From https://sourceforge.net/p/poco/bugs/611/

From latest stable code base, 1.4.6p1 at time of writing, the file HTTPClientSession.cpp has a function proxyConnect. This function creates an HTTPClientSession with the proxy details and then proceeds to generate a proxy request using the end target details. It does this by constructing a SocketAddress with the host and port, this has the side effect of performing a DNS lookup if the host is not already an IP address only to then obtain a string representation of the SocketAddress. This results in failures on systems that cannot resolve hostnames, this may well be the reason such a system is proxying in the first place. This will occur if the client is requesting an SSL tunnel through the proxy using the CONNECT HTTP method and generally results in a CONNECT request of the form
CONNECT a.b.c.d:p HTTP/1.1
Host: actualhost.com
Proxy-Connection: Keep-Alive
When really it should produce a request of the form
CONNECT actualhost.com:p HTTP/1.1
Host: actualhost.com
Proxy-Connection: Keep-Alive
Allowing the proxy to perform the DNS lookup rather than the client which may not be able to do so.