That's an interesting protocol choice, especially given the purpose. SMTP is probably the most filtered protocol on residential networks, SMB being a runner-up.
Interesting idea though, spoofing other protocols than HTTP/HTTPS are probably a good idea for censorship evasion in countries with incredibly strict national firewalls.
Ending up on any DNSBLs shouldn't be a problem unless you have a static home IP you plan on running an actual SMTP server from after this though.
Any residential dynamic or static IP with this port opened is definitely going to get flagged. Most ISPs already prevent these ports from being open, either by policy or by residential routers.
It would probably very quickly end up on something like SpamHaus's PBL, which looks for this kind of thing.[1]
I would imagine you would also find yourself on Shodan pretty quickly getting hit with constant nmap & login attempts from malicious actors. Spam bots are always looking for insecure servers to send emails from.
I feel like ssh, SFTP, or even a secure DNS server would probably make more sense as something to hide traffic from DPI than an SMTP server.
Any time you have any externally open TCP port (home or VPS) you should expect to get scanned to shit by Shodan and millions of other bots. It doesn't matter if it's the default port for SFTP, DNS, SMTP, HTTP, Minecraft, or whatever - all of them are great targets for malicious actors and as soon as the bots detect one open port they'll scan everything on that IP harder. I once forgot to disable certain default enabled login types and failed connection/authentication logging when exposing SSH/SFTP externally and ended up with GBs of logs in just one week.
>The PBL detects end-user IP address ranges which should not be attempting to directly deliver unauthenticated SMTP email to any Internet mail server. All the email originated by an IP listed in PBL is expected to be submitted - using authentication - to a SMTP server which delivers it to destination
Means in practice port 25 (unauthenticated) and port 587 (authenticated)
That's not what the referenced website says and it does not make sense at all.
For me RTP+rateless erasure codes come to mind, but I’m feeling Rube Goldbergy today.
https://blog.torproject.org/introducing-webtunnel-evading-ce...
Tangential: but I recall reading about a similar technique used on SRTP packets to guess the phonemes being uttered without needing to decrypt the traffic.
RTP isn't a bad choice, especially the WebRTC flavor of it:
- it's UDP; there's no need to worry avoid the TCP meltdown
- it's most commonly used for peer-to-peer and PBX communication; packets going in and out, from and to random IPs are expected
- high bandwidth RTP traffic is normal, so are high irregularities
- it most often carries video; huge room for steganography
- WebRTC makes encryption mandatory
I've come across corporate networks that do block non-intranet WebRTC, however this probably isn't feasible at the Internet scale.
Other good choices are QUIC and WebSockets (assuming your network doesn't do MitM), and SSH, which by default comes with strong protection against MitM and actually has SOCKS5 tunneling built into the most popular implementations (try `ssh -D`). SSH is what some of my friends successfully use to bypass the Great Firewall.
That being said, the shift of client-to-server SMTP from a common part of everyday internet traffic to something rather esoteric may have created some potential for firewall misconfigurations, and those might result in it being passed with minimal inspection. All depends on your particular firewall in the end.