5 ms·

You can read the SNI without decrypting[0]. You can see it in the ClientHello when I connect to news.ycombinator.com: Internet Protocol Version 4, Src: xxx

by vngzs 6y ago

You can read the SNI without decrypting[0]. You can see it in the ClientHello when I connect to news.ycombinator.com: Internet Protocol Version 4, Src: xxx.xxx.xxx.xxx, Dst: 209.216.230.240 Transmission Control Protocol, Src Port: 64962, Dst Port: 443, Seq: 1, Ack: 1, Len: 517 Transport Layer Security TLSv1.2 Record Layer: Handshake Protocol: Client Hello Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 512 Handshake Protocol: Client Hello Handshake Type: Client Hello (1) Length: 508 Version: TLS 1.2 (0x0303) Extension: server_name (len=25) Type: server_name (0) Length: 25 Server Name Indication extension Server Name list length: 23 Server Name Type: host_name (0) Server Name length: 20 Server Name: news.ycombinator.com Extension: padding (len=172) The server name is right there, in the packets, even for TLS! Since the Great Firewall sits between you and the website, it can know the sequence number and send an RST as soon as the connection is initiated. For a less destructive use of this packet introspection, I've been using Istio to inspect ClientHello SNIs to control egress traffic from Kubernetes[1]. The SNI extension is useful, but it can leak some metadata to adversaries. [0]: https://en.wikipedia.org/wiki/Server_Name_Indication https://en.wikipedia.org/wiki/Server_Name_Indication [1]: https://istio.io/docs/tasks/traffic-management/egress/egress-gateway/ https://istio.io/docs/tasks/traffic-management/egress/egress...

bogomipz 6y ago
Did you use Tshark for outputting this TLS handshake? I ask because it's so nicely formatted and legible.
- vngzs 6y ago
  Wireshark -> [select the field you want] -> Right Click -> Copy -> All Visible Items
MaxBarraclough 6y ago
But that's SNI's fault specifically. For comparison, SSH doesn't do that (to my knowledge at least), but the attacker can still see the destination IP, and might well be able to map that back to the domain. I agree though that ideally a bad guy intercepting your traffic should not be able to tell where it is headed.

by vngzs 6y ago

bogomipz 6y ago

Did you use Tshark for outputting this TLS handshake? I ask because it's so nicely formatted and legible.

vngzs 6y ago
Wireshark -> [select the field you want] -> Right Click -> Copy -> All Visible Items

MaxBarraclough 6y ago

But that's SNI's fault specifically. For comparison, SSH doesn't do that (to my knowledge at least), but the attacker can still see the destination IP, and might well be able to map that back to the domain. I agree though that ideally a bad guy intercepting your traffic should not be able to tell where it is headed.