Tests if addresses are online or offline by briefly connecting to them. These short lived connections are referred to as feeler connections. Feeler connections are designed to increase the number of fresh online addresses in tried by selecting and connecting to addresses in new. One feeler connection is attempted on average once every two minutes.
This change was suggested as Countermeasure 4 in
Eclipse Attacks on Bitcoin’s Peer-to-Peer Network, Ethan Heilman,
Alison Kendler, Aviv Zohar, Sharon Goldberg. ePrint Archive Report
2015/263. March 2015.
9e9d644 net: fixup nits (Cory Fields)
8945384 net: Have LookupNumeric return a CService directly (Cory Fields)
21ba407 net: narrow include scope after moving to netaddress (Cory Fields)
21e5b96 net: move CNetAddr/CService/CSubNet out of netbase (Cory Fields)
1017b8a net: Add direct tests for new CSubNet constructors (Cory Fields)
b6c3ff3 net: Split resolving out of CSubNet (Cory Fields)
f96c7c4 net: Split resolving out of CService (Cory Fields)
31d6b1d net: Split resolving out of CNetAddr (Cory Fields)
9d4eb9a Do diskspace check before import thread is started (Pieter Wuille)
aa59f2e Add extra message to avoid a long 'Loading banlist' (Pieter Wuille)
0fd2a33 Use a signal to continue init after genesis activation (Pieter Wuille)
1a5a4e6 Randomize name lookup result in ConnectSocketByName (Pieter Wuille)
f9f5cfc Prevent duplicate connections where one is by name and another by ip (Pieter Wuille)
1111b80 Rework addnode behaviour (Pieter Wuille)
6ee7f05 Allow disconnecting a netgroup with only one member in eviction. (Gregory Maxwell)
5d0ca81 Add recently accepted blocks and txn to AttemptToEvictConnection. (Gregory Maxwell)
* Use CNode::addeName to track whether a connection to a name is already open
* A new connection to a previously-connected by-name addednode is only opened when
the previous one closes (even if the name starts resolving to something else)
* At most one connection is opened per addednode (even if the name resolves to multiple)
* Unify the code between ThreadOpenAddedNodeConnections and getaddednodeinfo
* Information about open connections is always returned, and the dns argument becomes a dummy
* An IP address and inbound/outbound is only reported for the (at most 1) open connection
eebc232 test: Add more test vectors for siphash (Wladimir J. van der Laan)
8884830 Use C++11 thread-safe static initializers (Pieter Wuille)
c31b24f Use 64-bit SipHash of netgroups in eviction (Pieter Wuille)
9bf156b Support SipHash with arbitrary byte writes (Pieter Wuille)
053930f Avoid recalculating vchKeyedNetGroup in eviction logic. (Patrick Strateman)
6182d10 Do not increment nAttempts by more than one for every Good connection. (Gregory Maxwell)
c769c4a Avoid counting failed connect attempts when probably offline. (Gregory Maxwell)
This reduces the rate of not founds by better matching the far
end expectations, it also improves privacy by removing the
ability to use getdata to probe for a node having a txn before
it has been relayed.
If a node is offline failed outbound connection attempts will crank up
the addrman counter and effectively blow away our state.
This change reduces the problem by only counting attempts made while
the node believes it has outbound connections to at least two
netgroups.
Connect and addnode connections are also not counted, as there is no
reason to unequally penalize them for their more frequent
connections -- though there should be no real effect from this
unless their addnode configureation is later removed.
Wasteful repeated connection attempts while only a few connections are
up are avoided via nLastTry.
This is still somewhat incomplete protection because our outbound
peers could be down but not timed out or might all be on 'local'
networks (although the requirement for multiple netgroups helps).
The ability to GETDATA a transaction which has not (yet) been relayed
is a privacy loss vector.
The use of the mempool for this was added as part of the mempool p2p
message and is only needed to fetch transactions returned by it.
