EC
EPROTO
Linux / POSIXERRORNotableNetworkHIGH confidence

Protocol Error

Production Risk

Indicates incompatible protocol versions or server-side protocol bugs.

What this means

EPROTO (errno 71) is returned when a protocol-level error occurs. Common in STREAMS, NFS, RPC, and some socket operations when the remote peer violates the protocol.

Why it happens
  1. 1Remote peer sent an invalid protocol message
  2. 2STREAMS protocol state machine error
  3. 3NFS/RPC protocol violation from server
  4. 4TLS/SSL handshake protocol error
How to reproduce

RPC call where the server returns a malformed response.

trigger — this will error
trigger — this will error
// RPC or socket read that receives a protocol-violating response
ssize_t n = read(sockfd, buf, sizeof(buf));
// errno = EPROTO if response violates protocol

expected output

read: Protocol error (EPROTO)

Fix

Check protocol version compatibility

WHEN When EPROTO is returned from RPC or NFS

Check protocol version compatibility
# Check NFS server version compatibility
mount -t nfs -o vers=4 server:/export /mnt
# For RPC, check rpcinfo
rpcinfo -p server

Why this works

Protocol errors often indicate version mismatches. Specify a compatible protocol version explicitly.

Sources
Official documentation ↗

Linux Programmer Manual errno(3)

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