Intercept TLS SNI without connecting to remote server #519
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This change modifies the VPN packet handling to catch TLS SNI (Server Name Indication) without actually connecting to tracker hosts, enabling better tracking detection by faking the server response. Key changes: - Added SNI_PENDING state for TLS sessions awaiting SNI extraction - tcp.c: For port 443 connections, delay real socket opening until SNI is checked. Send fake SYN-ACK immediately to receive ClientHello. - ip.c: When SNI check fails (tracker detected), send RST to the app without ever making a connection to the tracker server. - netguard.h: Added sni_state field to tcp_session structure and SNI_PENDING/SNI_RECEIVED constants. Flow comparison: Before: SYN → connect to tracker → get SNI → block (tracker sees connection) After: SYN → fake SYN-ACK → get SNI → block (no connection to tracker) Co-authored-by: kasnder <5175206+kasnder@users.noreply.github.com>
- Handle write_syn_ack failure by cleaning up and returning early - Don't update remote_seq in SNI handler - let check_tcp_socket handle it - Remove redundant SNI_PENDING checks - Remove unused SNI_RECEIVED constant - Keep state as TCP_ESTABLISHED instead of resetting to TCP_LISTEN Co-authored-by: kasnder <5175206+kasnder@users.noreply.github.com>
Co-authored-by: kasnder <5175206+kasnder@users.noreply.github.com>
Co-authored-by: kasnder <5175206+kasnder@users.noreply.github.com>
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[WIP] Explore catching TLS SNI without connecting to remote server
Intercept TLS SNI without connecting to remote server
Jan 28, 2026
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Currently, tracker detection via TLS SNI opens a real socket to the tracker before extracting the hostname. Trackers see the connection attempt even when subsequently blocked.
Changes
netguard.h
SNI_PENDINGstate andsni_statefield totcp_sessiontcp.c
socket=-1, send fake SYN-ACK, setsni_state=SNI_PENDINGip.c
Flow
The existing SNI extraction in
ip.ccallsis_address_allowed(). For blocked connections, the new code inip.csends RST to the app. For allowed connections,tcp.copens the real socket and relays the queued ClientHello.Warning
Firewall rules blocked me from connecting to one or more addresses (expand for details)
I tried to connect to the following addresses, but was blocked by firewall rules:
dl.google.com/usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED --add-opens=java.xml/javax.xml.namespace=ALL-UNNAMED -Xmx4608m -Dfile.encoding=UTF-8 -Duser.country -Duser.language=en -Duser.variant -cp /home/REDACTED/.gradle/wrapper/dists/gradle-8.13-bin/5xuhj0ry160q40clulazy9h7d/gradle-8.13/lib/gradle-daemon-main-8.13.jar(dns block)If you need me to access, download, or install something from one of these locations, you can either:
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