net: cap inbound connections per address

A single IP could hold every inbound slot up to MAX_CONNECTIONS -- which
is what a connection-exhaustion flood did to the rendezvous relays, one
address parking hundreds of sockets and starving real peers. Add
MAX_CONNECTIONS_PER_IP (32): the accept path counts existing connections
from the same address and refuses a new one past the cap. The relays'
firewall connlimit does the same a layer lower; this makes every build
carry the defense, including future poll()-based relays with no FD ceiling.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/net.h b/src/net.h
index 984588b..4fdd1b3 100644
--- a/src/net.h
+++ b/src/net.h
@@ -112,6 +112,15 @@ static const int          BTF_INCOMPLETE_MESSAGE_TIMEOUT_SECS = 2 * 60;
 // 125 is Bitcoin's number and remains a conservative public-node default.
 static const unsigned int MAX_CONNECTIONS = 125;
 
+// Cap inbound connections from any single address. Without it one source can
+// hold every slot up to MAX_CONNECTIONS -- exactly what happened to the
+// rendezvous relays, where a single IP parked hundreds of connections and
+// starved real peers. A legitimate peer, even a relay talking to a relay,
+// needs only a handful; 32 leaves generous room while denying any one address
+// more than a quarter of the table. The firewall connlimit on the relays does
+// the same thing a layer lower; this makes every build carry the defense.
+static const unsigned int MAX_CONNECTIONS_PER_IP = 32;
+
 // A node marked for disconnect is normally held until its buffers drain, so
 // a last message still goes out. But vSend cannot drain through a socket
 // whose far end is gone, and that made such a node immortal: it stayed in