On 2024-05-29 05:01, Rik Theys wrote:
acl allowed_clients src "/etc/squid/allowed_clients"
acl allowed_domains dstdomain "/etc/squid/allowed_domains"
http_access allow allowed_clients allowed_domains
http_access allow allowed_clients CONNECT
http_access deny all
Please note that the second http_access rule in the above configuration
allows CONNECT tunnels to prohibited domains (i.e. domains that do not
match allowed_domains). Consider restricting your "allow...CONNECT" rule
to step1. For example:
http_access allow allowed_clients step1 CONNECT
squid doesn't seem to validate that the IP address we're connecting
to is valid for the specified name in the SNI header?
That observation matches my reading of Squid Host header forgery
detection code which says "we do not yet handle CONNECT tunnels well, so
ignore for them". To validate that theory, use "debug_options ALL,3" and
look for "SECURITY ALERT: Host header forgery detected" messages in
cache.log.
Please note that in many environments forgery detection does not work
well (for cases where it is performed) due to clients and Squid seeing
different sets of IP addresses for the same host name. There are
numerous complains about that in squid-users archives.
For example, if I add "wordpress.org" to my allowed_domains list, the
following request is allowed:
curl -v https://wordpress.org --connect-to wordpress.org:443:8.8.8.8:443
8.8.8.8 is not a valid IP address for wordpress.org. This could be used
to bypass the restrictions.
Agreed.
Is there an option in squid to make it perform a forward DNS lookup for
the domain from the SNI information from step1
FYI: SNI comes from step2. step1 looks at TCP/IP client info.
to validate that the IP
address we're trying to connect to is actually valid for that host? In
the example above, a DNS lookup for wordpress.org would return
198.143.164.252 as the IP address. This is not the IP address we're
trying to connect to, so squid should block the request.
AFAICT, there is no built-in support for that in current Squid code. One
could enhance Squid or write an external ACL to perform that kind of
validation. See above for details/caveats.
Similar question for the server certificate: I've configured the
'ssl_bump peek step2 https_domains' line so squid can peek at the server
certificate.
Peeking at the server certificates happens at step3. In many modern use
cases, server certificates are encrypted, so a _peeking_ Squid cannot
see them. To validate, Squid has to bump the tunnel (supported today but
problematic for other reasons) or be enhanced to use out-of-band
validation tricks (that come with their own set of problems).
Is there a way to configure squid to validate that the
server certificate is valid for the host specified in the SNI header?
IIRC, that validation happens automatically in modern Squid versions
when Squid receives an (unencrypted) server certificate.
HTH,
Alex.
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