Opensmtpd fails to start when IPv6 is semi-disabled

[birdie-github]

Under Linux if you disable IPv6 support this way:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

Opensmtpd will fail to start:

smtpd[]: pony express: smtpd: bind: Cannot assign requested address
smtpd[]: smtpd: process pony socket closed

This is a bug, please fix.

ifconfig lo
lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 13847  bytes 1118878 (1.0 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 13847  bytes 1118878 (1.0 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

So technically IPv6 support is there but the loopback interface doesn't have an IPv6 address assigned.

[poolpOrg]

Can you provide more details, such as the distro you're using ?

I have just tested on an Ubuntu and:

root@vultr:~/OpenSMTPD# cat /etc/issue.net 
Ubuntu 20.10
root@vultr:~/OpenSMTPD# uname -srm
Linux 5.8.0-29-generic x86_64
root@vultr:~/OpenSMTPD# ifconfig lo
lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 164  bytes 13510 (13.5 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 164  bytes 13510 (13.5 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

root@vultr:~/OpenSMTPD# grep ^listen /tmp/opensmtpd/etc/smtpd.conf 
listen on lo
root@vultr:~/OpenSMTPD# /tmp/opensmtpd/sbin/smtpd -d
info: OpenSMTPD 6.7.0-portable starting
^CExiting
root@vultr:~/OpenSMTPD# sysctl net.ipv6.conf.default.disable_ipv6=1
net.ipv6.conf.default.disable_ipv6 = 1
root@vultr:~/OpenSMTPD# sysctl net.ipv6.conf.all.disable_ipv6=1
net.ipv6.conf.all.disable_ipv6 = 1
root@vultr:~/OpenSMTPD# ifconfig lo
lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 164  bytes 13510 (13.5 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 164  bytes 13510 (13.5 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

root@vultr:~/OpenSMTPD# /tmp/opensmtpd/sbin/smtpd -d
info: OpenSMTPD 6.7.0-portable starting
^CExiting
root@vultr:~/OpenSMTPD#

At least on that Linux, it works as expected

[birdie-github]

You haven't disabled IPv6 like I've shown by using two sysctl parameters.

Compare your ifconfig lo interface and mine - mine is missing inet6 ::1.

The distro doesn't matter as we are talking about core networking which is the same between distros.

[poolpOrg]

Read the entire sequence I pasted 🙂

I setup a brand new Linux with inet6, showed the ifconfig with ::1, started the daemon in foreground successfully, THEN disabled inet6 with both sysctl, showed the ifconfig without ::1 and started the daemon in foreground again successfully.

Did I do it wrong and not see ?

As for the distro, I don't want to invest time in tracking a bug if I have partial information, let me decide if the distro matters when I can reproduce the issue on the same environment as you. I recently spent days tracking a bug in something that wasn't supposed to be different between distros but ended up being subtly different between a glibc and a musl distro.

[birdie-github]

My bad :-( My distro is Fedora 33. Loopback is not the only interface that I have, the other has a link-local IPv6 address attached to it but not lo. E.g.

enp3s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.0.100  netmask 255.255.255.0  broadcast 192.168.0.255
        inet6 fe80::redacted  prefixlen 64  scopeid 0x20<link>

Strace shows this:

1141613 setsockopt(9, SOL_SOCKET, SO_REUSEADDR, [1], 4 <unfinished ...>
1141610 pipe2( <unfinished ...>
1141612 connect(7, {sa_family=AF_UNIX, sun_path="/var/run/nscd/socket"}, 110 <unfinished ...>
1141609 recvmsg(7,  <unfinished ...>
1141613 <... setsockopt resumed>)       = 0
1141611 geteuid( <unfinished ...>
1141610 <... pipe2 resumed>[7, 8], O_NONBLOCK|O_CLOEXEC) = 0
1141613 setsockopt(9, SOL_IPV6, IPV6_V6ONLY, [1], 4 <unfinished ...>
1141612 <... connect resumed>)          = -1 ENOENT (No such file or directory)
1141611 <... geteuid resumed>)          = 983
1141609 <... recvmsg resumed>{msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="-\0\0\0\20\0\0\0\0\0\0\0mk\21\0", iov_len=65535}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 16
1141610 getuid( <unfinished ...>
1141613 <... setsockopt resumed>)       = 0
1141612 close(7 <unfinished ...>
1141611 getgid( <unfinished ...>
1141609 sendmsg(8, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="-\0\0\0\20\0\0\0\0\0\0\0ik\21\0", iov_len=16}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0 <unfinished ...>
1141613 bind(9, {sa_family=AF_INET6, sin6_port=htons(25), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "::1", &sin6_addr), sin6_scope_id=0}, 28 <unfinished ...>
1141610 <... getuid resumed>)           = 983
1141609 <... sendmsg resumed>)          = 16
1141612 <... close resumed>)            = 0
1141611 <... getgid resumed>)           = 976
1141614 <... poll resumed>)             = 1 ([{fd=3, revents=POLLIN}])
1141613 <... bind resumed>)             = -1 EADDRNOTAVAIL (Cannot assign requested address)
1141610 geteuid( <unfinished ...>
1141609 poll([{fd=8, events=POLLIN}], 1, 10000 <unfinished ...>
1141614 socket(AF_INET, SOCK_DGRAM, IPPROTO_IP <unfinished ...>
1141612 socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0 <unfinished ...>
1141611 getegid( <unfinished ...>
1141613 write(2, "pony express: smtpd: bind: Canno"..., 59 <unfinished ...>

So for some reasons the daemon sees ::1 which is not there.

[poolpOrg]

Thanks, will try to reproduce !

[vgratian]

I had the same issue as the OP, disabled IPv6 on Debian and OpenSmtpd failed to restart. Then I looked at my server network settings (VPS): I have IPv4 and IPv6, and IPv6 is tagged as "first IP".

I looked again in my SMPT config, I saw this line:
listen on localhost port 10028 tag DKIM_OUT

I changed it into:
listen on 127.0.0.1 port 10028 tag DKIM_OUT

Restarted OpenSmtpd and it worked.

Btw., I disabled IPv6 because outgoing email to Google was rejected because "Our system has detected that this message does not meet IPv6 sending guidelines regarding PTR records and authentication".