obfs4proxy cannot bind to <1024 port with systemd hardened service unit
I was running an obfs4proxy Debian Wheezy bridge with such configuration in torrc:
ServerTransportListenAddr obfs4 0.0.0.0:443
When I dist-upgraded to Debian Jessie, obfs4proxy could not bind to :443 any more, while tor logs had such messages:
Feb 21 22:51:09.000 [warn] Server managed proxy encountered a method error. (obfs4 listen tcp 0.0.0.0:443: bind: permission denied) Feb 21 22:51:09.000 [warn] Managed proxy at '/usr/bin/obfs4proxy' failed the configuration protocol and will be destroyed.
Mind that I have already set the appropriate capability to the obfs4proxy binary:
getcap /usr/bin/obfs4proxy /usr/bin/obfs4proxy = cap_net_bind_service+ep
I took some time moving things around and I think the problem resides on the systemd service unit: https://gitweb.torproject.org/tor.git/tree/contrib/dist/tor.service.in and specifically the option introduced in b4170421cc58d8c57254f4224ba259e817f48869 .
I assume so because flipping 'NoNewPrivileges=no' results in obfs4proxy binding to 443. Also, 'PR_SET_NO_NEW_PRIVS' section in 'man 2 prctl' implies so:
PR_SET_NO_NEW_PRIVS (since Linux 3.5) Set the calling process's no_new_privs bit to the value in arg2. With no_new_privs set to 1, execve(2) promises not to grant privileges to do anything that could not have been done without the execve(2) call (for example, rendering the set-user-ID and set-group-ID permission bits, and file capabilities non-functional). Once set, this bit cannot be unset. The setting of this bit is inherited by children created by fork(2) and clone(2), and preserved across execve(2). For more information, see the kernel source file Documenta‐ tion/prctl/no_new_privs.txt.
I understand that 'NoNewPrivileges=no' is a system security drawback but I also consider a regression to not be able to bind obfs4proxy to ports <1024. Could we find a middle ground?
If that helps, I'm running:
tor: 0.2.7.6-1~d80.jessie+1 deb.torproject.org/torproject.org/ jessie/main amd64 Packages obfs4proxy: 0.0.4-1~tpo1 deb.torproject.org/torproject.org/ obfs4proxy/main amd64 Packages
cat /etc/debian_version 8.3 cat /proc/version Linux version 3.16.0-4-amd64 (email@example.com) (gcc version 4.8.4 (Debian 4.8.4-1) ) #1 SMP Debian 3.16.7-ckt20-1+deb8u3 (2016-01-17)
Thanks for your work.