Unable to start endlessh on port 22

[nbraud]

When trying to start endlessh on port 22, on a machine using systemd, the service fails to start due to getting EACCESS on the bind(2) call to open the socket.

As endlessh only reports fatal: Permission denied, I took a recording of the process' activity under strace, running as the systemd service:

Jan 30 22:19:10 neon.citronna.de systemd[1]: Started Endlessh SSH Tarpit.
Jan 30 22:19:10 neon.citronna.de strace[2060944]: execve("/usr/bin/endlessh", ["/usr/bin/endlessh", "-p22"], 0x7ffe5c745d68 /* 5 vars */) = 0
Jan 30 22:19:10 neon.citronna.de strace[2060944]: brk(NULL)                               = 0x55809d08c000
[loading libraries...]
Jan 30 22:19:10 neon.citronna.de strace[2060944]: openat(AT_FDCWD, "/etc/endlessh/config", O_RDONLY) = -1 ENOENT (No such file or directory)
Jan 30 22:19:10 neon.citronna.de strace[2060944]: rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[], sa_flags=SA_RESTORER|SA_INTERRUPT|SA_NODEFER|SA_RESETHAND, sa_restorer=0x7f5f98187100}, {sa_handler=SIG_IGN, sa_mask=[], sa_flags=0}, 8) = 0
Jan 30 22:19:10 neon.citronna.de strace[2060944]: rt_sigaction(SIGTERM, {sa_handler=0x55809b5eddd0, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f5f98187100}, NULL, 8) = 0
Jan 30 22:19:10 neon.citronna.de strace[2060944]: rt_sigaction(SIGHUP, {sa_handler=0x55809b5edde0, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f5f98187100}, NULL, 8) = 0
Jan 30 22:19:10 neon.citronna.de strace[2060944]: rt_sigaction(SIGUSR1, {sa_handler=0x55809b5eddf0, sa_mask=[], sa_flags=SA_RESTORER, sa_restorer=0x7f5f98187100}, NULL, 8) = 0

[we are now in server_create]
Jan 30 22:19:10 neon.citronna.de strace[2060944]: socket(AF_INET6, SOCK_STREAM, IPPROTO_IP) = 3
Jan 30 22:19:10 neon.citronna.de strace[2060944]: setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
Jan 30 22:19:10 neon.citronna.de strace[2060944]: setsockopt(3, SOL_IPV6, IPV6_V6ONLY, [0], 4) = 0
Jan 30 22:19:10 neon.citronna.de strace[2060944]: bind(3, {sa_family=AF_INET6, sin6_port=htons(22), inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=htonl(0), sin6_scope_id=0}, 28) = -1 EACCES (Permission denied)

[BOOM!]
Jan 30 22:19:10 neon.citronna.de strace[2060944]: write(2, "endlessh: fatal: Permission deni"..., 35
Jan 30 22:19:10 neon.citronna.de systemd[1]: endlessh.service: Main process exited, code=exited, status=1/FAILURE
Jan 30 22:19:10 neon.citronna.de strace[2060949]: write(2, "endlessh: fatal: Permissi
Jan 30 22:19:10 neon.citronna.de strace[2060944]: write(2, "endlessh: fatal: Permissi
Jan 30 22:19:10 neon.citronna.de strace[2060944]:  = 35
Jan 30 22:19:10 neon.citronna.de systemd[1]: endlessh.service: Failed with result 'exit-code'.
Jan 30 22:19:10 neon.citronna.de strace[2060944]: exit_group(1)
Jan 30 22:19:28 neon.citronna.de systemd[1]: Stopped Endlessh SSH Tarpit.

I was unable to find immediately what causes this, though I expect it is related to the security-related settings in endless.service.

I didn't search very hard, though, as I have a more satisfying solution via socket activation...