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fix: send the SMTP greeting line and its CRLF in a single write #3599

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fix: send the SMTP greeting line and its CRLF in a single write #3599

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7 changes: 6 additions & 1 deletion app/lib/smtp_server/server.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -129,7 +129,12 @@ def run_event_loop
end
# We know who the client is, welcome them.
client.logger&.debug "Client identified as #{client_ip_address}"
new_io.print("220 #{Postal::Config.postal.smtp_hostname} ESMTP Postal/#{client.trace_id}")
# Write the greeting line and its terminating CRLF in a single write. Using
# #print emits the string and the "$\" output record separator ("\r\n") as two
# separate writes, which can be sent as two TCP segments. Some clients read the
# greeting text before the CRLF arrives, issue EHLO early, and then mis-handle the
# stray CRLF prepended to the EHLO reply. Sending the line atomically avoids this.
new_io.write("220 #{Postal::Config.postal.smtp_hostname} ESMTP Postal/#{client.trace_id}\r\n")
end
# Register the client and its socket with nio4r
monitor = @io_selector.register(new_io, :r)
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