SSH key forwarding not working between MinGW and Ubuntu VM

Question

I've read a great deal of similar issues, but haven't yet found the answer.

SSH forwarding between my MinGW (Windows 7) host and an Ubuntu Precise VM, provisioned with Vagrant, is not working.

Firstly, all is fine and well on the MinGW side:

$ ssh-add -L
ssh-rsa [SNIP!] /j/.ssh/id_rsa

So I know my key is currently loaded.

Next up, my Vagrantfile has the right config for forwrding the keys:

config.ssh.forward_agent = true

Finally, inside my Ubuntu Precise VM ~/.bash_profile looks like this:

SSH_ENV="$HOME/.ssh/environment"

function start_agent {
    echo "Initialising new SSH agent..."
    /usr/bin/ssh-agent | sed 's/^echo/#echo/' > "${SSH_ENV}"
    echo succeeded
    chmod 600 "${SSH_ENV}"
    . "${SSH_ENV}" > /dev/null
    /usr/bin/ssh-add;
}

# Source SSH settings, if applicable

if [ -f "${SSH_ENV}" ]; then
    . "${SSH_ENV}" > /dev/null
    #ps ${SSH_AGENT_PID} doesn't work under cywgin
    ps -ef | grep ${SSH_AGENT_PID} | grep ssh-agent$ > /dev/null || {
        start_agent;
    }
else
    start_agent;
fi

Sadly logging in with vagrant ssh -- -A doesn't yield the key being available, and nor does SSHing in directly:

$ ssh vagrant@127.0.0.1 -p 2222 -i /c/workspace/precise64/.vagrant/machines/default/virtualbox/private_key -Avv
OpenSSH_6.6.1, OpenSSL 1.0.1i 6 Aug 2014
debug1: Reading configuration data /j/.ssh/config
debug2: ssh_connect: needpriv 0
debug1: Connecting to 127.0.0.1 [127.0.0.1] port 2222.
debug1: Connection established.
debug1: identity file /c/workspace/precise64/.vagrant/machines/default/virtualbox/private_key type -1
debug1: identity file /c/workspace/precise64/.vagrant/machines/default/virtualbox/private_key-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_6.6.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_5.9p1 Debian-5ubuntu1
debug1: match: OpenSSH_5.9p1 Debian-5ubuntu1 pat OpenSSH_5* compat 0x0c000000
debug2: fd 3 setting O_NONBLOCK
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug2: kex_parse_kexinit: [e-mail],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: [e-mail],[e-mail],[e-mail],ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],ssh-ed25519,ssh-rsa,ssh-dss
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,[e-mail],[e-mail],[e-mail],aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[e-mail]
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,[e-mail],[e-mail],[e-mail],aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[e-mail]
debug2: kex_parse_kexinit: [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],hmac-md5,hmac-sha1,[e-mail],[e-mail],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[e-mail],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: [e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],[e-mail],hmac-md5,hmac-sha1,[e-mail],[e-mail],hmac-sha2-256,hmac-sha2-512,hmac-ripemd160,[e-mail],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,[e-mail],zlib
debug2: kex_parse_kexinit: none,[e-mail],zlib
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: kex_parse_kexinit: ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1
debug2: kex_parse_kexinit: ssh-rsa,ssh-dss,ecdsa-sha2-nistp256
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[e-mail]
debug2: kex_parse_kexinit: aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,aes256-cbc,arcfour,[e-mail]
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[e-mail],hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96,hmac-ripemd160,[e-mail],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: hmac-md5,hmac-sha1,[e-mail],hmac-sha2-256,hmac-sha2-256-96,hmac-sha2-512,hmac-sha2-512-96,hmac-ripemd160,[e-mail],hmac-sha1-96,hmac-md5-96
debug2: kex_parse_kexinit: none,[e-mail]
debug2: kex_parse_kexinit: none,[e-mail]
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit:
debug2: kex_parse_kexinit: first_kex_follows 0
debug2: kex_parse_kexinit: reserved 0
debug2: mac_setup: setup hmac-md5
debug1: kex: server->client aes128-ctr hmac-md5 none
debug2: mac_setup: setup hmac-md5
debug1: kex: client->server aes128-ctr hmac-md5 none
debug1: sending SSH2_MSG_KEX_ECDH_INIT
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ECDSA 11:5d:55:29:8a:77:d8:08:b4:00:9b:a3:61:93:fe:e5
debug1: Host '[127.0.0.1]:2222' is known and matches the ECDSA host key.
debug1: Found key in /j/.ssh/known_hosts:6
debug1: ssh_ecdsa_verify: signature correct
debug2: kex_derive_keys
debug2: set_newkeys: mode 1
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug2: set_newkeys: mode 0
debug1: SSH2_MSG_NEWKEYS received
debug1: Roaming not allowed by server
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug2: service_accept: ssh-userauth
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug2: key: /j/.ssh/id_rsa (0x10031d58),
debug2: key: /c/workspace/precise64/.vagrant/machines/default/virtualbox/private_key (0x0), explicit
debug1: Authentications that can continue: publickey,password
debug1: Next authentication method: publickey
debug1: Offering RSA public key: /j/.ssh/id_rsa
debug2: we sent a publickey packet, wait for reply
debug1: Authentications that can continue: publickey,password
debug1: Trying private key: /c/workspace/precise64/.vagrant/machines/default/virtualbox/private_key
debug1: key_parse_private2: missing begin marker
debug1: read PEM private key done: type RSA
debug2: we sent a publickey packet, wait for reply
debug1: Authentication succeeded (publickey).
Authenticated to 127.0.0.1 ([127.0.0.1]:2222).
debug1: channel 0: new [client-session]
debug2: channel 0: send open
debug1: Requesting [e-mail]
debug1: Entering interactive session.
debug2: callback start
debug1: Requesting authentication agent forwarding.
debug2: channel 0: request [e-mail] confirm 0
debug2: fd 3 setting TCP_NODELAY
debug2: client_session2_setup: id 0
debug2: channel 0: request pty-req confirm 1
debug2: channel 0: request shell confirm 1
debug2: callback done
debug2: channel 0: open confirm rwindow 0 rmax 32768
debug2: channel_input_status_confirm: type 99 id 0
debug2: PTY allocation request accepted on channel 0
debug2: channel 0: rcvd adjust 2097152
debug2: channel_input_status_confirm: type 99 id 0
debug2: shell request accepted on channel 0
Welcome to Ubuntu 12.04 LTS (GNU/Linux 3.2.0-23-generic x86_64)

 * Documentation:  https://help.ubuntu.com/
Welcome to your Vagrant-built virtual machine.
Last login: Wed Mar  4 13:29:51 2015 from 10.0.2.2
vagrant@precise64:~$ ssh-add -L
The agent has no identities.

Am I doing something wrong with the way ssh-agent is started? I can imagine that if it's in .bash_profile then it gets reloaded, thereby resetting the keys it has stored?