net: open p2p connections to nodes that listen on non-default ports

By default, for mainnet, the p2p listening port is 8333. Bitcoin Core
has a strong preference for only connecting to nodes that listen on that
port.

Remove that preference because connections over clearnet that involve
port 8333 make it easy to detect, analyze, block or divert Bitcoin p2p
traffic before the connection is even established (at TCP SYN time).

For further justification see the OP of:
https://github.com/bitcoin/bitcoin/pull/23306

2 files changed, 115 insertions, 0 deletions

diff --git a/doc/README.md b/doc/README.md
index 2800937d30..c200ac3753 100644
--- a/doc/README.md
+++ b/doc/README.md
@@ -79,6 +79,7 @@ The Bitcoin repo's [root README](/README.md) contains relevant information on th
 - [Init Scripts (systemd/upstart/openrc)](init.md)
 - [Managing Wallets](managing-wallets.md)
 - [Multisig Tutorial](multisig-tutorial.md)
+- [P2P bad ports definition and list](p2p-bad-ports.md)
 - [PSBT support](psbt.md)
 - [Reduce Memory](reduce-memory.md)
 - [Reduce Traffic](reduce-traffic.md)
diff --git a/doc/p2p-bad-ports.md b/doc/p2p-bad-ports.md
new file mode 100644
index 0000000000..0dd7d36cf4
--- /dev/null
+++ b/doc/p2p-bad-ports.md
@@ -0,0 +1,114 @@
+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)