Funny netcat issue with additional X characters send over UDP protocol and why this is not a problem.
The funny issue
Send something over network using UDP protocol.
$ netcat -q0 -u remote_address 2000 <<< "test message"
Receive it on the other side.
remote_address$ netcat -k -u -l 2000
test message
Everything seems fine, but use a verbose mode to see something not expected.
$ netcat -q0 -u remote_address 2000 -v <<< "test message"
Connection to remote_server (172.16.0.1) 2000 port [udp/*] succeeded!
Receive it on the other side.
remote_server$ netcat -k -u -l 2000
XXXXXtest message
Notice the additional X characters.
This is not a problem
This is not an issue with your network stack or an application itself.
The strace will clearly show that this data is sent deliberately.
$ strace netcat -q0 -u remote_server 2000 -v <<< "test message"
execve("/usr/bin/netcat", ["netcat", "-q0", "-u", "remote_server", "2000", "-v"], 0x7ffd64fa26e8 /* 31 vars */) = 0
[...]
connect(3, {sa_family=AF_INET, sin_port=htons(2000), sin_addr=inet_addr("172.16.0.1")}, 16) = 0
fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0
write(3, "X", 1) = 1
write(3, "X", 1) = 1
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7ffcb539cff0) = 0
write(3, "X", 1) = 1
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7ffcb539cff0) = 0
write(3, "X", 1) = 1
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7ffcb539cff0) = 0
write(3, "X", 1) = 1
newfstatat(AT_FDCWD, "/etc/nsswitch.conf", {st_mode=S_IFREG|0644, st_size=510, ...}, 0) = 0
openat(AT_FDCWD, "/etc/services", O_RDONLY|O_CLOEXEC) = 4
newfstatat(4, "", {st_mode=S_IFREG|0644, st_size=12813, ...}, AT_EMPTY_PATH) = 0
lseek(4, 0, SEEK_SET) = 0
read(4, "# Network services, Internet sty"..., 4096) = 4096
read(4, "\ntinc\t\t655/tcp\t\t\t\t# tinc control"..., 4096) = 4096
read(4, "cd\t6445/tcp\tsge_execd\t# Grid Eng"..., 4096) = 4096
read(4, "es\namidxtape\t10083/tcp\t\t\t# amand"..., 4096) = 525
read(4, "", 4096) = 0
close(4) = 0
write(2, "Connection to 172.16.0.1", 24Connection to 172.16.0.1) = 24
write(2, " 2000 port [udp/*] succeeded!\n", 30 2000 port [udp/*] succeeded!
) = 30
poll([{fd=0, events=POLLIN}, {fd=3, events=0}, {fd=3, events=POLLIN}, {fd=1, events=0}], 4, -1) = 1 ([{fd=0, revents=POLLIN|POLLHUP}])
read(0, "test message\n", 16384) = 13
poll([{fd=0, events=POLLIN}, {fd=3, events=POLLOUT}, {fd=3, events=POLLIN}, {fd=1, events=0}], 4, -1) = 2 ([{fd=0, revents=POLLHUP}, {fd=3, revents=POLLOUT}])
write(3, "test message\n", 13) = 13
shutdown(3, SHUT_WR) = 0
shutdown(3, SHUT_RD) = 0
close(3) = 0
exit_group(0) = ?
+++ exited with 0 +++
This is because I am using an OpenBSD version of netcat.
$ netcat -h
OpenBSD netcat (Debian patchlevel 1.217-3ubuntu1)
usage: nc [-46CDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl]
[-m minttl] [-O length] [-P proxy_username] [-p source_port]
[-q seconds] [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit]
[-w timeout] [-X proxy_protocol] [-x proxy_address[:port]]
[destination] [port]
Command Summary:
-4 Use IPv4
-6 Use IPv6
-b Allow broadcast
-C Send CRLF as line-ending
-D Enable the debug socket option
-d Detach from stdin
-F Pass socket fd
-h This help text
-I length TCP receive buffer length
-i interval Delay interval for lines sent, ports scanned
-k Keep inbound sockets open for multiple connects
-l Listen mode, for inbound connects
-M ttl Outgoing TTL / Hop Limit
-m minttl Minimum incoming TTL / Hop Limit
-N Shutdown the network socket after EOF on stdin
-n Suppress name/port resolutions
-O length TCP send buffer length
-P proxyuser Username for proxy authentication
-p port Specify local port for remote connects
-q secs quit after EOF on stdin and delay of secs
-r Randomize remote ports
-S Enable the TCP MD5 signature option
-s sourceaddr Local source address
-T keyword TOS value
-t Answer TELNET negotiation
-U Use UNIX domain socket
-u UDP mode
-V rtable Specify alternate routing table
-v Verbose
-W recvlimit Terminate after receiving a number of packets
-w timeout Timeout for connects and final net reads
-X proto Proxy protocol: "4", "5" (SOCKS) or "connect"
-x addr[:port] Specify proxy address and port
-Z DCCP mode
-z Zero-I/O mode [used for scanning]
Port numbers can be individual or ranges: lo-hi [inclusive]
Inspect source code of openbsd/netcat at GitHub to understand this behavior.
[...]
if (vflag || zflag) {
/* For UDP, make sure we are connected. */
if (uflag) {
if (udptest(s) == -1) {
ret = 1;
continue;
}
}
[...]
[...]
/*
* udptest()
* Do a few writes to see if the UDP port is there.
* Fails once PF state table is full.
*/
int
udptest(int s)
{
int i, ret;
for (i = 0; i <= 3; i++) {
if (write(s, "X", 1) == 1)
ret = 1;
else
ret = -1;
}
return ret;
}
[...]
This behavior is expected in OpenBSD version of netcat when using a verbose mode.
An alternative
If this bothers you then you can use the traditional version of netcat.
$ apt search ^netcat
Sorting... Done Full Text Search... Done netcat/impish 1.217-3ubuntu1 all TCP/IP swiss army knife -- transitional package netcat-openbsd/impish,now 1.217-3ubuntu1 amd64 [installed,automatic] TCP/IP swiss army knife netcat-traditional/impish 1.10-47 amd64 TCP/IP swiss army knife netrw/impish 1.3.2-3 amd64 netcat like tool with nice features to transport files over network
Install netcat-traditional package.
$ sudo apt install netcat-traditional
Display information about the nc link group.
$ update-alternatives --display nc
nc - auto mode link best version is /bin/nc.openbsd link currently points to /bin/nc.openbsd link nc is /bin/nc slave nc.1.gz is /usr/share/man/man1/nc.1.gz slave netcat is /bin/netcat slave netcat.1.gz is /usr/share/man/man1/netcat.1.gz /bin/nc.openbsd - priority 50 slave nc.1.gz: /usr/share/man/man1/nc_openbsd.1.gz slave netcat: /bin/nc.openbsd slave netcat.1.gz: /usr/share/man/man1/nc_openbsd.1.gz /bin/nc.traditional - priority 10 slave nc.1.gz: /usr/share/man/man1/nc.traditional.1.gz slave netcat: /bin/nc.traditional slave netcat.1.gz: /usr/share/man/man1/nc.traditional.1.gz
Set nc.traditional as alternative for nc.
$ sudo update-alternatives --set nc /bin/nc.traditional
update-alternatives: using /bin/nc.traditional to provide /bin/nc (nc) in manual mode
Display updated settings.
$ update-alternatives --display nc
nc - manual mode link best version is /bin/nc.openbsd link currently points to /bin/nc.traditional link nc is /bin/nc slave nc.1.gz is /usr/share/man/man1/nc.1.gz slave netcat is /bin/netcat slave netcat.1.gz is /usr/share/man/man1/netcat.1.gz /bin/nc.openbsd - priority 50 slave nc.1.gz: /usr/share/man/man1/nc_openbsd.1.gz slave netcat: /bin/nc.openbsd slave netcat.1.gz: /usr/share/man/man1/nc_openbsd.1.gz /bin/nc.traditional - priority 10 slave nc.1.gz: /usr/share/man/man1/nc.traditional.1.gz slave netcat: /bin/nc.traditional slave netcat.1.gz: /usr/share/man/man1/nc.traditional.1.gz
Display netcat version and help information.
$ netcat -h
[v1.10-47]
connect to somewhere: nc [-options] hostname port[s] [ports] ...
listen for inbound: nc -l -p port [-options] [hostname] [port]
options:
-c shell commands as `-e'; use /bin/sh to exec [dangerous!!]
-e filename program to exec after connect [dangerous!!]
-b allow broadcasts
-g gateway source-routing hop point[s], up to 8
-G num source-routing pointer: 4, 8, 12, ...
-h this cruft
-i secs delay interval for lines sent, ports scanned
-k set keepalive option on socket
-l listen mode, for inbound connects
-n numeric-only IP addresses, no DNS
-o file hex dump of traffic
-p port local port number
-r randomize local and remote ports
-q secs quit after EOF on stdin and delay of secs
-s addr local source address
-T tos set Type Of Service
-t answer TELNET negotiation
-u UDP mode
-v verbose [use twice to be more verbose]
-w secs timeout for connects and final net reads
-C Send CRLF as line-ending
-z zero-I/O mode [used for scanning]
port numbers can be individual or ranges: lo-hi [inclusive];
hyphens in port names must be backslash escaped (e.g. 'ftp\-data').
Send something using verbose mode.
$ netcat -vv -q0 -u remote_server 2000 -v <<< "test message"
remote_server [172.16.0.1] 2000 (?) open
Receive a message on remote server.
remote_server$ netcat -u -l -p 2000
test message
Although, this solution has its own drawbacks.
