Connection to AWS Transfer SFTP: Connection reset by peer

Question

How to connect to AWS Transfer SFTP endpoint from an EC2 instance?
Currently from EC2, the connection is interrupted with Connection reset by peer message:
sftp -i  -v user@.server.transfer.ap-southeast-2.amazonaws.com
OpenSSH_7.4p1, OpenSSL 1.0.2k-fips  26 Jan 2017
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 58: Applying options for *
debug1: Connecting to .server.transfer.ap-southeast-2.amazonaws.com [ServerPrivateIp] port 22.
debug1: Connection established.
debug1: permanently_set_uid: 0/0
debug1: key_load_public: No such file or directory
debug1: identity file sftp_id_rsa type -1
debug1: key_load_public: No such file or directory
debug1: identity file sftp_id_rsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.4
debug1: Remote protocol version 2.0, remote software version AWS_SFTP_1.1
debug1: no match: AWS_SFTP_1.1
debug1: Authenticating to .server.transfer.ap-southeast-2.amazonaws.com:22 as 'user'
debug1: SSH2_MSG_KEXINIT sent
Connection closed by ServerPrivateIp port 22
Couldn't read packet: Connection reset by peer

Answer

From the client output provided, it seems you are able to establish a TCP connection and start the protocol negotiation though something is failing at the start of key exchange.

This could happen for a number of reasons but I'd first suggest checking the encryption algos/ciphers supported by the client vs what is configured in the security policy for the server or if you have logging configured for the server but the IAM role is not able to be assumed by the service.

https://docs.aws.amazon.com/transfer/latest/userguide/security-policies.html
https://docs.aws.amazon.com/transfer/latest/userguide/monitoring.html#monitoring-enabling

If you are continuing to not be able to connect after this, I would recommend opening a case with  support as more information regarding the server and client may be needed to troubleshoot.

Connection to AWS Transfer SFTP: Connection reset by peer

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