You are asking socat
to connect to a HTTP proxy, but the ssh tunnel you set up is a SOCKS proxy. Tell socat
to connect to the SOCKS proxy:
host HOST_A
ProxyCommand socat - SOCKS4:127.0.0.1:7070:%h:%p,proxyport=7070
(Other SOCKS options may be available --- check man socat
for the particular socat you have installed.)
But usually you don't want to have to setup the ssh tunnel in advance. The usual way is to use netcat on HOST_B:
host HOST_A
ProxyCommand /usr/bin/ssh username@HOST_B /bin/nc %h %p
(Changes pathnames to ssh and netcat as necessary.)
demure's ssh -t HOST_B ssh HOST_A
method also works, but cannot be configured in ~/.ssh/config
.