I use 6tunnel 0.12 from Debian.
The "-f" option is supposed to make 6tunnel start even if the specified endpoint is not resolvable. And I suppose it would retry resolving it on each connection from clients. However this does not work at the moment:
# strace 6tunnel -f 1234 -6 nonexistent.example.com 80
execve("/usr/bin/6tunnel", ["6tunnel", "-f", "1234", "-6", "nonexistent.example.com", "80"], [/* 14 vars */]) = 0
brk(NULL) = 0x558d64af4000
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=21639, ...}) = 0
mmap(NULL, 21639, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fa6f9918000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0000,\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1808440, ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa6f994e000
mmap(NULL, 1821408, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa6f9758000
mmap(0x7fa6f977a000, 1335296, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x22000) = 0x7fa6f977a000
mmap(0x7fa6f98c0000, 307200, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x168000) = 0x7fa6f98c0000
mmap(0x7fa6f990b000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b2000) = 0x7fa6f990b000
mmap(0x7fa6f9911000, 15072, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fa6f9911000
close(3) = 0
arch_prctl(ARCH_SET_FS, 0x7fa6f994f540) = 0
mprotect(0x7fa6f990b000, 16384, PROT_READ) = 0
mprotect(0x558d6404c000, 4096, PROT_READ) = 0
mprotect(0x7fa6f9947000, 4096, PROT_READ) = 0
munmap(0x7fa6f9918000, 21639) = 0
socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
connect(3, {sa_family=AF_UNIX, sun_path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
close(3) = 0
socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0) = 3
connect(3, {sa_family=AF_UNIX, sun_path="/var/run/nscd/socket"}, 110) = -1 ENOENT (No such file or directory)
close(3) = 0
brk(NULL) = 0x558d64af4000
brk(0x558d64b15000) = 0x558d64b15000
openat(AT_FDCWD, "/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=494, ...}) = 0
read(3, "# /etc/nsswitch.conf\n#\n# Example"..., 4096) = 494
read(3, "", 4096) = 0
close(3) = 0
stat("/etc/resolv.conf", {st_mode=S_IFREG|0644, st_size=117, ...}) = 0
openat(AT_FDCWD, "/etc/host.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=9, ...}) = 0
read(3, "multi on\n", 4096) = 9
read(3, "", 4096) = 0
close(3) = 0
openat(AT_FDCWD, "/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=117, ...}) = 0
read(3, "# preset = home\nsearch home.roma"..., 4096) = 117
read(3, "", 4096) = 0
close(3) = 0
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=21639, ...}) = 0
mmap(NULL, 21639, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fa6f9750000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\3602\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=55784, ...}) = 0
mmap(NULL, 83768, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa6f9738000
mmap(0x7fa6f973b000, 28672, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x3000) = 0x7fa6f973b000
mmap(0x7fa6f9742000, 12288, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xa000) = 0x7fa6f9742000
mmap(0x7fa6f9745000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0xc000) = 0x7fa6f9745000
mmap(0x7fa6f9747000, 22328, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fa6f9747000
close(3) = 0
mprotect(0x7fa6f9745000, 4096, PROT_READ) = 0
munmap(0x7fa6f9750000, 21639) = 0
openat(AT_FDCWD, "/etc/hosts", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=199, ...}) = 0
read(3, "127.0.1.1\tumi umi.romanrm.net\n12"..., 4096) = 199
read(3, "", 4096) = 0
close(3) = 0
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=21639, ...}) = 0
mmap(NULL, 21639, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fa6f9730000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300\21\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=26960, ...}) = 0
mmap(NULL, 28896, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa6f9728000
mmap(0x7fa6f9729000, 16384, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1000) = 0x7fa6f9729000
mmap(0x7fa6f972d000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x5000) = 0x7fa6f972d000
mmap(0x7fa6f972e000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x5000) = 0x7fa6f972e000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libresolv.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240C\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=88904, ...}) = 0
mmap(NULL, 100992, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fa6f9708000
mprotect(0x7fa6f970c000, 69632, PROT_NONE) = 0
mmap(0x7fa6f970c000, 49152, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x7fa6f970c000
mmap(0x7fa6f9718000, 16384, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x10000) = 0x7fa6f9718000
mmap(0x7fa6f971d000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x14000) = 0x7fa6f971d000
mmap(0x7fa6f971f000, 6784, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fa6f971f000
close(3) = 0
mprotect(0x7fa6f971d000, 4096, PROT_READ) = 0
mprotect(0x7fa6f972e000, 4096, PROT_READ) = 0
munmap(0x7fa6f9730000, 21639) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "@\255\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 41, MSG_NOSIGNAL, NULL, 0) = 41
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [97]) = 0
recvfrom(3, "@\255\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 97
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "G;\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 58, MSG_NOSIGNAL, NULL, 0) = 58
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [101]) = 0
recvfrom(3, "G;\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 101
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "OL\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 53, MSG_NOSIGNAL, NULL, 0) = 53
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [96]) = 0
recvfrom(3, "OL\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 96
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "\373\22\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 56, MSG_NOSIGNAL, NULL, 0) = 56
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [99]) = 0
recvfrom(3, "\373\22\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 99
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "\255\327\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 55, MSG_NOSIGNAL, NULL, 0) = 55
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [98]) = 0
recvfrom(3, "\255\327\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 98
close(3) = 0
stat("/etc/resolv.conf", {st_mode=S_IFREG|0644, st_size=117, ...}) = 0
openat(AT_FDCWD, "/etc/hosts", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=199, ...}) = 0
read(3, "127.0.1.1\tumi umi.romanrm.net\n12"..., 4096) = 199
read(3, "", 4096) = 0
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "\3l\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 41, MSG_NOSIGNAL, NULL, 0) = 41
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [97]) = 0
recvfrom(3, "\3l\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 97
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "\350}\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 58, MSG_NOSIGNAL, NULL, 0) = 58
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [101]) = 0
recvfrom(3, "\350}\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 101
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "}N\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 53, MSG_NOSIGNAL, NULL, 0) = 53
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [96]) = 0
recvfrom(3, "}N\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 96
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, " \17\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 56, MSG_NOSIGNAL, NULL, 0) = 56
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [99]) = 0
recvfrom(3, " \17\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 99
close(3) = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
sendto(3, "x\351\1\0\0\1\0\0\0\0\0\0\vnonexistent\7example"..., 55, MSG_NOSIGNAL, NULL, 0) = 55
poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
ioctl(3, FIONREAD, [98]) = 0
recvfrom(3, "x\351\201\203\0\1\0\0\0\1\0\0\vnonexistent\7example"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("127.0.0.1")}, [28->16]) = 98
close(3) = 0
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0x18} ---
+++ killed by SIGSEGV +++
Segmentation fault
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