Hi,
I just encountered something that befuddles me:
On an AWS EC2 instance (ami-d0b520b8, FreeBSD 10.1-RELEASE), I noticed
that 'telnet www.google.com 80' first opens a UDP socket to google's
addresses on port 1, then closes it without sending any data before
opening the TCP socket.
Sample (trimmed) ktrace output:
1098 telnet RET socket 3
1098 telnet CALL connect(0x3,0xbfbfe8c8,0x1c)
1098 telnet STRU struct sockaddr { AF_INET6, [2607:f8b0:4004:807::2004]:1 }
1098 telnet RET connect -1 errno 65 No route to host
1098 telnet CALL close(0x3)
1098 telnet RET close 0
1098 telnet CALL socket(PF_INET,SOCK_CLOEXEC|SOCK_DGRAM,IPPROTO_UDP)
1098 telnet RET socket 3
1098 telnet CALL connect(0x3,0xbfbfe8c8,0x10)
1098 telnet STRU struct sockaddr { AF_INET, 172.217.12.228:1 }
1098 telnet RET connect 0
1098 telnet CALL getsockname(0x3,0x28c311dc,0xbfbfe8c4)
1098 telnet STRU struct sockaddr { AF_INET, 10.234.105.225:22661 }
1098 telnet RET getsockname 0
1098 telnet CALL close(0x3)
[...]
1098 telnet GIO fd 1 wrote 25 bytes
"Trying 172.217.12.228...
"
1098 telnet RET write 25/0x19
1098 telnet CALL socket(PF_INET,SOCK_STREAM,IPPROTO_TCP)
1098 telnet RET socket 3
[...]
1098 telnet CALL connect(0x3,0x28c0f0f0,0x10)
1098 telnet STRU struct sockaddr { AF_INET, 172.217.12.228:80 }
1098 telnet RET connect 0
I don't see this happening when the destination host in question has
either only an IPv6 record or only an IPv4 record. In those cases,
telnet will try to open the TCP socket to port 80. In the case of
dual-stack addresses, however, I see the above behaviour.
(I also see the same behavior in e.g. nc(1), so this is not a telnet(1)
specific thing.)
Anybody have any idea why this is done?
-Jan
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