bitcoin/pull/23542)
+
+[fetch.spec.whatwg.org](https://fetch.spec.whatwg.org/#port-blocking)
+
+[chromium.googlesource.com](https://chromium.googlesource.com/chromium/src.git/+/refs/heads/main/net/base/port_util.cc)
+
+[hg.mozilla.org](https://hg.mozilla.org/mozilla-central/file/tip/netwerk/base/nsIOService.cpp)